Underkeel clearance

STi?

:

THE JOURNAL

Underkeel Clearance

Captain A. F. Dickson

(Chief Marine Superintendent, Shell International Marine)

1. INTRODUCTION. The problem of underkeel clearance is not new;

since time immemorial navigators have been concerned to know the

minimum depth of water in which they can sail with a ship of given

draught. Until quite recently, underkeel clearance requirements were

determined almost entirely empirically, and in many cases the rule of

thumb values used can be shown to be greater than the requirement of

navigational safety would dictate.

In the years since the war the pattern of the oil industry has changed

and very large ships are now commonly used to carry crude oil cargoes

to a large number of ports around the world. It is obvious that use of

these ships, with their deep draughts, has meant that a number of

expensive dredging projects have been put in hand to provide adequate

access to the ports served. Continuing escalation in size means this will probably continue.

In determining the cicpth suitable for a ship of given draught, the

required underkeel clearance must obviously be known and the costs

involved in providing for each foot of draught have made it necessary to study much more scientifically the minimum underkeel clearance which

must be provided. However, at the outset it must be emphasized that,

despite the considerable amount of work which has been done on the

subject over the last few years, we still do not know how to reduce all

the elements which affect underkeel clearance to an exact formula. 363

r-._,_,

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Techn*ke HogesdiooL

Deft

364 A. F. DICKSON VOL. 20 It has often heen'said that navigation itself is an art and that the navi-gator, having determined as accurately as he can the answer to a particular problem, must rely in the final analysis on his judgment; and so it is with

undcrkccl clearance. However, it is the author's contention that by

logical analysis of' all the factors which affect underkeel clearance, a navi-gator can now make a much more sensible assessment of the underkeel clearance which should apply to a given set of circumstances for his ship than would have been possible years ago, and this is of very considerable commercial value.

It is not intended in this paper to deal with financial considerations in

any detail, but as an indication of the magnitude of the figure involved

for a 200,000-ton tanker, the additional earning capacity per foot of

draught would be about £25,000 per annum, and there are already some 8o ships of', or around, this size on order from the world's shipyards. This

figure is cjuoted simply to show the degree of commercial importance

which attaches to a correct assessment of the maximum draught for a

given port or a given ship.

2. UNDERKEEL CLEARANCE. Most charter parties require the charterer

to exercise due diligence to ensure that a ship chartered to proceed to a

given berth has sufficient water to allow her to reach the berth and there lie safely afloat. It is, therefore, necessary for the charterer to ensure that

the depth of water both in the approaches to the berth and at the berth

provides 'safely' for the ship which he proposes to charter. It is common

thesc days to talk of underkeel clearance of

ft. to 4 ft. in the right

circumstances for even the largest tankers, and it is perhaps interesting

at this stage to look at a simple drawing showing a tanker of200,000tons

deadweight at her full draught of, say, 6i ft. 6 in. with an underkeel

clearance of 3 ft.

Fig. i illustrates that such

an underkeel clearance,

200.000dwt 6'b'dnift which may well be

practic-me , bewn

able in the right

circum-stances, does not allow any

margin for appreciable

change in the ship's draught due to motion or unknown siltation of the channel.

It is

well known that

i'ic. i. Outline of'200,000d.w.t. tanker ft.

clear-ships under way change

their underkeel clearance,

compared to the value when the ship is stopped. The change results from the combined change of the ship's trim and the water-level due to motion, these facts usually being combined under the termSquat. To examine the problems it seems logical first to consider underkeel clearance

require-ments in still-water conditions, and then to deal separately with the

effect which wave conditions may have on the ship.

ance from sea-bed

NO. 4 3. STILL-W. account has tc are known. 11 survey to dete anticipated sir water for larg

of the tide, a

stand of the

whether the

in the approa

face port auti

assessments of

in draught of

to seaward of

the port ten y

It is well kr

into the Port

very \visely si depths in the

are now bein

of water in ci

achieve this a intervals of or

sultation witi

gauges which given time ag

It is intere

used by surve

be in error b

Authority ha

inaccuracies high precisior weekly surve incoming shi1

one foot, pr

indicate any d predicted va! The forego determining

which the dc

need for impi

areas of the w

water would

One has oni

navigation of draught of 6

-NO. 4 UNDERKEEL CLEARANCE 36ç

3. STILL-WATER CONDITIONS. When examining any sea area orchannel

account has to be taken of the accuracy

with which navigational depths

are known. This is, of course, dependent on the accuracy of the original

survey to determine the depth, and the change of depth which must be

anticipated since that survey. In the majority of cases where depths of

water for large tankers are to be considered, account is taken of the stand

of the tide, and here again assessment must be made of the predicted

stand of the tide at a given time, if necessary

checking to determine

whether the actual stand is in accordance with prediction. The depths

in the approaches to the Port of London illustrate the problems which

face port authorities, pilots and shipnusters required to make accurate

assessments of the depth of water

available, particularly as the increase

in draught of tankers has made it necessary to

consider shoal areas far

to seaward of what

would have been regarded as the practical limits of

the port ten years ago.

It is well known that the depths in many, if not all, the channels leading

into the Port of London are unstable, and the Port of London Authority

very wisely set up a committee some time ago to study the problem of

depths in the approaches to the port. One result of this work is that efforts

are now being directed to

afford navigators information on the depth

of water in critical areas to within plus or minus one foot. In order to

achieve this accuracy, re-survey of the most critical areas is necessaryat intervals of only six weeks. The Port of London Authority, in close

con-sultation with the Admiralty, has also set up

distant recording tide

gauges which will provide information on

the stand of the tide at any

given time against prediction.

It is interesting to note that a close examination

of survey methods used by surveyors indicates that depths shown on navigation charts may

he in error by as much as 12 in.; the Admiralty and the Port of London

Authority have at present under examination ways in

which these

inaccuracies can he reduced, It seems reasonable to suppose that with

high precision sounding equipment and more accurate tidal data, the

six-weekly survey checks mentioned should provide the navigator of an

incoming ship with accurate knowledge as to depth within plus or minus

one foot, provided always

that the distant recording tide gauges can

indicate any difference between the rise of tide on a particular day and the predicted value for a given time.

The foregoing serves to indicate that the first essential

requirement in

determining a suitable underkeel clearance is to assess the accuracy

with

which the depth of water is known. There is no doubt

that there is a

need for improvement in survey accuracy and tidal information over large areas of the world's seas where, until the advent of present-day draughts,

water would have been considered more

than adequate for navigation.

One has only to look at a chart of the

North Sea and consider the

navigation of ships already being built to sail with a fully laden summer

draught of 6z ft. 6 in. to realize that accurate knowledge

of depth and

t I I t 1 V e

t

El e n 1, Le IC

366 _{A. F. DICKSON}

VOL. 20

undcrkcel clearance requirement will have

_{to be taken into account far}

outside the confines of ports to which a ship may bc bound.

It is fortunate that the greater part of the passages on which the

_very

large tankers now being built

_{are expected to trade, will lie in really}

dccp water, but in addition to the North Sea, already

_{mentioned, depths}

arc critical in the Malacca Straits and some areas of Japanese coastal

waters. The optimum operation of large tankers in the future is bound to give rise to a need for extensive re-survey of quite large areas of relatively shallow sea, and close re-examination of the accuracy of tidal predictions.

4. SQUAT. The expression 'squat' in this paper refers to the total

de-crease in the clearance under a ship's keel which occurs due to changes in

thc ship's trim and depression of the

_{water-level in her immediate}

vicinity due to her forward motion. The following propositions are now generally widely understood and accepted:

(i) When a large tanker is under

_{way and laden approximately to an}

even keel condition, she will change her trim by the head

_{by an}

amount dependent on her speed.

Squat infeet

NO. 4 (2) Ar of de! ch The au and pract of squat The subj.

The Hon

the authc illustratic channels

Squat uij

These I speed is a

a large pr

past the s canal SpC(

power, a:

sufficient squat grea

practice t

2.0 1.5 I.0

Ttdol rise: neczps

sprrngs

14O'165'

II

/

/

4.

/

/

Jsfeaps,,/

Springs

2 4 6 8 10 12 14

Speed through water in. knots

FIG. 2. 47,000d.w.t. tanker on ft. draught

Yantlet Channel mile east of sea reach No. _{buoy at i hours before local H.W.}

The squat which calculation shows would be experienced in the Sea Reach Channel in the Thanies

NO. 4 UNDERKEEL CLEARANCE 367

(2) Any ship under way suffers apparent sinkage due to a

depression

of the water-level in her immediate vicinity. The amount of

the

depression increases with confinement of the ship in a narrow

channel and the speed of the ship.

The author's company has been associated with model experimentation and practical tests over a number of years to determine the total amount

of squat to which a tanker will be subject in any given circumstances.

The subject has been very adequately covered in a paper presented to

The Honourable Company of Master Mariners on

i January

1967 by

the author's colleague, Captain J. D. Rendle, but it is worth here, for

illustration purposes, looking at the effects of squat in three

typical

channels (Figs. 2, 3 and 4).

9 10

4 5 6 7 8

Speed in. knots

PIG. 3. Squat calculated by Sogreah method for channel

squat in an estuary channel

11 82

These figures illustrate that for a given ship in a given circumstance,

speed is a significant factor affecting squat. However, in confined channels

a large proportion of the ship's power is used up in pushing the water

past the ship between her sides and the bank of the channel, so that her

canal speed is always substantially less than her sea speed for a given

power, and there are many practical cases where ships do not have

sufficient power to proceed at speeds which would give rise to values

of

squat great enough for the ship to ground. In fact, it is generally true in

practice that ships in confined channels are much less likely to

ground

p 1

Squat htJct

Tanker

no'B.35'D. Tanker

_A

90'B.35'JJ.

AA

4 70'13.3 iD. 8 9 10 II 12 Speed in kzwts

HG. 4. Squat calculated by Sogreah method for channel

squat values for given speeds of varying tankers in a cornplctcly enclosed channel

due to squat than are ships with small underkeel clearance

_{in relatively}

unconfined waterways.

Squat values can be calculated accurately for a given channel by a

number of methods. The close comparison of various methods_{is apparent}

in Fig.

,

where values calculated by the methods adopted by the

Rotterdam Port Authority and the author's

company arc illustrated. At low speed values, there is very close agreement, with no great disparity at high speeds.

Mention has been made previously of the tendency which large tankers

have to trim by the head when under

way trimmed to even keel at rest,

and it could be argued that a small stern trim in the stopped condition

would counteract the ship's tendency to trim by the head when underway.

However, in the author's company, the view is taken that

even keel

trim is the best practical condition for

a ship to navigate shoal areas,

because if for any reason she

uses up her underkccl clearance it is better

to ground the forward part of the ship than the after part, and also in

very soft bottom conditions ships can be navigated satisfactorily with

the ship's forefoot in the mud.

It

is

_{obviously necessary when considering underkeel clearance}

requirements to take account of the effect of small clearances on the

manceuvrability of a given ship. The two relevant characteristics for ships

4 in confin

tention t

effective keel clear ing purpc moved bc basin or a considc

ship thr

allow a f

account I There reduces

It used 1

368 A. F. DICKSON _{VOL. to} NO. 4

Squat 1n

7.

6 4 3 2

NO.4

7

Squat infect

6 4 :3 2 TJNDERKEEL CLEARANCE 5 10 Speed inknots

FIG. g. 'Open water' squat, 200,000d.w.t. ship

369

15

in confined waters are steerability and stopping. It is the author's

con-tention that large tankers steer perfectly adequately even with very small effective undcrkeel clearances, and it is axiomatic that the smaller under-keel clearance a ship has, the easier it is to reduce her speed for

manuvr-ing purposes. A consideration of interest is that where ships have to be

moved bodily through the water by tug power, e.g. swinging in a turning

basin or breasting into a berth, reduction of underkeel clearances

makes

a considerable difference to the amount of effort necessary to push the

ship through the water. It is the custom in the Shell Company to

allow a final underkeel clearance for manceuvring reasons of z ft. after

account has been taken of all other considerations.

There is some evidence to show that reduction in underkeci clearance

reduces the velocity at impact which a tanker has when berthing.

It used to be argued that larger tankers berthed

with less velocity

' , - - f

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370

A. F. DICKSON

_{VOL. 20}

_NO.i

than smallcr vessels because more care was taken by Masters and pilots

a rca

of big ships, but in the author's opinion it is

_{more logical to assume that when}

bigger ships berth at a given berth with less velocity than smaller

_ships

because they have less water under the keel and consequently

_{move to- It}

wards the berth more slowly than smaller ships.

_that

Until quite recently it was customary in the oil tanker industry

_{to bilge}

increase the unclerkeci clearance for any given navigational circumstance _Until

with increase in size of ship. However, it

seems more logical to look at _nioth

each particular case giving due weight to the effect of squat and the final

of th

underkeel clearance required for manuvrabiIity by

_{a given ship}

_{in rule}

particular circumstances, _{to da}

The nature of the bottom has to be taken into account when deter-

moth

mining a satisfactory underkeel clearance, because where ships have _to I doub

cross sea areas or navigate channels with a rock bottom, the underkeel

_been

clearance allowance must ensure against the possibility of the ship taking clean

the bottom. On the other hand, if the bottom of the channel

or shoal shalk

area to be crossed is very soft silt, then it may be sensible to navigate in _As

a way that allows the ship to sail through the soft silt on the bottom,

_{to cv}

To conclude, it is suggested that the factors to be taken into account

a par

to determine undcrkcel clearance in conditions where no movement of

effcci

the ship due to wave action can be expected, can be set down as follows: take

value

(r) The degree of accuracy with which the depth of

a given sea area

there can bc cstab]ished.

h

(2) The accuracy with which tidal height can be determined for a given

t C

until time over a given shallow area.

() The change in the ship's underkeel clearance due to squat, which

is dependent on her speed and the confinement of the

_{area or}

channel in which she is sailing.

moti (n.) TI-ic mana-uvres which the ship has to carry out.

then

() The nature of the bottom, and therefore the final degree of

under-is da

keel clearance which must be allowed after due weiiht has been

bott

given to I, 2 and

above.

It

. WAVE MOTION AND UNDERKEEL CLEARANCE. When a ship _{is any}

affected by wave action she

_{may roll, pitch and heave, and all these}

_clear

motions must affect the underkeel clearances which the ship requires for solut

navigational speed. In the section of the paper dealing vith underkeel

with

clearance considerations in still water, mention was made of account

which has to be taken of the nature of' the bottom;

it is immediately

obvious that when ships have

a vertical movement in relation to the

bottom due to wave action, grounding will result

_{in much more severe}

damage. In fixing underkcel clearances it is absolutely

_{necessary where}

the bottom is hard to take adequate insurance

_{against the possibility of a}

ship striking the bottom whilst moving in

a seaway. TI

Again, in the section dealing with still-water conditions, it was

that

suggested that required underkeel clearance could be determined with

parti

(a' (b

-

-NO. 4 UNDERKEEL CLEARANCE 371

a reasonable degrec of accuracy.

Unfortunately, this is not yet thc case

when movement of the ship due to wave action

has to be taken into

account.

It is, perhaps, worth referring again to Fig. i,

which showed clearly

that with large ships a small angle of roll or heel will cause the ship's

bilge to take bottom. This, of course, is also true of small angles of pitch.

Until recently underkcel clearance allowances to take care of ship

motion in shoal areas were made entirely

by rule of thumb, but because

of the serious risks of damage which have already been mentioned, the

rule of thumb values tended to be very much on the safe side. Fortunately,

to date there have been very few cases of ships grounding due to wave

motion when crossing shoal areas, but on

the other hand there is no

doubt that the profitability of a very large number

of voyages must have

been decreased because of over generous allowance made for underkeel clearance to take account of anticipated wave conditions when navigating shallow water.

As a first approach to the problem it

might appear sensible separately

to evaluate roll, pitch and heave, which a given ship might experience in a particular sea area with a given wave condition, and simply add these

effects to determine the incremental underkeel clearance required to

take account of wave motion. This, however, does not give the correct

value. Although large modern tankers have very high block coefficients,

there is none the less a considerable rounding of the hull at the ends of

the ship which make it unnecessary to

add the effect of roll to pitch

until a certain value of roll has been reached. Figs. 6 and illustrate the

combined effects of roll and pitch on tankers

of

o,000 and 115,000

tons deadweight respectively.

The modifying effect of shallow water on waves and swell reduces ship

motion from that experienced in

deep-sea conditions. Furthermore,

there is substantial evidence to suggest that in shallow water ship motion

is damped by the cushioning effect of water trapped below the ship's

bottom and that this effect increases with decrease in clearance.

It is immediately apparent that the ideal solution to the problem for

any given ship in a given sea area

would involve relating the underkeel

clearance required to a given wave

condition specified; however, a

solution along these lines makes it necessary to

be able to determine

with a sufficient degree of accuracy:

The motion of a ship in terms of her roll, pitch

and heave related

to sea-level, which must in turn be related to a known datum.

The analysis of wave conditions, bearing in mind that wave length,

height and direction are all important and have differing affects

depending on the course and speed of the ship.

The number of variables involved make it

immediately apparent

that an enormous number of observations would be necessary in each

particular sea area to provide adequate data for analysis from which

- -

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.

* .---.-

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04-Increase in draught infeet

11 q 8 7 6 S 4 3 2 372 .25 .1- - -.

A. F. DICKSON

VOL. 2o.

0

40

- 30Tol

15 1.75

5 .75 10 125

Pitch angle in. degrees -bow down

FIG. 6. 50,000 d.w.t. tanker: combined effect of roll and pitch on draught

,c, -:

:.

a 10 9 8 7 6 5 4 3 2 NO. 4 lncrCcLS 11 V V V V 0

V

Fl

.1...

.. ., ' LL ;. :. .._ -.. .. - -' --- .. --'.. -ITt 11 10 9 8 7 6

PitcIt angie in degrees -bow down

FIG. 7. 120,000d.w.t. tanker: combined effect of roll and pitch on draught

15

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11 ...

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2 373 UNDERKEEL CLEARANCE NO. 4 .25 .5 .75 10 1.

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374

A. F. DICKSON

_{VOL. 20}

to prc(lict accurately the ship's

_{motion in relation to a particular}

set

of wave conditions. The idea that the sea conditions prevailing

_might

be related to some index between o and io, against which underkeel

clearance could be determined, remains attractive, but in all

_practical

circumstances when efforts have been

_{made to treat the problem in}

this

way, it has become apparent that it

_{would be necessary to devote}

a

very considerable amount of tanker time to practical

_{experimentation}

before any analysis along the lines envisaged could be made.

Fortunately, it is quite possible to demonstrate with known

instru-mentation and techniques, wave conditions under which ships will have

no significant movement,

_so

_{that in practice still-water}

underkeel clearances can be used, and

_{practical tests have further indicated}

that a given underkeel clearance can be regarded as satisl'actory

_{up to a known}

sea condition specified in terms of wave height. It must, however, be

borne in mind that ship motion is not dependent simply on wave height,

but will vary with changes of period and direction. It further must be

emphasized that the foregoing

_{treatment depends on ability to monitor}

wave conditions adequately, and in

_{some areas this has proved to be}

much more difficult than at first envisaged.

Notwithstanding what has been said, it is not unreasonable to suppose

that if a large number of ships were instrumented in a way which would

indicate their effective underkeel

_{clearance when navigating particular}

sea areas, and observations from these ships

_{could be coordinated with}

adequate wave monitoring

_{equipment over a period of years, it would}

he possible to determine practically the relationship between

_sea

con-ditions and unclerkeel clearances required.

Consideration has been given

_{to suitable instrumentation for tankers}

in the author's

_{company to allow these problems to be studied,}

and a

number of ships have

_{now been equipped. Basically,}

_{two types of}

instrumentation are involved:

Echo sounders. These

_{operate on exactly the same principle}

as

naviga-tional echo sounders but four transducers are used at the extremities of

the length and beam of the ship, so that if the ship is sailing

_{over a flat}

sea-bed, the four sounders will indicate the underkeci clearance under

her flat bottom.

At the beginning of the

_{paper, it was argued that adequate}

under-keel clearance was a matter for navigational judgment and if _{Masters and}

pilots are to exercise this

_{judgment sensibly they}

_{must have adequate}

instrumentation to build up experience on what happens to the ship in

given circumstances. For this

_{reason all new large ships in the fleets}

with which the author is connected will be fitted with

_{specialized echo}

sounding equipment of this

_{type. The echo sounder as}

_{a navigational}

instrument is now of far more value in shallow water than in deep

water, and it has been obvious for some years the manufacturers are now

directing their researches to improving the shallow-water performance

of the equipment.

NO. 4 A lot) design ment ( mcas UI

with it

tion nc

this in

flat bot contint as a wi

tion, b

pitch a howev

be dev

one bc

the shi1 C been th at buoy

to quit

berths introdu attentic clearan To d. such as

for vot

of 'a ft

berth c,

sensible

tion wh

Cleararl( bad we

to short

keel dc

The i evaluati and eflo equippe Pi been cx

with the

Bonny Bonn a

which i

torically

---

..H.- 4.__ --

-NO. 4 UNDERKEEL CLEARANCE ₃₇

Motion recorder. The ship motion instrumentation used to date was

designed by the Sierra Research Corporation to determine the

move-ment of aircraft carriers' decks. It uses a stabilized vertical gyroscope for

measurement of pitch and roll angles and an accelerometer mounted

with its sensitive axis perpendicular to the deck for measuring

accelera-tion normal to the deck. Attempts have been made to feed the output of

this instrument into a small computer so that the lowest point of the

flat bottom of the ship's hull relative to the sea-bed can be determined

continuously and thus avoid laborious calculation. This instrumentation as a whole has been of very considerable value in practical experimenta-tion, but there are difficulties when measuring very small angles of roll, pitch and values of heave. The author's technical colleagues are satisfied,

however, that a suitable relatively inexpensive (Ei ,000) instrument can

be developed which will determine with an accuracy of plus or minus

one foot changes in underkecl clearance relative to a flat bottom under

the ship due to the ship's motion under the effect of waves.

OFFSHORE TANKER MOORINGS IN UNSHELTERED LOCATIONS. It has

been the practice of the oil industry for a number of years to berth tankers at buoy moorings offshore so that the tanker when berthed may be subject

to quite severe wave conditions. The underkeel clearance required in

berths of this type is obviously a matter for serious consideration, and the introduction of ships at ever larger draughts makes it necessary to devote attention as a matter of urgency to this particular aspect of the underkeel clearance problem.

To date it has been common to assume that ships in unsheltered areas

such as the Eastern Mediterranean need an underkeel clearance to cater

for worst expectable weather conditions of about JO ft. But, provision

of jo ft. of water under the keel of a ship at maximum draught off the

berth can be very expensive indeed, and it would certainly not appear

sensible to spend a very large amount of money to cater for a sea

condi-tion which might occur very seldom. For example, if a i-ft. underkeel

clearance could be shown to he sufficient for all reasonably expectable

bad weather conditions in a loading berth, it might be cheaper for ships

to short-load in extreme weather conditions rather than provide

under-keel clearance sufficient for such conditions.

The instrumentation described above would appear to be suitable for

evaluation of the underkeel clearance problem in offshore moorings,

and efforts are being made in Shell to study the problem with suitably

equipped ships in berths where appropriate.

PRACTICAL EXPERIMENTATION. To date, four particular areas have

been examined using tankers equipped with the instrumentation described with the following results:

Bonny River. Oil from the Nigerian fields is loaded into tankers at

Bonny and ships then navigate a dredged channel over the Bonny Bar,

which is affected by typical West African coast swell conditions.

His-torically, the Nigerian Port Authority have recommended varying

under-.-.,---

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'r

I

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376

A. F. DICKSON

VOL. 20

keel clearances to take care of changes in swell conditions expected at

different times of the year. In June 196ç, a 5o,000-ton ship, laden to

suitable draught in ballast, made repeated transits of the channel for five days; underkeel clearance and ship motion were measured continuously

with ccho sounders and the Sierra motion recorder. The latter showed

that the ship did not move to any appreciable extent in swell conditions

whcre the wave height was under 6 ft., the highest experienced during

the trials. As a result of these tests a 2 ft. 6 in. underkeel clearance will

be used for tankers leaving Bonny as soon as instrumentation can be

introduced to monitor the wave conditions offshore and ensure that

swell conditions are 6 ft. or less. It may be that further experience will

show that the 2 ft. 6 in. underkeel clearance can be maintained with

swell conditions even higher than 6 ft.

Thc Rip survey. The entrance to Port Phillip Bay, known as The Rip, is

notorious for erratic wave and tidal conditions. The bottom is hard rock

and consists in the shallowest area of needle-like pinnacles. The damage which a ship would suffer if she grounded on these rock pinnacles is

self-cvidcnt, and consequently it has been customary to allow very large

unclerkeel clearances.

Practical tests were made in September 1965, when a 5o,000-ton ship was navigated over 200 times across the critical area ballasted to draughts

up to 42 ft. The wave conditions were measured during all these runs

and the results of the ship's echo sounder and her motion recorder were

analyscd. It was found that in all but the most severe weather conditions

the draught of the ship for this particular area could be increased from

the present value of 38 ft. to at least 42 ft.

Another finding of interest was that the motion of the ship in The Rip in a given wave condition is substantially greater when the ship is heading

to seaward, that is into the sea, than when she is navigating the passage

inward running with the sea, due to the speed of the ship varying the

frequency of the encounter.

Durban. The draught for Durban has for many years been restricted to

38 ft. and all concerned with the operation of large ships have been

conscious of the possible effects of roll which a ship may experience

immediately to seaward of the entrance to Durban Harbour.

In September 1966, tests were made with a 50,000-ton tanker over

five days, when the experience used in Bonny and in The Rip survey was

deployed in examining over 42 transits the behaviour of a ship at 38 ft.

draught on all states of tide. The underkeel clearance determined

indi-cated that ships can navigate the entrance to Durban Harbour at 42 ft

draught in perfect safety providing the entry is made at high water and

the swell conditions are not too severe. Delays in awaiting suitable

conditions to navigate the entrance would be virtually insignificant.

Fig. 8 indicates the underkeel clearance measured, resolved to standard

of conditions (height of tide 4 ft. above datum, ship's draught 38 ft.). Europoort. Europoort is already suitable for ships of well over 500,000

FIG. 8.

Underkeel clearance trial

results for Durban, 1966

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378

A. F. DICKSON

VOL. 2o

tons deadweight and the authorities are energetically examining what

must be clone to make Europoort suitable for the large number of ships

in the 2oo,000-tOfl range which users of the port plan to trade to Euro-poort from the end of this year onwards.

The dredged channel will have to extend far out into the North Sea to

natural deep water, and it is obviously essential that the authorities and

the users of the port must be satisfied as to the underkeel clearances

which will allow maximum draught to be used for any given depth.

A team of very highly skilled people was set up by the Netherlands

authorities to study these matters, and they examined the possibility of

doing full-scale tests in an area of the North Sea where the bottom is flat, the intention being that ship movement could he determined in relation to wave conditions. These measurements could then be used to find the

cor-rect correlation to model studies to allow the Europoort entrance to be

studied on model scale. However, to date this has not proved to be

feasible but the team are studying the motion of a dredger operating

continuously in the entrance, and in addition tankers with suitable

instrumentation will be boarded on each entry into Europoort so that

their underkeci clearances related to wave conditions prevailing can be

determined with a view to assessing more accurately the underkeel

clearances which the very large tankers will require.

8. CoNcLusioNs.

(i) The underkeel clearance required for any

given tanker to navigate a sea area unaffected by wave conditions which cause ship motion can now be calculated with a degree of accuracy suit-able for all practical purposes.

(2) Where ship motion must be expected, known techniques do not

yet enable required underkeel clearance to be derived by calculation.

() The effects of small underkeel clearance in damping

ship motion

are of very considerable importance, and it is incorrect to add the

trigonometrical effects of pitch, roll and heave expectable in deep water

to determine the underkeel clearance required in a shallow area.

() Highly sophisticated echo sounding equipment is now

available

which will detennine small underkeel clearances over the whole of the

ship's flat bottom area to an accuracy of plus or minus 6 in., and in

addition, instruments are available which will indicate ship motion

relative to a fixed plane. Practical tests using the foregoing equipment in

tankers can be undertaken to determine:

Wave conditions which will not result in any significant wave

motion of the ship.

Underkeel clearances which will provide adequate safety up to a

wave condition which can be monitored. ORDINARY MEETING

Captain Dickson's paper was presented at an Ordinary Meeting of the Institute held in London on 19 April, 1967, with Captain M. E. Butler-Bowdon in the Chair.

NO. 4 Captai tious ab does thi5 or two p but the r I were to paper m plexity 0 to operat draught i Captai first sect and the is due to have put even to lying to squat wo realities The Se The resp At the ri to the se against a imposed emphasiz experien& British .1 this pape most of graphy Oi patch, w be a bette the effect from this the papci data can 1 is a prope The dcfir. range of tions. Tb tions wot would rc take year problem. more qul accurate1 feasible i5 fully in d

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,.,-NO. 4 UNDERKEEL CLEARANCE 379

DISCUSSION

Captain G. A. B. KING (B.P. Tanker Co., Ltd.): It is difficult to bc conten-tious about a paper which contains as much good sense and practical value as does this one. I am sure that discriminating professional navigators may have one or two points of detail with which they might like to argue in a friendly fashion

but the main points put forward by Captain Dickson are virtually irrefutable. If

I were to voice one criticism however, I would say that in its simple lucidity this

paper may not adequately emphasize to all who

hear it or read it the

com-plexity of solving a problem which can be so simply stated. What we, who have to operate large ships, wish to know is how to estimate the increase in a ship's draught in shallow water. This I emphasize is easily stated.

Captain Dickson seems to me to deal with the problem in two main parts. The first section on squat has been, as he points out, very thoroughly investigated

and the squat problem is now well understood. That it is so well understood

is due to the activities of a comparatively few people like Captain Dickson who have put considerable effort into both practical and model tests. Nevertheless,

even to one who claims to appreciate the squat problem it is somewhat

horri-fying to look at the top end of the profile on Fig. s and see what the value of

squat would be in a 2oo,000-tOfl ship at 14 kt. This is a salutory reminder of the realities with which we have to live.

The second part of the problem, however, is a good deal more complicated. The response to wave action cannot be quantified as easily as the results of squat. At the risk of over simplifying these two aspects, one might say that squat is due

to the self-generated dynamics of the ship which can be reasonably measured

against a predictable datum such as a flat river bed. The second is due to externally imposed dynamics which defy easy measurement. Captain Dickson rightly

emphasizes the difficulty of measuring ship movement in a seaway and we have experienced this ourselves in our own modest experiments. When we fitted our British Argosy with four echo-sounders identical to the installation described in

this paper, we had an idea which now seems rather naive that this would give us most of the answers, It very soon became apparent in practice that the topo-graphy of the sea-bed, unless someone was going to lay for us a flat concrete patch, was not going to be suitable as the primary datum. Mean sea-level might be a better primary datum and the problem would then become one of measuring the effect of roll, heave and pitch in increasing or decreasing the distance away from this arbitrary datum. The instrumentation needed to do this is described in the paper and with this equipment feeding into a small computer meaningful data can be collected and processed. What cannot be so easily laid on, however,

is a proper range of experiments covering all required conditions of wave motion.

The definitive answer would need a very large number of experiments in a wide

range of conditions and in varying water depths over different bottom configura-tions. The range of experiments could be specified, but the full range of condi-tions would have to be waited for. To carry out this work in fullscale terms would require a very considerable act of faith since the ultimate results would

take years to compile. Captain Diekson, in his description of the Europoort

problem, suggests, however, the way in which this work might be carried out more quickly if the initial range of measurements done in the full scale can be

accurately related to model scale work. That this study has not yet proved feasible is no reason for supposing that it cannot and will not be tackled success-fully in due course.

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-380 DiSCUSSION _{VOL. 20}

What I will say again for the author, is that what is needed is a long-term

investigation using a large number of ships. It might be argued that this is an in-dustry problcm and not a company one. It might also be argued that it is

pecu-liarly an oil-tanker problem at this time. But the results of fundamental research

into ship performances must inevitably be useful to operators of all kinds of ships.

Navigators like Captain Dickson and Captain Mee from my own organization, arc constantly under pressure to provide the commercial sides of their organiza-tions with information and advice enabling more efficient use to be made of all facilities both ashore and afloat.

It is of little value to a major group for the responsible navigator when asked the answer to problems as complex as the subject of this paper, to say, 'I don't know'. But, in fact, until a few years ago this would have been the most honest

answer. It is not so now because of the pioneer work done on this particular

problem, but the questions daily become more complicated and the economic pressures more over-bearing so that our navigators must become surer and surer of the principles upon which they base their answers. We all have considerable admiration for the way in which the author's company and the author himself tackles so energetically such fundamental problems. From his paper it is clear that they arc prepared to go ahead with a long-range and comprehensive pro-gramme of their own. What has been done already is most interesting. At Bonny,

for instance, at least five days' work with a fo,000-tonner was carried out in

which my own company was also involved. At Port Phillip Heads, 200 passages were made with a o,000-tonner, and in Durban, 42 transits covering five days' activity were also carried out. The cost in ships' time alone is considerable but

probably has been recovered many times over in the ports investigated. The

data, however, is particular to these ports. The data for the London River and the Thames estuary, comprehensive as it is, is also particular to the area and may have only limited application to the general problem.

One of the great imponderables with which we arc dealing in the oil business, is in handling very large crude carriers at sea terminals on single-point moorings in what amount to open-sea weather conditions. Rigorous investigation with suitable instrumentation in a number of ships related to a specific site can be carried out with advantage, but the general problem, as I have already said, is more complex and may have to be approached on an industry basis.

Following the disaster to the Torrey Canyon last month, there has been a good

deal of talk about traffic control of large ships and about setting an upper

limit on the size to which such ships can be built. There may very well be more

talk both in official and unofficial circles and it is of vital importance that

the navigational advisors to the shipping industry be as sure as they can of their ground when it comes to discussing the performance of large ships in shallow waters and congested areas. The esoteric nature of the navigational art in the past will not do now. The spotlight is upon us and the knowledge and experience derived as a result of properly controlled experimental work such as this paper

describes is the best evidence we shall have. Because of the illumination it throws

on one part of our own black magic, I welcome Captain Dickson's paper without reservation.

Vice-Admiral D. VAN HOOFF (Director-General of Pilotage and Aids to Navigation in the Netherlands): There are, I think, two different approaches to this problem, one that of port authorities and the other that of the Master and

pilot. The authority's problem is how to provide for underkeel clearance for

NO. 4 ships () of the the M water under anothe The should corsdit Con would he sak ships. pared state ti the mi able tI solely if the vailiag My admir inwar reason which nique dead them Captai partici depths which approa are re when Hence chanrn Last and fe proble at or I produt which marks, embar in crr detaib. Cap a vcss existil

NO. 4 UNDERKEEL CLEARANCE 381 ships of the size he may expect so as to keep his port open 8o, 90 or ç per cent of the time and at thc same time run it on an economical basis. The problem of the Master and pilot is that at a particular moment there will be a given depth of water and certain weather conditions and they have to decide with the draught

undcr them, whether to enter or not. Their decision may involve waiting for

another tide, or even another day, and cost the company money.

The problem of detecting movement on giant ships is an important one. I

should like to support Captain Dickson's plea that we must establish the limiting conditions in which a ship starts to have any significant movement.

Commander C. V. PARMITER (Harbour Master, Port of London Authority): I would like to comment on three points. The first from Admiral van Hooff when

he said that Port Authorities should state at what draught they would accept

ships. The Port of London Authority is resolutely against this and arc only pre-pared to announce what depths are available in the approach channels. Thus we state that depths up to 4c ft. are available on neap tides and 48 ft. at

springsi.e.

the minimum depth at or near the high water or a particular day which will en-able the ship to enter the port. The amount of underkeel clearance is a matter solely for the ship owner in the person of his Master and pilot who must decide if the ship is to accept that channel, and to determine themselves under the pre-vailing conditions if one, two, three or four feet underkeel clearance is necessary. My second point is that Captain Dickson has given you an impression inhis

admirable paper that the Thames is a narrow, shallow ditch from the North Sea inwards. This is not so, and in fact most of the estuary in between the banks is reasonably deep, but there are three or four short stretches of shallow water in which underkeel clearance is important. As I have observed, the normal

tech-nique is to proceed at about i 2 kt. in the deep water parts and then ringfor dead slow engine speed on approaching the shallows. Thus the ship rides over

them without engine power and then goes on to 'ahead' when clear. Does

Captain Dickson agree that this reduces 'squat' over these critical areas? This is

particularly relevant to the new channel through the Black Deep where the depths are in the order of 9-10 fathoms, except for the short passage iii the middle

which we are dredging to 38 ft. at low water. This is in the middle of a straight approach and should present little difficulty, but it is another problem when there are restricted depths on a turning point such as in the North Edinburgh Channel

when rudder and consequent heel are factors to be taken into consideration.

Hence the Port of London Authority's decision to provide the straight line entry channel for deep-draught ships.

Lastly, I would like to make one general comment: in all this talk of inches

and feet, of splendid expensive instruments and the scientific analysis of the many

problems involved, I would be most happy to see the day when the ship arrives

at or near the draught she has announced byradio from sea. Cannot someone produce an instrument which will provide an accurate forward and aft draught which will not necessitate a man being lowered over the side to read the draught

marks, nor for the pilot to make a circuit of the ship to do likewise before he

embarks. In our experience some ships report not only inches in error but feet in error: until an accurate draught can be ascertained in all stages of trim, the detailed parameters of underkeel clearance seem somewhat pointless.

Captain J. ANDREW (Harbour Master, Southampton): The Master and pilot of

a vessel in the approaches to a port will require to know fairly accurately the existing swell conditions and an estimation of the height of swell by visual

______ - --'.---. NO. 4 for sur Captain that dcj When i are ma1 able to meas1tr accUrat vidual s whole ings ant of then appears to be vcys; ir reducti the act' deeper the sur after th The Dicksoi me.asur The att the shi he migi the por tion in I thij within more s of whic echo S( may ha ton sur burgh particu talk ab no sou: accura( is an id Capt tam Ki West A hclpin underk I woul were n The Surge a 382 DISCUSSION VOL. 20

vation would not be accurate. If swell measuring instrumcntation is established. in the approaches of a port, the height of swell can be automatically transmitted and recorded in the port radar and information room. The information would be continuously available and passed to a vessel on request using the port v.h.f. r.t.

communications system.

Captain DICKSoN: As regards your first question, in the Bonny River, we have put a wave metre offshore which is distant recording by radio to what is, in

effect, the port information centre, so that the man in the port information

centre has a graph of the wave conditions outside the bar. We have not yet deter-mined, although we shall have to determine in the light of practical experience, what is the cut-off point for the minimum underkeel clearance and I think we

shall have to deal with this information in the same way as 1 suggest we should deal

with information related to your second point. Already port authorities are com-monly using, and your own amongst them, instrumentation that will determine when the water-level is not up to prediction so that navigators can be warned. Captain J. ANDREW: The effect of storm surges can considerably reduce the

predicted height of low water in the North Sea particularly adjacent to the

Thames Estuary. I understand that, amongst others, the Tidal institute, Liver-pool, have investigated the possibility of forecasting the cf!ect of the height of tide by storm surges.

Admiral VAN HoofF: Captain Andrew has made two points. One concerns the actual height of tide, which of course you get from the tidal gauge and trans-mit to ships. The other is the height of the waves and the actual swell, which is even more important than the waves, to the big ships, and that is what we are now trying to correlate with the ship's movement in an arbitrary way and get all

this worked out. We cannot give, at this moment, wave heights to ships; they do not know the relationships. We might be prepared to do it in due course when it can be used. So far we have not been able to make real use of it. When we tell the pilot and the Master of a ship that there are now waves of 3 ft. or 4 ft., or

ft., they will say, 'yes, but what do we do about it?'

Lt.-Cmdr. GREEN (Port of London Authority), referred to the P.L.A.'s

distant-reading estuary tide recorders which enabled pilots to be informed (by

v.h.f.) of differences from the predicted tidal heights due to meteorological

causes.

Commander D. L. GoIuoN (Hydrographic Department): A point that has not been stressed is the difficulty of obtaining accurate tidal heights offshore and

re-laying them to ti-ic ships. We know that we can obtain predicted tidal heights and

also that actual tidal heights can be obtained from distant-reading recorders but neither predictions nor distant reading is the same tiling as observations on the

spot; this is definitely a problem in the Thames Estuary and is, or will be, a

greater problem in the open North Sea. It seems to mc that one answer wouldbe

to erect permanent tide gauges offshore at vital points. Tue cost of these gauges couJd be balanced against the saving of some of the £25,000 per year at present

being lost through the necessity for having a large margin of safety at these points.

I feel that the cost of erecting these gauges be recovered from this saving within

a few years.

Lt.-Cmdr. C. G. WEEKS (Hydrographic Department): One of the principal factors in determining underkeel clearance is the charted depth and I would like

to say something on this aspect. I should perhaps declare an interest since mylast

NO. 4 UNDERKEEL CLEARANCE 383

for surveying some of the critical areas of the Thames Estuary. In his paper Captain Dickson says that a close examination of survey methods indicates

that depths shown on navigation charts may be in errorby as much as r 2 inches.

When I read that 1 felt that he probably had his tongue in his cheek: if you

are making a large number of measurements of a quantity it would be

reason-able to assume that any one measurement might be in error

by ± i unit of

measurement. Thus as a navigator might expect a single sextant altitude to be

accurate to ± i minute of arc, so a surveyor would never claim that any

indi-vidual sounding had a greater accuracy than ± i ft., when they are measured in whole feet. A lull survey of course would have some tens ofthousands of sound-ings and one would hope that if it were possible to determine the standard error

of them all, this would indeed be a matter of inches, but each sounding that

appears on a published chart is an individual sounding and so should be expected

to be ± i

ft. at the least. This is the best that can be hoped for in normal

sur-veys; in practice various safety factors are applied, particularly in respect of tidal

reductionsand I would think that the realistic attitudewould be to expect that the actual depth in any spot might be between i ft. shoaler and 2 or even ft. deeper than shown on the chart. This of course is at the moment of carrying out the survey and you may be looking at your chart months, years, or tens of years

after the survcy was completed.

The importance of this is in connection with precision surveys, which Captain Dickson has referred to and which might be defined as ones in which depths are measured in feet and quarter feet, aiming at accuracies of similar dimensions.

The attained accuracy is likely to be between - 3 in. and + 6 or 9 in. Thus

the ship operator will have lost about 2 ft. of safety margin which I suspect

he might be banking on at the moment. The people who will gain of course are the port authorities since the increased accuracy will permit a significant reduc-tion in their dredging bill.

I think it shoold be pointed out also that precision surveys areonly possible within certain limited areas, and these areas cannot be extended just by using more sophisticated equipment. There are three basic requirements, without alt of which it is a waste of time attempting a 3-in, accuracy. These are awide-scale echo sounder, a tide gauge in the area and calm conditions. Now a 6-ft. swell may have no appreciable effect on a ço,000-ton tanker but the effect on a i 50-ton survey craft is definitely perceptible. When you are working in the Edin-burgh Channels it is possible to restrict sodmding toreasonably calm conditions, particularly now electronic aids permit sounding in low visibility. When you talk about the North Sea however, this ceases to be practicable or you will get no sounding done at all. I think there is scope for research to investigate what

accuracy has in fact been achieved in theNorth Sea but I would think that ± i ft.

is an ideal to be aimed at rather than an indication of present accuracies.

Captain W. L. MANSON (Palm Line): I wouldfirst of all like to correct

Cap-tain King who said something about the Shell Tanker Company being pioneers in

West Africa. As a matter of fact many years ago Palm Line vessels and others were

helping to keep bar draughts to something reasonable by using down to i ft.

underkeel clearance in some cases. This was because of a lack of dredgers.

I would like to ask what underkeel clearance you would propose if there

were no dredgers available for and noefficient surveys of Bonny Bar?

The second point is that around the British Isles we have a considerable tidal surge and this allows any large tanker that has come in laden at high water to

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384 DISCUSSION VOL. 20 N'

negotiate the same channel outwards in ballast at something less than high water

and possibly at low water. There could be underkeel clearance of the same order, qu

in each case the overriding difference being laden versus ballast condition. Has ta

anyone paid any attention to the wind-drift effect that is experienced by mammoth m

tankers in ballast condition with particular reference to navigation through sc

relatively narrow channels at slow speed?

Captain DICKSON: Our underkeel clearances have been fixed for the Bonny Bar Ca

quite unrclatcd to depth. In other words, you start by accepting a datum depth ac

for the Bar as presently j ft., you add to that the tide for a particular loading and ru

then determine the draft by subtracting the underkeel clearance. Obviously if is

a couple of fcct is lost on the Bar the datum goes down to ç ft. and draft tables so

for particular tides become 2 ft. less. The ships we are commonly loading in bu

Bonny have a summer deadweight of 70,000 tons and [imagine you are

com-paring these with some rather smaller ships. un

I do not see the relevance of the question about what we do in ballast, and rn

we have not seen ballast condition as presenting any problem in the matter of ali

underkeci clearance because if a given ship can use a port in the loaded condition, th

then underkeel clearance ceases to be a problem when she is in ballast, It is, of 2q

course, true that the wind eFfect on a ballasted tanker is of very considerable ml

importance, and in the particular case of the Suez Canal the 200,000-ton ships Cu

that we propoc to operate from the end of the year will have a ballast draft tli

ship side area 8o per cent greater than the 8o,000-ton ships they are going to re- _(P

place. It will be important to give due regard to this fact. frc

Captain MANSON: I would like to remind Captain Dickson that if a vessel th

arrives loaded on high tide she may be required to sail in ballast at low water, Co

using the same channel but probably of less width than it had at high water. This IaC in turn means increased navigational precision to maintain position vis-à-vis md

mid-channel; in terms of manauvring or slow speed this leads to the require- PC

ment for a better understanding of wind drift elf cct. gei

Captain DICKSON: We ballast ships to a minimum draft, because ballast time Wi

is expensive and therefore it is our intent to sail ships at minimum draft from fl1

port. The facts arc that in most cases the tidal effect is less than the difference in tat

a tanker's draught in the sort of size of ship we arc talking about between laden sh(

and ballast condition. Tankers in the ballast condition, therefore, are not usually pl(

dependent on the tide. ge

J. LAMEYER (Holland-America Line): Captain Dickson explained in his paper ho

that the behaviour of a very large tanker under wave conditions with regard to mt

underkeel clearance, is virtually unknown. The emphasis in his paper is laid on

measurements made under actual conditions, but one wonders whether scale Sot

model tests would not give faster results. However, Captain Dickson indicated

that scale model tests are diflicult, first because of the lack of knowledge of the ex

actual sea-wave conditions prevailing in the various sea areas which are of inter- C1)1

est, and secondly because of the lack of knowledge about how to relate the scale model tests in sea-wave conditions to the actual life conditions. I thought there

was considerable knowledge of sea-wave conditions of some areas, for instance, Di

of some areas in the North Sea, so one could start with the study of these areas _P° where the data is known. As regards the second point, the lack of knowledge of a

how to relate scale model tests to actual behaviour of deep-draught tankers, I Ste

would like to know whether there have been any experiments made to repro- 10

0 as 'Ii h (I if n ci f kft C-;eI r. 1is ,is e in n ly er to n Ic ed he r-le re C, as of

'I

0-NO. 4 UNI)EItKEEL CLEARANCE 38S

Captain DlcssoN: As far as 1 know, the answer to the last part of your question is that there have not been such experiments. Everybody who has

tackled this problemand there have been a

number of peopic, including

myselfagrees that it seems sensible, instead of spending a lot of money in

full-scale experiments, to do all your experimentation in model and then find a

relationship between the model and full-scale. The idea has come to grief

be-cause it is very difficult to determine what happens in the model with sufTicient accuracy related to wave conditions so that, even assuming that you knew all the rules, if you translated this to full scale you would not get the right answer. That is one problem. The other is how to measurewhat happens to the full-scale ships, so that you can get the correlation. I do not think that anybody has solved this, but it certainly does remain a sensible approach to the problem.

Captain I. S. S. MACKAY (Specialist Navigator, Royal Navy): We have heard of

underkeel clearance of 3 ft. and i ft. mentioned, and I should like to mention my own experience of zero feet clearance. When I was navigatorof a 27,000-tOfl aircraft carrier, we were sent up the River Plate but not before we had stipulated the following conditions, namely, the ship was to be ballasted to 24 ft. (normal 29 ft.), the channcl to have a minimum depth of 27 ft. and, as we had bottom inlets, we were to be escorted by three tugs who were to tow us if we blacked out due to our inlets becomingchoked. For various reasons we had to dowithout the escorting tugs for the first four hours. When we were well up the channel (proceeding at i kt. and allowing i ft. for squat) a strong wind started to blow from the port beam which gave us a list of about 2° to starboard thus decreasing

the depth under the starboard inlets; also to allow for drift we had to steer a

course which pointed the ship's head at the port-hand channel markers. This, in

fact, effectively decreased the relative width of the channel. Shortly after the wind

increased, the starboard engine revs dropped and this was soon followed by the port engine losing power as the inlets became choked. We changed to diesel generators and anchored. Our escorting i7,00o-ton tanker drawing 23 ft. but with ship side inlets, passed us a few hours later using revs, for is kt. but only making good g kt. through the mud while the breakfst cutlery slid across the table with the ship's motion. Further investigation by the Argentine authorities

showed that a super tanker which made the passage a few days before had

ploughed a furrow the ridge of which had chocked our inlets. I am not

sug-gesting that our tanker friends might plough furrows irs the sand and harder

bottoms of European ports, but perhaps some

of these points may be of

interest.

H. J. SpleEn (Marine Superintendent, Mobil Shipping Company): In the

south-western United States in ports such asBeaumont, Texas and Sabine Neches Waterway, it is not unusual for vessels to sail with drafts which are slightly in excess of the least depths reported in the river. This indicates that it is not

un-common for vessels to have to push their way through a few inches of silt in some waterways.

Captain S. L. MEE (Marine Superintendent): One of the points that Captain Dickson deals with briefly and one which, in the future, will become more im-portant is the steering, stoppingand manceuvrability of very large tankers having a small underkeci clearance in confined channels. So far it has been said that steering is relatively unaffected which presumably refers to tankers of about

100,000 d.w.t. I do not know whether this has been determined from

opera-tional conditions or whether it is a result of model tests but it will be of

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386 DISCUSSION VOL. 20

siderable interest to determine the effects of small underkeel clearances on

tankers of 2oo,00o d.w.t. and larger.

A further point Captain Dickson has made is that reduction of underkeel

clearance reduces the velocity of impact of tankers when berthing. Ultimately, if this is substantiated, difficulties may be experienced with the larger tankers presently envisaged. It would again be of great interest to determine whether

reduced underkeel clearance does have this effect or whether the vast increase in underwater hull form plays a more important role iii reducing approach speeds when berthing.

Captain DICKSON: The expression of opinion in my paper is related entirely

to practical experience. We have done a lot of tests, as you know, on the manuvrability of these ships in deep-water conditions. The expression of opinion on behaviour in shallow water results from many discussions with Masters

and pilots of the biggest ships that we have got in commission now. Of course, you are quite right to bring us back to the fact that we must always be careful when talking about the manouvrability of ships that we have not even seen. All that one can do is to say that, as of now, the trend upwards in ship's size does not seem to have outweighed the skill of the naval architect in designing good

steer-ing ships. There is no evidence at present to indicate that big ships lose their

steerability in shallow-water conditions and therefore I would suggest that as there is no evidence to indicate that even bigger ships will be adversely affected. J. H. BEATTIE (Decca Radar): We should note that the river navigator has faced for many years the problem of underkeel clearance as a regular day-to-day problem at all stages of his voyage. He navigates mammoth integrated tows with an undcrkcel clearance of 6 in. to i ft. continually and a low water-level affects the viability of his whole transportation system. The deep-sea navigator may find it worthwhile examining the experience of the river navigator who has had, for more than 15 years, his own specially designed shallow-water echo sounding

gear with such innovations as high-sounding rates and portable switchable transducers to put at the forward corners of the tow, as much as 1200 ft. in

front of his wheelhouse. This enables him to gauge the slope of the bottom and navigate by soundings as he feels his way through the bars on the river Missis-sippi. On the Rhine the water-level is known very accurately and is broadcast on the ordinary radio like a weather forecast. This is not surprising when it is con-sidered that a single Rhine push tug is capable of pushing nearly one million tons of cargo per year over, say, a i 3o-km. stage and reduction in the water-level of

2+ ft. reducing the permissible maximum draught Iroin 3 m2o (io4 ft.) to 2 mo (8 ft.), will reduce the cargo capacity of the tow by more than 25 per

cent. When the instrumentation for a port was discussed earlier, the question of

taking the ship over an underkeel-clearance range was not raised. For many

years ships have been taken over ranges to establish their magnetic or acoustic signatures. Would it not be more economic to establish a range in the entrance to a port to establish the parameters for underkeel clearance for particular ships?

Captain DICKSON: A straight answer would be that it would not. We looked

at this very closely in the particular case of Rotterdam, and it seemed at first sight to be rather attractive. The idea was to put down a mattress, as it were,

over which the ship would sail, but when we got up to 2c transducers and 25 recorders, and visualized the difficulty of trying to deal with the problem of the ship's movement over all these 25 recorders, it was just impossible. Of course, there is the dredging problem, as Vice-Admiral Van Hooff points out, that you

NO. arc fl( lerns Caj the f draug find r gener ing, a] creas certai own c lating the sc

- , A2--- - -.,J -

--NO. 4 UNDERKEEL CLEARANCE 387

are nearly always doing this in an area which has some depth maintenance prob-lcrns attaching to it.

Captain BUTLER-BOWDON, at theconclusion of the discussion, commented on

the fact that, anyway until reccntly, the existence of this hazard of increased

draught had not generally becn realized in the Royal Navy and certainly did not find mention during training. He suspected this was probably an equally true generalization in the case of the Merchant Navy although he did remember

see-ing, about 20 years ago on the bridge of one of the Queens, a table showingthe

in-creased draught due to speed/rudder

combinations. Captain Dickson has

certainly macic a very great contribution towards solutions of this problem in his own company and everyone present would surely be grateful to him for stimu-lating interest in this important subject in the course of his valuable paperand the subsequent discussion.

DEPUTY SECRETARY OF THE

INSTITUTE

Applications are invited for the post of Deputy Secretary,

to assist the Executive Secretary in the administration of the

Institute's affairs. Some familiarity with navigation is desirable

and a knowledge of editorial work

would be an advantage.

Applications should be made in writing to

the Executive

Secretary.