k; q
.
-JANUARY. 1968Íarbour entrances,
:.hanneIs and
1Ihjt1g
basins
Figw-a 1.-Typical oil tanker dimensions.
Figar. 2.Typical ore carmer dimensions.
40 O
CEADwEI3WrIN ThOUSAND IONS
ISO
9e
Duncan Hay
a-.,
Jn this article, the author, who was recently appointed Regional Coastal
.Fizgineer, Pacjflc Region, Department of Public Works of Canada,
fancouver B.C., describes how harbour designs are influenced by ships'
-jizes.
This factor has become increasingly important as a result of
he accelerated trend towards the construction of bigger and faster shzps.
-i 'The des-ign of harbour entrances, channels
'and turning basins is dictated by the size
shows a plot of overall length against dead-weight tonnage for the three general classes 'Of the largest vessel anticipated to enter the of ships.
The largest vessel afloat in 1964 was the iarbour. Although meteorological and
Nissho Maru at 132.000 tons deadweight.
-oceanographical factors are important in
and the largest afloat today is the harbour design, it is of primary iniportance
ldemitsss Maru at 210,000 tons. It has been
loi the designer, the port authorities and
estimated that if a 500.000 ton vessel was .tipping companies to determine the size of
built it would require 70 ft. of water at
he largest vessel expectedoften called the
berth.' The increase in size of the bulk car-.4esign vessel. The determination of the size
riers and general cargo ships has been less of the design vessel should be realistic,
bear-dramatic. Howver. there is a general trend Jog ¡n mind the recent trends in ship
con-to increase sizes in these classes con-to reduce tnsction towards larger and faster ships.
the cost per ton of cargo handled. To facilitate the determination of the size
This paper relates the dimensions of chan-t chan-the design vessel. Figures 1.2, and 3 have
nels, entrances and turning basins to the
bcen constructed from dimensions of ships
dimensions of the design vessel. It is neces-at present in service to show the draught
sary, in many instances, to take into account
r &nd beam against deadweight tonnage of
the xnanuvrability of the ship, the local
typical oil tankers, bulk carriers and general
winds and currents. A detailed study of the eargo vessels, respectively. The draught
influence of winds and currents is beyond -shown is the midships mean summer salt
the scope of this paper. The criteria
pre--water static draught Similady, Figure 4
° 30
65 1
20 30
Flgure2 0ADWEIGHT
sented are for ideal conditions and are based upon navigational requirements
-
r
Enfrance and Channel Depths
The differentiation between entrance and channel depths is made to suggest that these
depths need not be the same. A harbour
entrance is usually exposed to larger waves than those which occur within the harbour, consequently, the scend or pitching of a ves-sel may be larger at the entrance to a chan-nel than within the chanchan-nel. The anticipated scend is a fáctor which is included in the determination of the required depth.
The Permanent International Association of Navigational Congresses recommends that the minimum design dept-h should be the static summer salt water draught of the de-sign vessel pius 5 to 8 ft. (1.5 to 2.5 m.).2 While this criterion is useful in estimating the required channel depth, a detailed calcu-lation of the required depth could be based upon a summation of the following factors: - Loaded draught
Tide - - . -
-Density change -:.
-Squat .
-Pitching and rolling - -.
Trins -
-Empirical factor
(a) Loaded DraughtThe loaded draught
is the depth of water tbe.design.vessel draws
when loaded to the load1ine or plimsoll
mark at midships while stationary in mean summer salt water. 1f the design vessel is an actual ship in use, or under construction, the loaded draught may be obtained from the ships' owners, lithe design vessel is a hypothetical ship, the loaded draught may be obtained from Figures 1. 2 or 3. It would appear that loaded draughts of 32, 38 and 40 ft. (9.75, 11.58 and 12.19 m.) for general cargo, bulk carriers and ofi tankers, respec-tively, would represent ith some leeway.
the maximum draught of the majority of vessels in service today. However, the trend towards increased draughts must be taken into account. The Nasser Plan for
BEAM.,
- - - -
-
9e I I 65 40 50 60 IN THOUSAND TONS 269 60 19750-
- 164 I-I- I.-IL 40 DRAFTe°°',
131I
4 w10 APR. 1918
lab. y. Scheepsbouwkunde
ARCHIEF
Technische Hogeschool
4CEAM._o_L.. __!
DEADWEIGHT tN 'rHOUSAND TONS Figure 3.Tyj*a! general cargo vessel dimensions.
3' 98
65 z
32 b-Ib. 1200 u-k. IOOO o z usi -s -I -i 600 400 IO 3050 -70
90 110 130 150 DEADWEIGHT IN THOUSAND TONSITO
Figure 4.Typical lengths for tankers, bulk carriers and general cargo vessels Vs deadwesht.
190 210
the Su Cabal calls for provision by 1972 for ships drawing 58 ft. (17.68 m.).5
(b) Tide.The depth of the channel may
be designed to facilitate the enxrance of the design vessel at all stages of the tide or at
only the higher stages of the tide. The datum to whio.h the design depth is referred should be carefully established as it is pos-sible to have tides below the local low water datum. Any minus tide that is anticipated
in the channel should be included in the
determination of the channel depth if the design vessel is to pass at ail stages of the
tide.
A harbour entrance or channel which
relies upon tides to produce the necessary depths is becoming less attractive to ship-owners. The larger and faster modern ves-sels rely upon a short turn around time to make their operation economical. If the
amplitude of the tide is large at the site and
the largest anticipated vessel calls
infre-quently at the port, it may be economical to utilize a portion of the tidal range to pro-duce the necessary depths. The American policy at present is that when the cost.bene-lit ratio of providing access at all stages of the tide equals unity the provision of this depth is considered justified.'
(e) Density Cbange.A vessel leaving salt water and entering brackish or fresh water will increase its draught. due to the density -. difference of the water (64.0 lb. per eu. ft. to 62.5 Ib. perCu.ft. (1.025 g. per eu. cm, to
1.000 g. per cu. Cm.) from salt to fresh
water). The additional draught in fresh water is usually assumed to be 2 to 3 per cent of the salt water draught, depending slightly upon the hull shape. . A ship
drawing 351t. (107m.) in salt salt water
would draw approximately 36 ft. (11.0 m.)
in fresh water. Shoreline harbours are normally not concerned with density change, but estuary, river and off-channel river har-bours should take this factor into account.
(d) Squat.When a ship enters shallow
water there is a rapid increase in the height of the waves produced by the ship.
Accom-panying this increase in the wave height there is an average decrease in the water surface along the profile of the ship, rela-tive to the still water level. This surface depression causes the ship to sink or squat relative to the channel bottom. Sorensen1 in his study on ship waves predicts the con-dition of shallow water when:
- vs
->0.7
gd where,
V=velocity of ship in feet per second, rela-tive to the water,
g=aceleration due to gravity. d=water depth in feet.
Other factors which affect the amount of squat are given by McAleer, Wicker and iohnston as: (a) the distance between the keel and bottom. (b) the trim of the vessel, (c) the cross-sectional area of the channel. and whether the channel is located in a wide or narrow waterway. (d) whether the vessel is passing or overtaking another vessel. (e)
the location of the vessel relative to the
centreline of the channel, and, (f) the characteristics of the vessel itself.
There are two methods available for
detemiining the squat of the design vessel as it traverses the centreline of the channel.
One method has been developed by the
David Taylor Model Basin ° and Schijf't - and the other method by the Sogreah Laboratory fr the Dutch Shell Group of
Companies.15
The basic equation used by Schijf was derived from the Bernoulli equation. The
equation is:
-I2d(l-d-s)\ 4
F=(l(ld)5) -.
V (2)vjir
where. F=Froude number.h5=the undisturbed mean depth of water,
dthe dimensionless
squat=-h5
-THE DOQ( & HARBOUR AtYfl!OR?Y
t
-h,=the depth of water in the cross-section occupied by the vessel,
s=the ratio of the midships cross-section to the channel cross-section,
Vsthe velocity of the ship relative to the water,
g=the acceleration due to gravity.
. )
A plot of equation (2) is shown in Figure 5 for various values of s. shown in Figure 5 is Schijf's limiting velocity above which any increase in power theoretically does not increase the ships' speed due to increased resistance and decreased propeller efficiency. The asymptotic lines are similar,if not related to. the plots of ship wave
heights versus speed.'.
-With values of V, h5 and s for design con-ditions, it is possible to determine a value
of squat from Figure 5. If a channel is
bejng designed, a minor reiteration is usually
involved as a channel depth has to be
assumed and made equal to h5. The value of the squat is h5-h2.
-The graphical method of thc Sogreah
Laboratory is shown in Figure 6. The results were obtained from model tests on tankers 18.000 and 33.000 deadweight tons. The i channel depth for the tests was between 1.1 and 2.8 times the ships' draught, the mean width of the channel was between 23 and 10.0 times the ships' beam and the channel side slopes were 1:3. Dickson15 has suggested that the channel width to ships' beam ratio of 10 could he used for open water channels
thus providing a factor of safety.
The methods of determining the squat
from Figure 6 is as follows:
(I) Knowing sand h5 (as dedned previously), enter Figure 6(a) and determine the limiting velocity,V5,.
Calculate the ratio V:VL and the ration of the undisturbed water depth at the vessel
to the draught of the vessel.
With these two ratios enter Figure 6(b)
and determine z.. which is the squat for
'5
JANUARY,1968
alo
0.05
Figure 5Dimensionless squat number.
Figure 5
0 0.2 0.4 0.8 0.8 1.0
Vs Froide
\
Figures 6 (a). (b) end (c).Sogreah Laboratory squat curves. NE 0.1 5 u Ui 0. +10 NIM
IO
Figure 6c0
0.2 Figure 6b WATER DEPTH DRAFT WATER DEPTH DRAFT I.' 2.8 0.4 0.6 0.8 1.0 V --VL)
I I i 4 6 8 IO WATERWAY WIDTH BEAM 2.1-I,
Isa waterway width to vessel beam ratio of 6. (4) II the waterway width to vessel beam ratio is not equal to 6. enter Figure 6(c) to find the positive or negative per cent cor-rection for as determined from Figure
6b).
McAlecr. Wicker and Johnston5 com-pared the two methods mentioned to actual squat measurements. They found and sug-gested that it was best to use the Sogreah method when s is less than 0.080 and use equation (2) when s is greater than 0.080.
Using this criteria they plotted observed
versus computed squats for ships in various channels. Their results are shown in Figure 7. It would appear that by combining the two methods, with the above criteria, a good approximation of the value of squat may be obtained for a ship traversing the centreline of a channel
The amount of the squat increases as a
G..
ship departs from the centreline of the chan-nel. Data from the David Taylor Model Basin10 appears to be the only information available for determining this additional
squat. A plot of the data from the David
Taylor Model Basin is shown in Figure 8. The date and the plot is for a canal of fixed dimensions as noted ¡n the figure. The figure shows that the additional squat due to being off the centreline of the channel is small for slow speeds but is approximately 50 per cent above the centreline vaihe for higher speeds.
The most probable reason for a ship being off the centreline in a channel is that it would be passing another vessel. The effective cross-sectional area of the channel is reduced
by the cross-sectional area of the ship being passed. Therefore, if the squat of a ship is to be determined in a channel where vessels
will be passed, the effective area of the
channel must be used in determining the value of centreline squat for the design ves-sel. Added to the centreline squat will be the additional squat due to being off centre-line. This would give the total squat for the design vessel in a channel where another ship will be passed.
(e) Pitching (Scend)
and RollingThe
pitching and rolling of a vessel when subject to wave action has to be taken into account when determining the required depth for the design vessel. This factor is important at harbour entrances where the wave action is usually severest.There is very little quantitative informa-tion on the magnitude of pitching. Quinn" suggests that half the wave height to which tbe ship is. subjected, be considered as the amplitude of pitching. If the amount of
pitching in degrees is known for the design ship under design conditions, the amplitude 271
¡4
t0.25-0.20
.0.I5
SCHIJF LIMITINGVELOCiTY .
00
01 1 odd
-01e
d
0
01 o d O 04e
d
0.2T.
I I272
of pitching can be determined knowing the ships' length.
A 5 deg. amplitude of roil is not uncom-mon at harbour entrances. Therefore a
ship having a beam of 100 ft. (3048m.)
would increase the mid-ships draught
ap-proximately 4ft. (122m.) due to a roil of
5 deg. New1and states that duc to the
pitching and rolling of ships a keel clear-ance of 10 to 12 ft. (3.05 to 3.66 in.) would be desirable for large vessels in the open water prior to reaching the lee of a break-water or protected channel.
(f) Trim.Often a vessel is not loaded to
an even keel in an attempt to improve its steering ability. Eisiminger observes that the vessel is usually set down at the stern approximately 3 in. for every 100 ft. (25 mm. per 10m.). Others report that the trim
down at the stern is usually I or 2 ft. (0.3 to 0.6 m.).
When the vessel is underway the trim cn change, though the amount of change is un-certain. The tests carried out by the Sogreah laboratories indicated that ships in channels at slow speeds trimmed down at the bow and for faster speeds trimmed down at the Stern.
(ej Empirical FactorAn empirical factor
is required in addition to those factors dis-cussed previously to facilitate manoeuvra-bility, an economic propeller efficiency and a factor of safety. The empirical factor is usually 2 to 4 ft. (0.6 to 12m.), the lesser
value being for sand bottoms and slow
speeds and the higher value for rock bottoms and fast speeds. The empirical factor
re-duces the chance of the ship's propeller
striking a sunken log or debris and also re-duces the possible displacement of material
which could be piled up in the path of a
following ship. s., a
t
o.0
Figure 7Obserred Vs computed squat.
Figure 8.Effect of ship's location in canal on squat.
I I I
2 3 4 5 6 7 8 9
Figure 8 VESSEL SPEED - KNOTS
In a channel which is subject to shoaling by sediment transport or littoral drift it may be wise to use an empirical factor of 4 ft. (1.2 in. ot greater to facilitate channel main-tenance.
The total depth required for the design vessel at a harbour entrance or in a channel is the summation of the pertinent factors (a) to (g), which are applicable to a speci case. It should be emphasized that local experience should be utilized in assigning a value to each factor.
-Thc depth required for the design vessel may require substantial dredging to provide this depth. This has led to the development
of harbours for large draught vessels in
areas where natural deep water conditions exist.
Channel Widths
The channel width is usually measured between the toe of the side slopes or at the design depth. The channel width depends upon the following factors: (a) the beam,
speed and manoeuvrabiity of the design
vessel, (b) whether the vessel is to pass
another vessel. (e) the channel depth. (d) the channel alignment and whether the channel is in a restricted or wide waterway, (e) the stability of the channel banks, and, (f) the winds, waves, currents and cross currents in the channel. There are no formulae Which explicitly include all these factors, but some criteria have been established based upon the beam of the design vessel which include
CENTER LINE
-I ¡ I
IO II 12
S
O
ThE DOCK & HARBOUR AUTHORITY
j
these faciors implicitly. P.I.A.N.C. recoin-i mends that if there is no passing of vessels the channel width should be 3 to 4 times the beam of the design vessel, if vessels pass, the channel width should be 6 to 7 times the beam of the design vessel.' They suggest these criteria would be for ideal conditions
and that cross winds and cross currentS
should be considered.
Another method of determining the re-quired channel width is based upon investi-gations made during the studies of the sea level Panama canal during which model and prototype vessels were observed in motion.I.. The opinion of pilots and navigators were included in the criteria pre-sented. This method divides the total chan-nel width into:
width of the manoeuvring lane. width of the ship clearance lane (e) width of bank clearance.
(a) Width of the manuvring lane.The
tmanuvring lane is analogous to a car lane
on a highway. Experimentally, a vessel navigating within this lane will not be ad-versely hindered by the channel banks or another vessel. The width of the manmuv-ring lane for a vessel depends upon the con-trollability of the vessel. The contrcyllabil. ity of various vessels was defined as follows: "Very Good", for naval fighting vessels and freighters of the Victory ship class.
"Good", for naval transports and tenders -T-2 tankers, new ore ships and freighters of
the Liberty ship class,
LEGEND
COMPUTATIONS DY SOSREAH METHOD - 6< 0.000 -
'
DY EOUATION 2 - SA 0.0902 3 4 5 (AFTER REPLI
Figure 7 COMPUTED SQUAT - FT.
CHANNEL IDTH LT GOTtOM 500 FEET PTH 45 FE ET
SIDE SLOPE I-I
/
- SIDE 0F SHIP FLOH EDGE 0F CANAi. RAFT-7
/
r
-Figure 9a
YSASEL HANDLES Ut ISFACTOR IL! NORMAL DEED B KNOTS flLATIbS TO 80110M FREGUINT BHOALI,G £1.080 COSES z C z E = C Dg C g.lØ I _ 50 410
Figures 9 (a) cud 9 (b) show
-
calculailOils. CURREPITS BUOYS paRAL.t.CL TO 51401E NDDER*TC TO STRONG WØLOS M AN ANG&.E OCCUR FICOIJENTLY typical width Figure 9b RCVETrED RAMES VESSEl. HANDLES SATISFACTORILYNORMAL SPEED S KNOTS
RELATIVE TO BOTTOM z z 'C g 'C W z )I.I
o-
-
-
W a 2 0 2 o 2 Ø e BEAM BEAM 1B0 ISO 760' CURRENT - 4 KNOTS PARALLEL TO SHORESTRONG WINDS RARE
(AFTER REF. R I
(3) 'Poor'. for old ore ships and damaged vessels.
- Based upon this classification the criteria shown in Table I were recommended for a
ship navigating the quarter point of the
thanneL A nsanuvring lane equal to 140
TABLE i
Manuvring lane width as percentage of vessel beam. vessel at quarter point
per cent of the vessels beam was recom-mended for a ship on the centreline of the channel, regardless of controllability.
The criteria presented for the width of the manuvring lane are for ideal conditions. They should be considered as minimum re-quireenents. Allowance must be made for the yaw of a ship if cross currents or cross winds occur in the channel. A vessel 700 ft. (213 in.) long with a beam of 90IL (27 in.)
)siwing 5 deg.. would require a channel
width of approximately 180 ft. (55 m.) just for yawing. A yawing of 5 deg. is reason-able for a vessel of this size in a semi-pro-tected waterway subject to cross winds and cross current. lt is suggested that the man-uvring Jane width be the sum of the yaw-ing width plus 60. 80. or 100 per cent of the vcssel's beam for Very Good. Good, and Poor controllability, respectively.
(b) Width of the ship clearance laneThe
width of the ship clearance lane is measured Iween manuvring lanes. The hydraulic
phenomena associated with ships passing in
a channel creates suction and repulsion
forces between the ships. The width of the ship clearance lane is established to mini-mize the hazards of these forces. The mini. mum width desired by many pilots and navi-gators is 100 ft. (30.5 rn).
(c)
Width of bank clearanceWhen a
vessel departs from the centreline of the
channel and approaches the banks, the
suc-tion and repulsion forces create yawing
moments. A rudder angle has to be applied to compensate for these forces in order to maintain a straight course. The rudder angle necessary for a vessel to maintain a straight course at a given speed, water depth
and distance front the bank is called the
equilibrium rudder angle.
The studies carried out by the Panama Canal engineers led them to conclude the bank clearance should be based upon an equilibrium angle of 5 deg.1' This criterion would permit an additional rudder deflection of 30 deg. on most ships. Based upon an equilibrium angle of 5 deg. and upon the results of the sea level Panama Canal
studies. McAleer, \Vicker and Johnston
state.' 'It appears unwise to accept a bank clearance lane width of less than 60 per cent of the beam of the vessel and unduly con-servative to provide more than 150 per cent of the beam of the design vessel without
additional evidence to support lower or
higher values".
Factors which would necessitate increas-ing the bank clearance over 60 per cent of the beam of the design vessel are: (a) poor manuvrability of the vessel. (b) speed of the vessel, if greater than 5 knots, (c) cross
currents and cross winds. «1) erodible banks. (es) wide waterways not confined by visible banks which define the approximate toe of the channel side slopes.
Typical designs of channel widths based upon the criteria resulting from the sea level Panama Canal studies are shown in Figure 9. Using a bank clearance of 150 percent of the beam ql the vessel would appear to give results which are slightly greater than the widths which would be obtained by applying the P.I.A.N.C. criteria.
The widening of channels at bends is dis-cussed in the following section.
Channel Alignment
A channel should be aligned to provide navigation without subjections to difficult manoeuvres and strong cross currents. Con' sideration should also be given to the align-ment of the channel with respect to shoaling and littoral drift. The physical factors of waves, currents and shoaling wiU not be
dis-cussed as they are covered frequently in
harbour and coastal engineering literature. The alignment of channels with respect to the requirements of navigation is discussed in this section.
The ideal channel should be free from
curves. This is rarely obtained in rivers and harbour areas where the topography or lay-out often requires a change of direction in the channel. The general conclusions of the XXth session of P.1.A.N.C. with respect to channel alignment were that the channel should: (a) be reasonably straight, (b) be free f rom S curves, and, (c) be perpendicular to the shoreline unless there is a predominant atoms direction; in that case, head into the
1eiy Good 160
Good 180
Poor 120
Figure 10
Figure lic
R
Figure 10.Radius of curvature and deflection angle in achannel bend.
(CINON-PARALLEL BANKS
Figurrs II (a), (b) and (r) sho.v methods of widening channels at bends.
Figure ita
a
(a) CUT- 0FF METHOD
-w
2
storm direction.2 The suggesnn that the
general conclusions of the congress include that the channel should not follow a com-ponent of a current was negated.
When a change of direction is necessary in a channel, many navigators prefer a series of short tangents connected by sh,rt curves. It has been suggested that for a maximum deflection angle of 30 deg. the length of the tangents should not be less than 1.000 ft.
(305 m.) and the radii of the connecting
- curves should not be less than 3.000ff, (9l4m.).'° The radius of curvature. R.. and thg deflection annie, a. are shown on the unwidened curve in Figure 10. It is evident in Figure 10 that for a given radius the length of the curve will increase with an increased deflection angle.
. The ease with which a long curve may be navigated depends upon the controllability
Radiusof curve at
channel centrelinc
TABLE 2
of the vessel. When a vessel turns under its own power, the centreline of the vessel is almost tangent to the curve which the bow follows. Usually a constant rudder angle cannot be taintained to navigate a constant radius. The varying degrees of controll-ability of ships coupled with the individual techniques of navigators has led to the pre-sentation of varying criteria for minimum radius of curvature and maximum deflection angles desirable in channels. Some existing canals have widely varying maximum deflec-tion angles, for example: Gaillard cut, Pan-ama Canal. 30 deg.; Suez Canal. 63 deg.;
Cape Cod Canal. 75 deg.; Houston Ship
Canal. 109 deg. The maximum deflection angle for the proposed sea level Panama Canal is 26 deg. with a radius of curvature of l2.SOOft. (3.810m.).'T
In 1926, F. V. de Miranda Carvalho
pee-Required widening. w, in ft (and m.) according tu equation
sented to the XiV International Congress of -Navigation the following criteria":
() When the angle of deflection is 25 deg. i or 1es. the minimum radius should be at least equal to three times the length of the r, largest vessel passing through the canal.
When the angle of deflection is between 25 and 35 deg.. the minimum radius should be equal to five times the length of the vessel. When the angle of deflection exceeds 35 deg., the minimum radius should be equal to ten times the length of the vessel.
The above criteria are for vessel speeds not exceeding 10 miles (16 km.) an hour. Making the radius of curvature dependent upon the angle of deflection would appear to be a reasonable and rational approach to establishing criteria for channel alignment. There are various opinions as to the maxi. nsum allowable radius of curvature apart from the angle of deflection. The Dock and Harbour Authority, p. 249. Dec. 1958, re ports that. "a canvass of a number of water-ways disclosed that operators were dissaiis-field with curves with radii of 4.000 ft.
(1.219m.) in which ships up to SoOft.
(152m.) were operating. The desideratum is a radius of not less than 7.000 ft. (2,134 m.)
for ships with length of 500 ft. (152m.). Radii up to lO,000ft. (3.048m.) should be considered where the transiting vessels are up to 700 ft. (213 m.) in length". The U.S. Army Engineers recommend a radius of cur-vature of not less than 5,000ft. (1.524m.)
ft. (m.) (3) (4) (5) 2,500 (762) 60 (18.3) 440 (134.1) 54 (16.5) 5.000 41.524) 35 (10.7) 216 (65.8) 28 (8.5) 7.500 (2,226) 10 (3.5) 144 (43.8) 20 (6.1) 10,000 (3,048)
- (-)
108 (32.9) 14 (4.3) 72,500 (3,810)- (-)
88 (26.8) 11 (3.4) 15.000 (4.572)- (-)
72 (21.9) 9 (2.7)274 THE DOCK & HARBOUR AUTHORITY
Figure lIb
I:
JANUARY. 1968
Figure 12e
(o) TURN AHEAD
Figure 12.Typical turning areas.
Figure 12b
r
Figure 12e
lin
(b) TURN ASTERN ô AHEAD
'
(\
..-s-'I
D01.PHIMS (cl WARPINGfor vessels over 500 ft. (152 m.) in length.' Abbott2' recommends the radius of curva-ture should be equal to approximately 8.5 times the length of the design vessel.
The minimum sight distance required by ßavigators while traversing channel bends has never been clearl' established. The pro-posed sea level Panama Canal was designed for a minimum sight distance of 1.52 miles (2.44 km.)" In many areas a sight distance of half a mile (0.8 kn'.) would be adequate. It is common practice to widen a channel in a bend to allow for the swing of the
ves-sel and to provide increased manuvring
width. The three methods commonly em-ployed for widening a channel at a bend are: (a) the cut-off method. (b) the parallel banks method, and, (c) the non-parallel banks method. These three methods are shown in Figure 11. The St. Lawrence seaway uses the cut-off method by increasing the width
at the point of intersection of the inside
tangents by lOft. (3.05 m.) for every degree of deflection." The cut-off method requires less dredging than the other two methods mentioned, but it was observed during the model studies for the sea level Panama
Canal that the cut-off method produced un-desirable current patterns. s
The amount of widening at a bend should depend upon: (a) the length of the ship. (b) the radius of curvature, (e) the deflection
angle, and (d) the velocity of the vessel.
There are no formulae which include these four factors. There are three formulae
which -elate the amount of widening to the where,
w=increased width at bend,
R=radius of curvature at ch5nnel centreline, L=length of vessel.
Equation (3) and (4) were used for the
de-sign of the Kiel Canal and the
Ghent-Terneuzan Canal. respectively. Equation (5) was propoecd by F. V. de arvalho.'°
The rationale for the derivation of these
equations is not evident. Also, these equa-tions do not include the angle of deflection. Table 2 shows the values of w for varying radii of curvature and a 733 ft. (223.5 m.) tanker as determined from formulae (3), (4), and (5).
Controllability Width ofmanasi'ring tuneas
ofthe vessel percentage of ships' beam
McAleer, Wicker and .fohnston' reduced the model study data obtained during the study of the sea level Panama Canal into criteria for the amount of widening required in channels. The criteria are shown in Table
275
3 as a function of the controllability of the vessel and the angle of deflection. The models were tested in a channel curved with a radius of 12,500 ft. (3.810m.). The model speeds were 5.0. 7.5. and 10.0 knots with head and the following currents of 3 and 5 knots. The derivation of these criteria is based upon limited data and averaged re-suits for all velocities. The operation of a self-propelled model in a curve is very sensi-tive to the techniques of the operator. This raises the questions as to how can one scale the human reflex or reaction time and how
accurate are the results of a model study
where human reflexes are involved? lt is evident that research is required into the desirable alignment of a channel with respect to allowable radii of curvature, de-flection angles, sight distances and amount of widening at bends.
Harbour Entrance Widths
The harbour entrance must be wide enough for access to shipping but at the
same time limit the entrance of wave energy. The width of the entrance is often a
com-promis between the navigational require-
-ments and the degree of protection desirable
in the harbour. The entrance width is
usually measured at the design depth. The navigational requirements are related to the size of the design-vessel, the density of traffic, the number of entrances, the depih of water. and, the height, direction and frequency of winds, waves and currents.
Entrance widths vary from 400 ft. to Deflection 26 deg. 40 deg.
Very Good 325% 385%
Good 370% 440%
Poor 415% 490%
radius of curvature and the ship's length. These are:
w=4 [R(R'-L')fl
(3)w=85R/l00
(4)276
l.000ft. (122m. to 305m.) depending. upon local conditions. Quinn" states that the
following widths have been found satisfac-tory; for small harbours, 300 ft. (91 rn); for medium harbours, 400 to 500 ft. (122 to 152m.); and for large harbours, from 500 to 800 ft. (152 to 244m.). Minikia2' suggests
that the entrance width should equal the
length of the largest ship expected to enter the harbour. Bailey" noted in 1949 that
the entrance width at H.W.S.T. for river
harbours and tidal basins was 1 fc. of width per 0.11 to 0.25 acre (1m per 0.15 to 0.33 ha.) of harbour area, the average for aine harbours being 1 ft. per 0.163 acrès (1 m. per 0.22 ha.). For harbours exposed to the open sea, Bailey noted that the entrance width varies from 1 ft. per 0.92 to 1.16 acres (1 m. per 1.22 to 1.54 ha.) and with double en-trances. 1 ft. per 0.431 to 0.545 acres (1 m. per 0.57 to 0.72 ha.). These dimensions, although arbitrary, give an indication of the width which is required for navigation.
Another method of determining the re-quired entrance width is to use the criteria for the design of channel widths which was presented in the preceding section. The yawing angle of a vessel at the entrance to a harbour channel would be larger than the yawing angle within the harbour. A yawing
angle of 10 deg. at the harbour entrance
would reasonable for a large yr-wet. Assume a design Vessel. 800 ft. (244 m.) long, yawing 10 deg. approaches an entrance at right angles. Based .upon the channel width criteria if the vessel passes another vessel of comparable size, the desirable width
would .be.775 ft. (236m.).: Allowance must also be made for the effective width of the ship if it does not approach the entrance at right angles.
The effect the harbour entrañce width has upon wave and current conditions within the harbour is best studied by a model. The maximum allowable velocity for navigation in confined waters is usually considered to be 2 to 3+ knots.'.
Turning Basins
The size of the turning basin is dependent upon the available water area and depth, the length of the design vessel and the method of turning employed by the vessel. The three methods of turning a vessel are: (a) turn ahead, (b) turn astern and ahead, and (e) warping. The three methods of turning may beaccomplished with or without assist-ance from tugs. The size of the harbour area which can be economically provided will often dictate the method of turning to be employed. In large barbours it is pos-sible that all three methods of turning could be used. Once the method or methods of
turning are established, the size of the turn-ing ares will be determiñed based upon the size of the design vessel. A typical turning
basin layout for each of the methods of
turning is shown in Figure 12.
A natural deep water harbour will norm-ally be able to provide a turning circle as shown in Figure 12(a). This type of turning basin is most desirable as it allows a quick turn around, free from going astern or con-tacting dolphins. The turning circle is best located near the breakwater heads or situated so the main channel is tangent to the turning circle.
The turning circle should have a minimum
radius equal to the length of the design
ship' ' or 1.000 ft. (305 m.).'" A turning
circle with this minimum radius would re-quire careful manceuvring and possibly the assistance of tugs. The minimum radius that can be comfortably manuvred without the assistance of a tùg is equal to twice the length of the vessel.
lt is sometimes convenient to provide a trapezoidal shaped turning basin adjacent to a channel where a portion of the channel may jbe used for turning. For free turning. the total width of the channel plus the turn-ing basin should be twice the length of the design vessel. The length of the short side of the trapezoid should be twice the length óf the design vessel. The ends of the trape-zoid could make an angle of 45 to 60 deg. with the edge of the channel. These dimen-sions assume no assistance from tugs.
The methods of turning by going astern and ahead and by warping are commonly accomplished with assistance from tugs. lt
is suggested for a vessel turning by going astern and ahead, not assisted by tugs, that-its movements be represented by a series of arcs with radii twicè the length of the vessel. A radius equal to or less than the length of the ship could be used where tugs would be assisting. A swinging width of 120 per cent of the length of the design vessel should be the minimum allowed for a vessel warping against a dolphin, pier or bulkhead.' Conclusion
The criteria and dimensions presented in this paper are based upon the navigational requirements of the design vessel. In most cases the minimum requirements are pre. sntcd. lt is necessary for the harbour de-signer to take into account the local condi-tions of winds, waves, currents, shoaling, and so, prior to establishing harbour dimen-sions. Also it is important to allow for
expansion of the harbours' dimensions to keep abreast o! increased trade and vessel size.
C.
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