Shallow water effects

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Shallow water effects

Dr Adrian Miliward discusses the phenomenon of 'squat' and the anomaly of less resistance for high

speed craft in shallow water.

THE phenomenon of'squat' is illustrated

in Fig. i which shows a large ship entering

a port where the water depth is not much more than the draught of the ship. As the ship moves forward the water is pushed

out of the way, some going to the side and

some underneath with the result that the water flow under the bottom of the ship

can be pictured a bit like a venturi tube but with the water stationary and only the top

wall moving. The principle of the venturi

tube is that the flow speeds up as it is squeezed through the smaller area and

because of this the local pressure actually decreases. Then after going through the narrowest part, the throat, the flow slows down again as the area increases and the

pressure steadily rises back to the original

value. The water being forced under the

large ship's hull in shallow water behaves

in a more or less similar way, again

causing a reduction in pressure over the bottom of the ship. Although this change

in pressure is not great it acts over the

whole underneath of the ship which is a very big area and is enough to cause the ship to sink bodily deeper into the water until the additional buoyancy of the extra draught balances out the suction force on

the bottom. As with the venturi tube,

where the pressure reduction at the throat

becomes larger as the flow speed increases with speed.

The 'squat' effect would be present for

smaller ships as well but is not usually

significant because they are not normally

taken into the same depths of water

relative to their draught. There is however

another result of being in shallow water

which would be noticeablean increase

in the resistance of the hull which would

either cause the ship to slow down or

require extra power from the engines to

keep up the same speed. This added

resistance, which appears to be mainly related to a change in the wave resistance,

becomes measurable when the depth of

water is less than the length of the ship and

H metres

Fig. 2. Variation of 'critical wave speed' with water depth.

SUPPLEMENT TO THE NAVAL ARCHITECT

Lab.

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Technische Hogeschool

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Fig. 1. The phenomenon of squat is illustrated by this diagram of a large ship in shallow

water.

originally led to problems with the position of the official measured miles used in

trials before a ship was accepted by the owner from the builder.

All these shallow water effects so far

however are mostly relevant to larger

ships which move at relatively slow

speeds, particularly in shallow water, so

that they are much slower than the

'critical wave speed'the fastest speed

that a free wave can move in a given depth of water. This 'critical wave speed' is

related to the depth of water by the

formula:

V = (gH)½,

where Vis the critical wave speed, g is the

acceleration due to gravity and H is the

depth of water. A graph giving this critical

speed against depth of water is shown in Fig. 2 and it can be seen that at a depth of

15 m the critical wave speed is about

24 knotsmuch faster than most large

ships go, especially in shallow water. On the other hand many small ships, such as those which are used for pleasure and as fast patrol boats do go quite fast for their

size and may well be in water which is shallower than the length of the hulla rough rule of thumb to estimate when shallow water effects may become measurable. If they cali go this fast then it is possible for them to be moving at or

even faster than the critical speed for that particular depth of water and a different effect is found. Although at sub-critical speeds, as mentioned earlier, the resis-tance increases in shallow water and is at its worst just below the critical speed, at

super-critical speeds it has been found

that there is actually a drop in resistance compared to the deep water value.

This interesting phenomenon is il!us-trated in Fig. 3 which is a reproducticii of a graph published in the Transactions of

the Institution of Naval Architects in

1899 of measurements made on a Da ish

torpedo boat in the Baltic. The graph

shows curves of engine power (resistance

X speed) plotted against ship speed in

four different depths of watercurve 1) is

the curve for fairly deep water. Curve C is

for a water depth of 1 46 ni and show an increased power required throughout the

whole speed range since the ship only just

reaches the critical wave speed, which is

23 knots, for that depth of water. The lower part of Curve B shows a similar increase in engine power required but

more noticeable since the water depth is less. However at higher speeds the curve

changes until at 20 knots the engine power

is the same as for deep water and at even

higher speeds the engine power is reduced

below the deep water value. Curve A is for the shallowest water depth of all and

shows the same pattern as Curve Ban

increased power required at lower speeds but less power required at speeds above

21 knots in this casebut even more

noticeable than before. This indicates that both the sub-critical and super-critical

effects of shallow water are increased as

the water depth becomes

shallower-more recent work shows that it is the ratio

(continued on pag. 12)

2 4 6 8 10 12 14 16 18 20 22 24

Fig. 3. The effect of shallow water on the engine powerrequirec/to drive a torpedo boat

7

TORPEDOBOAT SÓWÒRNEN MAY 1898

DispIement 132 tons

Horsepower curves A 21 fotms depth of ter B 6t. 0 20

I/CA

Cur

/

Speed n knots 50 40 30 V knots 20 10 Super critical Sub critical 2400 2200

20

1800 1600 1400 1200 1000 BC 600 400 200 10 20 30 40 5°0

i

-8JUNI 1988

_ARCHIEF

(2)

A comparison of Watercra ft's P2000 midship section (the larger

volume) with two competing 20m craft

is flexibly mounted utilising eight rubber mounts. All cable ways and access points between the hull and superstructure are sealed and in addition a rubber gaiter is

fitted between the superstructure and deck outer perimeter. This feature

combined with the aft engine layout has contributed to the particularly low

super-structure noise levels. For example, in the wheelhouse a maximum of 50 dB was recorded.

The helm, navigator and CO positions

are sited at the forward console of the wheelhouse with all instrumentation easily to hand. Brostrom A100-U series

suspension seats are fitted for the CO and helm. Controls and monitoring equipment

is split between the forward console and

an overhead display spanning the full

width of the wheelhouse. Above the CO position are the main engine and

genera-tor control panels, navigagenera-tor log and

depth gauge. Above the helm

is a

Robertson SKR-82 gyro, Thomas Walker wind speed indicator and Greenham Marine fuel tank contents gauges.

On the forward main console are

Furuno FR-701 Radar, Sestral Magnetic

Compass, Decca 150 PC Autopilot,

Chart Table

and Vingtor Intercom. Additional equipment includes Furuno

Shallow water effects (Contd.)

of the length of the hull to the water depth

that is the most important parameter with

hull length to draught ratio having a much smaller effect.

Although the shallow water effect at the

super-critical speeds was found all those

years ago little notice has been taken of it

until recently when the greater use of fast

patrol boats has re-awakened interest.

Tests have been undertaken at the

National Maritime Institute in Britain on

a number of high speed hulls to obtain reliable data on the effects of shallow water at high speed; this data has been used to test theoretical prediction methods

being developed at Liverpool University. It is expected that as a result of the joint project it will be possible to predict the

effect of shallow water on all types of high

speed hulls. Fig. 4 shows one of the hulls being tested in this work at a particularly

interesting conditionthe model hull is

being towed at the critical speed and the critical wave can clearly be seen on the

wall of the towing tank. This critical wave

ADF model FDl71, FSN-70 satellite navigator and Sailor VHF unit

All major components are repeated on

the flying bridge along with an additional

Furuno 2400 Radar.

External fendering comprises a con-tinuous 4½" X 3' D' section rubber at

gunwhale level along the hull sides and

around the transom. A lower rubbing

strake of the same section runs

approxi-mately ½ of the ship's length from the

transom forward.

Worth mentioning is Watercraft's own

design hand rail system which is arranged to absorb impacts into the ship, but is rigid should anyone fall against it from the deck. This is achieved by pivoting the rail staunchion at the base against a rubber

block. Hence, should the rail be nudged

when coming alongside or when boarding

another vessel, it absorbs the impact

Dheeb al Bahar i roughly translated means, "Lion or Wolf of the Sea", the

actual mythical animal described being a

cross between a very large dog and a wolf.

The vessel was shipped to the Oman in late December to begin operations in the

New Year. The Omani crew, although

experienced with several smaller Water-craft patrol boats and other much larger

craft,

_{undertook a series of training}

courses at Warsash with a particular

Fig. 4. Model hull undergoing tank tests at the National Maritime Institute. is formed only at this speed and stretches

right across the towing tank, moving just

ahead of the modelthe front guide post

Profile of the Ministry of Defence P2 000 coastal training craft

derivative featuring a less dramatic moulded GAP

superstructure.

emphasis on high speed navigation and

activities.

The MOD Option

The fourteen 24 knot Ministry of Defence P.2000 vessels will differ considerably

from the Omani version. A lighter

scant-ling hull without transom wedges is being

used and the superstructure will be an all GRP moulded structure rigidly connected to the hull. In appearance it will be less

dramatic with only a slightly raked

wheelhouse front and propulsion will be

by twin 650 shp General Motors 7lTI diesel engines, driving through Twin Disc 'U' drive gearboxes. Accommodation will be for a crew of 10 with an additional

detk level cabin,

including w.c.

a.d

shower compartment forthe Commanding Officer.

The craft will be operated by the Ro al Naval Reserve, University Royal Naval Unit and Royal Naval Auxiliary Service.

Already three craft are well underway

and two further hulls are in the moulds.

All in all the contract will provide

employment at the yard into 1986, and a

number of additional P.2000 contracts

are under negotiation. A 40 knot hull is on stock readily available for completion

and plans are well advanced for an all aluminium variant subject to customer specification.

for the model is nearly immersed whereis in flat water it was about4 cm clear of the

surface!