Why do boat suck down in shallow water

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Ifraser

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At the risk of seeming not to have a life!! can someone explain the hydrodynamics that cause a boat to be sucked down when the clearance below the hull is reduced. I came up with the theory that the props are pushing a lot of water out behind the boat and it cannot be replaced fast enough so lowering the water density or is it just drag with the water on the bottom?? anyone have a definitive answer?? thanks Iain
 
Venturi effect.

Water passing under the hull is accelerated in same way as air passing over top of aeroplane wing.

Pressur reduced with increasing speed so wing is "pulled" up and hull is "pulled" down.

QED

I think
 
Is it as simple as that?

We are always told that water is incompressible, which would mean you can't increase the density. If that is so then you can't reduce the density either, and therefore the usual venturi effect wouldn't operate........I think! Don't know if it's true and would be interested in the definitive answer.
 
Blame it on the Italians!

'The Venturi effect is a special case of the Bernoulli effect, in the case of fluid or air flow through a tube or pipe with a constriction in in it. The fluid must speed up in the restriction, reducing its pressure and producing a partial vacuum via the Bernoulli Effect.

The Venturi effect is visible in the capillaries of the human circulatory system or in large cities where wind is forced between buildings. It used in gas jets that mix air and flammable gas in barbecues or gas stoves; in water aspirators that produce partial vacuum from a water spigot; in atomizers that disperse perfume; and in carburetors that use the effect to suck gasoline into an engine's intake air stream. A simple way to demonstrate the Venturi effect is to squeeze a flexible hose that is carrying water. If the flow is strong enough, the constriction will remain even if the hose would normally spring back to its normal shape: the partial vacuum produced in the constriction is sufficient to keep the hose collapsed.'

This is from Wikipedia but I remember it all from Fluid Mechanics @ Manchester Uni C.1972. This is what brought down the Ferrybridge Cooling Towers:-

4th November 1965
Three Tower Crash Quest Begins
Eyewitness Accounts Sought

Workers may take their part in assembling information about the crash-down, during a tense mid-morning hour of Monday’s gale, of three out of eight 375ft concrete cooling towers at Ferrybridge ‘C’ Power Station. For eyewitness accounts are being sought, meteorological and photographic records, and other data on the fall of the 8,000 ton giants which cost £290,000 each.

The disaster has tended to dwarf other damage and dislocation in local areas, where roof tiles were stripped and vehicles overturned by the wind. What people encountered however may be of value in the final analysis such as a sound "like a jet flying low" and a sight "like a shower of pepper" - as or before the towers collapsed into their shells. They might be the means of throwing light on the most spectacular disaster of its kind that local districts have ever seen, yet most remarkable for its absence of serious injury among the 2,500 workforce at the site.

Did some peculiar force develop with the position of the towers (they were to the leeward side of the five left standing) and the wind (96 miles per hour was recorded at the station)?

Answers that may be sought to such questions as this and others could have considerable effect when big constructions are planned in the future. The towers were the highest in Western Europe.

Though the area of the towers was sealed to all except specialist staff, it was back to work on Tuesday morning for those employed on the site. During the night C.E.G.B. and police security patrols had been maintained. At 10.30 am on Monday, the first cooling tower crashed, and the others followed at about half-hourly intervals. The first fell minutes after 200 men working on adjacent towers (not the fallen ones) had been recalled because of the weather conditions "not because it was thought that there was any danger of a collapse" said Mr. Leydon.

The three hurt but not seriously- and treated at Pontefract Infirmary, were Malcolm Wayne of Moorthorpe, Trevor Dillon, Lane End, Skellow, and 17-year-old Herbert Wilkinson, of Lander Street, Bentley, who was working at the bottom of the first tower to fall. As he went through a door it swung and knocked him out of the tower just seconds before the building fell, he said.

Described as the largest of its kind in Western Europe when work began on it four years ago, the still incomplete £88,000,000 station has twin 680ft concrete chimneys.

Yesterday it was stated that a fact finding operation onsite would investigate the condition of the existing towers and would also try to discover why the other two towers fell. A thorough steeplejack survey of the remaining towers seemed possible and would take about a fortnight. At headquarters a parallel investigation was being made into cooling tower design, and would make use of any findings of the investigating party operating on the site. Earlier it had been said that means of utilising one of the remaining towers so as not to delay bringing into operation the first generating set, might be considered. The station was due to be fully operative by 1967.



If you can remember any of these occasions write in and tell us about it.

As reported in the Pontefract and Castleford Express 1963
 
Aerodynamics treats air as an incompressible fluid up to about 500 knots, believe it or not. The theory of lift still works, even in water, that's why effective foils are shaped like wings.

So the cause of 'squat' or 'dynamic sinkage' is probably as Bergman states. On bigger ships it can be up to a metre, and depends on speed, the ratio of draught to depth, beam, length etc. I tried googling for more but there is not much detail available. I suppose 'bank effect' has the same cause?

The QE2 ran aground off Massachusetts some years ago and squat was found to be the cause.
 
could it be that your prop. is pumping gallons of water from under the hull which has to be replaced this causes the hull to drop as the water cannot replace quick enough and more noticeable in shallow water.
 
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could it be that your prop. is pumping gallons of water from under the hull which has to be replaced this causes the hull to drop as the water cannot replace quick enough and more noticeable in shallow water.

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Got it in one. /forums/images/graemlins/grin.gif
 
My sea kayak squats in very shallow water and the paddle does not displace water from under the hull so I'm not sure if this is right.
Andy
 
further to above I wascoming down the Trent a few weeks ago at low water,just before Dunham bridge there is a sharp left hand bend,the channel there is quite narrow,when I met a graveller barge coming the other way, just on the apex of the bend.Just before we passed one another he pulled all the water away from me and dropped me on the bottom! soon as he passed I floated again.Fortunately it is gravel at that section and no damage done,
 
yep, i agree.

static pressure (which gives buoyancy) + "dynamic pressure" is constant, dynamic pressure being 0.5 x density x velocity x velocity.

So, with small clearance, slowing down to "just moving" is vital, as the squat increases with the square of the speed so nine times the effect at 3 knots than at 1knot frinstance.

To demonstrate the effect, get a plastic straw like from macdonalds and cut allmost all the way through it about a third the way up the straw. Now bend the straw to open at that point so it forms a right angle, with the short end sitting in the drink. Blow hard, and the water/drinks gets sucked up the straw and then sprayed out with the airflow, which is excellent fun and highly memorable for people (esp small people) learning a bit of fluid dynamics.
 
Joe
Water is virtually incompressible so as you say the density is considered to be constant for constant temp. It is the pressure that varies as a flow passes through a venturi. The pressure is lowest at the point of minimum cross section.
 
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So, with small clearance, slowing down to "just moving" is vital, as the squat increases with the square of the speed so nine times the effect at 3 knots than at 1knot frinstance.

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Unless you can plane over the top? ... but then, with minimal clearance, do you risk it?!
 
One of the worst aspects of this effect is that the ability to steer becomes affected. I have experienced this in a big ship at speed (south of Thailand) and it is a horrible feeling
 
good point - altho of course the worst bit will be *just* before the hull rises. I suppose a better idea wd be to go somewhere else or wait for the tide :-)

This whole thing is noramlly an issue for big ships for whom the depth is super-critical, and where they often run down the same coastal route with fine clearance. I'm not sure that many if any leisure boats would get away with blaming this effect if they went aground!
 
The effect is termed SQUAT

This is a well known effect that happens to ships when transiting shallow water such as Dover Straits etc.
Basically water is displaced and cannot in-fill quick enough ...

You normally have to be pretty close to the sea-bed to get a real visible lowering .....

Race cars used to use similar effect - they called it Ground Effect - it was banned eventually .... and cars skirts etc. raised to limit it.

The effect also happens horizontally .... if you have two ships or yachts coming alongside each other while moving .... as they approach side to side the effect grows until actually just before alonsgide the effect can be strong enough to need substantial helm to counter ....

The actual full physical mechanics of it - I am unsure .... but yes it does and will happen ....
 
Re: The effect is termed SQUAT

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Race cars used to use similar effect - they called it Ground Effect - it was banned eventually .... and cars skirts etc. raised to limit it.


[/ QUOTE ]

Nigel, it can be used the opposite way to create lift. Didn't the Russians build a huge Ground Effect ferry to use on Lake whotsitsname (the huge one that's shrinking fast). Couldn't decide if it was a ship or a plane.
 
and here, courtesy of the Canadian Coast Guard, is a formula for it...

"Squat refers to the increase of a ship's draught as a result of its motion through water. It is a hydraulic phenomenon whereby the water displaced creates an increase in current velocity past the moving hull causing a reduction in pressure resulting in a localised reduction of the water level and, consequently, in a settling of the vessel deeper in the water. For various reasons-having to do with hull design, trim and other physical and operational factors-squat may be different at the fore and aft.

"Recently, a new equation was developed on the basis of extensive research by Waterways Development to specifically target commercial waterways with vessel traffic and conditions representative of most major Canadian waterways. This equation takes into account the vessel beam in relation to the channel width, contrary to earlier equations that supposed infinite width. This new parameter is of importance since most Canadian waterways have limited width. The equation, known as Eryuzlu Equation # 3, is therefore recommended as the one providing the most reliable results in waterways of limited dimensions. The equation is written as follows:



where:

Z = squat;

d = vessel draught;

D = channel depth;

Vs = vessel speed;

g = gravity acceleration;

W = channel width;

B = vessel beam; and

Fw = channel width factor.

With Fw = 1, where W > 9.61 B;

a, b, c are common coefficients:

a = 0.298, b = 2.289, c = -2.972

, where W < 9.61 B; and

"The equation is non-dimensional and therefore, can be used universally with any system of measurement units.

"Applications
The formula applies for vessels ranging from 19,000 DWT to 227,000 DWT, representing general cargo or crude carriers (block coefficient over 0.80);
a channel that is shallow and relatively straight;
the channel width may range from unrestricted to four times the vessel beam;
speeds ranging from about 2 knots to about 14 knots;
maximum trim of about 10 % of draft;
the predominant squat is fore squat; and
vessel loaded draft equal to or greater than 80% of the registered draft.
Formulae, by definition, tend to generalize the real situation. Therefore, good judgement, experience and common sense are required in the use of this and any formula."

You will note the assumption that the block coeefficient of fineness is 0.8 or greater; a state of affairs which applies to almost all bulk carriers and tankers but to almost no yachts or power boats (except for canal type boats like narrow boats and dutch inland waterways barges).

The effects of squat diminish sharply as the block coeffiicent of fineness reduces.

I have noticed an amusing instance of its effect when rowing in a four on a tidal estuary; if the cox takes us over a shoal we slow right down and he can't steer! The block coeeficient of fineness of a rowing four is probably about 0.5 with a draft of 8".
 
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