Why do boat suck down in shallow water

Re: The effect is termed SQUAT

Ground effect was used to "suck" the cars onto the ground and keep them stable and on track in corners etc.

It also affects Helicopters .... they need more power at low level than just a few feet off ground .... (know that from my model heli-days !!)

The russian craft you mention used the effect of trapping the underflow of air and water to lift the craft .... little bit different.

The inland sea you mention is a disaster area .... whole fishing fleets and communities stranded miles from water's edge ... I forget the actual %age of the sea that has disappeared but its substantial .....

Not only that but the Volga is often lower that it used to be by enough to restrict the Volga Ships ....
 
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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

[/ QUOTE ] You don't need a big ship or to go to Thailand to experience it. I have just come back from a week on a narrowboat in Wales and it was a very common problem. Pull too close in to the side to pass a boat and you can suddenly find the tiller becomes purely ornamental!
 
Otoh

shallow water is usually preferred when trying high speed records, in the south of france there is a long and straight strip of water barely covered by a few inches of water: when mistral wind is at its strongest one usually finds queues of windurfers trying to establish new speed records

in this case water becomes rather a mean of reducing friction with the ground than anything else /forums/images/graemlins/ooo.gif
 
Re: The effect is termed SQUAT

one of the reasons must be that a prop is not 100% efficient the hull is not friction free,so it pulls more water under the hull than it needs than if they were both 100% efficient hence you get squat.
 
i dont know the scientific explanation but suspect it is the same interaction that occurs when two vessels get close to each other they are sucked together.

The underwater hull is curved like an aeroplane wing,the water has to accellerate to pass under it creating an area of low pressure under the hull.

You can demonstrate this clearly using two desert spoons ,dangle them between your fingers with the spoon bowls back to back about 10mm apart then hold them so running water from a tap goes between the spoons.

You would expect the water to push them apart ,try itand find out.
 
If you do get the venturi effect with water, why aren't propellors blades shaped like an aerofoil? (at least mine aren't, I don't know about others)
Surely a propellor could pull itself through the water (due to an aerofoil effect) as well as "screw" itself through the water?
 
dont know about props but on modern bilge keel yachts the keels are aerofoil shape so the leeward one ,immersed into denser water by heeling, provides some lift to windward.
 
Re: The effect is termed SQUAT

The Ekranoplan was actually a Cold War design that was commissioned and intended to transport Soviet assault troops and their light tanks very quickly across the Southern Baltic, below radar cover from Bornholm for much of the way, so as to land virtually unopposed on the Danish Islands and over-run/seize vital miliatry targets such as airfields, radar sites, Command and Control centres - so that the massive but slower 'follow on' forces would be relatively undamaged and intact.

The aircraft worked, but they didn't build enough of them to be a real threat. The idea was, in its time, a good one.

Today, we use Special Forces and Royal Marine helicopter assaults.... so do the Yanks.
 
It's also known as "The Canal Effect", as in Suez, Panama, and Trent and Severn. Think of it like this: Water can only flow if there is a difference in pressure. When a ship passes over shallow water the water is displaced. The faster the water is displaced the lower the resulting pressure. The lower the pressure the greater the suction applied to the surface area of the ship's bottom. Ship squats.

Practial experiment: Take a soup spoon and hold it vertically with the convex side against running tap water. You can pull the spoon handle a considerable distance before the water "lets go".
 
Re: The effect is termed SQUAT

It is / was the Aral Sea. Now split into two salty lakes divided by desert. All due to the Soviet master plan for growing cotton, which took ALL the water from the rivers which used to feed the this sea.
 
Re: Otoh

I do not know, there are so many nautical bases (tiose for rowing, canoeing, etc) with miles of straight line water just a few feet deep and absolutely no waves, still people always prefer that place (?)

never liked windsurf anyway -.<)
 
Ekranoplan

I knew that! Twenty two years in the army.

Shhhhh. Thought that was still classified information. I will check with HQ, but I think I am supposed to go round and 'silence' you.

But, it will have to wait until later as we are going to Tesco after lunch. /forums/images/graemlins/crazy.gif

I actually spent eight months in Denmark, attached to their forces. At the time, 1975, they were seemingly resigned to thier fate.

What was that? I heard a noise downstairs..............................
 
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