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Less drag

Thread Status: Hello , There was no answer in this thread for more than 60 days.
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New Member
Sep 15, 2001
I have the impression that swimming just above the bottom of the pool is easier than somewhere at mid-depth.

Is it just an impression because floor tiles pass by faster, or is this really like that because of less drag ( a friend told me it is true for planes flying very low above the ground).

Has someone experienced the same ????

I think you are right

I read in some swimming-page (don't remember the adress) that on surface and beneath the surface the wave-effect is reducing speed and that I think everyone that trains in pool have noticed.

The most interesting was that if you have an object going in the water and the diameter of the object is "d" then you have to go down to a depth of "3*d" to get rid of the wave-effect.

In a human application it would be at about 90cm+!

That explains a part of why it's harder to do dynamic in a shallow pool. (of course it's because of a more variable boyancy too).

:D I found the adress: http://www.education.ed.ac.uk/swim/papers-ISBS2002/ht3.html
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For pool training i would belive. But planes?
Have you seen any plane flying at very high speeds very low?
It is almoust impossible because of drag.

my experiance also


Faster near the bottom is my experiance also, and I wondered if it was real. Dolphin kick seems to be where I feel it the most. It could be "surface effect" like what airplanes experiance at very low altitude, but I don't know for sure. Logically, something like surface effect should slow you down. Any monofin users care to comment? I'll try to get some pool times next week and report back.
US Airforce is planning a huge cargo plane that would use ground effect to reduce drag and facilitate fast deployment to far away destinations. But that one would obviously be a slow flying one.

Ain't it strange though, that planes carry CARgo and you can make SHIPments with UPS vans? ;-)
The Russians built a plane that traveled like 20ft off the deck to take advantage of the lifting effect created by high speed ground travel. As a matter of fact I think I have a link to it...i'll have to dig around. This plane was capable of flying across the ocean carrying the same weight as a 747 and use half of the fuel. Some kind of lift is generated.... weird.
Please do find that link. Maybe bruno's friend is wright.
I would like to hear an explanation.

a picture

This is a russian construction.


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When a body (be it in air or water or any other fluid) moves, it creates what's known as vorticity - that's what makes planes fly. The down side to this vorticity stuff is that it induces drag on the body. You know the winglets you see on the 747-400 series? They're designed to stop the flow of vorticity outwards, and reduce drag. When you're talking about airfoils, you can imagine the vorticity that adds to drag as two big tornadoes coming off the wing tips. Now when the planes closer to the ground, these 'tornadoes' are cut in half, which means you've got half the drag you used to have.

This isn't exactly technically correct, but close enough for these purposes. If you wan't any more detail, throw me a PM and I'll bore you with a tonne of aerodynamics :)

Looking at it from the point of view of a diver close to the bottom, there'd be a little decrease in drag, but you'd find it's not much - you'd get a bigger decrease by just sticking your arms out in front of you. It is pretty important though to get below the effect of wave drag as someone said before.
Re: link

Originally posted by teppo
[BI recall hughes built a big ground effect aeroplane long ago.
There was about 12 engines on top of the wing, each having its own propeller. [/B]

I think you're referring to the Spruce Goose, a wooden aeroplane that Howard Hughes built. It's in a museum in Los Angeles....very impressive. It was not intended to be a ground-effect vehicle; it just never made it above a few meters off the water!
Erik Y.

I just spend half an hour writing a beautiful piece showing that ground effect had zip to do with a diver on the bottom of the pool and that in fact he experiences more drag close to the bottom... but my connection closed and i lost the work.

So im not doing it again. I am convinced however ... Swim mid water.

Skin - can you pm me the basic idea behind your theory?
less drag near the bottom

I think that there are more drag near the bottom.
When sailing in kayak from deep water over to shallow 20cm water its becomes remarklebe harder to hold the speed.
This is beacuse the water pushed down by the kayak hit the bottom.

Johan denmark
I visit this forum too often.

Ok. Abreviated version.

Ground effect is a phenonemon experienced by birds and planes. The effect starts at a height roughly equal to the span of the wings above the ground.
The airfoil ( wing ) creates lift by virtue of the upper surface being longer ( curved )than the lower surface. The upper airflow speeds up to meet its mates that take the slow route under the wing.
( flat )
In speeding up, the air molecules spread out and the pressure drops relative to the under wing pressure. ( same amount of molecules occupy a greater space - vaccum effect )
The wing is literaly sucked up from the top ( LP ) and to a lesser degree pushed up from the bottom. ( HP )
The 'push up' accounts for 30% of the lift and is primarily created by angle of the wing ( angle of attack ) giving a compression effect under the wing and the relative pressure to the upper wing area.

Ground effect is a cushioning effect created by trapping air between the wings and the ground. Push up effect can increase by up to 75%.

Drag increases with speed. Usually the air taking the slow route under the wing tries to take an even shorter cut up over the end of the wing tip. It corkscrews up and is sucked in by the upper surface ( LP ) and strikes the wing on its top surface destroying lift and where there is no lift there is drag. This is the spiralling wingtip vortex or 'tornado' :) referred to earlier.

In ground effect however a plane can fly slower and still stay up thanks to the cushioning effect of ground effect. Ground effect decreases lift induced drag. The drag is reduced because the speed is lower and the exaggerated cushion of air under the wing plays a part in blowing away the wing tip vortex preventing it from curling around and striking the uppersurface of the wing. Thus better lift etc.

Water is incompressible and man does not look like a wing. Ground effect has no bearing on a diver at the bottom except for the negative effect of the dynamic cushion. lifting him upwards and backwards.

The diver drags a layer of water around with him and this layer may retard on the floor ( sucking him back and down ) and a wave may form on his chest striking the floor in a forward motion ( dynamic cushion )buffering and retarding his forward movement. The kayaker in the previous post has hit the nail in the teeth with his example.

A diver swimming close to the floor will likely suffer from Bernouli's principle ( lift, but in reverse ) where he is sucked down onto the floor behind the dynamic cushion formed in-front of his chest. He consistently drags his fins and bumps his knees due to the suction downwards. ( Like a boat travelling at speed in a canal close to the wall is sucked to the wall.)
The diver then adjusts his hands (angle of attack) upwards to gain altitude, rides up on the ground wave which moves backwards towards his waist but the lower body does not respond ( bernoulli ) so he swims at a positive angle of attack for many meters trying to escape the floor exposing a poor profile to the water, increasing drag and surfacing prematurely.


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Nice arguement Skin, though some of your aerodynamics is askew... the problem with dynamics though is that we're talking about shallow pools. and the wave drag created by swimming close to the surface is far more than any drag off the bottom. In a deep pool, I think you've hit the nail on the head, but for normal dynamics, I'd still be on the botttom...
Fun reading guys. At last a subject that I know a little about. Speaking of drag, do you want to try a puzzle? Two 747's are side by side at 35,000 and close the throttles. Planes are identical but, one weighs 500,000 and the other 400,000. Who touches down first?
Answer #1, the 400,000er. Once forward momentum from engine thrust is stopped the heavier will start to fall faster, but in doing so gains extra lift becuse of increased speed. Therefore the smaller plane w/ less lift will touch down first.

Answer #2, once to planes reach terminal velocity(9.8m/s2) I believe. They glide the same. Since the heavier one will reach it first it should touch down first.

I'm sure this is a trick question. :confused:
Bill - if this isn't a trick question, then they both touch down at the same time, because you max L/D ratio isn't affected by weight (that is if they fly at the same airspeed). They'll just have different angles to the horizontal.

Otherwise it's the one with only one wing :D
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