Yankee "expert" on scrapheat challenge just said his air boats can go 5-10mph faster in shallow water than deep.
Just confirms our previous thoughts about ground effect.

Could also be flatter water

Could be something in it. I remember reading some opinion about planing was riding over the top of your own bow wave. As I see constantly on the harbour boats bow waves get bigger as they get to the shore as they interact with the bottom like surf waves. So in theory it may be possible that the interaction of your own bow wave with the shallow bottom is giving more lift.

The theory would be best tested at a shoreline downwind as you can't say the water is flatter. Of course the safest thing would be to stay away from shallow water, but that is another topic...

got into an argument with 2 sailing buddies when i heard a commentator say something why Bjorn goes fast and does it in shallow water despite the threat of catapulting..i kinda foolishly just said that a board in shallow water kicks pressure downwards and ahead of the board thereby giving a bit of lift but all i got was..well it may be true who knows

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elmo said...

Could also be flatter water

Possible, but we're talking everglades here, don't think chop is often an issue.
As I understand it, theory does say that a planing hull will go faster in sufficiently shallow water. Just how shallow the water has to be, to have much effect is the question.

JP I think you're very close, in shallow water there is more resistance to the displaced water moving away, so it pushes up harder on the hull.
The opposite is true for non planning hulls, as before said, the effect of the water pushing up, is no longer under the hull but in front of it in the form of the bow wave.
So my mate who paddles our estuary, reports a significant slow down in shallow water.

It may be part of the reason, hardys run has so many WA PBs.

you know decrep i wish i could copy paste this and paste it to their foreheads for once[}:)]but it all started with why kiters do it in shallow water cuz it was flatter and i just added that's Bjorn does it because of....just wish you're at my beck and call when i get into arguments regarding boards, fins, just about anything why windsurfing is fastthese 2 homeboys won't give an inch ever and i just let go of the subject, i sail in peace

Of course, what do you gain by winning that argument?
If they don't want to listen, there's no point you stressing out trying to convince them.

That European kiter, got his records in very shallow water, think that's what started us thinking about ground effect.
Of course it also works in the air, that's where the term "ground" effect came from.

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decrepit said...

Of course, what do you gain by winning that argument?
If they don't want to listen, there's no point you stressing out trying to convince them.

That European kiter, got his records in very shallow water, think that's what started us thinking about ground effect.
Of course it also works in the air, that's where the term "ground" effect came from.

see! wise words that are hard to argue with!!!

Like the old Soviet Block seaplanes on the Black sea that only fly several metres above water - no higher. They get added lift from the ground effect of the air compressing against the water.

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TonyC said...

Like the old Soviet Block seaplanes on the Black sea that only fly several metres above water - no higher. They get added lift from the ground effect of the air compressing against the water.

hi tony in reference to a sailboat would a flying moth be faster in lighter wind because of this not just because it had hydrofoils

Don't think a moth hull is going to get much aerodynamic lift from surface effect, not going fast enough and not enough area.

Hi JP, I have very little knowledge of hydro/aero dynamics. I would think it is due to the lesser wetted surface area of the foils and the variable lift of the foils - such that they will be far more efficient for the wetted surface than the hull itself. That is why I will be very interested to see how some variable curve foils (instead of fins and the current "big" foils) will perform on sailboards in the future. Like some of the curved foils used on the giant trimarans. The lift characteristics change as more forward pressure is built so less lifting section of the foil in the water - whereas Moths have the same foil surface area in the water no matter how fast they go (except when they sometimes pop the surface) - and this limits/constrains their ultimate top speed.

In summary, and with my limited knowledge, Moth foils are somewhat antequated as they have horizontal foils that cannot have the area of the foil changed in flight (only the attack angle). The new big tris and speed machines like wot ?? etc have angled and curved foils so that the faster and higher they go the less foil is in, and required to be in the water (less drag) so top speed theoretically not as constrained.

I also think the future world speed records (maybe in 5-10 yrs) will be well in excess of current limitations and will comprise some form of foiling windsufer. Perhaps a tri-fin, with the forward curved foil/fins somewhat further forward and a tail fin much like used today, though lesser chord and width, all maybe 6-8" long. The question will be initially getting the things up on the foils.

All pure naive conjecture - but it will be interesting to see the direction WS speedsailing goes because everything I have read here and elsewhere indicates that speed WS is very close to being maximum speed constrained. Oh well looks like the weird machines will takeover again for a while, although L'Hydroptere is a beautiful machine - and performs in some pretty wild conditions.

nevertheless, i'd love to have a moth on lite breeze days they are in essence air boats and are

Agood example of a shallow water hull design that fails in deeper wateris the AVON DESCENT boat . they plane really well in shallows but when you get on flat deeper water they hobby horse as the pressure wave of air on water dissipates downwards , there being no bottom to force against. In Alaska they race a similar concept of boat on the YUKON with metal blades along the edge of the hulls to contain the air, but the job of the forward hand is to find the shallows and guage the depth (and spot the logs and rocks) . the helmsman tries to keep in as shallow water as possible as it is just plain faster. the best boats do 50+ knts with25hp outboards and a 3 man crew