Why no "Gurney Flap" for boat foils?

[Thorne]

http://en.wikipedia.org/wiki/Gurney_flap

Is water just too dense for this sort of thing to work well enough to be worth doing?

I know that some designers use a flat plate on the bottom of shallow rudders, but this design element calls for the small flat plate to be on the trailing/rear edge of foils.

No practical application in mind, just wondering...

[Canoez]

http://en.wikipedia.org/wiki/Gurney_flap

Is water just too dense for this sort of thing to work well enough to be worth doing?

I know that some designers use a flat plate on the bottom of shallow rudders, but this design element calls for the small flat plate to be on the trailing/read edge of foils.

No practical application in mind, just wondering...

Ought to work in water. The only thing that should change is the scale of the "fence" in the design to account for the difference inf fluid density and Reynold's number.

In college we did lots of fluid research (Air and water, mosty, beer too, but that was after hours...)

One of the big things we were working on was wake and drag reduction. Take a look and see if you can find information on "Rippled Trailing Edges" - basically they mix the down-stream wake out in both air and water.

Another thing we worked on was avoiding flow separation to get rid of stall and flow instability on control surfaces of submarines. One way this was done was with a slot on the "pressure" side of the foil parallel to the main axis of the foil. The slot was located at the point where the flow would turn turbulent. The flow from the slot was redirected with angled tubes to the "lee" side of the foil to prevent the flow from separating. Two benefits - one was the delay of the angle of attack where stall would happen, the other was the drag reduction from stripping turbulent flow.

In jet engines an annular "ring" at the aft end that has this rippled profile is being used in combination with a jet ejector to reduce drag and noise levels on older aircraft (727/737), increase thrust, and by using the jet ejector to mix in lower temperature air with the exhaust it reduces thermal signature of the exhaust. (Important if you want to avoid a heat-seeking missile, for example...)

Fun stuff to work on.

[Paul Pless]

I think the chief contribution of the 'gurney flap' was that it allowed the amount of rear down-force to be easily tuned. It would have been greatly time consuming to change the angle of attack of the rear wing during a pit stop, however it only takes about 3 seconds or so to change the flap. Incidentally, Bobby Unser claims he was the inventor of the flap, not Dan Gurney. Bobby and Roger Penske call them wickerbills. They are still used today.

[rbgarr]

Speeds are probably too low, mechanical device to allow for changing tacks too expensive for someone to experiment with, too difficult to prove it makes a difference (and is thus worthwhile) since conditions between one boat and another have too many variables.

Maybe AC guys would give it a whirl if it fit the rule....

[Canoez]

Speeds are probably too low, mechanical device to allow for changing tacks too expensive for someone to experiment with, too difficult to prove it makes a difference (and is thus worthwhile) since conditions between one boat and another have too many variables.

Maybe AC guys would give it a whirl if it fit the rule....

The speed of a boat in air would probably be too low, in water, the change in density could be worth it. Kind of testing that takes flow visualization and pitostatic pressure measurements.

In the whole scheme of things, you might only be looking at a single digit percentage improvement in angle of attack for the foil. For car racers trying to squeeze out the last ounce of efficiency, yeah. For a boat, you may induce more drag than it's worth with the fence. Anyway, it's not going to make the boat point higher, but just turn a bit faster. You cannot forget that it's a system and you've got hull shape, foils in the water and sails to contend with. The fence will not be a marked improvement. Better off to clean the crud off the bottom.

On the mechanism to move the fence for tacks? That would be fussy and would have to be strong to deal with the forces involved. One more thing to break...

[Bill Lowe]

I have seen a V shape on the back of work boat rudders

[rbgarr]

Those rudders help redirect the prop thrust for a tighter turning radius.

[JimD]

Gurney flaps have found their way onto junk sails as well.

[John Meachen]

The wing that the original Gurney flap was fitted to only had to work in one direction.The foils beneath a boat have to work on both tacks.We may be lucky enough to have a forumite with a good understanding of fluid dynamics take an interest in this thread and explain whether this is a factor.FWIW the book written by Gordon Trower on yacht design refers to an improvement in lift for a minor drag penalty if a keel has a slightly fat trailing edge.

[Ian McColgin]

In planing boats that even have rudders, it's pointless. In displacement boats it does work, as has been pointed out, but the drag makes it unsuitable for most applications. In sailboats where improved turning power is needed, a bottom foil is a better appendage.

[Woxbox]

In the race car application, wasn't there another factor driving it -- mainly that the foil area is carefully measured, so you can't just go with a bigger foil to get more downforce. So to "cheat" the rule, they came up with the flap.

Courageous did have that trim tab, perhaps a more efficient way to acheive the same goal?

[djyachting]

Mike Golding's new Open 60 Ecover 3 - not wood though, all carbon - has one of these flaps at her transom which moves vertically a few centimetres unlike the Farr designed Open 60s which have a heavy flap that can be angled down like a motor boat trim tab.

[merlinron]

as i understand a gurney flap, it is a device that actually tricks the airflow into acting as it would passing over a wider wing with a thicker chord. assuming the same phenomena on a sailboat's foil, what would happen to the dynamics of the sail's center of effort and foil's center of lateral resistance?