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Dr T posted 02-07-2002 08:53 PM ET (US)   Profile for Dr T   Send Email to Dr T  
Let me preface by saying I have always wanted to learn how to fly an airplane. After years of bringing heath issues under control, I have just passed my FAA medical.

This has rekindled an interest in how wings and planing hulls work.

Recently (say over the last 10 years) vortex generators (essentially small skegs on the wings and control surfaces) have been used to enhance the performance of light airplane wings in low speed conditions.

It seems to follow that the use of these on a planing hull would help a small boat come on plane faster and work more efficiently at speed. Such a hull would be a challenge to trailer, however.

Is any one aware of the use of this technique to enhance the performance of planing hulls?

BTW, I don't think that Chainsaw Whaler is a good candidate.....:-)


EasyE posted 02-07-2002 10:10 PM ET (US)     Profile for EasyE  Send Email to EasyE     
I still dount no what vortex generators are? Small skegs? You mean like a hydrofoil boat? Could you explain? I am interested on diffrent hull design. Whalers are good all around hulls...or at least the classic ones.-EasyE

PS..Why does my friend's 12' carolina skiff run at 30mph no matter what hp outboard or pitch prop goes on? Im still bustin my brain on that one.

jimh posted 02-08-2002 01:18 AM ET (US)     Profile for jimh  Send Email to jimh     
I think these "vortex generators" on aircraft are the equivalent of the winglets on the keel of sailboats.

The only application on a outboard boat would seem to be on the tip of the engine skeg (?).

Dr T posted 02-08-2002 07:31 AM ET (US)     Profile for Dr T  Send Email to Dr T     
Not quite.

Vortex generators are small (about 1.5 in. long) and are placed on the top of the wing several inches behind the leading edge. They act to smooth the laminar flow of the air over the wing and decreasing the turbulance when the wing is at a high angle of attack. (see picture ).

To me, this appears to be similar to the conditions (subtitute water for air and boat hull for wing) that occur on the bottom of the hull as the boat transitions from displacement mode to planing mode.

If you look carefully, as a boat comes up on the plane, the leading edge of the hull pushes a wave. This is a major source of drag. It would seem that a decrease in turbulance on the leading edge would help the boat step up faster.

Another place that you can see this turbulance is in the luff of a sail as it is being trimmed. Sails are more efficient at generating lift when they are about to begin stalling out.

I have thought that the winglets on the keel of a sailboat have served a different function. I thought that they served to generate an "inverse lift" that acted to decrease the angle of heel. This would keep the angle of the mast more closely perpendicular to the wind, and make the sail more efficient in transfering energy from the wind to the hull.

In looking at the hull of the 13, there are two little ridges on either side of the center part of the hull. I have often wondered exactly what role they play. Could it be that they act in a manner similar to the vortex generators and help the little thing get up on plane faster?

jimh posted 02-08-2002 08:36 AM ET (US)     Profile for jimh  Send Email to jimh     
Interesting! Maybe Dick Fisher discovered this effect in the 1950's and did not realize it.
SteveC posted 02-08-2002 12:05 PM ET (US)     Profile for SteveC    
I'm not sure that this type of thing would apply to a boat hull since the pressure side of the foil is in the water, on airplanes these devices are typically on the suction side. That said, don't many of the new chopper gun boats have some ridges on the bottem these days to improve performance?
Taylor posted 02-08-2002 02:19 PM ET (US)     Profile for Taylor  Send Email to Taylor     
Winglets on keels were famously used in 1983 by Australia II in the America's cup. She broke US lock on the cup since the America took in from England in the first place. Shocking!

I spent some time 1982-3 supporting an Aerodynamics staff, including one of the US keel designers. They looked at winglets for 1983 but felt that they did not measure in under the 12 meter rules.

Anyway, the point of winglets is to increase the 'lift' of the keel itself by keeping the high preasure on one side, low on the other and to keep preasure from leaking around the end. Lift here is horizontal, since when viewed from above or below, the keel is acting as a vertical wing in the water. With the winglets, the effective size of he keel is increased, reducing leeway when going to windward.

I'm afraid I did not quiz the same guy on lifting strakes of vortex generators. I really only cared about sailboats back then.

Jerry Townsend posted 02-09-2002 10:45 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Dr T -- I doubt that the vortex principle you are discussing is applicable for boats. The reason in part, the fluids are totally different as - air is compressible whereas water is incompressible. On the planes, the "winglets" are installed to "spoil" the tip vortex which is simply wasting energy.

Now, I could have slipped a cog too - so I will "revisit" some of my stuff on vortex flow. ------ Jerry/Idaho

Dr T posted 02-10-2002 01:38 PM ET (US)     Profile for Dr T  Send Email to Dr T     
Thanks. You comment on compressiblity is noted. If you have a good reference on vortex flow, I would appreciate it.

I have received some information about an interesting application of vortex generators to help provide aeration in flow-through live bait wells of high end sport fishers. The passage of the water over the vortex apparently causes a cavitation-type effect that frees excess gas.

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