Exploring the Benefits of Anhedral Rigging on Sailboats

[ken]

Hi,

There is some evidence that on a silboat going to windward in moderate to strong wind, heeling the rig over to windward (the reverse of normal) gives significantly increased speed. It works on narrow hull boats but only on wide hulls where the rig is heeled independantly of the hull.

The question is why?

This is like an anhedral wing. Does anhedral decrease induced drag?

Does anhedral increase lift?

Where can I find information on this?


Ken

 

[russ_watters]

My guess would be that with windward heel, air flows at an angle down toward the water, while with leeward heel, wind flows up toward the top. Since the sail is wider near the bottom, it would be better for more wind to flow over the bottom.

 

[ken]

My guess would be that with windward heel, air flows at an angle down toward the water, while with leeward heel, wind flows up toward the top. Since the sail is wider near the bottom, it would be better for more wind to flow over the bottom.

Hi Russ, nice to hear from you again.

That's a good thought.
I haven't experienced this myself but the claim is for as much as 10% increase. If your guess is correct, It will not work so well with eliptical sail plans as with triangular sails. Also should not work above the design wind as the boat is already at maximun power.

Are you aware of any significant effects on induced drag with changes in dihedral or anhedral, up top +-10 deg?

Ken

 

[Meir Achuz]

My guess would be that with windward heel, air flows at an angle down toward the water, while with leeward heel, wind flows up toward the top. Since the sail is wider near the bottom, it would be better for more wind to flow over the bottom.

I don't understand your last sentence.
I think heeling to wind'd means the sail in giving some lift, which is like
lightening the boat. Also sailboats are usually designed to sail faster when heeled, because the WL length is increased.

 

[ken]

Hi

I believe Russ is talking about the way cross flows change with heel.
When heeled to lee, there is an upward component of cross flow toward the peak of the sail there the chord is generally narrower. When heeled to windward, the flow is directed more downward toward the foot or boom. Now as all sailboats operate within the winds boundary layer, there is a velocity gradient with height so Russ is suggesting the whole veolcity gradient is moved downwards where the sail has more chord and therefore generates higher load.

Lets say my 14 foot boat. Total sailing weight = 250 kg including 2 crew
Sail area = 9.3 m^2
Max sail reaction upwind = 900 N

heeld 5 deg to windward, lift = sin (5) * 900 = 78 N = 8 kg

Boat now 96.8% of original weight.
By cubes and squares, that's 96% wetted or even Xsection area.
Take the inverse square for the new velocity 1.09 %

OK, that's very interesting confirming the lift theory.
I'd not have thought it that much. Is the calculation valid?

Ken

 

[ken]

I made a mistake, I squared the inverse change in area/drag, should have taken the square root. Velocity increase would then be 1.014% which is not enough to account for the reported speed increase.

Ken

 

[vivesdn]

By the way, if there is a lift (accounted by sin(5)), then you loose part of the power (cos(5)), that would decrease the increment provided by the lift.
Now, if one thinks that the power of the sail is not fully oriented to the heading of the hull (ie, ther is a drift component), most of the lift would come from the perpendicular, undesired component of the force: then drift. Then, by heeling, the boat experiments a reduced drag due to the lift but also reduce drift. Might this account for the increased performance?

 

[ken]

Agreed there is technically reduced power in the horizontal plan proportional to cos 5 but that's pretty insignificant.

Someone also pointed out that when a round hull with transom (most planing dinghies) is heeled to windward, the aft contact point with the water moves to windward. This has the effect or rotating the wetted hull to align with the leeway flow. Much the same effect as a gybing board. Don't know if this could reduce the drag much.

Maybe it's a combination of several effects.

Ken