Keel Trim Tab

I'm interested in the use of keel trim tabs, but having never sailed on a boat with one I'm interested to how they work and their tactical advantages.

I realise they increase lift to windward, but considering the increased drag would your VMG improve?

What are some ways people have faired the trim tab with the keel to reduce drag?

Does anyone know if they have developed keels yet that are able to change shape? So they are assymetrical but the shape can change from tack to tack.

Cheers
Mike

 

Aussie 2 had a plastic flap on her trim tab so it was a smooth surface. They are sometimes used for steering upwind (not for tacking of course) when straight line sailing so the rudder is nearly centered and not causing as much drag.

There is a Nelson/Marek(?) 36 called Diode that has a gybing keel. It is apparently very fast upwind.

 

Flexible Airfoil Section

A friend has a patent on a: "Flexible Tailored Elastic Airfoil Section" ; US Patent #5,181,678
Here is the abstract:
"A flexible airfoil section for a wing or blade comprising a streamlined shape and an elastic structure whose stiffness distribution along its chord and span is tailored to provide a desirable cambered shape with proportional increases in camber with increases in lift , mounted to a supporting structure such that the airfoil sections are free to pivot about axes near their leading and trailing edges. In operation, the foil derives much of its lift from elastic bending deformation of its flexible shape, thereby achieving a higher lift than a symetric foil at the same angle of attack while post poning the onset of flow separation and stall and, for operation in water,of ventilation and cavitation. A wing or blade of general planform comprising flexible elastic airfoil sections which can be used to stabilize, or to control the direction of travel of, as well as to provide side force for, a watercraft such as a sailboat or a sailboard ,or for application to a variety of aircraft components and configurations to provide lift and side force and as well can serve as the aerodynamic surfaces of various types of fluid machinery including fans, helicopter rotors and wind turbines operating in subsonic flow."
They've done a lot of experiments so far on windsurfer foils. Also in Pierre Gutelles book "Design of Sailing Yachts" and in Marchaj there are other references to experiments in changeable foils used for keels and rudders.In the latest issue(#96 I think) of Professional Boatbuilding there is an artcle by Eric Sponberg describing the engineering behind a trim tab used on a canting keel race boat. It was Project Amazon and in that race the boats were required to have emergency rudders. Project Amazon demonstrated to the race committee's satisfaction that it could be steered using JUST THE TRIM TAB. And trim tabs(of a sort) may be used on high cant angle canting keels to increase DOWNFORCE and thereby righting moment.

 

But the lift from the keel is dependent on the sail force, which is directly related to righting moment, so the trim tab does not actually increase the lift force.
It may be that the trim tab reduces drag by making the foil and hull operate at a smaller angle of attack for the same lift.
The effect may be observed as 'more lift to windward' by a reduction in leeway due to less drag.

 

Another example of why they are good to use is the 12 meter Intrepid. With her trim tab being able to be linked to the rudder, she was able to turn much faster that the other boats.

 

A keel trim tab gives benefits in a number of ways:

1) It reduces the keel's profile drag by shifting the keel airfoil's drag bucket up to the operating CL. If laminar flow is attainable, up to 50% profile drag reductions can be obtained.

2) It gives a near-zero leeway even when the keel is highly loaded, thus reducing hull drag.

3) It increases the keel's maximum lift (CLmax) capability, which is critical for accelerating out of a luff or a tack, or.....

4) ... If the larger lift capability is not needed, then the tabbed keel can be made narrower than usual. This gives less drag when running downwind.


To get the most benefit from the tab, it's essential that the hingeline be well sealed, so that water can't seep from the pressure side to the suction side. Such leaking fluid will wreak havoc on the suction-side boundary layer trying to overcome the adverse pressure gradient.

DIfferent types of seals can be used. One seal type is an internal "windshield wiper" attached to the front fixed part of the keel on the centerline, and pressing against the nose of the tab. Another type of seal is a pleated membrane permanently fixed to the front keel on one edge, and the tab nose on the other edge. The pleats in the membrane allow hinge movement between keel and tab.

Surface fairing flaps covering the hingeline gaps on the surface are much less important than the seal. They can be made superfluous with appropriate shaping of the hinge area.

 

I would add one additional point to Prof. Drela's:

5) Keel tabs change the angle of the sail rig to the apparent wind.

Instead of pulling the boat up to windward, which is how most people think of it, the tab really rotates the bow off the wind so that it is more aligned with the direction of travel. This is because of the reduced leeway angle.

This rotation of the hull "opens up" the foretriangle, much like trimming in a rig with a ballestron boom (such as an Aerorig). The mainsail is maintains roughly the same aerodynamic geometry because it is sheeted out to maintain the same angle of attack. But the forestay has moved to leeward compared to the boat without the keel tab.