Trim tabs on trailing edge of the keel

[rbgarr]

My question is: Which way is the trim tab cocked for upwind performance improvement? To windward so it 'steers the keel' up to windward- or to leeward so the keel 'acts more like an airplane wing with it's flaps down' (and thus has more lift ???)

From http://www.thedailysail.com about America's Cup designs-

TRIM TABS

From his position as Principle Designer for the South African team, Ker has been able to transfer some of his new found Cup design know-how to his latest IRC designs. Most apparent is the trim tab on the trailing edge of the keel found. It is of no benefit at all if you’re running but it is good for races where you have a decent amount of upwind racing. It is nice to have height and speed off the line and upwind generally. The trim tab has this effect and is a useful tactical weapon, although Ker acknowledges that it takes some getting used to, a sentiment echoed by many of the crews struggling with them at the Rolex Commodores' Cup. "You have to be set up right to get the full benefit out of it," the
designer warns.

While trim tabs on Cup boats are operated by a wheel within the main steering wheels, on the new Ker boats it is simply moved by a continuous line, the two ends of which emerge just forward of the backstay and pull the tab in different directions. Aside from the method of control the gear is exactly the same, although scaled down, to what is used on Cup boats. "Normally the tactician or someone else at the back of the boat controls the trim tab and obviously it needs to be controlled in the tacks and then sometimes small adjusts need to be made when you are sailing along in a straight line," says Ker.

[Presuming Ed]

The point of trim tabs is that you get an assymetric foil - like an aeroplane wing, rather than the ususal symmetrical foil. This gives you more lift for the same drag. So the trim tab goes to leeward to increase lift to windward.

On some 12 metres, you can link the trim tab to the rudder for manoevering - especially useful in the pre-start.

[Don Z.]

Hey, I have one of these gizmos...

Short answer: It depends.

Long answer: Keep in mind that most (not all, but most) boats go to windward better with a little bit of weather helm. The problem is that the big rudder you have for manuevering well can sort of slow you down (increased drag) as well.

The trim tab is smaller than the rudder. Cocking it a bit decreases weather helm on the main rudder, but gives you the hydrodynamic benefits mentioned by Ed. Keep in mind that if the tab or rudder goes to leeward, the helm is to weather. Hard to see on the wheel of a 12, but instantly recognizeable on the tiller of an 8.

If set up correctly, it will give you all the benefits of a slight weather helm, but the main rudder stays neutral, limiting drag. If you want to compare it to an airplane, think of it as the little tab the pilot adjusts by wheel, whose purpose is to balance out the plane with its actual load. The idea is to find the point where the controls are basically neutral, requiring little input from the pilot for straight and level flight.