Are you referring to the original or newer N20 rudders? The original are much slimmer than the later ones.

Didn't know that there were two designs. The only N20 I've seen in person is Dave Spain's boat, which I think is fairly old.

There are actually three different I20 rudders:

The original "pin head"
The "butterknife" (which came on boats for only three or four months - but was common on '04 and '05 F18s)
The "spade" - current fat, clover leaf looking rudder blade.

I use generation 1, 2, and 3 when describing them.

Spain's are generation 1, the narrow ones.

I still prefer those rudders - they do stall but because they are sensitive to proper sail trim...you can learn a little by learning to keep them from stalling.

how do you tell when a rudder stalls?

you move the tiller and nothing happens! <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" />

And your crew yells "W T F" as you careen out of control... Usually happened to me when reaching too hot with the spin up and trying to drive down too quickly..

You feel it - it's pretty dramatic...you turn and the only thing that happens is a really huge rooster tail at the back of the hulls. With it blowing about 12 to 15, sail upwind and try to bear away sharply without easing the mainsail and observe the stalling. Do this on a crowded start line trying to duck a boat and observe the odor coming from your crew's shorts. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

If you're really cooking - like downwind with the chute up in waves, you can feel the rudders start to lose traction if you turn too sharply.

...and you're heading for a spinout--been there done that.

Gotcha...

Our gen 1 rudders would loose it upwind in waves. Very annoying. Take a wave, then rooster tales and no helm. Step to the back of the boat, goof a little with the main sheet, they recatch and you retrim.

We're using the gen 3 rudders right now. So far I like them. They change the feel of the boat, it is far less twitchy than before. They haven't cavitate/ventilated yet, but we haven't yet hit the right conditions. Whether they are as fast is always the question in the back of our minds, but so far I don't have any hard evidence that they aren't.

Instead of grinding why don't you just use a spacer under the top gudgeon, washers or a delrin block. ( move the top back ). This way you can play around with rake easily. You can also re-drill the piviot hole.

The spacer might be a good idea, it never occured to me. It would tilt the tiller arms up, though, which might cause a problem.

I don't think the pivot hole can be moved, not enough meat in the casting.

I have re-drilled mine and your right the farther back you move the head the less support there is. However the Nacra 6.0 rudders I have are built quite strong. I have experimented with a few different rudder types in the Nacra castings and have ripped all of them off. Each time I put the old 6.0 rudders back on and go sailing.

Alum. rudder arm are very easy to adjust (bend) down once you find the rake your looking for.

But if you put spacers under the top pivot block, you're not changing the relationship between the rudder's center of lift and the pivot axis - the distance from the center of lift of the rudder and the pivot axis is what changes the load on the helm. Since this geometry would stay the same, it would have no affect on the helm since you're just angling the whole assembly.

I'm not so sure that's right. Here's how I see it. With the boat level and rudder down, measure half way up the rudder pin make a mark. Now with a level held vertically at that point draw a line down the rudder. Now space the top gudgeon and use the level again.

I think you'll find there is more blade in front of the mark on the rudder pin. You have moved the center of the pivot point back in relation to the blade and the blade forward so a little goes a long way.

If that doesn't make sense then I'll go by the fact that Robbie's T is set up that way. <img src="http://www.catsailor.com/forums/images/graemlins/smirk.gif" alt="" />

unless I'm misunderstanding something, it won't work - the pivot axis is still in the same relationship to the rudder's center of lift. Imagine the image below. None of the relationships between the loads have changed. It may accomplish something else - but it will not change the load on the helm.

There are several dynamics involved here. Traditionally moving more foil area ahead of the pivot makes the boat feel less heavy on the helm up to the point where it gets too sensitive and squirrely.

I’m not sure if that really changes the relationship of airborne and waterborne forces very much.

Think of a windsurfer where raking the sail forward or aft accomplishes the steering. There is a perfect place where everything is balanced and the boat (board) goes straight, with no helm pressure at all. I would think that this is what you are trying to achieve by changing mast rake. The placement of underwater foils plays into this equalibrium.

Both ways of moving the rudder accomplish the same when figuring points of lateral resistance. Tilting the pivot shaft with shims at the transom will not affect rudder balance and sensitivity. Sensitivity is what comes from tilting the blade forward or aft of the pivot point.

Of course mast rake also affects other things, such as the slot and sail angle of attack which may have a greater impact on absolute boat speed and ability to point.

What does Wouter say?