Rudder length

[slowpoke]

In light of the recent spate of rudder and gudgeon failures, has anyone looked into shortening the rudder?

[admin]

No, but I've looked into overbuilding mine!

On the other hand, a wipe out or ugly round-up is better than losing a rudd (unless there is another boat close by, on your weather hip). So maybe you are on to something?

[Kevin]

I think Andres point on his failure was the closeness of the top and bottom gudgeons to each each other. I'm planning to maximize the separation of the 2. I'll have a aluminum gudgeons on the transom, and then 4 on the rudder cassette going above and below the ones on the transom with a SS pin going through the whole lot. That should maximize the load separation about as much as possible. So from the bottom of the bottom one on the transom to the top of the top one on the transom would be that separation. That would be more than I had before which was only top to top of each. (I hope that makes sense.

I'll post up what I'm doing once I get some materials in house. Should be here by the end of this coming week.

Kevin.

p.s. Oh, with smaller kites you would create smaller loads. Andrew's big kite is 55+sm. That's almost 600 square feet. My big one is 485 at the moment.

[slowpoke]

I saw the post Andrew did about thinking the gudgeons were too close together, and he's right. I think the length of the rudder contributes to the amount of force being directed to the bottom gudgeon. Remember also that josh's rudder literally tore the carbon fibres apart where the rudder pin ran through it. AWhole lotta force there.

p.s. also; bigger [u]is[u] better, right?

[Kevin]

Yes, I think you are right rocky. I think what they both (Andrew and Josh) probably need is a smaller profile rudder for those heavy conditions. They can control the boat just fine when they are doing 15 knots. A puff comes and they try to control the boat with the rudder. It has more than enough lift to control the boat and counter the force of the puff. With a smaller rudder they would lose control and wipe out before they broke the rudder. You can either shrink the profile (9% instead of 12% for example) or shorten the amount of rudder in the water. I would probably opt for the former. They could also fly smaller chutes when it's gusting to 30 knots, but they just don't believe in chicken chutes down there. It's all out all the time and if you break something then you broke it.

That's my thought at the moment anyway. I'll let you know more when I have my own rudder repaired.

[slowpoke]

Profile as in chord, or width, or length, or all three? My biggest worry with shortening the rudder would be loss of control as the boat heels,which puts me back into the area of dual rudders.

[Kevin]

You can reduce the chord length which will reduce the width of the foil. That will reduce surface area and should maintain the lower speed behavior of the foil for the most part. It will still reduce the amount of lift the foil can create.
You can reduce the profile percentage and keep the chord length which will reduce drag and lift but won't reduce surface area. At low speeds you may not create enough lift to control the boat. That's why this is so tricky and fun to figure out.
You can reduce the length which will reduce surface area in the water and leave everything else the same. You point about when the boat heels makes sense. But remember, the most you are going to loose out of the water is 8-12 inches and that's with a lot of heel angle. The transom is only 4 feet wide at the waterline so there isn't that long of a lever to pull the rudder out.

I'm reducing my chord length from 12" to about 10" and I'm changing my profile away from a naca0012 to something different. I'm not reducing my width percentage because I'm in light conditions way too often to do that. I'm also going to a cassette so I could play with pulling the rudder up a little to see if the impact of the length on performance. I'd shorten in light conditions until I loose steerage and then make that my new length is probably how i would do that. But that's moons away from now before I have time to play with that.

Cheers, Kevin.