Rudder / Transon spacing

[styge]

After nearly three years of no progress on Tin Bin, I am finally able to spare the odd minute here and there to start making headway toward one day having an aluminum CS17 that actually sails!

I have been putting together the rudder, out of aluminum, of course - I'm trying to stick with a theme, plus I have so much of the stuff left over! Anyway, I am trying to figure out the correct spacing from the front edge of what would be the rudder cheek to the transom. As I don't have the same construction as a wooden one, there aren't too many clues from the plans. Some general consensus on this must exist already, so I would love to hear the feedback from everyone. I have cut the whole rudder from one piece of 3/8" sheet 5052. It looks pretty nice, I think!

Thanks

[Brent]

Styge,

I didn't measure, but I am thinking the spacing between transom and front of the rudder cheek is about 1 1/4" on my boat. This is totally dependent on the gudgeon/pintle arrangement. I bought commerically made gudgeon/pintle sets. I figure you are making your own, right? Make your gudgeons and pintles mate up so that there is just enough room for the rudder to swing without impinging on the transom.

BTW, make one of your pintles with a 1/4" longer pin. Mount that pintle in the lower position, so that you can thread the lower gudgeon on first, making it easier to get the rudder on when there is some chop. But you are probably way ahead of me on this already.

[JeffM]

Styge,

I get 1 1/8" with the Racelite pintles & gudgeons I bought as part of the hardware kit Graham & Carla sell. The longer lower pintle does ease the lining-up when you hang the rudder. By the way (it's probably too late to mention, but) you might want to do a search on this site for discussion of NACA foils for rudders. A foil-shaped rudder will stall less readily than your flat aluminum one. It's less of an issue with the cb, since it won't develop as much angle of attack.

[styge]

I've had more time to think about this and I am more concerned about the spacing of the pintle from the rudder than the distance of the rudder from the transom as this is what sets the pivot point and therefore the deciding factor regarding how the rudder will perform, I think, in relation to wether / lee helm. I'm a bit of a green horn!

Jeff, not sure what you mean by the rudder stalling, can you explain.

My rudder is indeed rather flat, but the CB has a nice foil shape to it thanks to a 1/2" rod running down the center top to bottom before being welded shut.

Thanks.

[JeffM]

Rudder (and cb) work a bit like an airplane wing. A wing lifts the airplane due to the wing's curved shape, and the angle at which it moves through the air. The plane is lifted as air moves over the top surface faster (generating lower air pressure) than over the bottom surface (generating higher air pressure). A sail works exactly this way, but lifts the boat forward. In the case of a rudder, as you move the tiller and change the angle of the rudder to the water, the water moves slightly faster (hence at lower pressure) over the "upper" side of the (e.g. left side if you move the tiller left) than the "lower" (higher pressure) side, generating force that "lifts" the stern to the (e.g.) left. This works fine unless you swing the tiller too hard: then the sharper angle of attack causes the smooth flow of water over the "upper" surface to become turbulent; the rudder stalls, loses lift, and you lose much of your control. I've experienced this when greatly heeled and pulling hard on the tiller to counteract a strong lee helm: the rudder merely drags ineffectively through the water. (Better than doing it in an airplane, though!) The fatter the foil shape to a rudder (within reason), the greater the angle of attack before the rudder stalls. A flat rudder, on the other hand, stalls readily, so it is less forgiving of hard ruddering. The centerboard, staying in line with the boat, is less likely to face an angle of attack that will stall it, though I suppose it would stall if you were sailing in thin water with too little board down to prevent substantial leeway. (Note that I learned much of the above right here on this forum!) Any book that diagrams airplane wings would make this easier to follow.

[styge]

Thanks for the explanation. That makes perfect sense.

I still need to figure out how far out from from the rudder cheek to set the pintle. I'm sure it it position will effect how much or how little weather helm the rudder will have. I have Schaefer pintles from West Marine.

Graham, any comments on this?

Thanks

[Peter HK]

Styge

I'm not sure I quite understand what you are asking but here goes.

Weather helm is really determined by the rig and centreboard i.e. where the centre of effort of the rig is compared to the centre of lateral resistance of the centreboard. Essentially it represents how much the rudder needs to be held one way or the other from dead central to hold a straight course. Google these terms if you are not sure.

The rudder balance is a different thing- if all of the rudder is behind the line of the pintles in a lateral view then helm is heavy as is the case if a lot of the rudder is in front of that line. Think of the rudder kicked up coming into shore - the helm is heavy. If the rudder were all angled forward then the opposite heavy helm would be felt.

Just go with the plans- pintle distance the same top and bottom and the rudder to have a slight lead in front of the line of the pintles. If you're worried, then allow a little more lead forward and adjust it by putting a block in later.

Peter HK

[JeffM]

I couldn't have said it better. Styge, the center of my pintles is 5/8" from the transom, and just over 1/2" from the front edge of the rudder cheeks. Originally, my rudder didn't swing quite far enough forward. Now I'm happy with it: if you look down the pintles, the leading edge of the rudder just touches that line. Remember that a little weather helm in most conditions is good, since it allows the rudder to earn its keep sharing a bit of the work of lateral resistance. Not only can the "weight" you feel on the tiller be adjusted, but so can the "weight" the boat feels on her ruddder. Having a centerboard means you can adjust the helm to increase or decrease weather helm pretty freely on the fly.

[styge]

Great, thanks guys. That helps me out alot. Things aren't overly obvis to me at first glance, and building in aluminum adds a little slant to things as well!

Hopefully I can post some pictures when I have it rigged!