Need advice on building spade rudders - SailNet Community
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post #1 of 5 Old 05-05-2010 Thread Starter
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Need advice on building spade rudders

I bent a spade rudder on my newly-acquired MacGregor 36, and inspection of both rudders makes me think they have been bent and straightened more than once in the past. I think they should be replaced, but can't afford to have them professionally fabricated.

That leads me to try building them myself. I have found some information on how to go about it, including an old thread from Sailnet, but I would appreciate any other advice or resources, online or elsewhere. I particularly need input on lamination schedules, designing the internal web structure, and how to prevent water penetration where the post enters the rudder.

Is there any reason to consider a different foil shape, or is MacGregor's design from 35 years ago good enough?

I'm considering carbon fiber posts, if (big if) I can find them or do a respectable job of fabricating them. I understand why many consider stainless steel preferable, but for my boat carbon might be a good idea for a couple of reasons:

1. With two rudders and two transom-mounted outboards, I have double redundancy in case a rudder is damaged. A shattered carbon post that can be completely removed might be preferable to a bent and jammed stainless steel post.

2. This lightweight catamaran is particularly sensitive to weight, and any savings, especially at the stern, are valuable.

Does my thinking make sense?
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post #2 of 5 Old 05-06-2010
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A recent edition of Good Old Boat had a good article on building a rudder using a method much like the methodology for building a wing-light weight ply "ribs" with solid closed cell foam between. It would be adaptable for your application and you could build them in your the garage without much difficulty.

I also suggest that you not try to use carbon fiber stocks. Attaching an armature (the framework within a semi-balanced spade rudder) to Carbon fiber that one can rely upon is very difficult for a person not thoroughly versed in composit bonding. By comparison, having an armature welded up to a Stainless stock is a fairly simple job that any compentant machine shop that works in Stainless can do. You can split and use your old rudders as a guide.


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post #3 of 5 Old 05-06-2010
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I would be wary of using carbon fiber for the rudder posts. Carbon fiber has more galvanic issues with metal fittings and are far more likely to fail without warning.

Unless you have reason to use a different foil shape, I would stick with what they designed for the boat originally.

I'd point out that most multihulls are better off if they're a bit aft heavy, rather than bow heavy... since most have more flotation in the aft end and burying the bows is one of the most dangerous things that a multihull can do.


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post #4 of 5 Old 05-06-2010
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Don't replace them, rebuild them! My Etap 26 rudder has an aluminum post that was badly corroded just above the blade. I used an abrasive blade in a dremel to split all the way around, and one half peeled right off. The other half was glassed to the armature with strips of roving. I just cut that away, cleaned up the armature, ground away the corrosion nearly a 1/4" deep and got it rewelded. I then hand filed and scraped it until it was smooth and round again. I glassed the armature back in, put the other half on and taped it with fiberglass all the way around.

Then put two pieces of black poly tubing about 3/4" in diameter down into the blade. I poured a medium density two part foam into the tubes so it would go all the way to the bottom, then withdrew the tube. I made a second pour of foam on top of the first, and finished with a third pour that expanded out of the hole at the top. I then glassed that over and faired the rudder. It looks like new, and should be good for another 25 years!

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post #5 of 5 Old 05-07-2010
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I've no experience, but I read a thread on another site (sailing anarchy) where famed designer Bob Perry chimed in recommending carbon fiber rudders. Basically, the thought was that you can essentially create a rudder that is monocoque (that is, the armature and the rudderstock, skin) is essentially one piece. Thus, the problems with water intrusion, expansion/contraction etc. and corrosion due to dissimilar materials are fully avoided.

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