New Rudder

[MQMurphy]

I have two photos here of the pattern that I've made up for my new rudder. One photo shows the "comb" that I plan to have a local fabricator make up from stainless steel and weld to the rudder stock. This is an idea that Don Casey sketches out in his book "This Old Boat". I plan to use two slabs of marine ply, probably 5/8" each. I'll rout out recesses for the "comb" on the inner face of each piece, then fasten them together, fair them to a foil shape and glass them over. At least that's the plan. Any comments/warnings? Thanks.

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[FloatingMoneyPit]

Are you going to fair the plywood itself, with a planer or grinder?
I guess if I were to do this, I would consider using successive layers of 1/8" ply. The innermost 2 layers would be the full rudder size, the next a few inches shorter on the trailing edge, et cetera, until the outermost layers which would be a narrow strip along the rudder stock. Then goop a bunch of fairing epoxy on the gap between each step, scrape a thin piece of ply the shape of your airfoil down the rudder, and you should have something approximating the shape. Another session of fairing and some encapsulating/strengthening glass, and I'd be done.
Then again, I think I do a lot of stuff pretty half-ass, so caveat emptor!
Food for thought.
I'm sure there are rudder rebuild examples in many publications, but I haven't had to deal with that so I skip the chapters. Look around.

[Tim]

Your plan will work as well as any. There's no hard and fast rule, or perfect way to do any rudder; each situation is different.

What you have proposed will provide a strong rudder. Be sure to coat all areas of the plywood well, and use only high quality plywood in this application. One potential avenue for future water ingress--which could damage your hard work--is where the rudder shaft enters the rudder. Do your best to seal this area very well, though I think this area is always susceptible to water ingress.

Beware that removing substantial amounts of plywood, depending on how much of a true foil shape you want, will take quite a bit of time and effort, as plywood sands and planes a lot harder than solid wood. But it's do-able. FloatingMoneyPit's suggestion regarding the stepped layers bears consideration, but the two thicker layers will work fine too.

Good luck!

[MQMurphy]

Thanks for the input - FMP's suggestion sounds interesting. Another friend suggested less plywood and a layer of foam as a top layer each side. He was afraid it'd be too heavy. I'll try a little test with the 24 grit disc in my 4" grinder and a scrap of ply. Tim, I haven't got the original rudder to work from so I don't know exactly how the plywood-steel-cloth detail is handled at the leading edge. I figured that I would shape the ply edge concave at that end to fit against the stock and then fair it with putty before wrapping a layer of cloth around that edge.

[krissteyn]

CAUTION CAUTION
I did almost exactly what you are planning on my Bristol 27 and ...

After doing a fine west epoxy and marine ply beautifully fared and fitted !
It was too light. On the hard, the rudder always went to centre but in water, the tiller always flopped to one side causing fun & games when single handing.
Suggest you include space for a lead weight on the bottom.
Also - suggest you drill a hole big enuff to take an eye bolt on the stern upper corner, seal it well, and fill with putty or similar before finishing.
This will give you a purchase point for an emergency rope operated tiller/steering mechanism. ie in the emergency, lean over the side and push the plug out with your finger or similar... and attach a line.

good luck
kris

[MQMurphy]

Thanks, Kris. That is a modification worth considering. I took the shaft to the fabricator on Monday and we discussed the plan. He suggested that I get a new piece of shaft - was suspicious of the areas of oxydation (spalling? those etched-out spots) he also laid a magnet to it and said that I should make sure to get a piece of 316L. The existing shaft is a little magnetic. To be continued . . .