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Old 19-12-2008, 11:37   #1
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Rudder construction

I'm going to be building a new transom hung rudder for a 22 foot sailboat. I'm planning on a wood frame skinned with marine plywood 1/4" and foam filled and glassed over. My question is, do I need to add weight to the bottom of the rudder? I would think it will want to float up out of the gudgeons. My existing rudder seems to be water logged.
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Old 19-12-2008, 12:22   #2
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Spooky—

The short answer is that, ideally, you should weight your new rudder so that you achieve essentially neutral buoyancy, whatever construction method you choose.

However, it seems to me (FWIW) that you are proposing a construction method involving more work than is really necessary. Consider that the rudder for your boat (a Reinell 22' per your Profile) really needn't be more than about 2-1/2 inches thick at its maximum. Given that, you could start with a "core" of 1/2" ply cut in the shape of your existing rudder and then contoured by laminating 4 successively smaller 1/4" plies stacked one upon another (on each side) in the manner of a contour map. The edges of the plies can then be sanded smooth to create the foil shape you'll want, epoxy sealed and finally covered with a layer of glass or two for moisture protection. If the plys for both sides are cut simultaneously—i.e. two at a time—and drilled with alignment holes at top and bottom (for alignment pins, say ¼” dowels, that extend completely through the blade) your contours should match pretty closely and, if you really want to get fancy, you can make up a set of chord templates for one side at intervals along the length of the blade that can be used for fairing the opposite side to match, although that probably wouldn’t be necessary unless you plan to race.

In any case, however, good luck.

s/v HyLyte
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Old 19-12-2008, 22:27   #3
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::nod::

svHyLyte has a good description.

A couple of additional points:

If you're using ply, as you shape the blade the plys and glue lines will create very useful lines showing where your shape is smooth or lumpy, and you can use it to compare the two sides. I used a hand plane to get the shapes, then electric sander to fine tune the shape.

You can calculate the weight of the materials and get a rough estimate of the total and its density - not easy, but it can be done. If you need to add weight, an easy way to do so is to cut a square in the middle layer of the appropriate volume for the lead, and pour it in (stick a couple nails into the sides of the cut out so there's something for the lead to grab onto.) Then glue on the cheeks and shape the blade.
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Old 20-12-2008, 04:49   #4
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Rolling sheer can be an issue with ply rudders and daggerboards, though on slower vessels it may not be as much of an issue

rolling shear gougeon - Google Search

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Old 20-12-2008, 07:59   #5
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David--

Good pick-up on the rolling shear issue. That thought occured to me but, truthfully, the Reinell is such a small yacht and the rudder accordingly so lightly loaded, I dismissed it out of hand. Perhaps I should not have but I don't think its an issue.

As for the weight of a laminated plywood rudder per Amgin's observation, I think that AA Marine Grade (or Aircraft Grade) ply can fairly safely be assumed to weigh in at roughly 32-35PCF. Assuming that only approximately 50-60 percent of the rudder is immersed (depending upon the rudder's shape of course) it would not seem that there should be much up-lift due to buoyancy. Moreover, I would expect that there should be a "keeper" on, at least, the upper grudgeon to hold Spooky's rudder in place. If up-lift does prove to be a problem, however, he/(she?) could simply bolt two slugs of elllipticly shaped lead weights to the bottom of the blade without ill effect and perhaps even to some advantage when the yacht's heeled.

FWIW...


s/v HyLyte
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