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Old 04-15-2003
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Building Rudder/Sealing Marine Plywood

Hi,I need some help as I have to build a rudder for my boat because my old one fell off some how towing it home. I wanted to just buy one but none of the salvage yards around here have one.

I was planning on using marine plywood and trying to design something similar to what I had before. It was a swing up rudder. My boat is 25 feet long and weighs about 4500 lbs. I figured I''d build something that goes about 3 1/2 - 4 feet deep and is about 18 inches long, and 3/4 inch wide. I''d round out the leading edge of the rudder and leave the back square. I''d have it swing down to the 3 1/2 or 4 level but when up only be about 12 inches. I''d probably just use ropes and blocks to lock it in postion.

Well there is my basic design idea, any feed back would be greatly apprieciated as I am not an extremely handy person. My question is what would you use to seal the marine plywood?
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Building Rudder/Sealing Marine Plywood

Rudders are usually built with a core of wood or other material, encased in fiberglass. The shape that you are suggesting, with a rounded leading edge and a squared-off trailing edge, doesn''t sound like an efficient foil.

What company manufactured your boat? For example, if your boat is a Catalina, replacement rudders can be purchased from the company, or, if you log onto the Catalina 25 owners website, you can get plans for building your own rudder. Plans for building rudders are probably available for boats manufactured by other companies as well. Sometimes you can find "classified ads" on the owner''s websites, listing used rudders and other parts for sale. I just bought a used rudder for my boat by logging onto the owner''s website. Tell us the manufacturer of your boat, and perhaps someone on this forum can tell you where to find the owner''s website for your boat.
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Building Rudder/Sealing Marine Plywood

My boat is a 24.6" Friendship Terripen, I''ve been unsuccessful in finding any info on it. Your right about the design idea, I probably need to taper both ends.
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Building Rudder/Sealing Marine Plywood

Mauidan

Do you have any pictures of your boat out of the water on her trailer? If you do that might help you with your rudder design.

Walt ----- waltw@unnet.com (above is wrong)
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Old 04-15-2003
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Building Rudder/Sealing Marine Plywood

I suppose I''ll put in my two cents.

Plywood is good for building rudders mainly because as you plane off the wood to make the foil shape, your progress will be shown on the glue lines between the plies. For a twenty-five foot boat, I''d suggest you plan to make it 1 1/2 inches wide by gluing two pieces of plywood together.

As far as the shape goes, there''s an old saying: ďIf it looks right, it is rightĒ. Cut the initial shape out with a jig saw, then get a good and sharp block plane and go to town! Make the leading edge round and try a 5 to 1 taper for the trailing edge. Leaving a small, perhaps ľ inch round or flat to finish the trailing edge wonít hurt anything. Iím sure there are some who would cringe at that shape but it will work!

What seems like a lot of work will turn to enjoyment as the curls fly away and the shape of the rudder starts to come out of the wood. Be careful here because you may find yourself actually starting to like this stuff and having thoughts about building your own boat!

Once you''ve shaped your new rudder and sanded it fair, you can coat it in two coats of fiberglass. I''d recommend epoxy instead of resin for this job. WEST, System 3, what ever will do. Paint it with two coats of epoxy then start laying down the fiberglass while the second coat is still wet.

Once the glass has kicked hard, sand the rudder fair again and paint it over. Hang it on the boat and ask her what she thinks! Just take your time and you''ll be proud of the results.

Pi
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Old 04-16-2003
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Building Rudder/Sealing Marine Plywood

Thanks Pi, that was about what I had in mind just not as thick. As far as the figerglass goes would you use a fairly thin sheet?
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Old 04-17-2003
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Building Rudder/Sealing Marine Plywood

Honestly, Iím not sure what would be the best thickness, and type of glass to use for this job. Iíll direct that question to the others out here but if it were me Iíd buy enough woven glass for two coats and lay it on!
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Old 04-18-2003
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Building Rudder/Sealing Marine Plywood

Pi is mostly right. I have ended up building a lot of rudders in my day including one for my old 25 foot Folkboat. 3/4" is no where near thick enough. The rudder should be laminated from two thicknesses of plywood to reduce the likelihood of warping. Minimally the blade should be 1 1/4" thick (two layers of 5/8" marine plywood) with 1 1/2" probably being a bit better.

The two thicknesses should be glues together using a high quality epoxy resin. My current favorite is MAS but any quality resin, WEST System or System 3 will do. To avoid laminating a warp into glued up blank, you will need to glue up the two halves with the bottom layer lying on a flat surface. My two current preferred methods of clamping the two halves together while curing is either to lay plastic below the bottom layer of plywood, apply glue to the two halves and then put the two pieces together. Then put plastic over the two pieces and then pile weight evenly over the top of the laminated piece.

My other favorite way is to drill 1/8" holes at 6 to 8 inches on center in the top layer. Lay plastic below the bottom layer of plywood, apply glue to the two halves and put them together. Using a drill with a Phillips head bit chucked up I clamp the two halves together with 1" or 1 1/4" drywall screws driven into the 1/8" holes. When the blank has cured I remove the screws and fill the holes.

A 25 footer is big enough that rudder shape does count. The blade should be 15% to 20% shallower than your keel or centerboard in the down position should have at least roughly a foil shape (teardrop shape). That means that in rough terms that the leading edge is a semi circle and that the trailing edge has a long taper over perhaps something less than half of the horizontal fore and aft length of the rudder. For structural reasons the top of the rudder above the waterline is traditionally left square but is tapered down to the foil shape by the time the rudder is at the waterline.

Plywood is pretty easy to shape. I typically have used a heavy-duty disk grinder with a 15 or 30 grit floor stripping heavy-duty sandpaper to rough cut the shape. The glue lines allow you to see if you have humps or hollows and the glue lines should be fair and true. The key here is to keep the grinder moving rather than to stay in one spot. When you are close to the right shape you can knock down any bigger humps with a sharp, low angle, block plane and do a final faring vertically with a sharp jointer plane if you have one.

I would test fit the rudder and drill any holes for hardware that is required. This last item is a bit of a pain in the neck but well worth the effort. Plywood rudders want to float up. This makes them want to self steer if there is any rake to your transom and it means that you need a downhaul on your rudder which sort of defeats the purpose of having a kick up rudder. I typically pour a puck or two of lead into the bottom of the rudder. To do so, I either get discarded balancing weights at a tire shop or spent rounds from a shooting range. I have melted the lead in an old iron put in a gas grille but that can be pretty slow. I would take a look at a traditional boat building book for safety precautions because you do need to be careful working with molten lead.

I typically drill the holes for the pucks with a 2" hole saw and then take a rasp and taper the hole outward on either side. I then screw stainless steel screws radially around the hole so that the screw heads are below the face plane of the plywood. I screw sheet metal (with a light plywood backer) over the bottom of the holes and then pour the holes full of lead. Once cooled lead is easy to shape with the grinder. Because the wood scorches I fill the void around the lead with unthickened epoxy. Lastly, I fair the face of the lead and plywood with thickened epoxy.

Once the rudder blank is faired and ready I would follow the manufacturers instructions and apply several coats of unthickened epoxy to seal the plywood and then fill any voids, screw holes etc. with thickened epoxy in a syringe. I would apply a single coat of fiberglass cloth in epoxy and then do a final fair and coating with epoxy. The rudder will need to be painted to protect the epoxy from UV damage. While this all sounds like a lot of work, it can actually be accomplished in two of three weekends.

Good Luck,
Jeff
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Old 04-18-2003
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Building Rudder/Sealing Marine Plywood

Jeff & Pi both excellent!
Final coat of epoxy could use a "Barrier Coat Additive" such as WEST System 422 or 425.
Good luck!
Gord
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