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post #1 of Old 08-19-2004 Thread Starter
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Building a New Ruuder

I''ve built a new rudder for my boat a 77 Newport 30. Old one was indeed not working like I had assumed (Stock slipping in the rudder). The bracing on the stock inside the rudder had corroded and was sheared off. The rudder post turned about 60 degrees through the rudder before engaging and actually moving the rudder. After a bit of thought about what to do, I decided Id try my hand at building my own. Quotes for new ones (with the same 27 year old design) ranged in the high $2K to mid $3Ks. Too much for my blood.

So, after a bit of research, I chose to design a new one and build from scratch. The foil shape I decided on is the NACA 0012 foil. It''s a narrow elliptical shape and has undergone thousands of hours of testing by the aero industry. Turns out that its the best foil shape for both lift and the absence of drag for air speeds in the range of MACH 1.0 to 1.2. Also turns out that the fluid dynamics of air speeds in that range are very similar if not identical to underwater fluid dynamics at 4-7 knots. Lucky for us sailors. Its the foil design going into almost all new production boats out there (Hunter, Catalina, Beneteau, etc.)

So, based on width and length of the rudder (20 x 51) the midrange for a boat of my size, I was able to get the cross section measurements of the foil. Some website out there which I cant seem to find now had a shareware program which kicked out the cross section numbers for a given set of measurements. A lot of RC airplane people fiddle with this as well as a number of aero engineering people, so its not too hard to get hold of the numbers.

I hand shaped a blank of cheap poly foam from home depot. I knew the depth I needed to cut into the blank from the numbers generated by the program. At each inch (from leading edge to trailing edge) I ran a router down the length (top of rudder to bottom) of the foam at the desired depth. I then used a 12 rasp to cut the foam down to the grooves made from the router, this gave me the general shape to within a few 1/8s of an inch. I cut a piece of plywood 24 long by 3 wide and cut a female pattern (reverse of the foam) to do the final detailed shaping of the foam blank. I hand sanded in front of my plywood pattern board to get the perfect shape. I then ran the entire foam blank with a board 18 long covered in 100 grit paper to get a final smooth surface and to take off another 1/8 to account for glass and fairing compound. The blank cost about $10. I figure I''m about 96-98% accurate to the designed foil shape which is good enough for me. This only took a few hours.

I then laid out 3 layers of 6oz fiberglass with 2-part West System epoxy in laminate layers (I left the top of the rudder open so I could later carve out the foam blank, leaving the fiberglass shell). Cost about $150 for all the supplies and another $30 for the various tools (squeegees, rollers, pots, pumps, etc.) and such. I bought everything from the US Composites website which is about half the price of West Marine or any other place around here. I spent about 1 hour a day on each layer and filleted the edges between each layer using the West additive 403 filleting compound for epoxy. After I completed the glassing I filleted the voids and any seams still showing. Another 3 days for about an hour each. Then into the fairing stage. A few coats of epoxy with the West System 410 fairing compound additive to really give the whole board a smooth finish 2 days for an hour each, very easy to sand down with a long board (3 inch wide and 18 inches long 1/2 thick) with 3M 100 grit sticky back paper (the best sandpaper in the world, it lasts forever without losing its bite, but very expensive up front. I bought a 45 yard roll a few years ago for about $45). I finished with a few coats of pure epoxy to really seal everything up, another hour.

My next step was to carve out the foam blank around which I built the rudder. It was very easy and took less than an hour. I used a 6'' piece of 3/8 rebar sharpened to a chisel edge. I was left with the finished fiberglass shell.

I was going to order a new Stainless Steel 316 post (the old one was actually bent and had lots of pitting). The post was 50 by 1-7/8 O.D. but the damn things cost about $100. Plus I''d have to get the cross tabs welded on and I''d still have a future issue of corrosion and failure. So I opted for going with 100% composites. I ordered a Carbon Fiber (and Epoxy) post, out of MA (super pricing on any rolled composite) with the same O.D. as the stainless but with a wall thickness increase of about 100% (1/4"). This post will have its weaves and threads at 45, 90 and 180 degrees at various stages and have about 135% more torsional, compressive, and modulus strength as the old stainless measure, so I''m basically very comfortable with it. Post was $130. I just glassed the tabs onto the post for its entire length within the rudder to about 6 inches wide, it basically looks like a rudder without the fiberglass shell Ive made, another hour.

I lined up the tabbed post inside the finished fiberglass shell at the proper angles from the old rudder (i.e. post is longitudinally straight and about 9 degrees off the vertical looking at the face of the rudder, the angle between the top of the rudder and the post is about 81 degrees). I also moved the post toward the trailing edge at a split of about 11% to 89% leading edge to trailing edge. In other words I left about 2.3 of the rudder forward of the center of the post and 17.7 aft of the center of the post. This was so I could get more balance but not enough to get lee helm, the old rudder was not at all balnced and could get very squirrely when the wind piped up or was gusting. I poured a two-part closed-cell expanding Urethane foam into the shell. (Remember I left the top open for this). This foam is whats called 16 lb foam, i.e. it weighs 16 lbs per ft^3. It also has a buoyancy of about 50lbs per ft^3. It has a very high compression strength (in the neighborhood of 400-800 psi) in other words you can barley dent it with a swinging hammer, and it sticks like epoxy. Its a liquid during the mixing of the two parts and pours very easily. It begins to expand within about 30 seconds of mixing to about 4 times its liquid volume, fills any voids and then hardens like a rock. I believe its whats used in all rudder manufacture today (or maybe they use 20lb foam, whatever). I needed about 1.0 ft^3 which cost about $48. Picked this up at US Composites as well, this took another 2 days for about an hour each day.

I glassed over the top of the rudder where I inserted the post and poured in the foam (which has been left open all this time), faired it and completed the rudder with an epoxy barrier coat, primer, and bottom paint, another 3 hours over two days. All the paints and primers cost about another $75.

All said and done the whole project cost just less than $500 in materials and about 20 hours of labor over the period of a 2-3 weeks. The old rudder came out of the boat pretty easy (didn''t even need to dry dock the boat), just undid the tiller attachment and the quadrant from old post and the rudder basically slipped out. There is a fiberglass sleeve inside the hull that the rudder post slips into/out of and rides higher than the water line. The new rudder slid into place easily. I will eventually (this fall) install a lower bearing for the carbon post so as not to attract too much wear and tear (another $100 and some more fiberglass work when the time comes). So now I have a racing rudder with about ft^2 more surface area (turns on a dime now) but less fluid drag than my old rudder, has more lift, weighs about 20 lbs less (it actually still has quite a bit of positive buoyancy, I had to tie dive weights to it so I could get it into place), and is brand new. Plus Ive learned an awful lot about how rewarding some projects on the boat can be.
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