First up, I know nothing about rudder design. But, I want to play - LOVE to tinker with this stuff.
HOWEVER, I do have a few years experience designing, cutting and vacuum bagging high performance RC glider and jet wings (super critical to hit correct profiles, strength etc.). After a little research, it appears most high performance rudders are merely using a NACA airfoil of one version or another. It APPEARS to me, at first that most of those composite skills, programs and tools directly carry over.
Been looking around, but don't see clear cut conclusions as to what NACA foils would work well with beachcats. Anyone know? Also, what is better - elliptical or tapered planform? I can easily plot the rudders, build templates and hot wire cut rudders out of EPS, then reinforce and fiberglass (epoxy reinforced with carbon cloth). What I'm curious about, mostly is what kind of stresses these things see laterally?
I believe I can overcome tendencies to flutter/cavitate since I can calculate the center of pressure on the foil; just don't know if the starting point is the typical 25% of rudder chord or what (given rake is adjustable).
Anyone here done a full-up composite rudder set before?
And, there is the option to do a full-on hollow molded set...
After consulting with 2 Naval Architects I made 3 centerboards for my boat all based on the NACA 0009 airfoil and a 30% chord:
All were vacuum bagged multiple laminate schedule epoxy over 9lb high density foam that was hand shaped.
One was 12" longer and had less width but same square inch (wet) area than the stock board from the manufacturer. Another was same template as original but 25% less weight in the laminate schedule. The third was to stock laminate specs which I only use in high winds.
I use the long, thin, skinny board most of the time. It points 4-5 degrees higher than the stock board and is faster than the stock board on all points of sail. The stock board was designed in 1977.
If I did it again I would do the long board in a tri weave composite material at 75% of laminate schedule and possibly a less dense foam.
My next big project is replacing the stock rudders which are also designed in the late 1970's. I will be using the NACA 0009 foil and a longer, thinner and more squared shape for the rudders.
Hope this helps and good luck with your project.
Stiletto 27 CE
Awesome - thanks, Brad.
What IS your laminate schedule? I'd be using Dow high Load, I think and definitely hot wire cutting because my rig is automatic and makes perfect cores every time.
So, from what I can garner from pictures, a high aspect ratio rudder is preferred to the elliptical? That actually makes it WAY easier to build and more accurately. I was thinking on the order of: foam: 6oz satin, 6 oz satin, CF uni-strips run vertically, 4 oz, 1.5 oz on outside, vacuum at whatever mark that sharpie mark is at and post cure @ 110f for 6 hours. Lay out the layers of cloth across each other, 90 degrees for torsion control and put in hard points where the bolts come through (just hardwood dowels). My current favorite is Pro Set epoxies (have some that's 6 years old and STILL sets up as if new- but consider it "play material" now).
I pretty much know I'm going to fiddle with this at this point - will share pics and methods as I go.
Eppler E836 is the latest and greatest foil for boards, and will likely work well on a rudder.
Shape has little to do with the boats hull shape but a lot to do with board placement relative to the rig and exactly what helm feel you want. I prefer longer, skinnier rudders as they are more sensitive to sail trim and thus help you go faster. Checkout the Scorpion, Phantom and Cirrus R2 for the latest in high performance straight board thinking.
Delftship Free has a nifty rudder design wizard. Although the program is designed for ship/boat design and thus is pretty sophisticated, I've used it for a wide range of projects, including a kiteboard and a windsurfer. Designing a rudder using it would be easy peasy. ;)
Edited by gobigkahuna on Sep 07, 2018 - 07:53 PM.
I slightly modified the schedule I got from the original manufacturer. On the board that works best I did a 4" wide strip double layered on the bottom and running top to bottom centered on the chord using 6oz plain weave fiberglass mat and west system. Then 3x layers of 2 oz and West. Each layer was bagged. Then finished by hand fairing and white gel coated.
I am told 3x layers of 2oz properly vacuum bagged is 2x stronger than a single layer of 6oz.
That schedule is a complete overkill for most beach cats. The centerboard fits a Stiletto 27 beach cat that weighs around 1400lbs and it just has one centerboard, not 2 like most beach cats.
I think if I had used something like a tri-weave aramid/kevlar/carbon fiber composite material I would just do a 4oz strip in fiberglass and 2x layers of 2oz tri-weave with no gel coat.
Because of the 6oz strip on the chord the finished profile thickness is about half way between the Eppler E836 and the NACA 0009 shape in total thickness. That is still significantly thinner than the stock board.
Other specs: the centerboard is 7ft long, 14 inches wide and weighs about 65lbs. A replacement board costs 2K for that boat. I estimate that I made mine for about 500$.
The hardest part was getting the foam shaped. After two failed attempts I paid a local surfboard shaper to do it.
Stiletto 27 CE
Edited by bradinjax on Sep 07, 2018 - 09:50 PM.
Search Bill Roberts posts about foils on this site.
He is the designer of the Super Cat and Arc catamarans.
He goes on at length and in great detail.
1983 SuperCat 15
Very cool-thanks guys; exactly what I was looking for.
The technique I've used elsewhere is to wax a piece of thick mylar, cut to just wrap around 1/2 the rudder plan form, like a template, the wet out layers of cloth and reinforcement on the mylars. Then make a toxic taco out of two mylars/wet out cloth and your foam core and stuff it in a vacuum bag. When you peel off the mylars, the finish is glass smooth, requiring very, very little finishing. Alternatively, you could used peel ply before the mylar and get an easily sanded profile that grabs primer/gel coat.
I hope I get a chance to get to this project by about Christmas break...heated garages are awesome.