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post #1 of 47 Old 12-02-2007 Thread Starter
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Need to make new rudder

Our SJ21's kick-up rudder is trashed. Really not worth saving. (Lovely 3/4" twist down its length, too.) It's a mahogany blade about 40" long by 12" wide, thus:



So how should I construct the new one? This thing (as is) weighs a ton, so I'd like to do it the lightest way practical. I gots any amount of tools and some fiberglassing experience.

1) solid wood, probably mahogany, with spar or poly varnish. That's what it is now. And if it was good enuf for grandpa, by God it's good enuf for me.

2) wood core with glass mat and 2-part epoxy. In which case I'd probably consider painting the wood core before layup, or the final product, or adding pigment to the epoxy. Just because I like a pretty, white foil.

3) foam core with glass mat and 2-part. Should be nice & light. If so, what kind of foam to use, & how best to affix the pivot bushing?

4) carbon fiber. Never used it before. Over wood, foam, or air its own self?

Buccaneer18, Grainnia
SJ21, Diarmuid
Albin Ballad 30, Fionn
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post #2 of 47 Old 12-02-2007
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Choice 1) obviously good enough if you can find suitable stock to make it from.

2) Possible but may not end up lighter, but just paint the thing afterwards once it's all been faired.

3) Usually done with a two part mold and the halves sandwiched together... could shape a piece of rigid foam and glass it over, then fair and paint as above. You have to be sure it ends up strong enough for the forces applied and for the attachments too.

4) Pretty serious overkill for a SJ21 (but might be fun)

Last edited by Faster; 12-02-2007 at 09:36 PM.
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post #3 of 47 Old 12-02-2007
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Just make one out of wood.

You can make it in 2 or 3 tones of wood. Like his.

And to help the boat go fast, after varnis, sand it down with very very fine sand paper to remove the shining, that causes friction.



Don't bother with applying glass over it...its better looking and lighter in wood.

For foam, you need a lot of sandpapapr aftewrards, a vaccum system etc. etc.

CF, your rudder will cost over $800, and you need an oven.

Wood, simple good and beautifull.
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post #4 of 47 Old 12-02-2007
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I had to fix ours up not long ago... This is how I did it:

1. Take 5 or so strips of oregon (glued & clamped) - enough to make a plank about 12" wide by 3/4-1" thick.
2. Plane to shape roughly by hand, then use 40-grit sandpaper to smooth and final shaping.
3. One coat of epoxy with one thickness of glass mat laid up whilst still wet.
4. Final coat of epoxy over the mat when dry.
5. Sand out any imperfections, prime and paint.

Using a soft wood makes it easy to shape the blade, whilst epoxy coating gives it strength - true, you don't need to glass it, but if the wood is a bit thin in places it helps. You could use straight glass or carbon fibre, but you'd have to mould it somehow and, unless you're after really light weight, if you're only making one rudder that doens't make much sense to me.

You could use a polystyrene foam sheet, cut it to size, sand it and glass it, but making the pivot hole strong enough might be an issue and later reshaping is impossible (foam isn't as forgiving as wood).

Good luck with your project.

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Last edited by Classic30; 12-02-2007 at 09:41 PM.
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post #5 of 47 Old 12-02-2007
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I just made one for a friend. Got a piece of mahagany and cut it to shape. Planed it with my handheld power planer then a coat of epoxy and glass. One coat of epoxy and two coats of varnish. Epoxy has to be painted or varnished to protect it from UV rays. I wouldn't have epoxied it at all, but I couldn't get 5/4 and I needed to add an 3/16th of an inch to the blade.

!! WARNING !! The above information is to be used by intelligent people only. If you are Stupid, could be considered a moron, or otherwise. You are instructed to disregard this information and seek the help of a licensed and bonded professional.
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post #6 of 47 Old 12-02-2007
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post #7 of 47 Old 12-02-2007 Thread Starter
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Terrific advice. Thanks & keep it coming! Given the condition of our lakes, we WILL be grounding on rocks occasionally. The trouble with wood rudders or centerboards is that when the bottom does get nicked (whether varnish or glass-coated), it exposes endgrain to the hungry waters. Once water wicks up into the wood, you are truly hosed, as the wood will swell and shrink and hydrostatic force will begin kicking off the hardshell.

So let's say I use cedar or mahogany for the main blade, then inlay an aluminum or brass or stainless strip down the leading edge, &/or add a metal prosthetic tip the last 2" of the foil. Inlays faired into the shape. I can do this easily: 8-foot beltsander laughs at such tasks, mwahahhaha. Worth doing?

Guessing a max-thickness to major chord ratio of about 10% -- or should I go fatter? Which is more important in the drag vs lift compromise, chord length or foil thickness? I.e., if I wanted more steering response, should I go fatter, or wider? This boat is slow upwind but flies on reaches and runs.

ETA: Holy Cow, Timebandit! *faints* Dat's a bootiful thing. Hmmmm. I have access to any sort of industrial plastics, too. Hadn't even considered HDPE or PVC. Guess we should add that to the quiver. Urf. Anyone got six bills they can lend me?
(FWIW, I'm not concerned with class rules as I won't be racing one-design, so anything goes.)

Buccaneer18, Grainnia
SJ21, Diarmuid
Albin Ballad 30, Fionn

Last edited by bobmcgov; 12-02-2007 at 10:21 PM.
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post #8 of 47 Old 12-02-2007
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Originally Posted by bobmcgov View Post
Terrific advice. Thanks & keep it coming! Given the condition of our lakes, we WILL be grounding on rocks occasionally. The trouble with wood rudders or centerboards is that when the bottom does get nicked (whether varnish or glass-coated), it exposes endgrain to the hungry waters. Once water wicks up into the wood, you are truly hosed, as the wood will swell and shrink and hydrostatic force will begin kicking off the hardshell.

So let's say I use cedar or mahogany for the main blade, then inlay an aluminum or brass or stainless strip down the leading edge, &/or add a metal prosthetic tip the last 2" of the foil. Inlays faired into the shape. I can do this easily: 8-foot beltsander laughs at such tasks, mwahahhaha. Worth doing?
You probably don't need to do the entire bottom of the blade, but it's definitely worth doing.

I neglected to say that when fixing ours (it was suffering from the effects of the PO's tendency to hit rocks with the rudder blade), I hacked a piece of walnut from an off-cut in our firewood stack and shaped it in a wedge to fit the bottom-front corner of the blade - served the same purpose, but could be sanded to fit (just! ).

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post #9 of 47 Old 12-02-2007
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So how should I construct the new one? This thing (as is) weighs a ton, so I'd like to do it the lightest way practical. I gots any amount of tools and some fiberglassing experience.

Quote:
1) solid wood, probably mahogany, with spar or poly varnish. That's what it is now. And if it was good enuf for grandpa, by God it's good enuf for me.
Personally I'd choose this option.

Quote:
2) wood core with glass mat and 2-part epoxy. In which case I'd probably consider painting the wood core before layup, or the final product, or adding pigment to the epoxy. Just because I like a pretty, white foil.
Wood and fibreglass have different expansion rates. Eventually - not sure how long it will take, it will separate. If you should happen to ding your rudder and the water gets under the outer resin layer, it's going to start to decompose. Also - it's harder to get this fair and congruent than you would think. The rudder is a fairly important piece, and your boat is "performance oriented" so I'd avoid anything that had the potential to disturb flow.

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3) foam core with glass mat and 2-part. Should be nice & light. If so, what kind of foam to use, & how best to affix the pivot bushing?
You need a stainless armature in order for this to be durable. If there isn't one in the rudder already it could cost you a few hundred dollars to have one made. Also, it's difficult to get both sides of a rudder exactly the same unless you are using a mould. Both sides the same is more important than you might think.

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4) carbon fiber. Never used it before. Over wood, foam, or air its own self?
Again - hard to mould and there have been more than a few documented cases of carbon fibre things failing at inopportune times. It seems that they are finding out you need specialised epoxies to establish great bonds. You need a stainless armature in order for this to be durable. You may well end up spending a pile of money for something that doesn't work as well as a nicely shaped wooden piece.
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post #10 of 47 Old 12-03-2007
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I usually resist throwing money at a problem, but I must say that plastic rudder is a nice piece of gear. I think the beauty of it is that you could pretty well make a spare blade from plastic or wood/epoxy before you even install the new assembly, and then if it gets damaged or just cruddy, you've got a replacement ready.
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