Instructables
Picture of How to Build a Sailboat Rudder from Scratch
This particular rudder is built off of the original rudder for a ~20' Bayliner Buccaneer sailboat.  The original had cracked and rotted pretty badly.  The owner of the sailboat cut the top of the rudder off and made a wooden 'boot' to cap the rudder.  However, it wasn't water sealed with fiberglass, and over time more and more moisture got in until it became so flimsy that it wasn't reliable.  

While this instructable is specifically for this Bayliner sailboat with a tiller-style rudder, the instructions should be general enough for you to modify it to work for many sailboats.  With that said, there are many many nuances to fiberglass/composite marine construction, so this type of build will require more research beyond what is covered here.
 
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Step 1: Previous Rudder

In these photos you can see the extent of the damage.  

The rudder was foam-core/fiberglass sandwich.  Think of it as a Big Mac; the three buns of the Big Mac were layers of fiberglass, and the meat was the foam (the yellow stuff).  The only difference was that the buns would have all been connected and fully enclose the meat.

First, I cut apart the rudder along its perimeter with an oscillating saw, so that I could use the pieces as templates for the build.

In the fifth image you are seeing a piece of balsa (I think) at the edge of the rudder where the mounting hardware was located so as to provide compressive stability for the tightened hardware.

In the last image, if you look at the top of the image you can see where the previous owner had chopped off the top of the rudder.  There was a rudimentary wooden cap on that, so you can see how easy it would have been for water to get in.  
Nice job !…
It's too bad that you let some spots unsaturated. Especially since you worked on a flat surface and it was not that large. I you have any demise from your rudder it will come from these spots.
What you should have done to be more than perfect (but it pays on the long run !) was to impregnate the wood with epoxy before putting the fiber glass. This is done by heating the wood with a heat gun, or better a hair drier). It goes loke this. You heat up the wood on a manageable surface (this one finds by himself depending on tools, location, type of spar, etc…) so that the space between fibers opens and you spread epoxy which is sucked by the wood : resin is inside the material not on it's surface only. A word of caution though : do not "reheat" the wood surface you've just impregnated as you will heat the epoxy resin and depending on its formulae its qualities may well be ruined.
Once you're familiar with the technique it goes pretty smoothly and a large surface can be done pretty fast.
It took me 3 days to make hull and deck of my 10m (33') cruise boat (then I applied 7 coats of epoxy with different additives on all surfaces bellow and over water line and that was an other story !!…) That was 17 years ago and all the surfaces did not show any sign of wear. Except for a soft spot that ruined one plywood panel on the deck, but that was due to to a hammer that fell from the mast and we failed to care properly for the dent it made… before noticing rain water had seeped under and rotten a somewhat large area that we had to replace … A case of overconfidence !).

PS. I should add that the surface heating process was not a personal invention but a recommendation from the epoxy manufacturer.
Thank you for your suggestions! The heating to increase the space between the fibers of wood makes a lot of sense. I did go back and re-saturate all those areas of which that you can see in the pictures before moving on, so I am pretty confident is is sealed.

Thanks again!
I'm sure you have plenty of time for uneventful sailing (regarding rudder I mean !) ;)
Anyway, this vital spar has to be checked quite often just to be on the safe side : attachments (nuts, bolts, forks, etc… can be very prone to electrolysis
The problem with electrolysis is that it's full of mysteries : you never know why a bolt here is affected when another 20 cm from the 1st is not… 
I doubt fix a zinc anode on the rudder.
If both forks do not connect do the connection with a copper braid. Works perfect.

Enjoy sailing !…
(the most wonderful pleasure in this world !…)

Vincent


BTW : one way to check that wood has sucked enough resin is that it should slightly "overflow" from the wood : surface shines a little bit. However do NOT re-heat the surface once it is covered with resin. If you feel that patches are more shiny (where the epoxy "overflows" - sorry my english is not perfect) than others leave it at that. I wouldn't want ruining epoxy properties by blowing heat directly on it.
r_harris21 year ago
Nice job. If you go through a hurricane, you might want to strap yourself to the rudder. It is probably stronger than the boat now :-)

Regarding building in two layers: Besides the construction advantages you mention, having two layers will decrease the tendency to warp, because warping in the two layers work against each other.