[Public-list] My Rudder is Broke and Needs Fixin

[FINNUS505 at aol.com]

In a message dated 12/20/04 10:27:11 AM Eastern Standard Time, 
sousa_stephen at emc.com writes:
Anyone want to chime in on rudder design - a topic that I can guarantee
would be of practical use to one reader (although I'd have to attach the
usual legal disclaimer).  I've tracked down a rudder shoe, gonna use the j24
gudgeon/pintel thingers and gonna take that really strong type plywood and
slap two peaces together "sealed" in epoxy.  

One mystery that seems to ping around in my brain (there are one or two
others) is -- how the heck does the that bent poll that sticks out and below
the hole in the boat get inbetween those two peaces of really strong type
plywood i slap together?

Casey has a diagram for one piece of metal, shaped more or less like a comb,
which is welded all along the rudder post.

Hey,
The easiest way to make a new rudder is to have that guy in Florida do it for 
you. I never like to have anything built for the boat that I can do myself, 
though. Money is one big issue, but it is the principle of the thing too.

Many production rudders are foam cored, skinned by layers of glass laminate. 
I don't have experience with this method of construction, so I cannot offer 
any advice. If I had to build a new rudder for my Alberg 30, I would go with 
layers of marine ply epoxied together, and faired to the necessary thickness at 
forward and trailing edges. Glass the plywood well when done, and you would 
have one strong rudder.

I like the 'Comb' idea from the Casey book for marrying the rudder blade to 
the rudder stock. If you do not want to make a custom extrusion for the comb, 
then you can make your own comb with 1/4 inch, or 5/16 inch bronze threaded 
rod. We would drill the rudder stock to take the threaded rod. There is no room 
for nuts to protrude on the forward face of the stock, so we tap the stock so 
the rods will thread into it, and then peen over the ends of the rod, 
effectively riveting them to the stock. 

The rods extend the full length of the blade, into notches cut into the 
trailing edge of the blade that would allow nuts to be placed, as the ultimate 
guarentee of the structure staying together. The notches would be filled with 
thickened epoxy and faired before glassing.

A concern would be that drilling holes in the stock to take the threaded rod 
might weaken it. I would have to see all the components apart to make that 
determination. If I felt it would be weaker, than I would make a new rudder stock 
of larger scantling material.

The rudder is about 2 inches thick at it's forward end, maybe a bit more, 
actually, so we will be making the blade up of at least two pieces of 1" marine 
ply. The two pieces will be bonded with epoxy along the center line of the 
rudder blade. We route out grooves on the inboard faces of these two pieces, a 
half round groove on each inboard face, to take the threaded rod. 

We want the glue seam between the two pieces of plywood to be very good; 
avoid any voids, or areas of pooled glue. The best way to do this is with alot of 
clamps. Clamps holding two by fours across the blade, six inches apart from 
eachother, or closer, would assure even pressure, and a good glue seam. Also 
important, we want to make sure the blade is absolutely flat while clamping and 
gluing, so the final blade is not glued  in a warped shape.


To counter the extreme torque forces that will be trying to seperate the glue 
line, I would use copper rivets, athwartships, through all the thickness' of 
plywood, above and below each threaded rod, along their entire lengths. Though 
the glue line would probably not give, the wood may splinter away from the 
glue in an extreme incident, and that must be avoided.

The two pieces of 1" marine ply would make up the largest part of the rudder 
blade. Additional layers of marine ply could be added to bring the thickness 
up to the necessary dimensions. At least three or four layers of glass will be 
necessary to make sure the structure is waterproof and strong, and that will 
add significant thickness, too.

That's how I would do it. 
It's fun thinking about these things. I'm no engineer though, so consult 
someone who knows something before you proceed. :)
Lee
Stargazer #255

 1103591366.0