Gap between rudder blade and cheeks

Greets

I'm rebuilding a kickup rudder for a 17' AFC. The rudder blade is something like 3/8 alum. stock. I've routered a gap into both sides of the (3/4 plywood) cheeks but I'm not sure how much free space there should be for the blade to swing on impact. The original just had a thru bolt with oversized washers that provided enough compression to keep the blade down absent stumps.

I suppose the gap needs to be great enough to allow the blade to swing free until the bolt provides it's compression - and free swing is free swing...the interior parts of the cheeks shouldn't be in contact.

Right?

many thankx
--steve...

 

And to save the inevitable followup...

I'm varnishing the plywood (cabinet-grade) with standard (non-spar) varnish...3 coats. Should I anticipate any swelling and size the gap accordingly?

 

kickup rudder

Most kickup rudders that work well have either a rope or wire run to a cleat to hold them down or an adjustable clamp on the pivit bolt. They need something, just a little angle back will make the helm very heavy. Catalina 22s have a simple plate and lever that can usually be fitted to almost any rudder, or the rope system is not too hard to set up. I would try to do something while you are building it. Bruce

 

Buy a "broken back" jam cleat and install a down haul. The broken back jam cleat (some call them clam cleats) has a pivot pin and a small catch so that shock loads wil dump the line that's engaged in the cleat.

Another option is a shock cord arranged "over center" so it always pulls the blade back down, but doesn't prevent it from bouncing over obstructions.

You want the minimum gap between cheeks and blade as small as possible. A gap will vibrate and hum. No gap will prevent this.

Varnish alone will not prevent your cheeks from delaminating or swelling. Cabinet grade plywood usually doesn't have waterproof glue and will delaminate with exposure. A high quality varnish will permit about 25% moisture vapor ingress. Shellac is slightly better around 22%, which is on par with polyurethane and polyester coatings. Epoxy will be the best at about 1%, depending on formulation.

Cabinet grade plywood also typically has a paper thin out veneers, which contributes nothing to the strength of the panel, further weakening it. With paper thin veneers, you'd be best advised to epoxy the crap out of the cheeks and hope for the best on the veneer adhesive. A 4 ounce 'glass sheathing would further protect these paper thin outer veneers and wouldn't be visible once the weave was filled.

I'd also strongly advise making an epoxy pivot bolt bushing, to further protect the plywood. Drill and over size hole for the pivot, say 3/4". Fill the hole with 50/50 mixture of milled fibers and silica set in epoxy. When cured drill for the pivot bolt say 3/8", through the epoxy bushing.

For what it's worth, cabinet grades of plywood are for looks and indoor use only.

 

plywood

I agree, I have a cabinet shop with lots of scraps- and I never use them in boats. Plain exterior grade construction ply will hold up just fine if you epoxy coat it, or it does ok properly painted. Most work boats use some of it (above the waterline) and expect it to last, but everything gets plenty of paint. I would use some kind of thin plastic washers between the case and the blade- I have used very thin "cutting board" sheets that are available from cooking stores. About 3-5 dollars for two of them in a pack, cheaper than the plastic supply store and they last several seasons. Keep it simple where you can Bruce

 

Shelmarine

 

Kis

Remember what Steve is working on- It is a small simple boat! Everything above is correct, but maybe not all necessary. I would not use the cabinet ply, but I would not buy a whole sheet of 3/4 marine ply either. If the rudder housing is stored indoors, I probably would not glass it, just paint it as necessary. It will last a long time. Bruce