add length to rudder shaft or shorter blade?

I'm building a 30' catamaran and realize my transom came out a bit taller than the 5' rudder shaft allows for, given the specs. of the blade. The shafts are 1.5" diameter 304 stainless tube, with 3/8" thick walls.

The rudder shaft as it is would go through a 34" kickup cassette with fiberglass bearings at top and bottom of cassette, and a 2" tall tiller head on top of that (total 36"). That only leaves 24" of shaft for embedding in blade, which is spec'd out at close to 36" - and specs have a bit under 30" of blade embedded. So I'm about 6" short on the shaft.

So I have inquired of the designer but in waiting for a response thought I would throw it out here as well. If I were to add length to the top of the shaft, I would imagine one would press into the 5' piece a stainless dowel a few inches in and sticking proud a few inches, then drill a hole or two thru tubing/dowel and pin it with small stainless dowels. Then press on the 6" tube onto the protruding dowel from the 5' piece, and pin it. The join would have to be fairly smooth as it is inside the cassette fiberglass tube, but the top 6" piece would be at the top bearing area of the cassette tube.

So the question is if that would hold up to twisting stresses. If what I said above would work, is there a better metal for the dowel like bronze or something a bit softer to allow pressing in if that is a concern. Wondering if anyone has done something like this - design ideas are appreciated. Or a better way altogether?

Otherwise, I could make that blade a few inches shorter and just have 6" less shaft in blade. I suppose I could make the blade the full length of 36" too, but might worry about the bending strength with the too short shaft.

I could also buy longer tube, but these cost me over $400 as is - I'm way up where shipping is way expensive - only by air.

thx, jim

 

Simple answer, no!

 

Thanks for the reply Ad Hoc!

 

New plan: I think I can get necessary strength by, when adding the 6" top piece with dowel (pressed in), each mating surface of tube is chamfered and welded. Double checking with designer, but seems he may be ok with that. He does say it is better not to shorten the rudder blade any for a sailboat. /jim

 

Other opinion

J, I had the same need to extend a rudder shaft- I used a recycled Catalina 27 rudder as a base for a better rudder for my 33' tri, it is almost the same size as your shaft.
I live in an area that uses a lot of stainless in the chicken processing industry, so there are some very competent welders available.
I added a 14" piece of equal size stainless pipe butt welded to the top of my shaft, it came out straight and strong, and less than $100 total. I used it hard this season, just inspected it, and all is well. I trust it.
A properly certified pipe welder should have no problem with yours.
Bruce

 

thanks for the reply Bruce! Data points like yours lend confidence. Now to find some tube.

(this is a good distraction from sanding daggerboards.)

 

sanding

I HATE sanding, and I am about to start a new dagger for my boat.
Enjoy!
B

 

old guy having fun sanding (not)

 

I've also done the same, a properly backed and welded joint should be fine, but I would question using 304 in salt water, especially passing through bearings. If you are in fresh water, then you should be OK.

 

thanks Hump101! /jim

 

No problems at all

On my 38ft cat I initially had a cassette system with the shaft poking out the top by about 100mm. I had a short tiller arm which went through a bulkhead into the stern compartment. I didn't like the steering system because it was complex. I liked the cassettes.

I had only 100mm poking up through the cassette top so got a guy to weld me up some tight fitting tube that slid nicely over the rudder stock 100mm. Bolted it on and this tube carriers my tiller arms above the back step. Much better arrangement. Great steering.

Did that about 15 years ago and used pretty lightweight stainless. Never had a problem, even when the tide went out once and there was a lot of load on the rudders.

Keep all new work above the bearings and all you are dealing with are torque loads. You can work them out. How hard can you pull? Let's say 1000N with all your might on a 1 metre tiller. That is 1000Nm. This is the torque the extra piece will have to deal with. So you can test the piece you get made by putting the sleeve over the rudder when complete, putting a long lever on the thing and going to the load and beyond.

Weld a nice new piece on or sleeve it. The welded piece will need to have enough wall thickness to cater for the shear load at the weld. There is a nice shear stress calculator here.

http://www.amesweb.info/Torsion/TorsionalStressCalculator.aspx

I get around 16 MPa. The yield strength of ordinary 304 is 215 MPa. So do it for yourself and check but yeah, I think you have a bit of extra for the weld there.

cheers

Phil

 

Just re-read the post - inside the cassette - uh-oh. I am now very scared.

Why is the cassette so big? Why not just pull the cassette top down some and make a step in there? Get creative but don't splice the shaft BETWEEN the bearings.

cheers

Phil

 

Gotta stop posting so quickly

Or you could reduce the spacing of the bearings WITHIN the cassette. 3ft is a huge cassette and bearing spacing - mine is about 300mm between bearings. So put a loaded bearing 6 inches from the top and weld away.

 

Thanks for the input catsketcher. The "bearings" are just a reinforced fiberglass sleave with some carbon flour embedded for friction reduction. Here's a post showing pictures of the transom and cassette slot:
http://www.boatdesign.net/forums/bo...-catamaran-repair-log-53898-4.html#post790152

http://www.boatdesign.net/forums/at...30-cruising-catamaran-repair-log-transom1.jpg

 

A pic inside the cassette and the short rudder pre-glass. The blade was only about 3" short but the shaft inside the blade must have been close to 5" short of design. I can say it held up very well when the barge company that smashed the cat dragged it on the rudders, bending the shafts (where they exit at the bottom of the cassette) and breaking some of the glass. They held up very well despite the short shaft length within the blade. And the glass sleeve inside the cassette was embedded with glass fiber enhanced bog after this pic was taken.