Alum rudder shafts

[jaydh]

Hi guys,

Anyone out there have direct experience with making rudder shafts? I'm thinking about this....and please know I'm not an experienced builder. Just interested in real-world hands-on results someone else has tried. We need to take a good guess at issues and unknown factors to us right now.

We're building a 14.4m sailing cat with kick-up rudders. Everyone else has been using 60mmx5mm SS steam pipe from the scrap yard for the shafts. They weld on the SS tabs for the rudder itself then bolting the alum tabs to this for the rudder foil sides. The SS pipe is slotted and bent down into a taper also to compliment the foil shape, too. It's gooped up for the two metals touching and does work, but......

I was reading about how the Dashews use solid alum rudder stock/shafts. 6xxxsomething and/or 7xxxxsomething?? Can't recall right now. I was wondering it we could just forget the SS shaft and go with the solid alum. No dis-similiar metals, easy to cut/taper and weld it all up (not set up for SS welding at the moment).

So my question is...how the heck does one determine what diameter of alum would be 'right'? How much does solid alum round bar flex when you get up into the 50-60ish diameters? It has to be somewhat stiff for the kick-up and the distance between the top and lower bearings. Anyone know how much more this stuff weighs per meter at a certain diameter?

I have more questions, but will try starting with this. I appreciate anyone's opinion. We're in south-east Queensland, btw...I think a few of you are around this area??

Thanks!
Jay

[lazeyjack]

its normal, i have seen alloy rudder stocks on 150 footers, sailing yachts, normally on my boats I use 2025 ss, because I can use a thinner section, and with a big spade rudder you need big stocks The thing that is detrimental to spade rudders using alloy stocks is the weld around the top of the rudder, weakens the stock by buggering the tensile That why you go up in size
I have also seen a lot of cats with ss stocks and ss plating the downside even on sandblasted plate is that its hard to keep paint on the lead edge
I will pm you I am in SE qld