Rudders and Rudder Stock Stainless 316 329 or Aquamet(aqualoy) 22

You need to use hollow shafts to reduce weight. Then 316 is perfect.

 

This is for a 40mph power boat. Solid 2" aquamet 22 is the recommendation of the calculations in Dave Gerr's "Boat Mechanical Systems Handbook" If I were to go to hollow it would likely have to be 3-4" and I don't have that option.

I think dulplex 2205 might be the winner. Its is stronger than 316 and I can get plate, round, and welding rod of all matching material. It also has good corrosion resistance.

I'm not sure what the difference between 2205 and 17-4PH (630) are. Other than it looks like the 2205 is cheaper.

 

That is very important. That's why we were all asking what type of boat it is for. The 2" hollow is scantling for a 24mph, 48' sailing catamaran. Different boats, different forces. The rudder are is a factor too.

 

Normal rudders for my model are "kick-up-cassets" and because I went saildrive I fitted mini-keels to facilitate some level of protection - also opting out of kick up rudders - and retain an ability to beach unhindered - requiring 3 ft of water to move off - - (that does not preclude other problems like freeing my stern anchor from a heavy home made crab-pot - driving the boat to free the anchor resulted in the damaged propeller... (see http://www.boatdesign.net/forums/boat-building/my-little-piece-peace-25962-142.html#post556919 )

 

Hollow is fine, there is many thickneses . When im talking hollow Im talking 6-7 mm walls.

The middle bit does not do anything top stop bending,-- the strength is in the outside tension and compression.

You may also want to consider a weaker shaft than the bearing its inside of and the fibre glass surounding it.

Ide rather rip the rudder out than rip the back of the boat out.

 

Rudders are not used to support a vessel, just strong enough to turn the boat. It is better that they break if grounded than they tear a boat apart. They also provide a lot of drag , so making them thicker for no reason is a bad idea. Rudders usually don't break, the mechanisms do.

 

But if you are changing a design don't forget shear, you cannot just match extreme fibre stresses, you have to make sure you have the shear area as well.

 

Bollocks.
Look at bent tube , the walls have collapsed and deformed.
A solid insert would help prevent these deformations and the load required to bend the tube and solid insert would be far greater than the tube alone.

 

My rudders used to support the vessel to some degree every time I dried out on a compacted sand beach

 

I agree, this seems pretty standard practice to internally sleeve the rudder tube/stock at the lower bearing on a lot multihull spade style rudders that are only supported by two bearings & Cantilever? out of the hull bottoms ready to stop the boat when it grounds.

 

Like I siad , you want it to fail, before the boat does.

Shear????? its a rudder shaft.

 

Come on Frosty...... shafts....... Shear........ ring any bells yet ????
Are you getting out of practice in paradise?

All the side force of the rudder is present as shear in the shaft at the bearing(s). Just sketch the shear and BM diagram. Then there's shear due to torsion.

 

A guillotine is shear. Rudders break off.

 

You got a blunt guillotine..

 

If the rudder was a supported rudder with bearing support then the force on the shaft below the upper bearing would be shear. But this is a unsupported power boat rudder, any force will bend it offering both tension and compression in the opposing walls of the shaft.

Chopping off some ones head for instance is shear, bending it till it work hardens and breaks is not shear.