rudder shaft help

Hello all,
I am doing a refit including a new rudder.
The old shaft is 60mm by 3mm wall ss tube.
I have had no trouble with this setup but I am told its to light.
Boat is 8Mts and 1300 Kgs and rudder size is .3sq. Mt approx.
What size should it be?
Thanks,
Peter

 

Without a fair bit more information about your boat and it's intended operational area/conditions, any suggestions would be just a guess. Can you post some pictures of the boat? A 2 3/8" shaft with an 1/8" wall is pretty stout on a 1.5 ton 26' boat, that I'll assume it's a sailboat, so additional shaft diameter probably isn't an option. This means if you want stouter, step up to a 1/4" (6 mm) wall stainless tube (not pipe). 304 in fresh, 316 or 316L (preferably) in salt or brackish.

 

Don't forget also to tell us the boat's speed.

 

And also:
- the boat type (power, sail?)
- the rudder type (spade, skeg, transom-hung?)
- the rudder profile (flat plate, foil section, schilling or else?).
Cheers

 

Agreed, I'm making assumptions based of the general size of this rudder, it's shaft and the boat's length. I'm reasonably confidant it's a sailboat, as even a displacement powerboat (of this length) wouldn't have a rudder this big. A transom hung rudder wouldn't need a shaft, so I'm guessing it's under belly and through a port and probably a spade, though could be on a skeg. Considering the shaft diameter, it's probably not a plate, though it could be a schilling.

 

sail
spade
foil

Quarter tonner

I would actually like to decrease the shaft diameter to get a better rudder profile

Peter

 

Sorry for the, probably, stupid question but could you do the contrary: change the blade profile (NACA or similar) and keep the shaft diameter?

 

Blade is already an approx naca00 profile I would like to bring the percentage down to 10 or 12 %

Peter

 

I see. But reducing the shaft diameter can be tricky.
Just curious, what advantages do you get the finest blade?.
If the blade is designed well, there should not be advantages to change.

 

Post a photo of your setup or tell us which 1/4 tonner you have and if it's the stock rudder.

You can decrease the shaft diameter, but you can run into issues. Is there an armature attached to the shaft (within the blade)?

 

Its a Mull, don't know the history of the rudder, Its over 30 years old.
I will build a new blade and shaft.

 

Your shaft dimension (60x3 mm) sound about right to me, based on general data you have provided. The diameter could be decreased by using a round solid bar instead of a hollow tube, but the weight of the rudder stock will significantly increase (by 3-4 times the weight of a hollow tube).

 

Shouldn't be a problem getting the required strength and going to a smaller diameter. You will have to go thicker wall or possibly even a solid bar.
Here are the formulas required to calc the loads and strengths. Start on about page 80. The formulas are pretty conservative but it's a place to start.
Dave Gerr also has the required formulas in one of his books and they were reprinted in Professional Boatbuilder

 

Is this the early "Ranger 26" version (1968 - 1975):




or is the fairly rare "Ranger 26-2" version (1980 - 1982)



The early version was known to have issues with compression under the mast, over the forward bulkhead.

 

neither of the above although similar to 2. Its a Sonata produced in Australia