Advise on Rudder Shaft coupling

[rustypirate]

I would like to hear your opinions regarding options for a coupling between my rudder and rudder shaft.

To provide some background I have a 38' steel ketch with a full-length skeg-hung rudder. The shoe at the bottom of the skeg is welded on permanently and not removable. This rules out having a permanently fixed shaft in the rudder, and I like the option to remove the rudder for service without leaving an empty hole in the bottom.



The current coupling is a simple four bolt flange that connects the shaft to a matching flange at the top of the rudder. This allows the shaft to be inserted into boat, then the rudder to be inserted into the shoe and rotated up to mate with the shaft flange.

My concern is that if the rudder should be struck by something at speed, there is the potential that the coupling bolts could shear off, and the rudder could fall free. This would leave me with NO rudder, which I would not like to contemplate.

On my last boat, a Franz Maas Calypso 43, the rudder shaft was tapered and keyed like a prop shaft, and mated with a tapered and broached hub welded into the rudder. this way, even if the shaft key failed, at the worst the rudder would be useless for steering, but would still stay connected to the boat by the shaft. The worry I have with this arrangement is that the current shaft log is sized for 1.25" shaft, and the maximum contact surface that I would be able to get for the hub would only be about 3.5" long. That would be a LOT of twist force to excert on a very short area of contact.

Any ideas, suggestions, or insights you would have will be very welcome.

[CDK]

If it eases your mind, use a short loop of stainless steel wire or chain, so the sheared off rudder will stay with you.
This rudder position is of course very exposed from the stern only, so the chances that it is hit at high speed are slim. But if it should happen and you would have succeeded in finding an unbreakable construction, the rudder shaft will bend or snap off, or the hull will suffer damage.

[rustypirate]

I am planning a trip to the Antartic, so ice would be my major concern.

I imagine that if the force were enough to shear 4 1/2" bolts, then some damage to the hull or shaft is going to occur as well, not to mention the remainder of the steering gear.

My real fear is loosing the rudder entirely, anything else can be repaired or jury rigged.

[gonzo]

The rudder can be jury rigged too. In my opinion, the safest rudder setup is a solid shaft that goes through a tube. There is a bushing on top and a stop collar on the shaft. If the rudder falls off, the tube won't let water in. Also, you can tie the rudder, take the collar off and remove the rudder while in the water.

[rustypirate]

I agree, the rudder can be jury rigged, there are even eyes attached at the top rear of it to jury rig ropes to use for steering, but if the rudder falls off and sinks to the bottom, then there is very little you can do besides trying to use warps to steer with.

I may just weld some gudgeons to the transom and fabricate an emergency rudder to use in the event of a loss of the main one to cover myself.

[CDK]

Assuming that the bolt position is well outside the shaft dia. (it must be), this is not the weak link in the rudder mounting, so you have nothing to worry about.
I think metric, so I won't try to make the calculation here, but the distance between the shaft center and the bolt(s) determines the load on each bolt.
The breaking strength of a .5" stainless bolt is well over 6 metric tons.

The critical area is the weld between the flange and the shaft above the rudder.