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Old 18-10-2009, 02:40   #1
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Question Regarding Rudder Shaft Coupling

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.
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Old 21-10-2009, 00:05   #2
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Old 21-10-2009, 00:54   #3
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Do you know a good surgeon?

It's a steel boat, and out of the water, right?

So why not have a chat with your local machine shop and local welder and get them to fabricate something you like, then cut and weld it into place?

Little bit Lots of aluminium vinyl primer, couple of extra anodes and you can be running aground with the best of us.

Of course, you must check with the designer first!
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Old 21-10-2009, 13:40   #4
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I have the ability to fab anything I want, I was just asking for alternatives.

Maybe I am overthinking the whole situation and the four-bolt coupling is plenty strong, I dn't know, never having dealt with one before.

I know that prop shafts endure a lot fo twist force and don't appear to have any problems, but the length of the rudder blade gives a lot of leverage to increase the forces exerted at the shaft.

Again, I would like to hear form anyone who has had nay experience with rudders with removable shafts for some input.

I have had skegs with removable shoes before and even had one shoe fall off, so I am inclined to keep the existing skeg design with the fixed shoe.
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Old 21-10-2009, 14:12   #5
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How big are the bolts now?
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Old 21-10-2009, 14:21   #6
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I believe that the bolts are 1/2" or 5/8"
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Old 21-10-2009, 14:51   #7
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I have seen the flange like you have work fine on many commercial boats for years. If you are worried about loosing the rudder weld a safety chain to it and the boat. Personally think the simpler the better if the bolts are the right size I am guessing you would have a lot more damage before they sheared off.

Good luck, nice looking project

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Old 21-10-2009, 15:11   #8
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Thanks, the more I look into this, the more I am leaning to just leaving it the way it is.
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Old 21-10-2009, 15:33   #9
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On most decent designed full keeled boats the bottom of the rudder is placed a little higher than the trailing end of the keel with the with the idea of protecting the rudder. Chance of doing rudder damage on full keeled properlay maintained boats is generally pretty low. I am confused, your posted picture shows a fin keel with spade rudder?
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Old 21-10-2009, 16:14   #10
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I believe that's just the skeg you see...
With the shear strength of 1/2" bolts at maybe 17,000 lb each you should be good to go! Fine thread bolts are usually rated a little higher if you want to go there....
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Old 21-10-2009, 17:16   #11
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We had friends with the bolted flange arrrangement, and it failed two or three times going across the Pacific. I'm pretty sure the failure was at one of the flange welds, and they ended up having it rewelded in exotic places like Easter Island and Pitcairn. When it failed, the rudder didn't fall off, but they couldn't put any torque on it. There was a hole drilled in the top aft part of the rudder, and they had to steer with ropes tied to the hole--pretty difficult and slow.
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Old 21-10-2009, 17:17   #12
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Yes, I think I will just use the existing configuration and weld gudgeons onto the transom for an emergency rudder that I will fab up.

This way I am covered for "what if".
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Old 21-10-2009, 18:22   #13
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Good thought and you might be able to incorporate that into a self steering system.

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Old 22-10-2009, 09:59   #14
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Cut the rudder in half horizontally about 1/3 to 1/2 or so down from the top, and make a pivoting joint that allows the lower end of the rudder to fold back if it impacts something without damaging the main connection. The rudder pivot would be held rigid by the addition of a sacrificial (plastic, nylon, or whatever) shear bolt next to the pivot bolt. Just fab up some side plates for the pivot point, thru-bolted to the top portion of the rudder, with one hole in the bottom section for the pivot bolt, and one hole for the sacrificial shear bolt that locks the rudder down.
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