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Old 28-06-2008, 16:32   #1
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Catamaran: Retractable Rudders

I am interested in different designs of retractable rudders, specifically for 10 to 12 metre cats. How does yours work?
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Old 28-06-2008, 17:26   #2
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On my boat there is a conventional rudder set in a hinged cassette. http://www.cruisersforum.com/gallery...00&userid=3477

They can be held down by a wooden or fibreglass dowel, which will break on impact and release the rudder. The dowels can be removed to manually lift the rudders. They will still offer some steering ability when partially lifted (and allowed to float) too. I think it's one of the simpler retractable rudder set-ups to build.
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Old 28-06-2008, 20:08   #3
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Old 29-06-2008, 00:42   #4
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Thanks for replying, I really appreciate the detailed photos from you both. idrhawk, that's one of the best looking set ups I have seen, is that system cable or hydrolic driven?
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Old 29-06-2008, 05:41   #5
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Col

there the GBE style Lift up blade arrangement, The kick up arrangement like 44 said and that is evidenced on a lot of peter kerr's boats, and there is a section on this topic in Chris Whites book, the cruising multihull.

On kite its a swing up arrangement, but there is no steerage once you pop the rudder, at that stage its use the steerable outboard. Wheresas the dagger board style rudders offer steerage but dont have the advantage of being able to pop up if they hit.
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Old 29-06-2008, 15:01   #6
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Here is a photo of the Shuttleworth currently being built in Washington with a link to it's construction process. It's got a 'kick up' rudder design...

Shuttleworth 52 AeroRig Construction Pictures
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Old 29-06-2008, 16:23   #7
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A 47 ft >Shuttleworth was launched a couple of years ago in Capetown with this same lickup rudder set-up. Last year, he was fed up with all the problems and had the glassed in so they no longer can kick up.

Shuttlworth uses a transverse piece of foam/glass that breaks when overloaded, this apparently caused some inadvertent tripping of the rudder.

Ellen Macarthur had some kind of a rope "fuse" on the rudders of B&Q.


The Gunboats have a kickup system, but I know that some owners have changed this. I don't know the reason.

Unless your drives are retractable also, I would probably go for fixed rudders.

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Old 29-06-2008, 17:54   #8
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Originally Posted by Nordic cat View Post
A 47 ft >Shuttleworth was launched a couple of years ago in Capetown with this same lickup rudder set-up. Last year, he was fed up with all the problems and had the glassed in so they no longer can kick up.

Shuttlworth uses a transverse piece of foam/glass that breaks when overloaded, this apparently caused some inadvertent tripping of the rudder.
Wow! Am I understanding correctly? He uses cored glass on a point of articulation that receives the type of loads a rudder does? I can't even begin to guess as to why??? It would make absolutely no sense!
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Old 30-06-2008, 04:04   #9
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It is not as you think,

If you look from above, the hinged rudder stock can flip out backwards if something is hit.



Aft of the rudderstock there is a tranverse piece that gets broken. Even with the rudders up you still get some steering.

Looking at the photo above, you can see a space behind the 3rd step up, the tranverse piece could be mounted across the opening there.

I don't have a good picture of it, but all these lifting rudders add weight and complexity with limited advantages IMO.

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Old 30-06-2008, 20:29   #10
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I don't have a good picture of it, but all these lifting rudders add weight and complexity with limited advantages IMO.

Regards

Alan
The system on my boat adds very little weight. Less then 10 kg total, (both hulls) certainly. In fact, probably no more than than 5. It's a very simple system.

The advantages - if I hit something at speed the rudder will kick up. The only damage will be a broken dowel, of which I will carry spares.
I can lift the rudders to beach the boat.
I can partially lift the rudders for maneouvering in very shallow water, but still use them for steering.
I can lift the rudders to clean them, and the underwater lights.

With sail drives some of these advantages would be lost though.
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Old 07-12-2009, 15:12   #11
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The advantages - if I hit something at speed the rudder will kick up. The only damage will be a broken dowel, of which I will carry spares.
Any chance you could post the dimensions of your dowel and it's location relative to the hingr of the rudder cassette? I'm building a Shuttleworth 31 and I'm considering modifying the kick-up 'fuse' mechanism to make it even simpler and more accessible. It's hard to calculate how strong the fuse should be though.
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Old 07-12-2009, 17:26   #12
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my Hirondelle had rudders like this http://www.2hulls.com/images/catamar...nixRudders.jpg

worked OK. in a storm i managed to broke the both at the same time
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Old 07-12-2009, 23:50   #13
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Here is a Norwegian-built cat with transom-hung rudders which looks like a neat installation:
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Old 08-12-2009, 04:38   #14
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Originally Posted by 44'cruisingcat View Post
On my boat there is a conventional rudder set in a hinged cassette. http://www.cruisersforum.com/gallery...00&userid=3477

They can be held down by a wooden or fibreglass dowel, which will break on impact and release the rudder. The dowels can be removed to manually lift the rudders. They will still offer some steering ability when partially lifted (and allowed to float) too. I think it's one of the simpler retractable rudder set-ups to build.
A very interesting thread. I do however wonder what happens when a rudder kicks up. Does one suddenly loose steering in both as the kicked up blades rotation is now confined by the hull appeture. This may be very dangerous in marginal conditions where large helm changes are required to hold a course . I would imagine that the fused link is designed and tested ( not just guessed at) to fail at a point where no damage occurs to the rudder (this must involve some pretty interesting maths and materials testing,hardly repeatable with selected pieces of whittled fence post jammed in a hole), hence the whole rudder mechanism is built reasonably lightly as it no longer needs to be able to cope with grounding loads. Where as, a non kick up balanced spade will be engineered to take grounding loads (One of the late Lock Crowthers catch cry's) ,Hence the non kick-up will be so much stronger for the same weight. ( no case etc required)

There were some pictures of an Oram cruising cat kick-up rudders doing the rounds a while back that had failed when accidentally side loaded and hence the kick-up couldn't function. Were these "dog on cats"? I'm not sure. I'm sure someone here will remember the boat and maybe even dig up the pics.

How much draft does one actually save with the rudders up? Most moderately sized round bilge cruises are going to draw 600 to 900 mills anyway.
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Old 08-12-2009, 09:21   #15
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I used a dagger and cassette system set in a rotating drum on my boat. Pros and cons are as stated above plus it eliminates the stainless shaft.
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