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Old 08-12-2009, 11:34   #16
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my opinion is that you don't need rudders that can be pulled out of the water entirely. your hull will scratch the bottom anyway. in other words: your hull doesn't end on the waterline, there's underwater part too.

the draft is connected with density of the water and the weight of the boat.
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Old 08-12-2009, 13:52   #17
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my opinion is thatyou don't need rudders that can be pulled out of the water entirely. your hull will scratch the bottom anyway. in other words: your hull doesn't end on the waterline, there's underwater part too.
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True, unless you hit an underwater object. In the case of kick-up rudders damage will be minimized and you can sail on. In the case of daggers in a cassette you can replace them underway, assuming the rudders are weaker than the cassette.
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Old 08-12-2009, 15:51   #18
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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.
How much steering you have with a rudder kicked up depends on the steering system used. With one (or both) fully raised, I can still get full lock on both rudders. With one partially raised, I'm restricted to around 10 degrees of lock by the width of the slot.

To be honest I haven't seen anyone using whittled fence posts, so can't comment on this practice.

I do know the kick up system on Oram Boats works though, I've seen one in action. The boat still steered, but response was reduced. ie. I could feel the difference, but still had full control.

The system you say "failed" actually didn't. That was on "Outahia" when it was sailed onto the rocks at Keppel island. The rudders kicked up perfectly when they hit the rocks, but in the ensuing rush to get the boat off the reef, the rudders were not secured in the raised position before she was reversed off. One of the rudders caught on the rocks, and because the boat was going backwards it was dragged back into it's normal position, and damaged.

The kick-up system is really only designed to work when the boat is travelling forwards. I guess some could see that as a design fault.

It's easy enough to design and build rudders to support the boat when deliberately beached, in fact most of Bob's boat's rudders could do this.

But how heavily built would they need to be to withstand a collision with an object while sailing fast? What weight penalty would that incur?

I've heard of a Perry with heavy stainless steel rudder shafts (Bob's are composite) which hit something, bending a shaft to the extent that it jammed in it's tube, and disabled the steering altogether.

The cassette system is light, and protects the steering system from these types of impact. (But only when going forwards)

The 44C design draws 450mm with rudders and boards up, around 850mm with rudders down.
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Old 08-12-2009, 16:43   #19
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Here is a Norwegian-built cat with transom-hung rudders which looks like a neat installation:
But how do I now get out of the water with transom hung rudders in the way?

Also the measurement for a marina berth is now also longer requiring a more expensive berth and possibly more expensive registration
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Old 09-12-2009, 04:52   #20
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But how do I now get out of the water with transom hung rudders in the way?

Also the measurement for a marina berth is now also longer requiring a more expensive berth and possibly more expensive registration
There should be no problem to have a folding ladder made and attached to the transom steps which folds down over and behind the rudders.

Most marinas I know base their charges on full metre length intervals, i.e. 10m - 11m etc, and yes, this kind of transom rudder might just push you over the limit - tough luck!
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Old 14-09-2011, 11:52   #21
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Finn Cat ??

Quote:
Originally Posted by multihullsailor6 View Post
Here is a Norwegian-built cat with transom-hung rudders which looks like a neat installation:
Hi Roger,
Do you have any more info on this 'Finn Cat' that you posted a photo of??
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Old 14-09-2011, 15:17   #22
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Re: Catamaran: Retractable Rudders

Some further comments about the kick-up system I have, and some more advantages I hadn't anticipated.

The kick-up system works perfectly. I've hit a log at around 9-10 knots, with no damage except a broken dowel. I've hit the bottom a few times too, again only broken dowels. The rudders stay put otherwise, at speeds up to 18 knots so far.

Stuff I handn't anticipated - In tide affected anchorages I can lift the rudders and hang with the wind rather than the tide. More comfortable and better ventilation.

If there's a swell running into the transoms at anchor I can lift the rudders which removes a large slice of the flat underside and dramatically reduces slapping under the hull.

I can grease the rudder shafts with the boat in the water. Touch up the antifoul (after hitting logs) too. I can check the rudder alignment easily. Also the zero setting on the autopilot rudder transducer.
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Old 15-09-2011, 10:37   #23
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Re: Catamaran: Retractable Rudders

Hi Mike I have built a similar system with a 14" iner drum the top of the drop in rudder is a 2x10" rectangle that rest on a 1/2" shelf at the bottom I have yet to solve the bearing twist 1/2 x 14" delrin rods with a cage top and bottom. What did you use for bearings? Great to know there is a like minded about. I would be intrested in trading thoughts with you.
Mike conley S.V. Wampum
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Old 16-09-2011, 20:35   #24
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Re: Catamaran: Retractable Rudders

"I can lift the rudders to clean them, and the underwater lights." ==> What do you use your underwater lights for - fishing? If they are mounted on the rudder assembly, do they face rear? Did you need special lights? I have been thinking about doing this with a couple of sealed high-beam or spotlights from a truck. I was going to put mine on the front of the 'keel' (beach supports?) along with a small webcam so I could see in front of the boat in clear shallow water in low light conditions and perhaps attract fish at night. I was thinking that one could lie on the surface of the water in snorkel gear with a spear-gun, concealed in the dark between the hulls, and pick out an appropriate dinner. People think it odd when I mention underwater lights so I am glad that someone has actually done it. Sorry about the thread drift.
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Old 18-09-2011, 05:49   #25
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Re: Catamaran: Retractable Rudders

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Originally Posted by blkrk View Post
Hi Mike I have built a similar system with a 14" iner drum the top of the drop in rudder is a 2x10" rectangle that rest on a 1/2" shelf at the bottom I have yet to solve the bearing twist 1/2 x 14" delrin rods with a cage top and bottom. What did you use for bearings? Great to know there is a like minded about. I would be intrested in trading thoughts with you.
Mike conley S.V. Wampum
Hi Mike,

For bearings I use 2 long strips of 1/4" UHMW PE in 1/2" x 2" races at the top and bottom of the drum. I had to cut slits in the last 6 inches or so in order to make the curve. I experimented with 1/2" acetal rods in the races but they were noisy and tended to jam. I used graphite powder in the inner surface of the races for more slickness.

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Old 18-09-2011, 07:09   #26
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Re: Catamaran: Retractable Rudders

Look at the Stiletto 30 design. They are transom-hung and work with a simple lever system. They kick-up if they hit anything, are reset by lifting and lowering the tiller (though tis function could very easily be asigned to a separate lever), and have been proven over 30+ years.

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Do they need to lift all the way? Just yesterday I spent 15 minutes disengaging a crab float from a rudder, after towing it 1/2 mile. With the Stiletto it would have taken 5 seconds to raise and lower the rudder, without slowing.

Yes, you loose some efficiency by not having an endplate, but it is safe to have a higher aspect ratio rudder.

If I were to use these on a heavier boat I would certainly beef up the construction. That would not be difficult.
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Old 18-09-2011, 07:11   #27
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Re: Catamaran: Retractable Rudders

We have a transom-mounted kick-up rudder on our boat (mono). It uses one of these pressure-release clamcleats to hold the rudder down.

Auto-release Clamcleat

Actually my rudder blade is weighted, which usually holds it down just fine, but in a certain kind of seaway the rudder blade would swing backwards, and screw up my steering momentarily as much greater leverage was applied to the tiller from the blade flung out full length aft; and at worst it would wrench the tiller right out of my hands. Now, with a control line led from the front edge of the blade through a few padeyes up to the tiller stock, the clamcleat hold-down keeps the blade down in all sea conditions. And if we want to sail up onto the beach, which we often do with our shallow draft, you just hear a little pop as the cleat releases.

The great thing is it's easy to reset. Not sure if it could be adapted for use on boats as big and complex as some discussed here, but if it could, it would beat the broken dowel system for ease of reset, sort of like a circuit breaker vs. fuse. Very handy items.
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Old 18-09-2011, 07:19   #28
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Re: Catamaran: Retractable Rudders

Just to help visualize, here's an image of the Jim Michalak style of kick-up rudder. Ours follows this basic plan, with the addition of the hold-down line led from the top forward edge of the blade to the auto-release cleat. Needless to say, this isn't right for many boats, but elements of it can be adapted. . . .
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Old 22-01-2014, 23:00   #29
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Re: Catamaran: Retractable Rudders

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Originally Posted by Aussiesuede View Post
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
That old link for the Shuttleworth rudder doesn't work any longer. Does someone have some better illustrations, photos of his rudders that they might post here?
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Old 22-01-2014, 23:08   #30
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Gemini Rudder System

The First-Ever Lifting, Underhung Rudder System in a Cruising Catamaran

The rudders on the Gemini 105Mc were actually introduced to the boat in 1994 with the 3400 models. The revolutionary design has been so successful, there has been little need to alter the system. They are mechanically joined through the steering system and will steer the boat at 3' or 18" of water without putting any extra load on the helm. In fact, the Gemini exhibits little to no weather helm. Her steering is precise, balanced, and easy. Using two sets of lines, the rudders can be raised or lowered - and automatically raise if and when the boat enters shallow water.
A solid 1 " stainless steel post that runs through the middle of each rudder making it exceptionally strong. The rudders are very well balanced and easily kick up if they hit the bottom.

I'll have to find a photo/dwg to post
Brian
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