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Old 06-07-2015, 09:44   #16
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Re: Fast cats and bad rudder posts

Rudders (like other things on boats) should be designed not to fail ever. I know it's hard to make a rudder that is low cost, light weight and have the proper helm feedback. Nevertheless, rudders (and keels) must be designed not to fail as the #1 priority in my opinion.

In the case of a cruising cat with 2 rudders then a single rudder should ideally be designed to handle all the loads in case the other one fails. Otherwise it is not a redundant design. Also, grounding, gathering weeds, snagging fish traps and other obstructions are a fact of life in cruising. Rudders have to be designed beyond worst case sailing loads to the point they can handle cruising abuse. Otherwise they are not cruising designs. Maybe a good racing design but not for cruising.

There are several ways to make rudders 99.99% reliable. 44CC has kick up rudders as a way to address this issue. That is sensible I think.
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Old 06-07-2015, 11:39   #17
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Re: Fast cats and bad rudder posts

Ted Clements (Antares designer) has a practical approach to catamaran design which includes his opinion on rudders.
http://catamaranconcepts.com/
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Old 06-07-2015, 11:57   #18
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Re: Fast cats and bad rudder posts

Quote:
Originally Posted by Kim Klaka View Post
The reason most rudder stocks break is because either:
a) the designers won't believe the (correct) answer that comes out of the very simple formulae needed to calculate the load or
b) when the designer has given the correct very large stock size the "knowledgeable" builder decides it must be far too big and reduces the size below design specs.

It's not rocket science; as a very rough rule of thumb you should be able to suspend your boat in air from its rudder.
My guess is they don't know how to apply the fatigue factor, because they have no data for it... and as you say onthat type of boat they want it light, light , light.
Just because you may be able to lift a whole catamaran with one small boat in a perfect situation doesnt mean it wont break in the real world!
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Old 10-07-2015, 06:36   #19
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Re: Fast cats and bad rudder posts

Another Schionning G-force 1400 has lost a rudder. The boat is "Bulletproof", which is the first one made. Happened in Pittwater, in case anyone has more details.

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Old 10-07-2015, 13:03   #20
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Re: Fast cats and bad rudder posts

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Ted Clements (Antares designer) has a practical approach to catamaran design which includes his opinion on rudders.
Catamaran Concepts | Catamaran design with Ted Clements
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Thanks for posting that link. Very interesting information on his site.

Later,
dan
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Old 10-07-2015, 16:55   #21
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Re: Fast cats and bad rudder posts

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There are several ways to make rudders 99.99% reliable. 44CC has kick up rudders as a way to address this issue. That is sensible I think.
I don't understand why kick-up rudders are being considered an advantage here.

Kicking up is good when you hit something, or want to float in 6" of water. But I wouldn't want a rudder that has any chance of kicking up because I'm going too fast. That doesn't say reliable to me.

So you're hauling ass and the rudders kick up. What happens next? I bet it isn't pretty.

Maybe your mast is 99.99% reliable, too.
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Old 10-07-2015, 20:59   #22
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Re: Fast cats and bad rudder posts

dannc:
You're welcome, Ted certainly knows what he's talking about and pulls no punches.
And, after owning one of his designs for the past 6 years can attest to the practicability of his designs.
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Old 11-07-2015, 00:03   #23
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Re: Fast cats and bad rudder posts

JayH - I agree with you about the kickup rudders. When I'm surfing down waves at 14 knots, a rudder kicking up would likely be disastrous. My boat came with kickup rudders, but I made some little changes so that a big bolt locks each rudder down. If I'm in really shallow water, I just take the bolt out.

Journey on a Woods Sagitta catamaran: Uncharted waters
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Old 11-07-2015, 00:55   #24
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Re: Fast cats and bad rudder posts

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Originally Posted by JayH View Post
I don't understand why kick-up rudders are being considered an advantage here.

Kicking up is good when you hit something, or want to float in 6" of water. But I wouldn't want a rudder that has any chance of kicking up because I'm going too fast. That doesn't say reliable to me.

So you're hauling ass and the rudders kick up. What happens next? I bet it isn't pretty.
I can't say because it's never happened.

The rudders haven't kicked up except when hitting solid objects. Speeds up to 24.7 knots. On the GPS. Not once. Not ever.

They've never even remotely looked like kicking up when not required to.

So why are they an advantage? How about hitting a log at 10 knots? We've done it, with zero damage to the rudder. Didn't even mark the antifoul.

And yes, we can float in about 18 inches of water.

And back into a beach without worrying about rudders.

Or remove a rudder to lube the shaft, without slipping the boat, or getting in the water.

Or untangle a dinghy painter without getting into the water. Or fishing line...

Or untangle a crab pot float, without even needing to stop the boat....

Yeah, I consider them an advantage. A ****ing HUGE advantage.
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Old 11-07-2015, 01:58   #25
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Re: Fast cats and bad rudder posts

44cruisingcat:

Your rudders sound ideal! Mine aren't, though I know they won't kick up accidentally, and I can, with a little bit of work, pop them up - but not so easily as yours apparently.

Recently I had my anchor rope snag on rocks and break, and we were aground in surf. One rudder was wrecked and I had to rebuild it. I was in the Azores, and facilities and my time was a bit limited, so I just replaced what was bust and sailed on. But I was thinking all the stainless steel I have hanging on the back there is a lot of weight in the wrong place, and I was wondering about using carbon or fibreglass rudder stocks.

I'd really like to know the details of your rudder design, so any more info you could provide would be much appreciated.
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Old 11-07-2015, 03:51   #26
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Re: Fast cats and bad rudder posts

Maybe it's just me but as cruisers I would think its not a great idea to surf down waves at 20 knots in a storm. If you think the boat might get broken maybe it's time to put out a suitable drogue.
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Old 11-07-2015, 13:01   #27
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Re: Fast cats and bad rudder posts

transmitterdan: If you have a cruising multihull, you don't necessarily have to surf down waves to hit 20 knots (though I do in my 9m cat).

Here's my friend Tom Cox on a 30 year old Newick trimaran that he regularly cruises between New England and the Bahamas, doing 20 knots on a close reach:

Journey on a Woods Sagitta catamaran: North from Georgetown

I've since seen him doing 20 knots in stronger winds with just his mainsail up, on a close reach (I was doing 8-9 knots at the time and was more than satisfied with my speed till Tom shot past).
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Old 11-07-2015, 15:40   #28
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Re: Fast cats and bad rudder posts

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Originally Posted by transmitterdan View Post
Maybe it's just me but as cruisers I would think its not a great idea to surf down waves at 20 knots in a storm. If you think the boat might get broken maybe it's time to put out a suitable drogue.
We were surfing, but it wasn't a storm. I agree, doing that in a storm wouldn't be a good idea.
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