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Old 04-07-2015, 09:11   #1
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Fast cats and bad rudder posts

I really don't know which forum to post this in - the ones where catamarans can't sail or don't sail, or the ones that go on about the dangers of spade rudders.

Anyway: Latitude 38 - 'Lectronic Latitude

Synopsis: In the first case, someone built a very fast, if not odd looking, almost foiling carbon fiber cruising cat that does 25kts in 18kts of wind (and it is a cruising cat). Makes 44CC's boat performance look like an Oyster's.

The second case involves a brand new carbon fiber Schionning G-force 1400 that snapped both rudder posts on their maiden voyage (they were doing 19kts at the time and didn't hit anything).

The surprising thing about this to me was the picture of the broken post. It appears to be ~1.5" of SS tube. Our much smaller and slower boat has 2" solid SS posts, and I would have expected larger ones on this boat.

The article says that a preliminary examination indicates that the "rudder fittings" were not built to spec. If this includes the rudder post, it seems like that would have been noticed very quickly and easily.

Mark
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Old 04-07-2015, 09:32   #2
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Re: Fast cats and bad rudder posts

Ouch. Yeah 38mm tubing seems lightweight. I guess these guys have the boat bucks to sort it out. Maybe they're trying to keep the rudder thickness down, but reducing the diameter in the rudder would probably give a better result.
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Old 04-07-2015, 14:37   #3
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Re: Fast cats and bad rudder posts

Quote:
Originally Posted by colemj View Post
I really don't know which forum to post this in - the ones where catamarans can't sail or don't sail, or the ones that go on about the dangers of spade rudders.

Anyway: Latitude 38 - 'Lectronic Latitude

Synopsis: In the first case, someone built a very fast, if not odd looking, almost foiling carbon fiber cruising cat that does 25kts in 18kts of wind (and it is a cruising cat). Makes 44CC's boat performance look like an Oyster's.


Mark
Meh. They've probably got 50 hp engines.
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Old 04-07-2015, 16:19   #4
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Re: Fast cats and bad rudder posts

Hollow SS stock, same **** happen again!! Catana try also with Hollow Aluminium stock with horrendous results!!
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Old 05-07-2015, 11:49   #5
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Re: Fast cats and bad rudder posts

I build a copy (design 150 )of Lock Crowther 's Deguelo , and he specified 2 inch ss tubing for the rudder posts , but also required where it exits the hull ,30 cm of high tensile steel epoxied into it , 150mm up into the hull and 150mm ,outside of the hull . now this is where it's going to bend if you run aground , which is exectly what happened to a friend of mine when he hit a sand bar in his cat . But then Lock did not just draw up new designs , he was also an egineer I believe , and those rudders were designed to be able to beach the cat ,and partially sit on them .They perform pretty well , as I've had days of cruising from 12- 16 up to 20 knots of boat speed .(the fastest yet, 25 a couple of times And no ill efects from the acceleration , which I read in some guys blog on catamaran sailing , he can't handle going faster than 10 knots )
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Old 05-07-2015, 16:31   #6
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Re: Fast cats and bad rudder posts

Our rudder stocks are fibreglass. 60mm, with originally 10mm wall thickness.


I found at sustained speeds over 10 knots the steering got a little sticky. Put it down to the rudder stocks flexing and binding in the tubes. So I stiffened them up by squeezing rolled up biaxial glass into them. Now the effective wall thickness is about 25mm.


Steering is nice and light up over 15 knots.


We have kick-up rudders, so I don't worry about snapping the rudder stocks.
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Old 05-07-2015, 17:32   #7
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Re: Fast cats and bad rudder posts

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.
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Old 05-07-2015, 18:14   #8
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Re: Fast cats and bad rudder posts

Fatigue, corrosion and groundings are what kills rudder stocks through time, under specced will probably fail the first kick ass session that comes along.
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Old 05-07-2015, 18:21   #9
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Re: Fast cats and bad rudder posts

Anyone have any links to a kick-up rudder design?
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Old 05-07-2015, 18:30   #10
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Re: Fast cats and bad rudder posts

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Anyone have any links to a kick-up rudder design?
Check out Gemini (3400 or 105 models). Smaller than the boats being discussed but the same principals could be applied to larger boats.
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Old 05-07-2015, 19:00   #11
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Re: Fast cats and bad rudder posts

"Fatigue, corrosion and groundings are what kills rudder stocks through time, under specced will probably fail the first kick ass session that comes along."

Yes, but then again no. What tends to happen is that the under-designed or under-built stock has minimal safety factor so it might survive a year or two of normal use but fatigue and/or corrosion eat away at both the rudder and the safety factor. Whilst acknowledging that fatigue weakening and corrosion should not be present in a well designed and built rudder, they will always exist to some extent and must be allowed for in the design process. it is a brave(?) designer indeed who does not allow for imperfect build or operation.
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Old 06-07-2015, 06:51   #12
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Re: Fast cats and bad rudder posts

The case in question was a new build on a maiden sail. I don't think fatigue or corrosion had much time to occur. Since the posts broke at 19kts without a collision, I surmise that under-engineering was the problem.

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Old 06-07-2015, 07:12   #13
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Re: Fast cats and bad rudder posts

I don't believe they were "going 19 knots" when the rudders failed, but had seen a peak speed in that neighborhood in the 24 hours before. One rudder broke first and the steering was noticeably affected. They initially thought it was autopilot problems until the second rudder failed hours later.


The boat was consider "new" by the owner but had been afloat in South Africa for about 4 years partially finished before completion recently.
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Old 06-07-2015, 07:22   #14
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Re: Fast cats and bad rudder posts

In the case of Guadeloupe. There's huge amounts of floating weed the at the moment. We have to head upwind and stop every 30mins or so to dump the weed from the rudders. When the weed wraps around one the AP or the helmsman struggles to maintain course and speeds drop by more than a knot. It's surprising how much a bit of weed effects balance. I'd guess the weed contributed to the disruddering.
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Old 06-07-2015, 09:14   #15
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Re: Fast cats and bad rudder posts

Quote:
Originally Posted by colemj View Post
The case in question was a new build on a maiden sail. I don't think fatigue or corrosion had much time to occur. Since the posts broke at 19kts without a collision, I surmise that under-engineering was the problem.

Mark
Both of them?

You can cut it a little closer with a cat because you've got two, but it seems many are cutting them far too close.

This is one thing which really should be "safety of flight" level of reliability, but which is obviously not with many boats. Easier to achieve if you have two of them, but still requires some effort.

I like Dashew's spade rudder designs. Maybe something like that, less extreme, would be the thing for these cats.
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