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Old 26-03-2009, 03:01   #1
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Bi-rig and accidental jibes

I'm pretty impressed with the biplane rig designs from Schionning, Nordic, and others. I have a question though about what I see as a potential risk with this design, and it concerns accidental jibes.

As I understand, these boats are sailed wing-and-wing downwind. Since the masts are unstayed, the two mains can be sheeted out arbitrarily far to reduce the risk of an accidental jibe. However we all know ... stuff happens. And with so much mainsheet out, an accidental jibe would result in a boom sweeping the deck through a 180 degree angle or more. Apart from being extremely dangerous to anyone on the side deck or near the mainsheet, the dynamic loads on the mast bearings could be difficult to engineer. You've also got a lot of mainsheet flying around, possibly snagging people or boat bits and causing a lot of trouble. Since the mast is stepped way out on the beams, there's no obvious way to mount a preventer, boom brake, or traveller to limit the energy from an accidental jibe.

Even worse, there are scenarios where the booms could impact each other. Consider a broad reach with both mains sheeted out on the same side. Then if the boat jibes accidentally and the (old) leeward rig happens to jibe before the windward rig, bang! you have damaged or destroyed both of your booms.

I can think of ways to limit these problems but not fully eliminate them:

1) Have a "stop sheeting out here" limit on the mainsheets to prevent boom interference -- can only be exceeded once the boat is wing-and-wing.

2) Stay the hell off the side decks when sailing downwind. Without headsails, there's not much reason to go up there anyway.

3) I guess you could rig preventers through turning blocks on the bows for sailing wing-and-wing. Interesting rigging challenge though -- the boom ends are completely inaccessible over the water once you are wing-and-wing.

What do y'all think about this -- particulary those of you who have sailed bi-rig catamarans? And for builders, what are thoughts on how to design for these problems? Where is your mainsheet anchored? Does it keep the mainsheet away from people who might get tangled/dragged/decapitated in an accidental jibe? Is the mainsheet anchor engineered for the load from an accidental jibe?

Just some food for thought....

Martin
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Old 26-03-2009, 08:06   #2
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I'd think they rig preventers BEFORE they are needed. Once downwind NO boom end can be reached.
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Old 26-03-2009, 12:26   #3
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It's no different to a traditional main, you rig a preventer...

On the Nordic Cat, the vangs are fixed, so the boom stays at a fixed height, so you can maintain sail shape at all times.

An accidental gybe is not likely to happen for both mains surely??

Finally, having a forward cockpit negates any reason to be up on the side deck for anything other than enjoyment


Alan
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Old 26-03-2009, 15:13   #4
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Thanks for the feedback guys, that definitely helps clarify.

Quote:
Originally Posted by Nordic cat View Post
An accidental gybe is not likely to happen for both mains surely??
In a normal broad reach the boat would not be wing-and-wing. Only one main needs to jibe in order to cause the booms to impact each other. This could happen even in a normal intentional jibe if it is executed badly. Certainly would be very easy to happen in an accidental jibe from a broad reach.

I think the foolproof solution is to never sheet out the mains beyond a certain point (where the booms could interfere with each other) unless a preventer is rigged.

Also seems like the preventer line would need to be quite permanent. It's not something you just rig up for the rare occasions you need it, it would be a permanent fixture & a frequently used sail control.

Randy: Sailing downwind on a monohull or catamaran it is usually possible to sheet in the main sufficiently to access the boom end -- perhaps 20-30 degrees off the centerline of the boat. Trying that on a bi-rig would be very dangerous because the main would have to be sheeted in almost all the way.

Martin
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Old 26-03-2009, 16:41   #5
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A preventer is not effective if the mast is too far forward, I know from personal experience (preventer was about 4 feet forward of the mast attached to the bow roller and bent the pulpit). So a preventer will only work on certain designs.... I would love a good solution to this problem also.
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Old 27-03-2009, 00:17   #6
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I don't know the design so I can't help there but I'm surprised the most used solution for rigging a preventer isn't mentioned yet!

The boom is out, you can't reach it's end anymore but want to set a preventer. Most boats accomplish this by attaching 2 lines to the boom-end which are a couple of feet longer than the boom and leave them there permanently. You tie them off at the mast-end of the boom. Now, when you want to rig a preventer, you only have to tie to these lines at the mast.

cheers,
Nick.
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Old 29-03-2009, 14:08   #7
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I sometimes crew on the Radical Bay 8000 by Schionning with free-standing masts. We HAVE seen both accidental and badly coordinated gybes, with the latter being easy to rectify through coordination. The mainsails are loose footed, the vangs fixed, the booms run at a rising level from the mast to the end. In the case of a bad gybe, the turning mainsail typically touches the other boom's end and, being loose footed, looses shape but does not tear. Boom touching boom virtually never occurs. We have used preventer lines but normally sail without them.

Roger
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Old 31-03-2009, 16:08   #8
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We always use a preventer or put the sheet-block on the toe-rail for long term reaching. With light winds, the preventer prevents the slamming of the rig that's caused by swells passing under the boat and it helps keeping speed up to a level that we don't need the engine.

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Nick.
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