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Old 02-09-2008, 13:54   #1
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Bi-rig

What advantages and disadvantages, is it with a bi-rig?

What type of sails to use? Is it possible to sail with spinnacker?

Is it more expencive than a traditional rig?

Can you sail closer to the wind?

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Old 03-09-2008, 20:44   #2
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Twin aka biplane rig

One advantage is that you can have unstayed masts, like the boat in your photo. That boat you have a photo of is probably very close winded, because it has wing masts. A standard marconi sloop rig on a catamaran is rather like a giant bow and arrow. The mast is under tension, just like an arrow is right before you let it go, and all of that downward force is right where you least want it-in the middle of the bridge deck. If the boat flexes, and they usually do, you have that tension varying quite a bit, as a wave passes first under one hull and then the other.

Another advantage is no gennys to crank or change.

I have a lot more about this subject on my website, including links at the bottom of the page.

Nordic Cat is designing one, and you can read about it on this forum at:

New Bi-Rig Performance Cruiser

You aren't very likely to find one for sale, though-
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Old 03-09-2008, 21:39   #3
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Quote:
Originally Posted by BigCat View Post
You aren't very likely to find one for sale, though-
Given a large enough offer there is nothing on this planet that is not for sale

Lots of advantages
First reef tends to be automatic if the mast stiffness is designed correctly.
Less loads on everything including sheets so smaller winches and effort required.
The main being divided into two makes each more easy to handle, much like the rational behind the ketch.
Less lines, only a halyard, outhaul, one reef and maybe a downhaul.
Can be totally depowered and reefed on any point of sail even without the fancy batcars.
Mast really has to rotate and the boom is quite likely to be rigid so no vang.
No standing rigging to fail or chaffe the sails.

Running a spinaker would be difficult without a backstay to transfer the load. One of those kites instead maybe.
On a beam reach there is the possible downside of the windward sail blanketing the leaward one although some suggest that easing the windward one alot to in effect feed the leeward one goes a long way to make up for the loss but somehow I find that a little difficult to fully accept.

Mike
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Old 03-09-2008, 21:51   #4
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pirate I made a drawing-

Quote:
Originally Posted by Whimsical View Post
Given a large enough offer there is nothing on this planet that is not for sale

Lots of advantages
First reef tends to be automatic if the mast stiffness is designed correctly.
Less loads on everything including sheets so smaller winches and effort required.
The main being divided into two makes each more easy to handle, much like the rational behind the ketch.
Less lines, only a halyard, outhaul, one reef and maybe a downhaul.
Can be totally depowered and reefed on any point of sail even without the fancy batcars.
Mast really has to rotate and the boom is quite likely to be rigid so no vang.
No standing rigging to fail or chaffe the sails.

Running a spinaker would be difficult without a backstay to transfer the load. One of those kites instead maybe.
On a beam reach there is the possible downside of the windward sail blanketing the leaward one although some suggest that easing the windward one alot to in effect feed the leeward one goes a long way to make up for the loss but somehow I find that a little difficult to fully accept.

Mike
Avast, matey! On my biplane rigged design, the masts aren't going to pivot. I've got a new drawing on my website showing beam reaching with a biplane rig. In overview, the two sails look much like one airfoil, when trimmed as you described, so I can see that it might actually work quite well.
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Old 04-09-2008, 00:47   #5
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Quote:
Originally Posted by BigCat View Post
Avast, matey! On my biplane rigged design, the masts aren't going to pivot. I've got a new drawing on my website showing beam reaching with a biplane rig. In overview, the two sails look much like one airfoil, when trimmed as you described, so I can see that it might actually work quite well.
Yes but yours would be a rare exception by not rotating.
I hope it does work well but I can't see it being as eficient as unblanketed sails, but then I have a 6.5 tonne boat with 12:1 hulls and 110 square meters of rag so I am not really bothered at all

Mike
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Old 04-09-2008, 03:10   #6
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MicroCat Construction

another tony bigrass boat, all of 16 ft long with a bi rig but pretty cute i thought
sean
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Old 04-09-2008, 05:24   #7
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I have seen this kind of "windsurfer" sail on a big cruiser but i canīt find the pic right now.
The mast was was very close close to the bow.

Then you have a very big sail! Bad??? Good???
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Old 04-09-2008, 08:58   #8
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Originally Posted by freetime View Post
What advantages and disadvantages, is it with a bi-rig?

What type of sails to use? Is it possible to sail with spinnacker?

Is it more expencive than a traditional rig?

Can you sail closer to the wind?
G'day,


Mike got the advantages and disadvantages spot on, except that if it is designed correctly, a running spinnaker is no problem. We are also using in boom roller reefing mains, so there is no reefing gear or lazy jacks required. Just a halyard and wnch, a multipurchase outhaul/leech tensioner and a mainsheet which is lightly loaded as it only adjusts the angle of attack, not the leech tension. Compare this to the deck gear, standing and running rigging of the conventional rig.

It is cheaper than a conventional rig if the boat is designed for it from the start. Even cheaper is to have just one mast, in one hull, but few people can get their heads around this sort of assymetry. If you add in the maintenance and replacement costs (virtually zero for the unstayed rig), it is way cheaper. Unstayed masts can also be built to a high standard by the home builder if you really want to save some money.

As an example, the professionally built 18m/80' mast in the video at cost $aus10,000, weighs 120 kgs/260 lbs and is on a boat with righting moment of 18 tonne metres, the same as a mm/20' wide (centreline to centreline) cat weighing 6 tonnes.

Windward performance is good, excellent if it is a wing mast.

Regards,

Rob
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Old 04-09-2008, 12:45   #9
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Originally Posted by freetime View Post


I have seen this kind of "windsurfer" sail on a big cruiser but i canīt find the pic right now.
The mast was was very close close to the bow.

Then you have a very big sail! Bad??? Good???
That's Cat2fold. It scissors together in the middle for trailering.
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Old 04-09-2008, 12:59   #10
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More biplane stuff

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Originally Posted by Whimsical View Post
Yes but yours would be a rare exception by not rotating.
I hope it does work well but I can't see it being as eficient as unblanketed sails, but then I have a 6.5 tonne boat with 12:1 hulls and 110 square meters of rag so I am not really bothered at all

Mike
Hi, Mike -

Maybe it won't be as efficient as one sail, but the bigger the boat, the more difficult that becomes. I have 1200 sq. ft. sails, roughly 118 sq. meters each, so I wouldn't want one sail, no matter if it is in the center of the bridgedeck or in one hull as in a proa. When you have two sails, you have a blanketing potential. I don't see why people complain about the biplane rig on that score, as the sloop rig has the same problem, but on a different point of sail.

If the boat is fast enough, of course, you never have the wind very far aft, but that may be too optimistic when speaking of cruising cats.

I, too, have a light boat with 12 to 1 hull beams, so, like you, I'm not too worried about performance. If your waterline is 44', then you would have a DL ratio of 76 loaded for cruising, which is about the same as mine, too-that being my boat's with all tanks full, about 1000 gallons (about 4,000 liters.)

As far as pivoting masts go, you could always have fixed stubs and have a wing shaped sleeve. I don't see why you couldn't have a boom vang, as long as it connects to the mast and not the deck.
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Old 04-09-2008, 13:08   #11
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You aren't very likely to find one for sale, though-
I can always buy a Nordic Cat or a Schionning bi-rig new.

Good site, BigCat.
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Old 04-09-2008, 13:13   #12
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Nordic Cat

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I can always buy a Nordic Cat or a Schionning bi-rig new.

Good site, BigCat.
Thanks for the kind thoughts. Schionning is a catamaran designer, and as far as I know, there aren't any stock boats you can buy. The Nordic Cat is a possibility, and close to you, too- Even if it doesn't become a stock boat, you might be able to get a fixed price considering that they will have built one already.
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Old 04-09-2008, 13:57   #13
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I have no plans to build a boat by myself. But maybe a yachtbuilder can build my boat.
Nordic Catīs.
This thread was based around bi-rig, and in my case Nordic Catīs bi-rig. And Nordic Cat is only 1 hour flight from me and thats matter. And i like the design of the new catamaran very much. With all steeringpositions, and the forward steeringposition with a chris white look but much wider.
And the ability to see the sails and that no mast is right in the front. And then the possibility to custombuild the interior with "my" style and accessories. But i donīt want hull #1, i would like to testsail one first.


Sailingperformance in different windconditions with a bi-rig if you compare to a standard rig?
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Old 04-09-2008, 14:10   #14
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Beam reaching

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Sailingperformance in different windconditions with a bi-rig if you compare to a standard rig?
The only difference would be on a beam reach. You can free the windward sheet and harden the lee sheet, or head up, or fall off. No difference, otherwise, according to those who have sailed them. I have researched this pretty extensively on the internet.
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Old 04-09-2008, 14:16   #15
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Sounds good.

It must be easy to single handle a bi-rig?
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