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  #16  
Old 01-09-2010, 10:16 AM
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gonzo gonzo is offline
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The box is a wrong analogy. Put the box against something that won't let it slide and it will tip either way. Do some force vector diagrams and it will be more clear to you.
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  #17  
Old 01-09-2010, 10:27 AM
chris14679 chris14679 is offline
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Quote:
Originally Posted by gonzo View Post
The box is a wrong analogy. Put the box against something that won't let it slide and it will tip either way. Do some force vector diagrams and it will be more clear to you.
If you find me some water that isn't slippery, I'll put the box against something.

... but the box STILL won't tip unless you cheat and push upwards - even if you put it against something you would squash the box before you would tip it, but you would tip it easily pushing on the inside edge. I suggest you do some force vector diagrams yourself, or just go get a box and try it! This assumes the box has some weight of its own, as a boat does. Maybe that's what you missed from your force vectors.
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  #18  
Old 01-09-2010, 10:32 AM
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They are the same. If you push a box at the same angle on both sides, the reaction is the same. Your pet idea is flawed.
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  #19  
Old 01-09-2010, 10:35 AM
chris14679 chris14679 is offline
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Originally Posted by gonzo View Post
They are the same. If you push a box at the same angle on both sides, the reaction is the same. Your pet idea is flawed.
Not my pet idea - it comes from the 'Schionning designs' website, the most popular designers and builders of biplane cats. I guess you must know more about biplane cat stability than them.

Quote:
In heavy weather, the most practical
sail configuration would involve reefing
in the windward sail and lowering the
entire sail on the leeward hull. The leeward
hull, then functioning like an outrigger,
doesn’t allow the windward hull to lift –
a great safety feature that makes it almost
impossible to tip the boat over.
http://www.schionningdesigns.com.au/...RadicalBay.pdf
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  #20  
Old 01-09-2010, 11:00 AM
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diagram
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Idea: Simple, cheap bi/triplane rig conversion using a 'yard' - would it work?-hull-torque2.jpg  
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  #21  
Old 01-09-2010, 05:37 PM
CT 249 CT 249 is offline
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Originally Posted by chris14679 View Post
Thanks Autodafe, glad you like the idea.

My intention was that this would be a conversion for an existing rig, so the mast would already be higher than necessary for a biplane, this would give the extra height needed to hang the yard without an extreme angle on the halyards. Compression loading on the yard wouldn't be massively more than on a very wide set of spreaders.

With respect, the angle between your upper stay and the yard is many times higher than the angle in wide spreaders, isn't it? And doesn't the load increase by the square as the angle decreases? I can't find the figures, but that's my recollection.

The leeward jib would not be set on a broad or beam reach. On a dead run the jibs could be backwards goosewinged (sheeted to the opposite bows) or a special downwind sail used as discussed above.

If you drop the leeward jib when on a reach, don't you lose power at a time when the apparent has dropped (compared to a beat) and therefore you can use more power?

My intention was that on the basic cruising form of the rig, the sail would be sheeted on the centreline of the hull, therefore self-tacking. Moving the traveller forward or back would give a lot of adjustment.

I have often sailed monohulls with a trisail which is sheeted to the centre of the boat and self-tacks. It works fine like that even on a very broad reach. The sail just needs to be shaped slightly differently for best performance with a centre sheet.

You still lose performance, because as the wind moves aft and the sheet is eased, the sail becomes excessively full while, and the sheeting angle is poor.


For maximum tweakable performance I suppose you could have two sheets on each jib with different leads for each tack, like on a conventional rig, or the sheet lead could be mounted on an athwartships traveller forward of the cockpit and companionway. Many cats already have a traveller there for the mainsheet. This would cause no more obstruction than the original main boom and sheet.

Dunno - conventional mainsheets, being multi-purchase systems running to booms (which provide a dampening effect on sail flogging) seem to flog around a lot less than jibsheets in my experience. That's why I'd prefer to have a mainsheet on a traveller (which is often down to leeward, out of the way) than the end of a jibsheet in the cockpit.

I've short-tacked up a narrow channel with the mainsheet on an athwartships traveller and moved it on every tack without problems. If you believe moving the jibsheets athwartships will make a big difference then it would be no more difficult or obstructive than a normal mainsheet traveller. Alternatively you could have two sheets on each jib and tack them, just like on a normal rig. Personally I wouldn't bother, I think for most purposes a centreline sheet lead will be a lot less hassle and will perform OK.

Sure, you won't lose a massive amount of performance and efficiency. But the performance you do lose (compared to another rig) will have to be made up for in increasing the rig size and therefore increasing cost, won't it?

I think this would only apply to racing boats. Most cruising boats just leave the main traveller in the middle and would probably do the same with a jib traveller if they had one. The only way to find out for sure is to try it (or ask someone who sailed a Hitchhiker rig which would be similar)

Surely it's the other way round - The shape of a normal main has been designed to work best with a boom, but a normal trisail is a different shape designed to work well without a boom.

The shape of a normal jib has been designed to work best with a centreline forestay and the jib sheet on the side deck. If the different sheet leads affect performance I'm sure a good sailmaker could do something about it.

They do do something about it - they cut a higher leach, and they tell you to move the lead outboard!

Many trysails around here are now set to be used with a boom, because it is more efficient. They can still be set without a boom for safety reasons, but it's not as efficient.


I never saw this as a racing rig, it's a cruising rig conversion which should in theory outperform the original rig significantly, both to windward and downwind. If it doesn't perform so well on a broad/beam reach (assuming you are lucky enough to get the wind in the right direction for that rare beam reach), then you could just put the original sails back up!
I just realised that one thing got accidentally erased from by earlier post. The load on the forestay would be a combination of static rig tension and the loading exerted by the sail. That means that the forestay of a jib in use would exert more pressure on the yard and therefore the yard would be under twisting forces, around the axis of the mast. This twisting force would vary routinely, according to the comparative pressure on each forestay, which is a combination of sheeting angle, blanketing, amount of sail set etc. Given the inherent stretch in any system, one would guess that there would be a lot of torsion in the rig, which would be hard to control without very stout gear and high rigging loads.


I just tried your cardboard experiment, using a wine box and a set of kitchen scales to measures the heeling moment.

If the box was allowed to slide sideways, of course no amount of heeling force on the 'leeward' OR 'windward' side caused it to heel. However, unless you have lateral resistance no boat can experience heeling forces, so that experiment is invalid.

And of course, the heeling force has to be applied above the part of the box that is restrained from sideways movement, just as the heeling force in a rig is applied above the keel. If you push directly opposite the area where the box is restrained, it doesn't matter where you push - if the heeling force was applied directly opposite to the restraining point, even an infinitely slim unballasted mono model won't heel.

Once I stopped the box from sliding (just like a centreboard or keel stops a boat from sliding) the measured power that was required to lift the box to a certain angle of heel was exactly the same (within the accuracy of the measurement system) whether I exerted the heel on the windward side or the leeward side.

Re "Not my pet idea - it comes from the 'Schionning designs' website, the most popular designers and builders of biplane cats. I guess you must know more about biplane cat stability than them."

Couldn't we just use the same response to your idea? We could just say "the idea (of using a conventional sloop rig) comes from Lagoon, Hobie, Seawind, Gemini etc, the most popular designers and builders of cats. I guess you must know more about cat rigs than them".

By the way, the 'rant' about the jib v main issue was in response to your claim that your rig would be more efficient as the jib luffs did not have a mast at the leading edge.

I expressed my views politely. If you feel that polite feedback is 'ranting', please do not ask people for their views.

PS - a lot of my thousands of miles offshore have been spent on a beam reach.
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  #22  
Old 01-09-2010, 05:55 PM
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Doug Lord Doug Lord is offline
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In the book, "Hydrofoil Sailing" by Alexander,Grogono and Nigg the "Exactly compensated foil stabilization system" is described on page 34. The description is for a low aspect single rig though I imagine a bi-plane rig would work better because the aspect ratio of the bi-plane rigs would be better than the single sail version of the system. According to the authors " no net roll moment shall exist when the center of gravity of the boat and crew is on the centerline of the boat. Theoretically such a boat will not heel under steady state conditions, and this will hold true on all points of sailing."
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  #23  
Old 01-09-2010, 06:25 PM
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Is that a theory from an alternative Universe? A rig that produces no heeling moment is a fantasy. This is another crackpot thread.
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  #24  
Old 01-09-2010, 07:07 PM
DarthCluin DarthCluin is offline
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While I would like to see this rig built, I question the validity of the box analogy. Perhaps you mean there is no lateral reistance if the centerboard/daggerboard/foil is up.

http://www.youtube.com/watch?v=f8j5FSCuJvI
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  #25  
Old 01-10-2010, 02:09 AM
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rayaldridge rayaldridge is offline
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Quote:
Originally Posted by chris14679 View Post
I have sailed tens of thousands of miles and only one or two percent of those have been truly reaching (with the APPARENT wind on or just aft of the beam).

.
This is a little hard to believe. In a fast multihull, reaching is very common. While speed causes the apparent wind to go forward, whether or not you end up sailing close-hauled depends on whicht point of sail you started sailing. For example, if you are broad-reaching, you may end up close-reaching in a good breeze. If you tack downwind, as you should in a fast multi, you'll be reaching.

The Schionning biplane cats do suffer from the lee blanketing problem.

As far as I know, the only biplane cat that has solved this problem ( according to the owner and designer) is Tony Bigras' Miss Cindy. He did this by sheeting the windward sail to the bow.

I don't know why you think a light yard would suffice. Even if you treat it as a diamond stay spreader, it will still have an enormous amount of downward pull, as it is holding up a whole rig on each end. To withstand the compression, it will need at least as big a section as a mast designed to hold up a similar rig, I think.

Another problem is what to do with the rigging needed to hold up a centerline mast on a cat. A freestanding centerline mast on a cat is impractical for a number of reasons. The shrouds needed to hold the mast up will interfere with sheeting the rigs that hang from the yard.

I have nothing against experimental rigs (I put a sprit sloop rig on a cat) but I'm a little dubious about this idea. You might want to try making a sailing model before you attempt a real boat.
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