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 jbarros 01-14-2010 05:53 PM

parallel sails?

I was looking at the Hobie Trifoiler ( Hobie TriFoiler History ) and it got me wondering, how do parallel sails like this work in practice? It appears they'd be fine until you're abeam the wind, and then you'd have only one sail.

I'm pondering duplicating a setup similar to this on one a kayak/trimaran I may be building soon.

Thoughts? suggestions?

Thanks.

-- James

 tager 01-14-2010 06:58 PM

I am pretty sure you would have to be abeam the apparent wind.

What do you think will happen when the apparent wind is abeam? If the leeward sail is sitting in turbulent air then the weather sail must be stalled. Sails are airfoils and airflow around them should be laminar on most points of sail. Even if flow is disconnecting from the weather sail a little bit, that dirty air moves aft as the boat moves forward at its ridiculous speed.

However I would expect the apparent wind on the leeward sail to be higher than on the weather sail, at least as you get close to wind abeam. That would be fun to see.

 mitiempo 01-14-2010 10:01 PM

You would never be abeam the wind because at the speeds of a boat like this the apparent wind will move forward as your speed through the water. Boats like this can sail almost as fast as the wind or faster. This is common to all fast multihulls and some of the fastest monohulls. So you would have 2 sails both working effectively in clear air. The concept would not work on a boat that was slower.

Sure you can have the apparent wind abeam in any vessel. You just need the true wind to be at an angle whose cosine is the negated ratio between boat speed and true wind speed (assuming my trig is correct).

I would say that even in that case you could have both sails operating in clean air, if properly trimmed.

 tager 01-15-2010 02:46 AM

Have to agree with AdamLein. A more simplistic approach to proving the existence of wind abeam would be this thought experiment: Say that you are going downwind, which direction is the apparent wind? Astern? Now suppose you are going upwind, the apparent wind is now coming from somewhere forward of the beam... Now consider how you change between the two. It is logically necessary that at some point the apparent wind is on the beam, but I agree with AdamLein that even then the flow would be laminar enough to allow the leeward sail to be useful, if somewhat less so than on other points of sail.

Interesting topic to research DDWFTTW just google it. I will give you a clue, it is possible to go dead downwind faster than the wind, multihulls and foilers do it all the time.

With this in mind, mitiempo is correct in some capacity, fast multis spend most of their time with the apparent wind forward of the beam, this makes sense when you consider that they can go dead downwind faster than the wind by broad reaching and jibing.

However, when jibing, the apparent wind will be astern at some point, and this is a sufficient condition to satisfy my previously stated "thought experiment." I have avoided a rigorous analysis of this subject just because it would reach a narrower audience, nobody likes a pedant.

(sorry pedants)

 mitiempo 01-15-2010 03:04 AM

If you're talking about "any boat" the wind can be abeam. But the tri-foiler in the link is a 40+ knot boat and if you look at it sailing in the video at the bottom of this link Hobie TriFoiler History
you'll see that the sails are sheeted in as much as possible whichever direction the boat is sailing in. That only works when the wind is forward of the beam. As it passes the speed of the wind and accelerates, which it does very quickly, the wind moves way forward. As long as the boat is going at speed it's close hauled.

Quote:
 Originally Posted by tager (Post 560098) nobody likes a pedant. (sorry pedants)
Yeah well nobody likes you either... j/k ;)

mitiempo: I watched that video and I admit that you do see boats mostly close-hauled even when the make wide turns. However I'm having trouble judging what tack the boats are on during those turns so it could just be that they're tacking.

Anyway, do you believe that if one of these boats is heading dead downwind, the apparent wind will still be in the close-hauled position? What do you think about tager's argument? If the boat is headed DDW and is going faster than the true wind speed, then apparent wind is dead ahead; otherwise it's dead astern.

 mitiempo 01-15-2010 02:18 PM

2 Attachment(s)
It wouldn't be from dead ahead but pretty close. In the video the boats are changing direction but as their speed is high even in the turns the apparent wind is still from way ahead of the beam. You are having trouble judging the tack they're on because at their speed they are close hauled all the time. The sails are cut pretty flat and sheeted in all the way. That rig wouldn't work on a slow boat. In their case one sail doesn't blanket the other but it would in a slower boat. The record breaking multihulls act the same way. They tack downwind with the apparent wind well forward of the beam. Notice on these 2 boats how they are sheeted in as well. After they reach a high speed they create their own wind.

 sailingdog 01-15-2010 04:15 PM

Bi-plane rigs, like that on the tri-foiler, work fairly well, provided the hulls are far enough apart for the sails to not interfere with each other a majority of the time. On a high-speed boat, like the tri-foiler, which uses hydrofoils to lift the hulls and reduce water drag, the wind coming from abeam is not a real problem. However, on slower designs, it can be a serious issue and effectively cuts the sail area of the boat in half as the windward sail shadows the leeward one.

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