Catamaran wing sail idea

Revise your ideas about monster outboards, as most sailing cats will be limited in speed, compaired to what a set of big outboards can produce. Size them appropriately, but don't get carried away with thinking much bigger is better, because all you'll do is beat the water to a froth, with little return on the investment, except with your fuel bill.

 

I actually second that, think David's client wants that power, and the linked thread is about David's struggle to find a way to make some use of it.

Hence the ‘‘Er... what?’’ emoticon at end of the line in my previous post.

(hold the cursor on the emoticon to see the text)​

 

Sailing performance is all about achieving a high lift/drag ratio for both the aerodynamics and the hydrodynamics. The problem with a power cat is the hull drag is already high. So no matter how efficient the rig is, the boat is not going to sail very fast or point very well into the wind, and will probably not go to windward at all without a daggerboard. So when you run out of fuel, your options may be very limited. And even dangerous if you are trying to stay away from a lee shore. Another issue is power cats have a narrower beam than sailing cats, so they are less able to stand up to the heeling moments produced by a sail rig. This greatly increases the chances of capsize, and it is another limit to the power you can get from the rig.

When you're under power, the apparent wind will be coming so much from ahead that the wingsail will not be able to contribute much in way of auxiliary power, either.

A better option than a wingsail might be a kitesail. It won't take you to windward, either, but it can be stored away in a bag and trotted out as emergency propulsion if you have engine failure. Limited options may be a lot more attractive than no options in that situation.

 

Zero Emission Catamaran pulled by a Dragon kite across the Atlantic​

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Mile-High Mega Kites Could Pull Giant, Floating Power Plants​

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The below quote comes from the first link in the above post...

Does anyone know what ‘‘a high-seasight’’ means in this regard . . ?

P.S. - Oh well, on second thought, I'll guess it means a ship or a boat that has seen the high seas . .

BTW, - ‘‘first time in the history of shipping’’ - ‘‘only with a wind-solar drive’’ - to keep the first sentence true, then don't cut ‘‘solar’’ out of that sentence . .

 

I'm reasonably certain that it is possible to get a boat with a draggy stern to sail upwind, as long as the stern is not ridiculously draggy.

You need a keel or a 'board of some kind that is effective at relatively slow sailing speeds. You also need sufficient sail area.

After that, the trick is to put the rig far enough aft to cancel out the stern drag vector.

Wing sails are probably not a good deal for this application. The reason for this is that they trade high lift for a high lift to drag ratio.

This is good, if you are sailing fast with a very low drag hull (like an AC foiling catamaran). But, if you are sailing with a power catamaran with immersed transoms, you need high lift, even if that high lift comes at the price of a lower lif/drag ratio.

This can come from low aspect ratio sails that can be furled.

To get long range under power, you probably won't be motoring all that fast (maybe 6 to 7 knots), so even low aspect ratio sails should be reasonably effective.

 

Pretty sure that a two or three element wing sail can get lift coefficients comparable to or better than a regular sail. However, it will be more complicated, harder to build, and harder to stow.

Since kiteboarders can sail upwind, I'm sure that a power kite can take a boat upwind. Plus they're much easier to stow than a wing sail. I'm sure they require more attention, though, than a wing sail. Can't just set the autopilot, unless you have a very sophisticated one that can also control the kite.

 

I agree. A kite works on pretty much the same principle as a sail.

You have to line up the drag vector with the thrust vector, for whatever point of sail you are on. This is not that difficult with a kite.

The problem is trying to get it to keep its alignment once it's set. Another problem is launching and retrieving the kite. This can get a bit dicey in heaving seas and gusting winds.

With 100% attention, this can probably be done, but I don't think it's all that good of a deal compared to other options. Figure maybe 8 hours of sailing to 16 hours of drifting, on any course except mostly down wind.

 

I just noticed the post mentioning two 19 foot rigs. That might be enough on a 20 foot boat, if you didn't want to go all that fast. But usually, masts are significantly taller than boats are long. A double rig would be more than 70 percent as high as a single rig, assuming it had the same proportions and the same total area.

On a power cat, sailing might be improved by putting a bit of water ballast up front to get the transoms out of the water. Just so the rig wasn't so powerful as to flip the boat over forward. The bows, if the right shape, might even provide enough sideways lift that a formal daggerboard wouldn't be needed, just a large rudder. There are sailing boats which use deep bows and rudders, and no other lifting surfaces in the water.
https://www.boatdesign.net/attachments/yorkshire-coble-jpg.47943/

 

Yes, it's true that a wingsail requires more accurate sheeting than a normal sail. But that problem is easily overcome by employing a tail to control the angle of attack exactly. So many people are
uncomfortable that you can't reef your wingsail. But is that really necessary? Surely the Walker Wingsail boats proved that you don't need to reef - whatever the conditions.
A successful rig nowadays that uses a tail and does not reef are the Saildrone autonomous boats which have survived the most severe conditions and crossed oceans with ease. A modified
Saildrone rig would surely make a good power source for an ocean cruiser. Peter Worsley has chronicled his progress in researching the self-trimming wingsail idea on a small catamaran. He tells the story here: Wingsail Rig Manager, Autonomous Windthruster Units http://www.windthrusters.com/story.html