Pepetual motion multihulls?

[Rob_Webb]

I've heard it said a few times that the giant multihulls are never truly becalmed because they are able to generate their own wind.

But surely this contravenes the laws of physics? Imagine a scenario with absolutely no true wind and a multihull starts out by motoring to generate an apparent wind and then raise it's sails and stops the engine. Surely it cannot keep going because of the effeciency losses caused by water drag etc etc. Otherwise wouldn't this represent the nirvana of perpetual motion?

So is the truth more that they can keep going in tiny amounts of wind rather than no wind?


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[Aeolus_IV]

Yes, you are right - you can't get something from nothing, and even achieving 100% efficiency is impossible. All that is meant by this saying is that the fast sailing craft (large multi-hulls and faster dinghies) can all travel so fast that even when sailing down wind, the apparent wind will be from ahead, thus apearing to generate thier own breeze.

Easy really.

Regards, Jeff.

<hr width=100% size=1><A target="_blank" HREF=http://users.swing.be/FDB/centurion/index2.html>Centurion 32 Web site</A>

 

[extravert]

> even when sailing down wind, the apparent wind will be from ahead...

But when sailing directly downwind, the apparent wind will still be from directly behind. The reason why the above is said is that performance multihulls do not usually sail directly downwind, but can attain a higher VMG downwind by sailing off on reaches and gybing every so often. (This is also true on monos as well, but is often much more beneficial on multihulls.) My multi is designed to be sailed like this downwind. I can't get my boom out far enough to sail well directly downwind anyway.

<hr width=100% size=1>One day, I want to be a real sailor. In the mean time I'll just keep tri-ing.

 

[Sybarite]

A similar notion is being explored in aviation. A "catamaran" plane filled with helium rises until there is an equilibrium with the outside thinner air and then air inside the plane is compressed to make it denser than helium. The plane then starts a long glide and wind turbines are used to generate power which will subsequently recondense air when the process is repeated. The idea is to create a fuelless plane.

http://www.fuellessflight.com/index1.htm

John

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[Aeolus_IV]

Yes, I didn't make myself clear - these faster craft zigzag down wind, never actually running with the wind. Whether they arrived at a down wind location before a boat simpy running with the wind I couldn't say. There was a thread on this a short while ago.

Jeff.

<hr width=100% size=1><A target="_blank" HREF=http://users.swing.be/FDB/centurion/index2.html>Centurion 32 Web site</A>

 

[snowleopard]

tacking downwind

i have experimented with the geometry of tacking downwind, starting with making a vector plot of boat speed v. course. (surprisingly difficult to do!)

i also tried various wind angles and plotted vmg on the tradewind crossing. in the end it turned out that best vmg was obtained by going straight downwind. we were more heavily loaded than normal so couldn't generate enough speed on a broad reach to bring the wind forward of the beam and really start to shift. also our rig works pretty well with the wind dead aft which cancels some of the benefits of reaching.

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[Gordonmc]

Surely the mechanic of a sail depends on air-flow over the windward surface. The efficiency of the foil is the key, upwind or down. By presenting a near-enough flat surface to the wind is the least efficient way of propelling a boat... or for that matter, an aircraft.
Think of it another way; of a boat was sailing directly downhill with sails at 90 deg to the wind and with a 0 friction coefficient (impossible) it would only ever be able to achieve the speed of the wind.

Er...!

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[wooslehunter]

Re: tacking downwind

In a completely zero wind, glassy flat sea with no tide, every wind driven vessel will become becalmed. If you just motor forward, sails can't generate any drive since they will be flogging. Whichever way you go, the apparent wind will always be dead on the nose. When sailing in some wind, as the boat speeds up, the apparent wind moves forwards and can certainly allow a vessel to sail faster than the wind. That's why windsurfers are almost permanently close hauled in anything above a breeze. They go so fast that the apparent wind moves as far forward as possible.

Now here's how a boat can appear to generate it's own wind using tide and waves. If there's some movement of the sails they have the potential to drive the boat.

Assume zero wind and put a boat across the tide & we generate some apparent wind - the boat moves forward.

Rock a boat in zero wind & tide & we get the same effect - hence rule 60 in the racing rules which dinghy racers do their best to just not break.

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[extravert]

Re: tacking downwind

My winter project has been to develop equipment to record data from my boat electronics, and plot the information on polar diagrams. I am just about to put it into service for a summer of logging and plotting. The result will be a set of measured polar diagrams for my boat, rather than the more usual theoretical calculated ones available for some boat designs.

The downwind theory (for my boat anyway) will proved or disproved later this year.

<hr width=100% size=1>One day, I want to be a real sailor. In the mean time I'll just keep tri-ing.

 

[Aeolus_IV]

Re: tacking downwind

Good idea - will you be able to correlate against sail plan in use at the time as well?

Jeff.

<hr width=100% size=1><A target="_blank" HREF=http://users.swing.be/FDB/centurion/index2.html>Centurion 32 Web site</A>

 

[extravert]

Re: tacking downwind

Yes. There is a menu item in the software where the current sailplan is set. This setting is then recorded in the logged data.

Obviously everytime the sailplan is changed, someone has to change the sailplan setting in the software. It's easy to do, probably harder to remember to do it.

<hr width=100% size=1>One day, I want to be a real sailor. In the mean time I'll just keep tri-ing.

 

[cameronke]

Re: tacking downwind

Sounds very interesting stuff Extravert, would love to hear how you get on

Regards
Cameron

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[extravert]

Re: tacking downwind

It has not been tried for real yet, only on the bench. If it is successful, I will post details here later in the year.

There's no laptop or Microshaft Windows on board. All the data recording is done by a phone. This is what the phone's screen looks like when in action...

Anyway, I'm in danger of hijacking this thread here, so more in a new thread later on.

<hr width=100% size=1>One day, I want to be a real sailor. In the mean time I'll just keep tri-ing.

 

[kds]

"recondense air" ? It requires more than 200 degrees below zero. Bring on Global cooling.
Ken

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[johnneale]

Re: tacking downwind

All these experiments & maths have been done before ---- See 'Design for Fast Sailing' by Ed Bruce - published by AYRS over 30 years ago.

Like many other tri sailors, I have sailed a circular course under a shy reaching spinnaker using apparent wind

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[Sybarite]

"Compact" was perhaps the word to use.. ie make more dense...

John

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[extravert]

Re: tacking downwind

Yes, I know it's all been done before, but it's just a bit of fun and interesting (for a computer nerd) to do. I doubt its been done using a phone before though.


<hr width=100% size=1>One day, I want to be a real sailor. In the mean time I'll just keep tri-ing.