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  #21  
Old 07-22-2006
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Quote:
Originally Posted by kokopelli9
Hey, SailorMitch...good to see you here again. How did the week sail with Denr go? Did you manage to get him sufficiently seasick?Kokopuff...
Hi Kokopuff!

Denr and I had a great week on the bay. Alas, he did not get sick -- the man has an iron constitution. But at least this time I didn't get sick!!!! My inner ear failed to grasp the intricacies of those Michi Gami square waves that first day back when. My home waters on the Chesapeake are kinder to my middle ear. Will send you more details in an email. Don't want to interupt the flow of this mesmerizing thread.
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  #22  
Old 07-23-2006
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I am not aware of too many boats that can beat faster than the wind.

pigslo
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  #23  
Old 07-23-2006
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I can't believe you're actually asking this. This has nothing to do with sailing. That's just a stupid question. I'm amazed that you can use a computer. A simple understanding of effort and resistance is taught in grade 10. Maybe you should drop in. I know I'm no rocket scientist but this is rediculous. Thanks anthony for the bootstrap line. I'll have to remember that/
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  #24  
Old 07-23-2006
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Quote:
Originally Posted by SailorMitch
Bob,

Have you even tacked a sailboat?
No, I'm going by what I've read about tacking into the wind:

The physics of sailing.
"How can boats sail faster than the wind? Lots of boats can---especially the eighteen footer skiffs on Sydney Harbour. Ask a sailor how, and he'll say "These boats are so fast that they make their own wind", which is actually true. Ask a physicist, and she'll say that it's just a question of vectors and relative velocities."
...
"The faster that the boat goes, the greater the relative wind, the more force there is on the sails, so the greater the force dragging the boat forwards. So the boat accelerates until the drag from the water balances the forward component of the force from the sails."
...
"Why are eighteen footers always sailing upwind?
In a fast boat, there's no point going straight downwind: you can never go faster than the wind. So you travel at an angle. But if your boat is fast enough, then the relative wind always seems to be coming mainly from ahead of you, as these arrows show. So the eighteen footers never set ordinary spinnakers: they have asymmetrical sails that they can set even when they are travelling at small angles to the apparent wind."
http://www.phys.unsw.edu.au/~jw/sailing.html

I gather from this that being able to sail at a speed faster than the wind speed is not the norm for the average sailboat, but only for boats especially designed for speed.
The method of tacking into the wind also works with ice sailing where the runners pushing sideways against the ice is what causes a force on the boat with a forward component that allows the ice boat to move at an angle *into* the wind. With ice boats the speeds can exceed more than 70 mph when tacking into the wind, much higher than the wind speed:

Ice Yachting.
"A course of 20 miles with many turns has been sailed on the Hudson in less than 48 minutes, the record for a measured mile with flying start being at the rate of about 72 miles an hour. In a high wind, however, ice yachts often move at the rate of 85 and even 90 miles an hour.
"Several of the laws of ice navigation seem marvelous to the uninitiated. Commodore Irving Grinnell, who has made a scientific study of the sport, says: The two marked peculiarities of ice yachting which cause it to differ materially from yachting on the sea are the ability to sail faster than the wind and that sheets are flat aft under all circumstances. Mr. H. A. Buck, in the Badminton Library, Skating, Curling, Tobogganing, thus explains these paradoxes. An iceboat sails faster than the wind because she invariably sails at some angle to it. The momentum is increased by every puff of wind striking the sails obliquely, until it is finally equaled by the increase of friction engendered. Thus the continued bursts of wind against the sails cause a greater accumulation of speed in the ice yacht than is possessed by the wind itself. When the boat sails directly before the wind she is, like a balloon, at its mercy, and thus does not sail faster than the wind. The ice yacht always sails with its sheets flat aft, because the greater speed of the boat changes the angle at which the wind strikes the sail from that at which it would strike if the yacht were stationary to such a degree that, in whatever direction the yacht is sailing, the result is always the same as if the yacht were close-hauled to the wind. It follows that the yacht is actually overhauling the wind, and her canvas shivers as if in the wind's eye. When eased off her momentum becomes less and less until it drops to the velocity of the wind, when she can readily be stopped by being spun round and brought head to the wind."
http://en.wikipedia.org/wiki/Ice_sailing

Apparently, these experts on ice sailing were not aware some sailboats can sail faster than the wind since that is not the usually state of affairs with the average sailboat. The greater drag of the keel in water explains why ice yachts are able to achieve higher speeds than sailboats.


Bob
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Old 07-23-2006
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You "people" have spent days answering the ridiculous question by a guy who admittedly has never tacked a boat. Now thats funny.
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  #26  
Old 07-23-2006
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Quote:
Originally Posted by anthonycolfelt
I have a better question... Can you lift yourself up by your bootstraps? What if you jump a little first to give yourself some momentum?

In all seriousness, the fan is going to act just like a propeller would in the water. It will give you some thrust if its big enough. You may even get a little extra from the sails, but no more than you would from equivalent thrust from a submerged propeller.

It's newton's third law of physics. Every action has an equal and opposite reaction. If there's thrust pushing the boat forward, its from the fan onto the sail and rig (pushing it equally backward). The fact that the boat moves forward because of the fan's thrust may draw a little extra air over the foil (sail) which will begin to cause some lift. But this isn't any different than if the 'fan' was underwater I would think.
I agree as far as trying to get the greatest efficiency from the power produced from an engine you very likely would be better off putting that propeller in the water rather than the air.
However, it is known you can get higher speed and higher efficiency with a propeller in water driven boat if in addition you use sails. You are in effect "pulling yourself up by your own bootstraps" since the apparent wind acting on the sails is coming from the power of the motor, which nevertheless acts to produce greater speed than from the propeller acting alone.



Bob
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  #27  
Old 07-23-2006
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What if you jump right before a falling elevator crashes to the ground?
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  #28  
Old 07-23-2006
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Quote:
Originally Posted by Surfesq
You "people" have spent days answering the ridiculous question by a guy who admittedly has never tacked a boat. Now thats funny.
That's why you ask questions to get information from more knowledgeable people.
Are you saying you personally are not aware of any sailboat able to travel faster than the windspeed?


Bob
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  #29  
Old 07-23-2006
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Wait Fan Man....You asked the question about the fan right? If you want to know about tacking...just get out there and sail.
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  #30  
Old 07-23-2006
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Quote:
Originally Posted by DynaMeme
For airplanes which have low speed performance requirements there is a concept called ventialted slots or blown flaps. In this case a blower, not a fan, blows air across the leading edge of the wing or flap. THis is done to insure the air flow stays attached -- preventing stalling -- and allowing the aircraft to fly much more slowly and land in shorter distances.

This concept has been experimented with on sailboats. Rigid sail catamrans in particular.

The issue is power requirements. Such a blower on a sailboat would have energy requirements that would be at least as demanding as a motorsailors.

However, boat's such as Cousteau's Alcyone did consider spinning their rigs using power until natural forces took over.
Thanks for the response. The question is not purely theoretical. I'm considering it both for sea cruises and transport and for aircraft to improve fuel efficiency and travel times.
It is known with motorsailers that you could improve fuel efficiency and speed by having the propeller in water operating at the same time you are using the sails.
Because of this fact if you used a fan not blowing over the sails but blowing rearward from the stern, you would likewise get better fuel efficiency and speed.
But it seems to me by placing the fan at the front so that it blew over the sails, the apparent wind speed would be even greater so the fuel savings and speed should be greater than for the fan at the stern.
I don't know how much greater though. Might it even be large enough to match or exceed having a propeller in the water?



Bob
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