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  #41  
Old 07-24-2006
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pearls before.....well, you know

Quote:
Originally Posted by pigslo
Once again, it does not matter how big the fan is, the force against the sail is exactly equal to the force that happens to be on the front of the fan blades but in the opposite direction and so is ZEROED OUT. Any low presure due to wind over the sail shape is canceled by the low presure behind the fan.

Pigslo
Pig -- Sorry, but we all are casting pearls before swine. No offense meant of course. Bob is intent on rewriting the laws of physics anyway. Can't wait for him to figure out that he has to consider drag in the formula. He's really hung up on this motorsailor making its own apparent wind thing, not realizing that a canoe also makes its own apparent wind.

Bob -- Have you ever seen the movie "What About Bob"? Rent it this weekend. Has a good sailing scene that made me think of you.

In closing: "It doesn't matter if the stone hits the pitcher, or the pitcher hits the stone. It's going to be bad for the pitcher." Sancho Panza in Man of La Mancha (speaking of tilting at windmills.)
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  #42  
Old 07-24-2006
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Quote:
Originally Posted by pigslo
Once again, it does not matter how big the fan is, the force against the sail is exactly equal to the force that happens to be on the front of the fan blades but in the opposite direction and so is ZEROED OUT. Any low presure due to wind over the sail shape is canceled by the low presure behind the fan.

Pigslo
The situation is made more complicated by what happens when tacking into a true head wind. In that case you would think that since the wind impinges on the front of the sail, though at an angle, and tends to move the sail backward, the boat should also move backward. However, it is the additional force of the water on the keel that makes the boat move forward.
Likewise it could be that the fan generated air flow impinging on the sail at an angle could also result in the boat moving forward because of the effect of the water on the keel.
In any case this is indeed something that can be resolved by experiment. Remember bathtub toys with boats? You could first have a little floating boat with a sail and use either air from a blown up balloon or those little battery operated personal fans held outside the boat to blow on the sail at an angle to see how fast the boat tacks into the wind. Your little toy boat would need a keel for this purpose.
Then attach the balloon or fan at the stern but with the sail removed to see how fast the boat can move from the momentum of the air flow alone.
Then attach the balloon or the fan at the stern with the sail attached forward, so it's not blowing over the sail, and see how fast the boat goes in this case. Change the angle of the sail to various degrees to see how tacking into the apparent wind can improve speed. Is the speed faster than in the fan alone case?
Finally attach the balloon or fan at the front so it is blowing over the sail and put the sail again at various angles and see if this results in an even higher speed than in any of the prior cases.
I'll let you know the results of my bathtub experiments.


Bob
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  #43  
Old 07-24-2006
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Quote:
Originally Posted by SailorMitch
Ok, I just reread this entire thread trying to understand where Bob is coming from. First Bob, yes, planing sailboats and iceboats can go faster than the true windspeed. That primarily is due to the lack of drag from a hull being in the water (not just the keel.) You now need to take your education one step further and read about displacement boats, theoretical hull speed, bow waves, etc. You can't take the "rules" for a planing hull and apply them to the motorsailor you keep talking about.

You also keep going back to your motorsailor concept that "it is known" that, while motoring, you can go faster with the sails up than with the sails down. Sometimes true, sometimes not. The propaganda you're relying on from that outfit that makes motorsailors fails to mention that "oh by the way, if the wind is on the nose, your sails will flog themselves to death, so don't try motorsailing into the wind." The only time that notion is true is if you are sailing at an angle to the wind ample enough to make the sails efficient so that they provide some forward assistance. If you're motoring into the wind and raise the main, for example, the sail will do you no good -- it becomes a very good windvane and flogs. If you try to move the traveler so that the wind fills the sail, that means the sail will be out so far it is actually working as a break and will slow you down.

It's not just motorsailors we are talking about either. Every sailboat with an engine becomes a motorsailor when a sail is up and the engine is on. Whether the sails increase speed depends on the angle of the wind. And if you insist on saying "the apparent wind will keep increasing if you keep tacking into it" -- think about what that means about your course and are you really going anywhere. You also need to learn about a little concept called velocity made good.

In short, Bob, get your nose out of the books and onto a sailboat and see how this all works on the water. You'll be amazed.

You made some good points here. I found this article from New Scientist while web searching on motorsailing:

The new age of sail.
26 February 2005
http://www.newscientisttech.com/arti...24881.600.html

The article reports on past and future efforts to improve fuel efficiency and speed on transport ships by attaching sails. It is notable that an earlier Japanese effort on this idea was discontinued; presumably it was not cost effective.
However, the article reports a Danish team believes it can be made to work by using efficient airfoils rather than sails.
This doesn't tell us though what would happen with a fan producing additional apparent wind.
Care to try your own bathtub experiments?


Bob
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  #44  
Old 07-24-2006
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Any forward movement is from the fan or ballon blowing against the air around the boat ( the same as an everglades boat moves forward). Any low presure (that is what moves a sailboat upwind) created around a sail by an object on the boat and therefore moving with the boat will be subject to an IDENTICLE counteracting low presure and there by canceling it out. That is just physics. File your drawings of this contraption at the patent office along with a perpetual motion machine.

Pigslo
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  #45  
Old 07-24-2006
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Any hybrid ships that use sails that increase efficiency of the engines do so by harnessing the wind that is provided by nature, not wind created on the boat. Those are two different things.
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  #46  
Old 07-24-2006
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I think i seen one of these at the dollar store.When i take a bath tonight i`ll flash it up and see if it can keep up to the rubber duck. A diet in beans could also be used for a natural form of propulsion as well.

Last edited by dman; 07-24-2006 at 09:11 PM.
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  #47  
Old 07-24-2006
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Quote:
Originally Posted by pigslo
Any forward movement is from the fan or ballon blowing against the air around the boat ( the same as an everglades boat moves forward). Any low presure (that is what moves a sailboat upwind) created around a sail by an object on the boat and therefore moving with the boat will be subject to an IDENTICLE counteracting low presure and there by canceling it out. That is just physics. File your drawings of this contraption at the patent office along with a perpetual motion machine.

Pigslo
Any explanation that does not include the force of the water on the keel is incomplete for explaining tacking into the wind.

This page provides a good explanation of it:

The physics of sailing.
http://www.phys.unsw.edu.au/~jw/sailing.html

Note especially the section that begins:

"Sailing close to the wind uses the shape of the sails to generate lift. To flow around the sails, the wind has to deviate in direction, as shown by the arrows for initial velocity vi and final velocity vf, which are given with respect to the boat. The change of velocity dv is in the direction shown. The acceleration aa of the air is dv/dt, so the force that sails exert on the air is in the same direction."

As you can see from the images accompanying this section, the lift produced only results in a force on the boat that has a rearward component, not forward. More precisely, the lift acts to move the boat sideways and rearward.
It is only the action of the water on the keel that provides a forwards force component that allows the boat to move forwards.

Here's another way to look at the scenario. Suppose you had your giant fan on a raft with no sail or keel. This raft is connected to the sailboat by a rope. Suppose this rope initially is slack. You turn on the fan directed towards the sailboat. The wind produced by the fan allows the sailboat to move forwards by tacking into the wind. The raft and fan also move forwards separately due to the momentum thrust of the fan. The instant the rope is about to become taut you turn off the fan. When the rope tightens, the sailboat will get a higher speed because of the forward momentum of the raft. The raft will also be pulled backward but it won't affect the forward speed of the boat as long as it does not contact the boat. You make the rope long enough so this doesn't happen. Once the rope is slack again. You turn on the fan again. The process repeats.
You see the result will be that the sailboat will wind up having a speed due to both the wind produced by the fan acting on the sails and from the momentum thrust produced by the fan.


Bob
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  #48  
Old 07-24-2006
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Bob, with all due respect to the professors I think that it would be better if you consider forces exerted on the sails and hull because not velocity moves the boat but FORCES. Then you would take into consideration ALL actions and reactions and result would show in which direction your boat would be moving. Don' forget including the mentioned Bernoulli effect and others as they have big influence on boat movement. Think about shape of sails. All together is a little bit more complicated than you think.
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Well why not beat the concept to death by studying Newton's Third Law of Motion which states: "For every action, there is an equal and opposite reaction." I guess that sums it up. Lets all buy fans!
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Old 07-25-2006
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