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  #31  
Old 07-23-2006
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Quote:
Originally Posted by Surfesq
What if you jump right before a falling elevator crashes to the ground?
Could you survive? No:

Elevator of Death.
http://en.wikipedia.org/wiki/MythBus...vator_of_Death

However, it is known that by using sails with a propeller driven craft you can get better speed than from the propeller alone. This increase in speed is coming from the wind created by motor itself:

Motorsailing - Some Thoughts.
"Based on my experience the motorsailer is the most practical and comfortable vessel for serious ocean passagemaking. However these days power-only long range cruising yachts seem to be the rage. We sell about 8 power boats for each motorsailer we build. I personally can not understand the long range powerboat skipper generating a beautiful 7 to 9 knot wind, at some cost, and than just throwing it away - while worrying about fuel?"
...
"Plus the sailing rig increases the speed of the vessel so less fuel capacity is needed. This weight savings coupled with the additional propulsion power available results in faster passages and excellent fuel economy.
"How can this be? Well most power-only trawler yachts cruise at around 6 to 8 knots per hour depending on power, weight, hull shape and water line length. All these vessels are creating, at some expense, a wind equal to their speed. This wind, a vital and reliable source of energy, combines with and adds to the true wind to create an apparent wind across the boat which is just thrown away. The motorsailer, on the other hand, uses its rig to regain this energy resulting in increased boat speed, up to 25% above a stabilized powerboat, without increasing fuel use."
http://www.seahorseyachts.com/seahor...emarine_3.html


Bob

Last edited by RGClark; 07-23-2006 at 10:51 AM.
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  #32  
Old 07-23-2006
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Bob,
In all due respect to your presumed intelligence, apparent wind and fan-generated wind are two completely different forces. My wife and I just returned from three days of both "pure" sailing and motorsailing - since we own a motorsailer, and that is what they're designed for.

Winds were variable in speed and direction. So, during those 5 knot lulls, I kicked on the 90 hp iron genny . . . at a low rpm - and put up full sail, 150 Genoa, main & mizzen. Where the other sailboats on the Bay were barely motorsailing at 4 knots, we kicked butt at 9 knots & 1,700 rpm.

Show me a powered fan which can get a 9 ton, 33 ft boat moving at 9 knots with that meager energy usage. Admit that this notion of yours is simply ridiculous.
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  #33  
Old 07-23-2006
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Of course, water is a far more efficient medium for a propellor, being far denser than air. Just thought I'd point that out. That said, the idea of a propellor on the front of a boat, is rather ridiculous, as the weight of the batteries or engine used to spin it...would far outweigh the benefit of the propellor itself, and how much thrust it would generate IMHO.
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  #34  
Old 07-23-2006
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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.
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  #35  
Old 07-23-2006
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Thats what I have been saying! Get out there and get wet!
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  #36  
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Quote:
Originally Posted by TrueBlue
Bob,
In all due respect to your presumed intelligence, apparent wind and fan-generated wind are two completely different forces. My wife and I just returned from three days of both "pure" sailing and motorsailing - since we own a motorsailer, and that is what they're designed for.

Winds were variable in speed and direction. So, during those 5 knot lulls, I kicked on the 90 hp iron genny . . . at a low rpm - and put up full sail, 150 Genoa, main & mizzen. Where the other sailboats on the Bay were barely motorsailing at 4 knots, we kicked butt at 9 knots & 1,700 rpm.

Show me a powered fan which can get a 9 ton, 33 ft boat moving at 9 knots with that meager energy usage. Admit that this notion of yours is simply ridiculous.
It is probably correct that you can't do better than using a propeller in the water.
However, that's not the only scenario I'm considering. Fan boats of course have to use fans. It is conceivable placing a sail behind the fan on those boats could improve efficiency and speed.
It's still not certain to me that you could get better speed by using a fan in front of a sail than one after the sail, but here's a start at something:

A motorsailer, with a water propeller, creates its own apparent wind. Then an obvious guess to make is that the increase in speed for the motorsailer is just like there was a true wind operating. So to find the increase in speed, calculate the total velocity as the vector sum of the velocity due the motor alone plus the velocity given to the sailboat alone tacking into a true wind of the same speed as that due to the motor alone.
IF that is true, then the obvious guess to make is that the speed of the fan in front of sail case, would be given by calculating the vector sum of the velocity due to the momentum thrust of the fan alone plus the velocity of the sailboat alone tacking into a true wind that is the same as the apparent wind due to the fan plus the speed of air flow through the fan.
That is, imagine the speed of the boat produced by the fan with no sail. Then the magnitude of the effective "true" wind you would calculate with would be this speed plus the air speed from the fan.
Under this hypothesis then, you see an increase in the fan air speed could result in a marked improvement in the boat speed. It would be just like an increase in the true wind speed.
Here's a start at understanding how much horsepower could improve the boat speed IF this hypothesis is correct:

This page gives a formula for calculating how much power a wind turbine or windmill can put out for a given wind speed and rotor size:

Practical Wind Generated Electricity.
http://mb-soft.com/public/wind.html

Then we may suppose an engine of that horspower could turn rotors of the equivalent size and generate an equivalent wind speed.

Here's the formula for calculating the horsepower of the wind turbine:

Practical Wind Generated Electricity.
"Any moving material carries kinetic energy and momentum. The basic laws of kinematics allow an easy analysis of a first approximation of performance. Essentially, any wind-power mechanism captures energy by slowing down the speed of the wind involved.

Undisturbed wind contains power from kinetic energy (energy flux) equal to:
E = 0.5 * (rho) * V^3 * (pi) * R^2.

Note that this is a simple application of the kinetic energy definition. Also note that the power is dependent on the THIRD power of V, the wind speed. A 20-mph wind has about 8 times as much power as a 10-mph wind, and a 40-mph wind has about 64 times as much power. (rho) is the density of air.)

In case you're curious, a 60-mph wind (88 feet/second) has:
E = 0.5 * (0.00237) * (883) * 12
or
E = 810 ft-lb/sec, about 1.5 horsepower per square foot of wind area!

You can probably see why strong winds can knock buildings down!

A 10-mph wind has far less power in it, around 4 ft-lb/sec, or about 1/150 horsepower per square foot. A ten-foot diameter farm windmill intercepts about 78 square feet of wind area, so that (10 mph) wind initially contained about 0.5 horsepower in it. At its maximum efficiency of 30%, the farm windmill could capture around 0.15 horsepower, a sufficient amount for pumping water."
http://mb-soft.com/public/wind.html

So a .15 horsepower engine turning a 10 ft rotor could produce a 10 mph wind. A 10 foot rotor would be small for sails though. But by the formula a 3*10 = 30 ft rotor for the same wind could be driven by a engine with 3^2 = 9 times more power or 1.35 horsepower.
Now again by the formula, this 30 ft rotor could produce a 4*10 = 40 mph wind by using a 4^3 times more powerful engine: 1.35*4^3 = 1.35*64 = 86.4 horsepower, within the range of your engine for your motorsailer
A 40 mph wind would be quite a significant wind for a sailboat and could give it significant speed.

This is assuming the hypothesis for how the fan air speed would contribute to the boat speed is the correct one.


Bob Clark
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  #37  
Old 07-24-2006
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Yes, but to effectively generate enough wind to cover a sail and have it generate lift, the propellor size would have to be almost the height of the sail. Using the wind generator scenario doesn't cut it...a wind generator has a blade that is two-meters at most in diameter... and six-feet of wind over a 35-foot tall sail isn't going to generate any appreciable lift.
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Old 07-24-2006
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G'day..
This question reminds me of when I was a kid (Last year) and I often thought of that.

BUT I was refered to another question by a wiser person. It is much the same as a fan to go faster but relates to a spaceship.
If a spaceship travels a little faster than the speed of light and turns on headlights, will that improve the drivers vision?

I bet there are dozens unanswered questions out there.
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  #39  
Old 07-24-2006
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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
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Old 07-24-2006
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Is this a joke?
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