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Old 22-04-2011, 14:41   #46
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Re: Motor Sailing Do's and Dont's

I believe Chris has got it right concerning 1+1=3.
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Old 25-04-2011, 01:04   #47
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Re: Motor Sailing Do's and Dont's

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I believe Chris has got it right concerning 1+1=3.
I am afraid that I can't let this one go. The resistance of a sailboat hull rises in proportion to the velocity squared. Therefore 1 plus 1 equals 1.41.
Also the apparent wind generated by any craft adds no component to propulsion. It only adds drag and slows vessel down. It's all basic physics and common sailing knowledge.
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Old 25-04-2011, 02:01   #48
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My 10 cents worth....... I've found 1+1=3 only up to and approaching our hull speed of 7.5 knts then 1+1=1.41 thereafter.
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Old 25-04-2011, 03:38   #49
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Re: Motor Sailing Do's and Dont's

When motorsailing in the ICW, a common practice for us, we choose the genoa and sometimes a mizzen staysail, but not the main. Too often, within the restricted channel, we need to depower our sails or lower them without the manuverability to head up into the wind to douse the main. This can be when approaching a bridge, other traffic or an area with shoaling concerns. Here is a specific case where there is a clear advantage of the in mast furling main and the ability to furl the main without changing course in a confined space.
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Old 25-04-2011, 05:08   #50
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pirate Re: Motor Sailing Do's and Dont's

Motorsailing..... hmmmmm
Only do it when the winds to light to power the genny then I furl/drop the genny and leave the main up to steady the boat if there's any rolling tendancy caused by the sea. If its flat calm I don't bother as then I'm only creating drag and increases fuel consumption...
I also technically motorsail when coming into a busy anchorage.... its nice to have the main up in case the engine dies at a crucial moment... maintaining propulsion through the crowd is useful...
Have also been known to use it as an 'assist while under full sail if I've just missed the tide somewhere like the Channel Isles or Ushant where the tides run hard to give me that little extra 'OOMPH' to get me through...
Otherwise I'll sail only.... even up the ICW...
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Old 25-04-2011, 22:32   #51
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Re: Motor Sailing Do's and Dont's

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I am afraid that I can't let this one go. The resistance of a sailboat hull rises in proportion to the velocity squared. Therefore 1 plus 1 equals 1.41.
This sort of thing is always at risk of degenerating due to arguments about exact wording. It's not clear to me exactly what you mean or how you arrived at your figures. I tried to be pretty careful with my wording in my post to indicate that enough thrust was given to increase the speed to two knots, independent of any extra synergistic apparent wind increases. Therefore, even if apparent wind didn't help your case, you would at worst have 1+1=2, simply by the fact that you increased your thrust until you got 2 knots of boat speed.

After that, you're right that the increased power from the apparent wind due to your boat speed would absolutely be offset at least somewhat by the increased drag from going faster. At two knots, your engine is giving you enough power to overcome all drag, but you have some extra new apparent wind to play with, so you have a surplus of thrust over drag and you will accelerate. As the drag increases, you will find a new equilibrium that will be somewhat greater than two knots where the total thrust due to wind and engine exactly equals the increased drag due to the faster water speed. This exact figure will depend on the precise wind angle, boat type, sail area, etc. It obviously isn't exactly 3, but the point is that it's bigger than 2.


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Also the apparent wind generated by any craft adds no component to propulsion. It only adds drag and slows vessel down. It's all basic physics and common sailing knowledge.
I can't agree with your assertion. The sails feel only the apparent wind. They can't tell that it's the vector sum of true wind and vessel velocity (be it from sail, engine, or current). Don't fall into the trap of trying to think of the wind as a superposition of the true wind and apparent wind. Superposition of winds doesn't work like superposition of forces. You have to do the vector sum of the two winds.

In fact, I have quite recently sailed along quite nicely in a situation where there was zero true wind. We were in a few knots of current and used the apparent wind from that to propel sail at about a knot of boat speed. Took us a while to figure out what was actually going on.

Chris
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Old 26-04-2011, 05:22   #52
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Re: Motor Sailing Do's and Dont's

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In fact, I have quite recently sailed along quite nicely in a situation where there was zero true wind. We were in a few knots of current and used the apparent wind from that to propel sail at about a knot of boat speed.
So your boat sails with wind from dead ahead? Could you explain the physics of that?
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Old 26-04-2011, 05:48   #53
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Re: Motor Sailing Do's and Dont's

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So your boat sails with wind from dead ahead? Could you explain the physics of that?
He was misusing terminology. "True wind" is wind in relationship to the water. He did have true wind. He meant that he had zero "geographic wind" (or "ground wind") -- without the current, it was a windless day. But with the current, he did have wind, both true and apparent, and was able to sail with that.

Your wind instruments, if they are networked like the Raymarine ST60's, will give you THREE kinds of wind speed -- True Wind Speed (TWS), Apparent Wind Speed (AWS), and Ground Wind Speed (GWS). TWS is apparent wind as measured by your masthead wind instrument, corrected for speed through the water as measured by your log impeller, and your heading. GWS is apparent wind speed corrected by Course Over Ground and Speed Over Ground as provided by your GPS.
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Old 26-04-2011, 06:21   #54
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Re: Motor Sailing Dos and Don'ts

See: http://www.panbo.com/assets_c/2009/1...dium-1135.html

For a graphical explanation of the difference between Apparent Wind, True Wind, and Ground Wind.
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Old 26-04-2011, 07:01   #55
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Re: Motor Sailing Do's and Dont's

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I am afraid that I can't let this one go. The resistance of a sailboat hull rises in proportion to the velocity squared. Therefore 1 plus 1 equals 1.41.
Also the apparent wind generated by any craft adds no component to propulsion. It only adds drag and slows vessel down. It's all basic physics and common sailing knowledge.
Then how does a Tornado do 14 knots in 10 knots of true wind on a beam reach? Sails operate on apparent wind as someone has already posted.

John
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Old 26-04-2011, 07:05   #56
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Re: Motor Sailing Do's and Dont's

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Then how does a Tornado do 14 knots in 10 knots of true wind on a beam reach? Sails operate on apparent wind as someone has already posted.

John
Right. And the physics are like this: you can make a watermelon seed shoot out of your hand at a large multiple of the speed at which your fingers are pressing against the seed. It's a matter of the amount of force and the angle at which that force is exerted, not the speed at which the force is exerted.

Likewise a sailboat's speed is not limited by the speed of the wind whenever there is aerodynamic lift being generated (it is otherwise on a run when the sails work purely by aerodynamic drag). The aerodynamic lift produces a force; that force applied at a certain angle produces motion.
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Old 26-04-2011, 07:09   #57
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pirate Re: Motor Sailing Do's and Dont's

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Then how does a Tornado do 14 knots in 10 knots of true wind on a beam reach? Sails operate on apparent wind as someone has already posted.

John
I've had similar results on a Tiki 21... only problem is its in bursts as once you pass a certain speed the fore sail starts luffing as you 'overtake' the wind and your boat speed moves the wind forward.... wierd...
Not sure I said that right... but any 'speed' sailors will know what I'm talking about
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Old 26-04-2011, 07:09   #58
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The most important thing to remember is the bank angle of the boat as it affect the water inlet duct for cooling the engine. The greater the angle, less water inducted causing higher than normal engine temperature.
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Old 26-04-2011, 08:22   #59
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Re: Motor Sailing Do's and Dont's

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I've had similar results on a Tiki 21... only problem is its in bursts as once you pass a certain speed the fore sail starts luffing as you 'overtake' the wind and your boat speed moves the wind forward.... wierd...
Not sure I said that right... but any 'speed' sailors will know what I'm talking about
You have to learn to anticipate. I used to race a Hobie sx18. It has an asymmetric spinnaker. You sail on a close reach, 60-70 degrees apparent to go downwind at 135 degrees to the true wind. It's pretty squirrelly driving as the apparent speeds and angles shift alot. Get a puff, boat accelerates, apparent goes forward, if you anticipate you've already started steering down to keep the sails from luffing. Now if you don't react quickly enough to a lull, the sails stall, the boat quickly decelerates, and you have the sails set for a close reach, when the wind has rapidly shifted to almost dead behind you. Very, very slow.

John
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Old 26-04-2011, 08:33   #60
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pirate Re: Motor Sailing Dos and Don'ts

Yeah... I got the hang of it soon enough but at first it was a frustrating PITA.... only having sailed mono's up till then....
But I'd still get caught out now and then when traffic precluded any changes in course....
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