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Old 08-05-2008, 08:16   #61
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Rremember, all of this is based on an assumption that the propulsive force of the sails is not affected by taking on more mass.
Dave
Dave, the power of a sail plan has nothing to do with the weight of a boat. The pull of a sail is the same regardless of teathering to you flag pole in the front yard or to your boat.
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Old 08-05-2008, 08:27   #62
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Dave, the power of a sail plan has nothing to do with the weight of a boat.
Of course it doesn't. Do you think I don't know my physics?

My point was that taking on more mass - as in the examples of taking on ballast to intentionally heel a boat over in light air or to prevent excessive heeling in heavy air - can alter the "sail plan" because the rig is heeled or less heeled vs not changing the mass.

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Old 08-05-2008, 08:44   #63
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Rremember, all of this is based on an assumption that the propulsive force of the sails is not affected by taking on more mass.
Dave

Dave, those are your words. You say the propulsive force of the sail is changed by weight. How?
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Old 08-05-2008, 08:47   #64
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This is what makes racing sailboats interesting. Is it better to give up speed for point? Is it better to close the leech and gain upwash? Is it better to add stability to keep flow but increase drag? And on and on and on........ Chess has nothing on racing sailboats.
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Old 08-05-2008, 08:47   #65
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My point was that taking on more mass - as in the examples of taking on ballast to intentionally heel a boat over in light air or to prevent excessive heeling in heavy air - can alter the "sail plan" because the rig is heeled or less heeled vs not changing the mass.

Dave
I think that's the point Joli was making. More mass can alter the sails to maintain a better shape in light winds.
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Old 08-05-2008, 08:50   #66
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BUT!!!

Theory aside, my heavy, full-keeled boat doesn't sail well in <5-6 kts no matter what I do. One of the reasons I bought an asymmetric spinnaker!
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Old 08-05-2008, 08:52   #67
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Dave, those are your words. You say the propulsive force of the sail is changed by weight. How?
As I said, taking on mass could affect the heel of the boat. Changes in heel can affect sail power, right?

Did you not read my post just above?

Wasn't it you who earlier provided an example of intentionally heeling a boat over in light air to keep the sail shape, thus affecting the propulsive force of the sails?

Why are you trying to argue? Because your meter boat example has been shown to violate the laws of nature?

Dave
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Old 08-05-2008, 09:31   #68
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Dave, I'll give it one last shot here. A couple reasons multis get sticky in very light conditions is because they have a quick motion and little weight to carry through the chop and lulls. It is tough to keep flow attached when the boat naturally has a quick motion, it is tough to keep boat speed with little momentum.

Steadying out the rig so flow can stay attached and keeping momentum through adding weight can in some instances be beneficial. I gave you an example of such a boat in post 33. Going to smaller sail when it is very light is another example.

Nothing in sailing is a zero sum gain. Every adjustment has an impact, some adjustments create surprising results.

Cheers,

Joli
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Old 08-05-2008, 09:33   #69
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BUT!!!

Theory aside, my heavy, full-keeled boat doesn't sail well in <5-6 kts no matter what I do. One of the reasons I bought an asymmetric spinnaker!
But Hud! It goes upwind in light air like hell with the throttle firewalled!

Thats a joke, don't chew me out again, I give.
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Old 08-05-2008, 09:44   #70
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Hey, that 56 hp Yanmar was put in the boat for a reason!
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Old 08-05-2008, 10:10   #71
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Mike - All other things being equal - a heavier boat can never be faster than a lighter boat in any air - assuming the lighter boat can attain the same propulsive force from its sails.

Never, never, never.

Very true a heavier boat (more mass) will maintain speed longer in a lull - it requires more force to slow it down >> F=ma works both ways.

If considering two otherwise identical heavy and lighter boats experiencing the same lull, the heavier boat will "coast" through the lull better for the reasons you state and be ahead. Once the wind returns the lighter boat will accelerate faster and attain a higher top speed sooner and, I submit, regain the lead due to being faster. Lighter means less friction and less wave making. This has to be the case. Physics demands it.

All of this is about differing mass. Moving ballast - shifting mass - is an entirely different story involving other variables.

Dave
2Hulls is right. What is scrubbing off speed is wetted surface area and windage. Adding more mass only increases wetted surface area given everything else remains the same. Its never advantageous to increase weight regardless of the wind conditions unless weight is added to weather to increase the righting arm....and we are not talking about this in light airs.

In class racing small boats like Lasers, it is always the light sailors who do the best in light airs for this very reason. Same boat, lighter skipper...the lighter guys win...always.

And conversely, the heavier guys win in heavy airs because they have the greater righting moment to keep the boat flat.
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Old 08-05-2008, 10:21   #72
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Dave, I'll give it one last shot here. A couple reasons multis get sticky in very light conditions is because they have a quick motion and little weight to carry through the chop and lulls. It is tough to keep flow attached when the boat naturally has a quick motion, it is tough to keep boat speed with little momentum.

Steadying out the rig so flow can stay attached and keeping momentum through adding weight can in some instances be beneficial. I gave you an example of such a boat in post 33. Going to smaller sail when it is very light is another example.

Nothing in sailing is a zero sum gain. Every adjustment has an impact, some adjustments create surprising results.

Cheers,

Joli
Joli, the reason multis do so poorly in light airs compared to monos is they have more wetted surface area compared to a mono with the same displacement and sail area. Once the wind picks up enough, a multis righting moment becomes a greater factor than its wetted surface area. The multis have a much greater righting moment and therefore more sail area left exposed to the wind, therefore leaving the monohulls behind. The monohulls heel more, therefore decreasing crossectional sail area that is exposed to the wind, not to mention an asymetrical hull, (as the water sees it), that even worse, induces weather helm, ...thus creating even greater resistance.

Yes I understand keeping the laminar flow across the sail but that is not the greater factor of the two factors.

So what monos are basically left with in higher wind is a less than ideal hull shape because they are heeling and less cross-sectional sail area exposed to the wind. For light winds the multis are at a disadvantage because of their greater wetted surface area and windage.

David
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Old 08-05-2008, 20:38   #73
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SA/D Comments

Gents - I do not recommend using SA/D for evaluating boat performance because it leaves out another important element, the waterline length of the hulls. I prefer to use Richard Boehmer's trademarked Base Speed, which is a formula he developed by analyzing actual best day's run average speeds for a large number of boats. The result of the analysis showed that sail area, displacement and waterline length are all important elements.

To illustrate this, I took the specifications for a new addition to the Multihull Dynamics, Inc. website, the Outremer 42, and stretched it one foot at a time twice, keeping the displacement and sail plan (area and height of the center of effort) constant. This shows a change in both speed and stability. Note in the attachment that SA/D remains constant, while Base Speed increases with each foot the hulls are stretched. Texel rating, the handicap calculation for racing, gets lower, reflecting the inceasing speed potential of the longer boats. None of this shows up in a SA/D analysis.

[See attached file]

Regarding stability, Hull beam, Bh, shrinks and beam between hull centerlines, Bcl, grows as the hulls are stretched in length while maintaining the same displacement. As a result of the Bcl increases, Stability Speed (sail reducing wind speed) and Hull Lifting Speed (well, hull lifting speed!) increase.

So, size matters!

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Old 08-05-2008, 20:41   #74
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SA/D Comments

Can someone tell me how to insert a table into a reply for this system? I'll correct my last post so it can be read.

Thanks, Cal
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Old 08-05-2008, 22:59   #75
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Red face Wetted surface, the DWL and the K factor

David is quite right, and Gideon is wrong if he means to imply that monos are slower than multis in light airs. A light multi has more wetted surface than a monohull, and it has even more when you add more weight to it. What David left out in his explanation for why multihulls are faster when the wind picks up is the "K" factor, which reflects the lower resistance of a much narrower hull. Cal is right about the effect of waterline length, but he is also leaving out the fact that the "K" factor gets higher if you stretch a hull to make it longer.

It is easier to design a narrow hull in a big boat, than in a small one, because the human body isn't scaling up as the boat gets bigger, and interiors are designed around the dimensions of the human body-if you are to have reasonable accommodations.
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