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Old 25-09-2007, 17:22   #31
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E-Gads! 21 knts and a hull flying?

After I blew the sheets, I'd be changing my pants!

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Old 25-09-2007, 19:15   #32
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E-Gads! 21 knts and a hull flying?

After I blew the sheets, I'd be changing my pants!

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Old 25-09-2007, 20:40   #33
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YES! I AM! And, a live one!

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Old 26-09-2007, 23:21   #34
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Derek Kelsall made a formula for this

There is a version at: http://www.sailmagazine.com/sailjermain.pdf
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Old 27-09-2007, 08:13   #35
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The answer is more complicated than just wind speed. Wave conditions, mast weight, mast steps and gusts (and a whole host of other variables) also play a large role. But a simple equation can be derived by multiplying the weight of the multihull by 1/2 the beam to derive the maximum righting moment (ft/lbs) at 10 degrees. The sails has a certain overturning power which is calculated by the area of the sail (sq.ft.) multiplied by the height of the center of effort (ft) which gives the moment of area of the sail (cubic ft). Then add the moment of area of all the sails at a given sail set and mast and hull moment of area to give the total moment of area. At 20 mph wind the maximum pressure of the wind is 1.22 lb/sq.ft. The maximum wind pressure of 100 mph wind is 30lb/sq.ft. You can then create a graph to show the maximum righting moment by putting wind speed on the x -axis and the moment of the sail area and hull and mast on the y-axis. Since the mast and the hull will remain the same you can remove these numbers from your graph to give the moment of sail area (cubic feet) for any given wind condition. If you stay under that moment you theorectically will not capsize.

Thus if your boat weighs 10,000lbs and is 20 feet of beam: the righting moment is 100,000ft/lbs. The maximum moment of sail area that can be carried in a 20 mph wind is 100,000 divided by 1.22 or 81,967 cubic feet.
Then do the same for 100mph and drawa line between the two values.

A chart can be created showing the possible sail combinations that would provide the maximum moment of sail area for each sail set combination. Thus you can sail in confidence that theorectically you will not capsize. Do not forget to consider that wind speed at the deck is lower than at the moment for the sails and it is suggested that you add about 13% to the highest wind gusts.

Please note that I claim no accuracy for these calculations and each person should do their own homework (liability statement :-) but they are based on some fine research by Charles Dennis and Hugo Myers Ph. D.
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Old 27-09-2007, 10:58   #36
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SSpd = Stability speed. Speed of wind when hull will begin to lift.

From here: Multihull Dynamics, Inc. - Sample Comparison

When/if I narrow down "my" boat list I will pay to see what these data are.
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Old 27-09-2007, 14:01   #37
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Has anyone emailed Pat at Multihull Dynamics and asked him to post the formula for SSpd here? I just shot him an email asking him to look in on this thread.
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Old 27-09-2007, 16:20   #38
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Stability Speed

Multihull Dynamics uses John Shuttleworth's formula for "Stability in the Wind" Multihull Design Considerations for Seaworthiness
Cal Markwood the Engineering Analyst for MDI modifies it just a bit so that the resulting answer is in knots, a result that is consistent with the rest of the website.
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Old 27-09-2007, 18:05   #39
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Not square feet Mike, CUBIC feet. 82,000 cubic feet of sail. Amazing. How big do you reckon they would be once you unfurled them?
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Old 27-09-2007, 18:18   #40
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Keegan - The formula for stability speed, that steady wind speed (knots) at which the windward hull is expected to lift for a given displacement (long tons), beam between hull centerlines (feet), sail area (square feet), and height of center of effort (feet) is 8.238x((0.5xBclxD)/(SAxHce))^0.5. This formula is from John Shuttleworth's Multihull Design manual, 1998, converted from MPH to knots.

Multihull Magazine July/Aug 2007 issue listed your boat weight at 19842 pounds, with main plus jib at 1172 sq. ft. At those figures, the Stability Speed is 20.51 knots. At 27500 pounds and 1230 sq. ft., the Stability Speed is 22.76 knots.

You can do these calculations for yourself on our website at Multihull Dynamics, Inc. - Home for a reasonable price.

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Engineering Analyst
Multihull Dynamics, Inc.
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Old 27-09-2007, 18:32   #41
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Quote:
Originally Posted by libellula View Post
Since the mast and the hull will remain the same you can remove these numbers from your graph to give the moment of sail area (cubic feet) for any given wind condition. If you stay under that moment you theorectically will not capsize.

Thus if your boat weighs 10,000lbs and is 20 feet of beam: the righting moment is 100,000ft/lbs. The maximum moment of sail area that can be carried in a 20 mph wind is 100,000 divided by 1.22 or 81,967 cubic feet.
Then do the same for 100mph and drawa line between the two values.
On first look this appeard to be sail area but it is moment of area so the hieght of the CofE needs to be used to come up with a meaningful number for us mere mortals
Shuttleworth's formulae may be useful.
This suggests I need to start reefing @ 23 MPH

Mike

D = displacement (lbs).
CE = height of the center of effort above the center of gravity (C.G.) in feet.
SF = windspeed in MPH that the boat has to reduce sail.
SA = sail area in square feet.
B = beam between the cenerlines of the outer hulls in feet.

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Old 27-09-2007, 20:56   #42
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Multihull Magazine July/Aug 2007 issue listed your boat weight at 19842 pounds, with main plus jib at 1172 sq. ft. At those figures, the Stability Speed is 20.51 knots. At 27500 pounds and 1230 sq. ft., the Stability Speed is 22.76 knots.

First off, Multihull Mag is quoting the manufacurers "Light Boat" weight at 19842 lbs. Dolphin Cats actually come in around 27,000-29000 lbs once you get all the stuff on them. I have spoken to several owners and they all are in that heavier range of weight.

I have also had a first hand account of sailing, unreefed at 27-28 knots of wind, with wind just forward of the beam. The hulls were both tracking in the water. I have also talked with a couple others that have sailed the Dolphin hard regularly and there were no reports of flying a hull. While I am certainly interested in the formula that you have suggested above, in practical experience 23 knots was not the point at which a hull lifted out of the water. I have also sailed her at 24-25 knots on wind angles just forward and just aft of the beam a few times and that did not do it either. Therefore I am left to believe that there is more margin for error than 23 knots.

I certainly dont want to ever find out where the break point is but it is not at 23 knots...

I wish there was a reliable formula that one could rely on.

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Old 27-09-2007, 21:02   #43
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This suggests I need to start reefing @ 23 MPH
I do agree that for safe sailing I should reef at around 21-23 knots but I think there is substantial margin left but just dont know how much.

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Old 28-09-2007, 08:25   #44
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I do agree that for safe sailing I should reef at around 21-23 knots but I think there is substantial margin left but just dont know how much.

Keegan
My guess is that number applies if the boat is sitting still in flat water sails sheeted tight and wind abeam.

Other factors then begin to apply ie: movement.

My guess.
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Old 28-09-2007, 15:37   #45
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I guess I would think that your sailmaker might have an opinion on when to reef that is substantially lower than what any formula would provide.
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