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- Thread starter EarthBM
- Start date

I'd like to see critique myself, as I am relying on below table (made with somewhat more careful measurements) in my own sail choices. Based on Regis' experience perhaps I should also laminate a 3x gust factor table for the night.

The wind force formula I use is

The wind force formula I use is

F=1/2∗p∗V^2∗A | ||||

Where F is the wind force (N) | ||||

p is the air density (kg/m^3) | 1.225 | at sea level | ||

V is the air velocity (m/s) | ||||

A is the surface area (m^2) |

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Seems you have used the full beam here- or at least distance between hull centres. It should be half beam...Let's look at the math...

The beam is 8.6m, but with the cabin and the mast effective righting leverage is probably 7m. So 5t (half the displacement) is weighing the boat down to counter the wind force that acts with 1.4x leverage (10m vs 7m). 5t =50kNrighting moment vs 32kN x 1.4 =45kNcapsize force. So the boat shouldn't have capsized.

Well, the first approximation for a cat is that its weight is equally divided between the two hulls. So when you begin to pivot on one hull, the other hull (1/2 of the total displacement) is pulling you down with the leverage of the full beam, but half the weight. Because some of the weight is in the center (cabin, mast), I used 7m instead of 8.6m.Seems you have used the full beam here- or at least distance between hull centres. It should be half beam...

For my tri I use the whole weight (minus the leeward ama) and half the beam, because the weight is centered. In practice these are similar calculations.

I don't take into account BSP and how AWS is affected. In my own stability table this is because focusing on AWS got me trapped several times in falling off "in gusts" to reduce AWS, which in practice were sustained TWS increases. Then you find yourself in the pickle of carrying too much sail at 170 TWA. Not the end of the world -- furl the foresail, ease the main a lot, turn into the wind, reef, but somewhat scary and uncomfortable.

I'd like to see critique myself, as I am relying on below table (made with somewhat more careful measurements) in my own sail choices. Based on Regis' experience perhaps I should also laminate a 3x gust factor table for the night.

View attachment 581920

The wind force formula I use is

F=1/2∗p∗V^2∗A Where F is the wind force (N) p is the air density (kg/m^3) 1.225at sea level V is the air velocity (m/s) A is the surface area (m^2)

In your formula you are assuming a force coefficient of 1.0 which can easily be exceeded by sails. Since it depends on the wind angle which can change quickly on a multihull, a conservative approach would be to consider the worst case and use the maximum transverse force coefficient.

Well, the first approximation for a cat is that its weight is equally divided between the two hulls. So when you begin to pivot on one hull, the other hull (1/2 of the total displacement) is pulling you down with the leverage of the full beam, but half the weight. Because some of the weight is in the center (cabin, mast), I used 7m instead of 8.6m.

For my tri I use the whole weight (minus the leeward ama) and half the beam, because the weight is centered. In practice these are similar calculations.

I don't take into account BSP and how AWS is affected. In my own stability table this is because focusing on AWS got me trapped several times in falling off "in gusts" to reduce AWS, which in practice were sustained TWS increases. Then you find yourself in the pickle of carrying too much sail at 170 TWA. Not the end of the world -- furl the foresail, ease the main a lot, turn into the wind, reef, but somewhat scary and uncomfortable.

Calculating maximum righting moment on a multihull is actually very straightforward. Just multiply displacement with the distance between centerline and the center of buoyancy of one of your hulls. No corrections for displacement or lever arm needed. For symmetric hulls the center of buoyancy will be in the centerplane of the hulls, for asymmetric hulls this is still a good approximation.