----- Original Message ----- From: "Richard Lane" <rqlhgl@ieee.org> To: "Keith Diehl" <kdiehl@xmission.com> Sent: Tuesday, December 28, 2004 10:21 AM Subject: Sail area & heeling moment
Kieth, I just came across something I did when I first had M23 Sadhana and I seem to remember a recent inquiry on the list about heeling, anyway here it is. Dick
Wind Strength and Sail Area 1) Measuring Righting Moment Using inclinometer, note heeling angle for 200lbs placed on each rail, note distance of each rail to center line (half beam). Place 200lbs about 2 off center line, note these inclinometer readings. If not enough heel to be accurately read can be induced in the above way, use a more sensitive inclinometer consisting of a bronze scrub pad immersed in a bucket of water hanging by a light line and a tape measure taped to a bulkhead. Righting moment is the product of the movable mass in lbs and the distance off center in ft needed to produce 1 deg heel. Plot a graph of heel angle vs righting moment lbs ft. 2) Finding Center of Pressure and Wind Pressure of available sail combinations. The center of pressure of each sail is where the bisectors of the angles intersect. The area of each sail is the luff x 1/2 L.P. LP.is the perpendicular from the clew to the lull When the individual sail C.P.s and areas have been calculated, the C.P. of each combination is lies on the line joining each individual C.P. and positioned towards the larger sail proportional to their areas. The Heeling arm is the height above water of the combined C.P. and the heeling moment is the product of the heeling arm and total wind pressure on the sails. Where wind pressure (lbs/sq ft) = .004 x velocity2 in knots. Wind velocity-pressure table Wind Wind velocity pressure (knots) (lbs/sq ft) 8 0.25 12 0.6 16 1.0 20 1.6 25 2.5 30 3.6 35 4.9
The heel angle = (heeling arm x wind pressure x sail area) / righting moment for 1 deg heel (This is a linear approx to -cos but is pretty close from 15 to 50 deg) Application of above results For the largest sail area, spindrifter and full main, calculate the wind velocity for say a 25 deg heel. Now plot the areas that give the same heel angle over the full expected wind range to be sailed in, say up to 40 kts using the inverse square law. Overlay this plot with the sail area combinations available (include all reefed down combinations). Check that the full wind range can be covered.