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post #89 of Old 11-10-2008
Join Date: Jun 2007
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Did anyone look at the spreadsheet I put up?
Is it useful to help find a cruising boat based on the Mahina, Ted Brewer and Johns Boat Stuff lists?

The one piece of data I haven't been able to figure out yet is Roll Acceleration, which requires Roll Angle. Where does one get that data on boats?

(7) ROLL ACCELERATION = (6.28/T)^2*RADIUS*(ROLL ANGLE*3.14/180)/32.2 Units of G's, where "T" is the ROLL PERIOD. From Marchaj's book, SEAWORTHINESS, THE FORGOTTEN FACTOR, chapter 4, "Boat Motions in a Seaway". The author presents a graph of roll acceleration Vs four physiological states; Imperceptible, Tolerable, Threshold of Malaise, and Intolerable. Malaise starts at .1 G, Intolerable begins at .18 G. Spending much time under these levels of acceleration reduces physical effectiveness and decision making ability through sleep deprivation. The radius term assumes an off center berth located 1.5 feet inboard from the maximum beam. The roll angle is 10 degrees. G levels above .06 are considered undesirable for offshore cruising conditions. Several light weight, large beam designs have G levels above .4, definitely "intolerable" for any length of time. The ROLL PERIOD is calculated from the equation: T = 6.28*( I /(82.43*LWL*(.82*beam)^3))^.5 , and has dimensions of seconds. The roll period is based on the moment of inertia, I, waterline length, and beam. The term (.82*beam) has been substituted for the waterline beam due to lack of data. The general rule of thumb is that boats with periods less than 4 seconds are stiff and periods greater than 8 seconds are tender. The MOMENT OF INERTIA is calculated from the equation: I = (disp^1.744 )/35.5 , and has dimensions of lb.ft.^2. An empirical term used by SNAME for analysis of the 1987 Fastnet race. The moment of inertia is very sensitive to the distance items are from the center of gravity. A heavy rig can greatly increase I, with little impact on displacement.
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