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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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Aquí están las curvas. Como puede verse, los límites de la aceleración dependen del periodo, estando el mínimo de tolerancia para un periodo entre 3 y 4 segundos, con periodos largos se toleran mejor las aceleraciones.
De lo que no se ha hablado, y es de vital importancia, es el efecto de la amplitud del balance sostenido durante largos periodos de tiempo. esta es la curva A. A partir de más o menos 6º el rendimiento de la tripulación disminuye rápidamente.
Que me corrija Guillermo si digo alguna burrada (uno es sólo un aficionado a estos temas):
En la navegación en portantes el balance producido por las fuerzas del viento se va amplificando hasta que el amortiguamiento producido por la obra viva absorbe toda la energía tomada del viento. El barco oscila con un valor máximo que entre otras cosas dependerá del amortiguamiento.
Aquí la pregunta para los sabios: ¿Existe algún ratio, indice... que nos de una idea aproximada del amortiguamiento?
Salud y buenos vientos
