| Design Basics |
| By John Holtrop |
Published
01/16/2006
|
Boat Design
|
Unrated
|
|
|
|
ROLL PERIOD (T)
2*PI*((disp^1.744/35.5)/(82.43*LWL*(.82*beam)^3))^.5
The roll period is based on the moment of inertia, waterline length, and beam. The moment of inertia, (disp^1.744/35.5), was developed by SNAME. Large values resist rolling forces. The moment of inertia is very sensitive to the distance items are from the CG. A heavy rig can greatly increase I, with little impact on displacement. This equation was developed to support the 1987 Fastnet race investigation. The term (.82*beam) has been substituted for the waterline beam due to lack of data. Using (.82) results in a close match for the few boats with measured periods. Simply stated, a sailboat’s roll period, in seconds, is inversely proportional to its stability. Tender boats have long periods, stiff boats have short periods. The roll period is very easy to determine, you simply grab a shroud and push / pull until the boat is rocking over a few degrees. Then measure the time it takes for ten full cycles , and divide by 10. 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 roll period is related to LOA and strongly related to COMFORT FACTOR.
|
|
|
