APPENDIX II
ADDITIONAL CONCEPTS
AND FORMULAS
The Three Primary Motions of a Yacht
Boats rotate on three primary axes: one runs fore and aft, a second runs athwart-
ships, and a third runs vertically. Any motion may act independently or combine
with another.
ROLLING
Step aboard any moored vessel and you induce roll motion. The roll axis lies along
the centerline of the boat, extending from bow to stern. A good indicator of vessel
stability is the roll period test.
- Stand with another person on the side of a vessel. Push hard up and down on the
gunwale to get the boat rolling. - Step back and time one complete roll period from right to left to right. Conduct a
second test and make sure it agrees with your fi rst test. Compare your results with
those shown below.
John Vigor, author of Th e Practical Mariner’s Book of Knowledge, gives handy formulas
for average roll periods:
Power vessels. Divide the vessel’s beam by 3.3. For example, if your power vessel
beam is 12 feet, 12/3.3 = 3.6 seconds. Compare this to your roll period test. Then
read the section following the sailboat formulas.
Sailing vessels. Sailing vessel roll period depends on vessel design and the ballast/
displacement (B/D) ratio. To determine the B/D ratio, divide the weight of the ballast by
the weight of the vessel. For example, if the ballast is 2,500 pounds and the vessel weighs
10,000 pounds: 2,500/10,000 = 25% (B/D ratio).
Heavy displacement sailboats
B/D ratio 25% or less
Average roll period = (beam/3.3) × .95308