"(3.) A measure or estimate of the height of the waves.
"(4.) The depth of the sea if it is known, but, at any rate, the position of the ship as near as possible, either by cross bearings of land or any other method, so that the depth may be got from charts or other sources.
"For a Ship at Anchor.
"(1.) The periodic time.
"(2.) The true direction from which the waves come.
"(3.) A measure or estimate of the height of the waves.
"(4.) The depth of water where she is anchored."
It is the opinion of scientists that when the period of oscillation of the ship and the period of the wave are nearly the same, the turning over of the ship is an approximate consequence, and thus the wave to such a ship would appear more formidable than to another ship with a different period of oscillation.—Nautical Magazine.
As an example, this table shows that waves 1,000 feet in length travel 43 nautical miles per hour in water 1,000 feet deep. The length is measured from crest to crest.