catch the swell on the rise


Senior Member
The scene takes place on a big sailing boat.
Skipper : Eighteen feet through the passage. And the ship takes fifteen.
Stand by, everybody. We're going through the passage. Find that jib sheet.
Catch the swell on the rise, we'll ride it through.

What does "catch the swell on the rise" mean ? Could you please rephrase it so I can understand it ?
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    Senior Member
    American English
    Swells have high points and low points -- if they didn't, they wouldn't be swells; they'd be flat water. :) So a three-foot swell will be offset by a three-foot trough. (I'm not a sailor, so it's possible that a three-foot swell is measured from crest to trough; if so, there would be only an 18-inch difference between level water and the crest -- but I think they're measured from flat water level, e.g. sea level, so a three-foot swell would be three feet high. My examples below use that approach and even if it's wrong, the theory is the same.)

    The aim here is to catch the swell -- in much the same way a surfer will catch a wave -- so that there is more water under the boat when they go through the passage. If it's a three-foot swell, for example, catching it right will give them another three feet of water between the boat and the bottom of the passage.

    If they miss the swell and catch the trough, there will be three less feet of water under the boat. In that case, 18 feet minus 3 feet equals 15 feet -- which means the boat, which needs 15 feet, might run aground.
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