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Porpoise, what is it?

On May 8, 2015, in Designer Articles, by Glen L. Witt
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Planing boats can have a condition where the boat “bounces” in the planing attitude and will longitudinally teeter totter back and forth. In severe cases the boat can leap clear of the water. In either case, the ride is rough and uncomfortable. A small amount of porpoise is desirable in faster boats as they shouldn’t be glued to the water for maximum speed. For most pleasure boats, a porpoise is a pain in the posterior both literally and actually.

Two factors tend to make a planing boat porpoise; weight distribution and/or a warped bottom.

 

Weight distribution

A planing boat that has excessive weight at the transom will tend to porpoise. Underway the excess transom weight makes the boat go down by the transom, but then forward speed overcomes the weight and the bow drops, most often slamming down. This condition repeats itself time and again. The boat is simply out of balance.

The cure is to remove weight from the transom. Shift any movable weight forward. Batteries, fuel or bait tanks, even passengers if sitting near the transom. Note in this case, weight is REMOVED from the transom, adding weight forward to bring the boat in balance is not desirable.

 

Warped bottom

The fore and aft bottom lines of a fast boat are straight in the aft section; they do not curve upward near or at the transom; commonly called a rocker. Our instructions and drawings generally emphasize the importance of straight lines in this area, but it’s easy to goof. We’ve erred, and we know better. It’s easy to let the transom drop down a little when building bottom side up. Don’t let it happen; check and recheck. A straight edge along the keel and parallel to it outboard must be straight without gaps between it and the bottom, particularly at the transom. Even a rounded corner at the transom/bottom junction can cause a porpoise. It is common practice to radius this edge when fiberglassing the boat, as the cloth will not readily form around a sharp corner. For top performance, this rounded edge must be eliminated. This is usually achieved by applying additional layers of scrap glass cloth and grinding the edge to a crisp corner.

If either of the above conditions are causing porpoising in your boat, the best solution is to correct it.

Trim tabs or cavitation plates can also correct the condition, if not too severe. These are often used on faster boats to control bottom defects or disadvantagious weight distribution.

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