Hkrause wrote:
> A friend with a new boat accompanied me yesterday for a little fishing. Every
> time he got his boat up on plane at mid-range speeds, it began to porpoise, even
> in absolutely flat calm water.
> We swapped boats for a spell so I could try to determine the problem. I thought
> it was trim. It wasn't. No matter how I trimmed the outboard, it would porpoise
> at mid-speed levels. AT WOT, the proposing went away.

> I noticed that his engine was mounted at the three hole mark, which, to me,
> seemed high.
> Anyone have a clue?


" Almost all plaining boats operate at a plaining angle of between three
and six degrees, if left to their own inclinations. Power trim and trim
planes can force the bow down below the three degree plaining angle but
can't lift it much above the six degree angle. The lift that holds the
boat up in a planing attitude is a vertical force against the bottom
that can be considered to be concentrated somewhere within the area in
contact with the water. Depending on the location of the boats center of
gravity, the boat may have a tendency to "porpoise." Usually the best
speed is achieved with a plaining angle just below that point, controlled
by adjusting the trim or moving the center of gravity ( passengers and
or gear forward ). The object is to present the after end of the boat
bottom to the water at the optimum angle for best efficiency: least
wetted surface consistent with the center of gravity location." [
quoted: Ralph Lambrecht, tech corner ; Boat & Motor Dealer, June 1998 ]
In anticipating consumer usage, some boat builders will design a boat to
be used fully loaded. When the boat is not used this way, the hook,
built into most hull bottoms, can cause the boat to porpoise. Keep in
mind, trim angle and mounting height of the motor do influence this
characteristic to some degree. I have found, as well, propeller style
and type can make or break a fine hull design.