A porpoise ‘porpoising’ looks smooth and energy efficient; a boat doesn’t and isn’t!
This action is constant, rhythmic pitching up and down of an out of trim planing craft at speed. Some claim that porpoising reduces the mean wetted surface and, hence, drag, but I feel the claim is to try to justify the porpoising already exhibited by particular craft.
The net wetted surface is a mean between that of the boat ‘up’ and ‘down’, and is unlikely to be less than the constant wetted area of a hull in steady planing. Added to this is the energy lost in the repeated oscillation. Porpoising is trim angle related. If the centre of gravity (CG) of a normal, non-stepped, RIB is slowly moved aft from amidships it gets faster for the same power.
The aft movement of the CG increases the trim angle and this increases the lift per unit area (because the water is deflected harder) which means that less area is needed and hence, less friction drag. As the CG continues to move aft it will reach a position at which the boat trims at a high enough angle to start to lift clear of the water.
This causes the wetted footprint to shorten from the leading edge and the lift centre moves aft behind the CG so the boat pitches forward at the same time as it lifts. Since the hull is starting to rise clear of the surface, the excess lift disappears and the boat drops back while still rotating bow down. It then lands flat and the wetted lift centre leaps a long way forward of the CG and pitches the bow back up. The whole process then repeats itself …
The critical trim angle varies between hulls depending on deadrise (vee) angle, beam, loading and speed. The oscillation can be constant, slowly reduce until restarted by a wave or increase to the point where the hull is actually leaping clear each time. The latter can be frightening but normally porpoising is just a nuisance. The most efficient trim is usually at the point just below the critical angle. Because porpoising is fairly dramatic those unused to it can overreact. It is a lot easier to fix than any other handling problems.
For a particular speed, the natural trim angle must be reduced. This may be done by moving or adding weight forward, by trimming the drive or motor in, dropping the trim tabs or by changing speed; usually a reduction but sometimes an increase can do the trick. The natural tendency is for trim to flatten as speed is increased – less lift is needed so the hull flattens out to keep the lift centre under the CG.
Interestingly, changing the pitch inertia of the hull does not change the critical trim angle it only changes the pitch frequency. Spreading the weight to the ends rather than concentrating it amidships will only make the oscillation slower; it won’t stop it.
Marine Craft Designers and Naval Architects