Porpoising in a seaplane is much like the antics of a dolphin - a rhythmic pitching and heaving while in the water. Porpoising is a dynamic instability of the seaplane and may occur when the seaplane is moving across the water while on the step during takeoff or landing. It occurs when the angle between the float or hull, and the water surface exceeds the upper or lower limit of the seaplane's pitch angle. Improper use of the elevator, resulting in attaining too high or too low a pitch (trim angle) sets off a cyclic oscillation which steadily increases in amplitude unless the proper trim angle or pitch attitude is reestablished.
A seaplane will travel smoothly across the water while on the step, so long as the floats or hull remain within a moderately tolerant range of trim angles. If the trim angle is held too low during planing, water pressure in the form of a small crest or wall is built up under the bow or forward part of the floats or hull. As the seaplane's forward speed is increased to a certain point, the bow of the floats or hull will no longer remain behind this crest, and is abruptly forced upward as the seaplane rides over the crest. As the crest passes the step and on to the stern or aft portion of the floats or hull, the bow abruptly drops into a low position. This again builds a crest or wall of water in front of the bow resulting in another oscillation. Each oscillation becomes increasingly severe, and if not corrected will cause the seaplane to nose into the water, resulting in extensive damage or possible capsizing. Porpoising can also cause a premature lift off with an extremely high angle of attack, resulting in a stall or being in the area of reverse command and unable to climb over obstructions.
Porpoising will occur during the takeoff run if the trim angle is not properly controlled with proper elevator pressure just after passing through the "hump" speed, or when the highest trim angle before the planing attitude is attained; that is, if up elevator is held too long and the angle reaches the upper limits. On the other hand, if the seaplane is nosed down too sharply, the lower trim range can be entered and will also result in porpoising. Usually, porpoising does not start until a degree or two after the seaplane has passed into the critical trim angle range, and does not cease until a degree or two after the seaplane has passed out of the critical range.
If porpoising does occur, it can be stopped by applying timely back pressure on the elevator control to prevent the bow of the floats or hull from digging into the water. The back pressure must be applied and maintained until porpoising is damped. If porpoising is not damped by the time the second oscillation occurs, it is recommended that the power be reduced to idle and elevator control held firmly back so the seaplane will settle into the water with no further instability.
The correct trim angle for takeoff, planing and landing applicable to each type of seaplane must be learned by the pilot and practiced until there is no doubt as to the proper angles for the various maneuvers.
The upper and lower trim angles are established by the design of the aircraft; however, changing the seaplane's gross weight, wing flap position, and center of gravity location will also change these limits. Increased weight increases the displacement of the floats or hull and raises the lower limit considerably. Extending the wing flaps frequently trims the seaplane to the lower limit at lower speeds, and may lower the upper limit at high speeds. A forward center of gravity location raises the lower trim limit at high speeds, and an aft location increases the possibility of high angle porpoising especially during landing.