posted 12-11-2001 01:30 AM ET (US)
I am working on what I call my GENERAL THEORY OF PORPOISING. It is still under development, but so far it goes like this:As a boat goes up onto plane, the wetted surface decreases and moves aft.
The center of gravity (CG) of the boat remains about the same.
The point where engine thrust is applied remains relatively constant, too. Call this Point the Center of Effort (CE).
Along the fore and aft line of the boat there is some point which is in balance, where the boat would tend to rotate ("pitch") up or down equally around this point. Call this the center of pitch resistance, or CPR for short.
When the boat is in the water in displacement mode, the CPR is near amidships.
As the boat comes on plane, the CPR moves aft along with the reducing wetted surface. This distance from the CPR to the CE decreases
The boat trim is determined by the vector that the thrust is applied thru the CE. This determines the bow lift and running angle.
Pretty soon the boat is running at a speed where there is marginal wetted surface, and the CPR is practically at the transom, just inches in front of the CE.
All the weight of the boat in front of the CPR ( a long lever arm of weight) is being balanced by the very short lever arm of the thrust being applied from the CE to the CPR. It takes very little force acting on the bow--a little wave--to lift the bow up and drive the stern down. The forces at the bow have much longer lever arms around the CPR.
The result is unstable pitching or porpoising--oscillations of the bow pitching up and down from small forces being applied at the bow through the long lever arm they have on the CPR. They cannot be controlled well by the engine thrust because it is acting through a very short lever arm.
The cure to suppressing them is one of two choices:
--change the angle of the thrust vector to force the bow down. This increases wetted surface and drives the CPR forward, bring more stability to the equilibrium around the CPR.
--or--
--use trim tabs, or what I call "stern lifters", to raise the stern slightly which will bring the bow down and increase the wetted surface and move it forward.
Notice that using an engine bracket should increase the distances between the CPR and the CE, giving the prop a longer lever arm and more control over the bow. This alone should reduce pitching or porpoising.
Thats my GENERAL THEORY OF PORPOISING.
As the Montauk owners have noted, heavy engines in the stern tend to cause porpoising. This is because the wetted surface when planing is driven smaller and farther aft.
The boats that don't have porpoising problems are boats that get some stern lift going. The boats plane with more of the hull in the water measured fore and aft, but less of the hull in the water measured keel to deck.
Both hulls achieve the same buoyancy by having about the same area of hull in the water, but on the boats with good stern lift the wetted surface is more forward than on boats whose sterns are heavy and dug in.