What causes Porpoising

[htillman]

I measured the motor and it seems too low. There is no marking on the prop except Rebel. I am going to take it off where I can get a better look at it. I confirmed the motor only turns 4800 rpm.

[Fr. Frank]

John C and Gillie are on the right track. Porpoising is caused by direction and placement of thrust and amount/type of thrust not being able to overcome PLANING rocker pivot angle. Direction and placement of thrust plus amount and type of thrust must overcome displacement CG to thrust hull past pivot angle. If the angle is too great or thrust too little once the hull pivots, the hull pivots stern-ward again off the planing rocker pivot, at which angle the thrust is once again able to force hull back into pivot, ad nauseum.

The are three possible ways to overcome this:
1. Change planing rocker pivot angle by changing hull CG.
2. Change direction and placement of thrust (Height and angle)
3. Change amount and/or type of thrust at speed. This is more about type and shape of propeller than amount of horsepower. (Yes it is possible to simply overpower porpoising by greatly increasing thrust, but it's really stupid as your actual control becomes minimal)

First, check motor mounting height by measuring distance from bottom of ventilation plate to keel when the two are as parallel as possible.
Start with ventilation plate no less than 1/2" above keel, and no more than 1 1/2" above. (This assumes a transom-mounted motor)
If your motor is at the right height, when trimming up/out you should ventilate the prop before porpoising under 3/4 or higher power. You still may porpoise slightly under acceleration, and perhaps more when slowly decelerating.

For a SeaCraft, make sure you have a stern-lifting prop. A bow-lifting prop (particularly when mounted at the wrong height) can seriously exacerbate a slight tendency to porpoise.

Monitor your trim angle. Does your hull still porpoise with the motor trimmed all the way down? Does adjusting the trim minimize or eliminate porpoising at various speeds? If trim does not ameliorate the porpoising at all, this can indicate an issue with static CG as well.

Whatever your power, you should be able to eliminate porpoising by adjusting thrust placement and direction, adjusting center of gravity, and adjusting type and amount of thrust.

[Beaver]

My 18 was porpoising so I moved the motor up three notches. Then it was cavitating so I moved it down 2 and now it rides great. I am still surprised that one notch up made all the difference.

Good luck.

[htillman]

Finally changed the prop, went with a 17 Viper. This made a big difference. Now turn up 5200 rpm and speed increased to about 42 mph. The motor may need to be raised a notch but the performance is dramatically improved. The porpoising is gone and the boat seems to hve a noticeable bump in acceleration.
Picture of completed boat is attached.

[Capt Chuck]

The boat looks great Eddie!

Did you end up using Signature Finish and are you happy?

BTW: No prop in this picture = I bet the RPM's are higher than my Glucose Count now

[htillman]

Thanks, this little boat was fun putting together.
The hull sides were sprayed with Imron. I am going to paint the bottom with Signature Finish dark blue. I removed the prop and had to re-rig the engine; the cables (controls, fuel and oil) were kinked when the motor was originally installed. During this process I bulk head mounted the hydraulic steering cables, replaced the water pump and tuned the engine. She really runs much better now with the prop and engine work. Found out this engine only had 85 hours.
There is a big difference (ride, storage, etc) between the 20 and 23. I really miss the 23 and have been looking for another one. Just not sure what model?

[bgreene]

Most unlikely, but finally, if hull has " hook " meaning running bottom is no longer flat, but has developed a curve, it could cause porpoising.