What is porpoising and cavitation???

[Boatist]

Re: What is porpoising and cavitation???

It could be that your stringer are also rotted and allow the hook to form while under way.

Trim your motor as far out as you can and still get a good bite on the water.
Raise your bow and make sure all the water is drained out.

In you case Hydrafoils will make you problem worse and so will Smart tabs and helm adjustable tabs.

Is you boat Fiberglass or Aluminum?

Can your motor be lowered by one hole deeper in the water?

Is the bow lower at WOT than at midrange speed?

 

[mtduckhunter]

Re: What is porpoising and cavitation???

the boat is fiberglass and it is lower at WOT.

 

[QC]

Re: What is porpoising and cavitation???

How fast are we talking when the bow starts pushing down? Is there anything that can be moved further aft? Fuel? You? Anything?

You say you didn't see the hook, did you use the string or a straight edge?

 

[mtduckhunter]

Re: What is porpoising and cavitation???

it starts at around 30 mph, which is the top speed. with the boat being 15 ft with a 70 hp i know it will go faster. i can feel the boat being slowed because of this.

i didnt use a string line. a little bit of hook may make a bigger difference than i am thinking.

 

[Boatist]

Re: What is porpoising and cavitation???

Without changing the trim on your motor, does the motor look like it is trimed in or down. In this case the prop would pushing water down not straight back or up. This would force the bow lower as you applied more power and would also create a lot of hull drag and slower speeds.

In a bow down condition the spray would come off the hull up in the bow area. Normally in calm conditions you would want the spray to come off the hull from midship to the back 1/4.

 

[NSBCraig]

Re: What is porpoising and cavitation???

If you can't get it at a good trim angle try shiming the motor. Doesn't Bob's sell transom shims?

 

[budsbud662]

Re: What is porpoising and cavitation???

I must be the only person to have the only boat that runs faster with hydrafoil, beyond the skeptics i had a small whale tail and it helped pull more boat out of the water at full trim. 65hp commercial tiller 16ft boat pretty heavy in the rear end if that means anything..

 

[QC]

Re: What is porpoising and cavitation???

I think the consensus here is that there are some boats that are positively affected by hydrofoils. They are usually smaller, and the weight you describe in the stern may be a factor in your experience. Generally though, most boats would also have benefited from some type of trim tab, and there are MANY boats that suffer very odd, scary and possibly unsafe handling conditions with the addition of a tale. Exceptions? Yes. Panacea? No.

 

[MikDee]

Re: What is porpoising and cavitation???

mtduckhunter, If you have no power trim, the starting point for setting your outboard trim angle, is setting the pin for the cavitation plate, so that it's perfectly paralell with the bottom of the boat. This should be the ideal spot to start with, and keep the hull at the proper angle on plane at planing speed, as long as there's no hook, or rocker in the hull.

Raising the trim one, or 2 more holes from here, may provide more speed, or a better, higher, ride, or,,, possibly just cavitation of the prop.

 

[mtduckhunter]

Re: What is porpoising and cavitation???

thanks MD i will try that.

the hull does have hook, but im working on fixing that now.