BOW is stamping up and down into water

[Nova]

I have a Novamarine RH630 RIB with a Yamaha 150 HPDI. The prop is 13 3/4x17. The boat weighs incl. engine and full tank, without persons, about 1,100 kg (2,245 lbs).

When I'm increasing speed to WOT, the bow is stamping up and down into the water. There's no reason to get sceared, but it's verry uncomfortable.

Who can give me advice to solve this problem.

Sorry for the bad English.

THANKS!!!

[AnthonySS]

...hope we can help you out..

The term used to describe your condition is "Porpoising".

Often porpoising can be overcome by adding more postive trim, triming the motor out.

If the trim is negative or in, it can have a tendency to pitch the bow down or make it porpoise at WOT for a hull that is trying to find its "sweet spot"

Most hulls try to find their optimum efficiency with some trim applied.

Another reason can be weight distibution with too much weight at the stern of the boat...but with your heavy hull, I don't think this is it.

Another reason could be a design flaw of the design...hopefully not!!

Hope this helps some

[Nova]

Thanks AnthonySS for the reply.

Is it also possible that the engine is to high or to low on the transom. Or the prop isn't the right one?
Or using stabilsators or something like that?

Thanks

[vector mike]

The term "PORPOISING" is from the prop losing grip with the water and the bow falling down. It is sometimes caused by prop design, hull design, engine height or just too much weight in the wrong place in the boat. You didn't say if this problem has been an ongoing problem or just started doing it. Try moving weight around to see if it changes. Did you change engine height or props? We need some feedback so we can try and help you out.

[Techno]

I have to disgree V Mike, the porpoising is from an imbalance. It could be caused from many things but all you have to do to fix it is jab the trim button. This is your on board balance adjustment tool.
I can't count the number of people that drove me insane from the constant hobby horsing of the boat when all they had to do is push a button.

They wouldn't listen which made it worse.


All you have to do is try the trim either way and the bouncing will go away, usually its a very small amount. It happens from a certain speed in a special wave type, these 2 just mean that it can change. A different speed tomarrow since the waves are different. Changing speed will also get rid of it. Changing speed and adjusting trim go together sometimes.

On an STV that porpoises at the transition speed all you have to do is increase the trim and lift the bow so it can't fall, porpoise stops. This is a special use though and your carrying the bow with engine power. This is different than the usual cause which is a speed sensitive zone, which the person always wants to stay in. Go faster or slower and it goes away- its trim time.

I see this all the time on the lake, that and someone running too slow for the waves. The boat pounds at just above planing speed when if they got it going faster it would bridge the waves- no pounding. I think that they assume it will get worse the faster they go when its the opposite.

[vector mike]

I don't want to get into an argument about this because everybody is entitled to their own opinion but I had a couple of boats that did this and triming the motor would not cure it. The only way I cured the problem was to move the weight around. If you get too much stern weight on a smaller light weight boat you need to counterbalance it. Like a V6 on a Viper. It would porpoise alot untill I reorganised the weight. Some of it was due to hull design because I have heard other people with Hydrostream's that had this problem. It would however flatten out under throttle above 50mph but when cruising at 30mph it was a problem.

[airide18]

Sounds like your not getting enough positive trim.You can use shims to add more positve trim.

[pyro]

My Hydrostream will porpoise with neutral trim at 20-25 mph when I have added passenger weight. the more passengers, the worse it gets, even with the trim buried. Above 30 mph, it goes away. When I drive alone, it hardly porpoises at all unless I gun the throttle repeatedly to make it jump out of the water.