very small jet boat bouncing

hi my very small jet boat on plain bounces rapidly on top of the water up and down real rough and every now and again it will be smooth for about 5 seconds then back to rough. I think i heard this being called porpising. my little jet boat would be smaller than a small car and is powered by a datsun a14 motor that is stock. no gbox just flywheel through to jet shaft impeller thing here is a photo. how can i stop it porposing. i was told i could extend cav plate. please not cav plate is currently not adjustable in water as it is a screw down one. would it be worth going hydralic to muck around with it on top of water or what?

 

Looks like good fun --I love the air filter.

You will have to get up out of your seat and move your wieght around to see what it wants, ie lean forward to see if it stops - or tie that spare wheel to the bow for a test run. Either forward or backwards will stop it.

If it likes wieght forward then trin tabs will help. If it like wieght back then you will have to shift weight back some how.

Ps theres some guys on this forum will go nuts when they see that car engine-- totally out of regs-- well for the Us any way.
I like it!!!

 

yeh I should be able to get a vid of it this w/e depending on whether i can fix the throttle cable

 

Short boats are always prone to porpoising. How long is the cav plate under your jet unit now? The full length of the jet unit to the end of the nozzle would help, and then a slight downturn may help.

Try with the plate exactly parallel to the bottom of the hull, then add a degree or two downturn. A tiny bit makes a big difference.

Assuming you have a nozzle steer, wedges may be available, depending on the make of jet, to vector the jet down a degree or two, and also provides a good solution.

Have you got a pic of the rear of the boat?

 

thats the best i can do for now but ill see if i can get one tommorow. we noticed when the plate was bent up it got to plane quicker. but couldnt get it to plane on down just front was alway up like it wanted to. would it be worth extending it to the back of the jet?

 

It looks to me like some one had tried to make that an adjustable sort of trim tab. Probably for the reason of porpoising. I would take some tools with you to the water and play with it.

I would loosen them up and clean the threads first though to make the job easy.

If you got it to perform better but not perfect it looks as though adjusting the angle of the jet would not be a big job.

 

so which way would i want to angle the jet as ill just whack some washers in there to muck around with.

 

Would probably help to make the plate twice as long as it is, ie where it ends now, make about half way. If you can set up a mount for it there that is adjustable like vertical plate welded to the ride plate, and vertical plate fixed to the jet, so you can drill thru both and bolt together. Make the plate from 6mm aluminium, same width as the jet unit with a slight taper to the end.

Like this one. Excuse the scratches from gravel...

 

And the wedge for the nozzle is like this. It angles the outlet down slightly to reduce bow lift, which is usually the cause of porpoising in a jet.

Too much angle will cause the bow to plough, and get bow steer, and unpredictable handling, so if you can adjust your nozzle, don't make radical adjustments.