Jet Jon build

[Stevenstout]

I'm building a jet Jon using a 1652 flat bottom Jon boat and a 1996 kawasaki zxi1100 jet ski. I have already had everything together and had it on the water but it has porpoising issues. I tried moving weight to the front of the boat which caused it to cavitate and when i moved weight backwards it porpoised more. I also tried trim tabs which didnt help either. I can only run at 1/4 throttle and go about 25mph before it starts to porpoise.The ski had power tilt/trim so when I installed it into the boat I put the pump nozzle at 0° Which with the tilt/trim nozzle gave me 15° up and 18° down. I pulled everything apart and am going to try it again but would like some advice. When I installed it the first time I had the pump sticking out of the back of the boat and mounted the intake flat on the bottom. My idea is to move the motor and pump forward so the pump is flush with the transom (better weight distribution) put more of a pitch on the pump, and build a tunnel to feed it. Does this sound like it might fix my problems or would yall suggest something else? Thanks in advance!
Stevie

 

[CedarRiverScooter]

Sounds like a very cool boat - 25 mph at 1/4 throttle = fun ride!

I think you are on right track. Move CG fwd & suck water from as deep as you can.

I got rid of porposing by adding a 'trim tab' plate over the ride plate & shimmed it down in back about 1/8 inch.

Good luck, post some pictures when you can.

 

[PSG-1]

I was able to take the porpoising out of my boat by making some spacers out of 3/16" plate. They are about 1X1" square, and they have a 3/8" hole in the center for the mounting bolt to pass through. This set of spacers fits between the ride plate and the ride plate mount, on the rear end. That little bit of pitch made a lot of difference.

 

[Stevenstout]

It's very fun! Everyone told me to leave it like it was and just put a thottle stop on it and that 25mph was plenty but I couldn't do it! Knowing that I had 3/4s worth of unusable throttle killed me! I didn't think about trying to trim the ride plate down though. Everything is back out of it now though so to late! Haha I was also using the steering cables from the jet ski which weren't very long so I was sitting right in front of the motor. I'm going to try to get some longer ones and try sitting up front and maybe move the gas tank up a bit too. I did try a set of the smart tabs as they worked well to get rid of the porpoising on my dad's checkmate but they didn't help much at all. Here are some picture before I pulled it apart. I didn't put any decking on it yet since its still a work in progress

 

[CedarRiverScooter]

I think your plan to move the pump fwd is good. Looks like the ride plate was not well supported, could have been flexing & may have gotten into an oscillation with the porposing.

 

[PSG-1]

Moving the pump/engine forward seems like a lot of work.

Let's try a quick fix and see if that helps. First thing, I would add some support brackets for that ride plate, as it does look like it's just hanging off, with not much support. Between the brackets and those rear mounting holes is where you would use the spacers. Don't put any on the front, you want to pitch the back end down. The idea is to use the ride plate itself as a trim tab.

Try it in 1/16" increments, starting with 1/4"

If that knocks out the porpoising, go down to 3/16" and see if it comes back. If not, step down to 1/8" One of these thicknesses will likely be the right amount of pitch to take out the porpoising. This worked for my boat, very simple fix.

 

[Stevenstout]

Kinda to late to try that, already stripped it all down. I had posted on another site a while back when I first built the boat and no one suggested that. That was before I had found this site. Oh wwell, what's done is done... so since it's already stripped down do you think moving the engine and pump forward and adding more pitch to it is the right thing to do? I'll post a picture of where I'm at with it now...

 

[PSG-1]

Ideally, you want the pump set in the boat so the only thing sticking out is the steering nozzle. Shifting it this much forward will have an effect on porpoising. If not, the spacers will most definitely take it out.

 

[Stevenstout]

Do you have any idea what degree of pitch the nozzle should be? When I first put it together a made everything level, so the pump nozzle itself was at 0° and having the trim nozzle gave me +15° and -18°. I'm thinking of making the pump nozzle -15° so with the trim nozzle I would range from 0° to -33°

 

[amk]

have you ran it with a buddy up front. Im sure with another guy infront of you that thing wont porpoise at least you can see what the other 3/4s is like.

 

[Stevenstout]

I had 2 very "healthy" friends sit in the front (at least 500lbs) and i got to 28mph before it started to porpoise. I also got 180lbs of plate weights and put the fuel tank (full 14 gallons) and 2 batteries in thefront and that didn't help either. I've moved weight all around the boat, installed trim tabs, got 4 people to ride with me thinking maybe the boat was just to light in general and nothing made much of a change.