Jet Jon project questions

[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

 

[Ranchero50]

I had problems with porpoising. Need to move the center of balance inside the center of stability bubble for it to go away.

I ended up just extending my trim tabs back until it went away. Check out my build thread for some better insight.

 

[Stevenstout]

So your pump is pretty much level coming out of the back of the boat? I'll post some picture of what I had. I pulled it all apart now because I couldn't get it figure out but it looks very similar to your with how you have your entire pump put of the back. Only difference being my boat is 2ft longer.

 

[Stevenstout]

Do you think adding pods and making trim tabs like yours would do the trick?

 

[Ranchero50]

It did on mine. What's your layout and balance now? Pics show a pretty empty hull. Gonna need to finish it out to figure the balance.

 

[badrider77]

Howdy,
new user here! I have a Lowe 2052 that I'm wanting to build using a 96 zxi 1100. I really like how you have your ski hull cut. I have a ton of hull left compared to yours! Have you had any luck with your fixing your porpoising? I'd really like to know how you secured your hull to the boat too if you don't mind.
Thanks!
Adrian

 

[Stevenstout]

Hey Adrian, sorry for the late reply. I used expansion joint caulk and 3/8" carriage bolts to secure the jet ski hull to the Jon boat. It worked very well. I am have installed the trim tabs but have not tested it out yet as I am trying to 100% finish the boat so all the weight is where it will be when I'm done so I don't get everything set right then add weight somewhere and screw with the balance of things.

 

[PSG-1]

I fixed my porpoising problem by making a ride plate that extends all the way back to the end of the steering nozzle, and then using a set of shims at the rear. Made the shims from some 1/8" X 1" flat aluminum stock, cut to 1" length, and with a 3/8" hole in the center for the rear bolts to pass through. I put the shims between the ride plate and the mounts, with the bolts going through them.

You wouldn't think that 1/8" would make much difference, but when I first ran the boat with the ride plate and no shims, it was porpoising at full throttle. I tried 1/4" shims and that took the porpoising completely out, but then it felt like the bow was digging in more, so I tried 3/16" and there was no porpoising, and the boat seemed to run a little quicker. So, I decided to try 1/8", and there was still no porpoising, so, I left it at that, and it works fine. =D>

 

[amk]

Have you ran it with someone else in it? My old boat porpoised but whenever someone sat towards the front it was fine

 

[Stevenstout]

I made a monster set of trim tabs for it, once I figure out where they work best I'm going to shim the ride plate down to match them. I tried everything you could think of (except shiming the ride plate) to get rid of the porpoising and nothing worked. I also moved my steering up front to get some distribute the weight better. I'll post some pictures. The only real purpose the platform on the bad serves is to keep the splash out of the boat. (Although it is sturdy enough to stand on, i guess could be a piss platform haha) It seems to splash off the trim nozzle and into the back of the boat

 

[Stevenstout]

Sorry for the pictures bwing upside down, I've tried to fix em 3 tines and it won't seem to upload them right.

 

[CedarRiverScooter]

Nice trim tabs, I hope they work out.

I have thought of adding 'pods' in back.

Seems like they could act like trim tabs.

A cheap way to make my boat bigger!

 

[Stevenstout]

I had actually fabbed up a set of pods and was going to do like Ranchero50 did and step them up off the bottom and have the trim tabs underneath but decided not to. The only downside of trying to use pods as trim tabs is they won't be adjustable. If your just looking to add length then they'd work great.

 

[CedarRiverScooter]

I was thinking the pods would reduce porposing just due to the jet nozzle being farther forward, relative to the wetted length. Probably just nonsense . . .

 

[Stevenstout]

If you made them flush with the bottom of the boat then it probably would help with porpoising but I had them set up an inch off the bottom to alow for the trim tabs underneath

 

[Stevenstout]

So here's an update on the build, I ditched the pods and made different mounts for the trim tabs. So far I've had it up to 39mph at fI'll throttle withe no porpoising. Which yes, is fast for a Jon boat, but with 120hp I think it should go much faster. Especially when I've seen some with 60hp motors going 35mph and 85hp motors going 55mph. I haven't had a while lot of time to mess with trim nozzle adjustment and trim tab adjustment because the river has been pretty high and swift but I can't see that getting much more mph out of it. Any suggestions?

 

[badrider77]

Hey guys welcome back! I was looking at my last post and that was in March??? Where did the time go. Sorry I didn't stay in touch. I haven't received any notification that there were replies... So to update you where I'm at. My boat is cut and I've had the hull dry fitted with hardware. Had some weld work done (mostly from me working the metal; had to split it in a few places) Hoping to get it glued in this weekend! There is still time left in the season to get it running. I'll post some pics soon. If you have watched Jocelyn Brucho youtube videos, I modeled after his build with my hull below the bottom of the boat. Plan on keeping the KATS trim system for porpoising. Working on how to have reverse. Thinking of welding a cut down STX ride plate to the existing plate, may also help with porpoising. I'm just wondering how skinny I'll be able to run with my hull sticking down 3"??? Guess we'll see....

 

[badrider77]

Lets see if I added them right.

 

[CedarRiverScooter]

Looking good! You should have any cavitation with your deep set intake. Great console control layout too. Lots of good skimming in the fall, the foliage is pretty.

 

[badrider77]

Got her glued in this weekend. Had a pal that owns a windshield company help me out. Used urethane as the sealant. Kinda messy but otta do the trick.