Jet Jon conversion cavitation

[wlockerb]

Okay guys so I need some help here. I've got everything all setup and even have my deck almost done but she cavitates so bad I can't get over half throttle at most! This is a flat bottom 14 ft Jon and maybe 36" wide. This thing would scream if it would hook! I've already tried a deep scoop intake grate because they don't make a top loader for it. Any other ideas? I'm tossing around the idea of a spoon I've seen some people talk about on here but I'm having second thoughts about drilling more holes in my boat if its not the issue. I've already got a make shift spoon built but waiting for feedback before I install it. I'll get some pics and video posted as soon as I can. Thanks all![attachment=-1]uploadfromtaptalk1400464323970.jpg[/attachment][attachment=-1]uploadfromtaptalk1400464340884.jpg[/attachment][attachment=-1]uploadfromtaptalk1400464354662.jpg[/attachment][attachment=-1]uploadfromtaptalk1400464364573.jpg[/attachment][attachment=-1]uploadfromtaptalk1400464428383.jpg[/attachment][attachment=-1]uploadfromtaptalk1400464442982.jpg[

 

[wlockerb]

Also, I forgot to mention that I already went through the pump and replaced the old stock impeller with a stainless 15 degree one for a little more low end. Here is a video of the progress:https://youtu.be/72-kW7paaHM

Here is one after the floor install: https://youtu.be/yNOAkJS1XlY

 

[wlockerb]

It is dark out so hard for a pic of the bottom of the boat but I will get one up asap. What do you guys think of the spoon idea? I've seen variations work for other people. What do you guys think of the one I made? Think that its worth a try? I just don't know anymore. I'm running out of ideas and time. I'm supposed to be going cat fishing this weekend so I need to try something!

 

[He Reigns]

Could you attach the spoon with some caulking for a temporary just to see if it works test? I would run a zig zag bead down the whole thing then finish off the edges. Let it dry to the next day and run it. If it works then bolt it on!

 

[wlockerb]

You think regular caulk would work? This was kind of a pain to make so I really don't want to lose it. I'll test it to see if it floats and if it does I will think about doing that. Good suggestion thanks

 

[wlockerb]

Anybody else have any input about the spoon I made? Think it will work?

 

[wlockerb]

I couldn't take it. I got some of the cheap marine adhesive from Walmart. E-6000 I think. I went this far so I think I'm going to go ahead and bolt it in. I'll give the sealant until tomorrow to cure and bolt it in after work tonight. Here is a pic from under the boat. Any predictions?[attachment=-1]uploadfromtaptalk1400518452296.jpg[/attachment][attachment=-1]uploadfromtaptalk1400518471722.jpg[/attachment][attachment=-1]uploadfromtaptalk1400518517869.jpg[/attachment][attachment=-1]uploadfromtaptalk1400518552076.jpg[/attachment]

 

[Ranchero50]

Yeah, that ain't gonna work... #-o

Well, let me say that I seriously doubt that will work. Go borrow a soup spoon and turn on your sink so the water runs out smoothly. Hold the bottom edge of the spoon vertically against the edge of the water stream. Notice how the water follows the curve of the spoon? That's what you want under your hull.

Now water is funky stuff. If you reduce it's pressure quickly it'll form air bubbles (cavitate). All you want your spoon to do it gently force the air you are ingesting between your strakes to the side and have the bottom of your spoon dip into the clean water and gently draw it up into your pump. If your back wall is too abrupt the pressure change will cause cavitation. If your spoon is too deep it will cause a huge pressure different in the water and cause cavitation. If you front wall is too harsh it will increase the pressure at the spoon making it easier to cavitate off the rear wall. These are all problems I see with you current design.

This is a functional spoon that tested to 50 mph. I built after talking to Kevin Turner and seeing the design from a video from racing 16 guys on youtube (Western jet boat racers). Minimal intrusion below the air zone and a smooth flat transition to the impeller. Mine is maybe 1.25" deeper than the hull (1/2" deeper than the strakes) and about 3' long.

Good luck and keep trying.

 

[wlockerb]

Ugh. Wasted time... Maybe I'll try it and see. I saw another thread where a guy did something similar to mine... Or maybe not I guess. So do you think it is too deep or just the overall shape is wrong or both? You think if I lower the profile maybe?

 

[wlockerb]

My front cut is a dual 45 degree cut and the back ramp down to the intake grate is about 20-25 degrees. I also went down each side of my stacked boards before i glassed it with my miter saw at 45 degrees. If I lower the profile and round everything out do you think that would work? Maybe flatten out the v a bit more?

 

[Ranchero50]

I would compare your pics to mine and reread my original post. I think your footprint is about the right size but even the side angles are too steep. I'd start over, same footprint just minimal depth.

Good for practice though.

 

[wlockerb]

Got it. Thats what I figured. I'll pull it off and hack it up tonight. I hit all the sharp edges with the grinder and cut about 2" at least off the depth. Also, I'll add some glass to smooth the transition from the sides to the hull.

 

[He Reigns]

I just posted a link to a YouTube video of my untested spoon in my build thread. Take a look and compare to yours.

 

[wlockerb]

Thanks! I see what needs to be done now! So I can salvage some of what i have it just needs to be about half as deep and everything needs to be rounded and swept back to the intake grate. Looks like I have my work cut out for me tonight...

 

[Ranchero50]

Just remember that you need it too hook up while in the turns as well. Jets skid through the water during hard turns. Make sure you don't have any flow disturbing walls on the sides.

 

[wlockerb]

Got it. I've been thinking about how best to do this. I am thinking I might just take some leftover plywood and cut out the general shape i'm looking for. Something like this(I know its crude but it's the best I can do while at work):

 

[wlockerb]

So I couldn't stand not knowing and took it out as is. It is definitely an improvement! Honestly, if I didn't think I could get more speed out of it I would leave it. My plan is to take about an inch off the top to get rid of the straight/flat sides and shallow it up by about 2 inches by taking what's needed off the bottom. Then, with what's left over in between I can cut 2 pieces of plywood, one for each side, to round out my spoon on the perimeter and re-glass over it all. Shouldn't take that long with the exception of the fiberglass. Hopefully it comes out like I see it in my head and she's even faster than she was tonight. I have to say I'm pretty impressed with it though. Before I couldn't even come close to getting on plane by myself it would cavitate so bad. The only way to get to barely 1/3 throttle was to put somebody in front. Now, by myself, I can get on plan with no issues and it seems to perform great from what I can tell. Hopefully with the suggestions from you guys I can only improve! More updates to come. Saturday is coming fast though...

 

[wlockerb]

New and improved guys. Was going to use pieces of what I had but I think starting over worked out much better. Give me some feedback here guys so I know I'm not wasting more time. Should I add one more level to it or you guys think its fine as is?[attachment=-1]uploadfromtaptalk1400598083352.jpg[/attachment][attachment=-1]uploadfromtaptalk1400598104403.jpg[/attachment]

Now obviously I need to glass over it all but I am waiting to see if you guys think I need to add 1 more layer or not. I think it will be fine but I'm not the expert.

 

[wlockerb]

Here's one more from the front. Don't mind the model... [attachment=-1]uploadfromtaptalk1400598284819.jpg[/attachment]

 

[He Reigns]

Great to hear you are going in the right direction. If it were me, I would attach the new one temporarily as well to test it. If it works great than bolt it on. If it doesn't work, rip it off.