# Alumacraft Jet Jon Project



## CedarRiverScooter (Jan 9, 2015)

My project is coming along, so I want to share some photos. I still need to add lateral braces across the bench seat top. I am currently busy untangling all the wires & hoses. 

I welded in ribs to bolt the plastic hull to. I used Marine-Tex as a prmer on the aluminum side of glue joint, then 2 tubes of 3M5200 to glue it in. I was going the through-bolt it but have decided it is strong enough without them. (My friend is trying to take his apart & he can't break the 5200 free). 

I am going to have 2 seats, side by side, just in front of the engine. I will have a stick steer on center, so I can drive from either seat.

The big problem will be having to wait for open water next month when I get it done!


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## PSG-1 (Jan 9, 2015)

Nice work so far! Keep us posted. 

Also, a bit of FYI, don't use 5200 on anything you may have to remove at a later date, as you have already surmised from your friend's experience with the stuff! :mrgreen:


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## amk (Jan 12, 2015)

that a 14ft or 16.


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## RiverBottomOutdoors (Jan 12, 2015)

I want one of these so bad. Like a 12 footer with nothing but a seat and steering.


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## CedarRiverScooter (Jan 12, 2015)

It is a 1648. I will have up to 4 peeps onboard at times.

The suspense of how it will perform is high.


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## amk (Jan 19, 2015)

I have a 1648 alumacraft mod v and it works well I can fit four but I have a smaller size engine so it only goes 23 ish with that load.


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## amk (Feb 17, 2015)

What did you fill the gap between the motor skid and the aluminum where the ski hul curves up? I used foam and lots of 5200 didn't know wether there's a better way.


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## CedarRiverScooter (Feb 17, 2015)

AMK - If you look at the 2nd picture down, you can see that I added some 'strakes' to the jet ski hull. These are positioned so that I had about 1-1/2 inches of surface area for the 3M5200. There is no filler between that & the stringers I welded in. I was going to thru bolt the hull to the jet ski insert, but i decided it was strong enough as is.

My buddy (who got me started on all this) tried to remove the jet ski insert out of his hull & he couldn't bust the 5200 loose. He was going to have to saw it out. So apparently the 5200 is doing it's job!


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## airbornemike (Feb 17, 2015)

I'd love to know how this works with a glass boat, I would think blending the jet ski bottom would work well.


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## amk (Feb 17, 2015)

does the second picture from the top of just what you cut out of the jetski include the motor mounting part. I don't see any joined pieces in the later picture but it doesn't look as long in the 2nd.


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## CedarRiverScooter (Feb 17, 2015)

]Yes, the 2nd picture is what was cut out of jet ski, it is just upside down so you can see the strakes I added (which were just for glue surface).

Not sure what you men by 'joining pcs' but here is a snip of the 'stringers I welded in, they are what the jet ski insert was bolted down to:
[attachment=0]Capture 4.JPG[/attachment

If I was to do this again, I would make the stringers from angle aluminum. 1 pc would be glued to hull & welded at the ends to the cross ribs, the other upside down & welded to the 1st. Then you can match the taper of the jet ski insert without as much work as I went thru.

A guy could just poo rivet all this together & then have a shop do the welding (I would have been better off with a shop).

Airborne - The would work fine with a fiberglass hull. Most guys are using aluminum hulls because the light weight requires less draft. Not to forget gives more speed!


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## Flat_Bottum (Mar 1, 2015)

Very nice so far. I made the mistake of using expandable foam to fill my ski to hull gap and regret it so much. I have ripped most of it out and used 5200 and fiberglass filler to fill the void but considering ripping it all out and starting from scratch. Will be watching your progress. Good luck!


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## Flat_Bottum (Mar 5, 2015)

Looks like a fuji 650? The 5200 is awesome stuff, dont care what anyone says. Builds looking good, keep up the good progress.


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## CedarRiverScooter (Mar 13, 2015)

Getting close to the finish line!

Got it started, idled @6000 RPM; had to sync the carbs better. Backed it into the river but pump wasn't sealed right & took on water right away. That's OK because icebergs were still floating by. Not sure I want to be the Titanic on 6 O'clock news.

Here are some photos. Still have some appearance items to clean up.


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## Flat_Bottum (Mar 16, 2015)

Well done so far. I would rethink your thoughts on through bolting the ski to the boat hull. Figure all the thrust pushing the boat around and with the bumps and vibrations, I would add some hardward. I used 1/4-20 stainless carriage bolts to go from under the boat and used stainless poly nuts to synch it down. Done 8 of these under the engine mounting plate so you cant see them. Glad to see it coming along, hopfully mine will be ready for water testing in a few weeks. All the small things are killing me haha.


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## tigfisher (Mar 17, 2015)

looks solid


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## PSG-1 (Mar 17, 2015)

Nice work! I like the cowling, the design is similar to the one on my jet boat.


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## CedarRiverScooter (Mar 18, 2015)

Better picture of operator station & controls. Switches are recessed to (hopefully) prevent unintential actuation:


[attachment=0]

Keep thinking about improvements, I am going to brace those grab handles down to the floor somehow. (They are made from a a bicycle handlebar).

I have a complete reverse system coming my way that I scooped up for $90.


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## Abraham (Mar 18, 2015)

Looks damn good. What kind of mph you expecting?


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## CedarRiverScooter (Mar 18, 2015)

Shakedown cruise scheduled for Saturday afternoon. Up until this point it's only run on the thrailer.

I got cold feet (sorry for the pun) last weekend with an engine miss, 35 degree water with icebergs floating by, & wicked current. Thought I'd wait a week to let mother nature mellow out.


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## CedarRiverScooter (Mar 21, 2015)

Shakedown cruise completed!

Results:

Carbs still not right, but at least not too lean so not damaging pistons, Low priority.

Getting cavitation, but could work through it. Going to have to add spoon. Will be looking at PSG's posts for sure. Medium priority.

Hate the fore-aft stick steering. Ran it for 15 minutes & never got used to it, have to think about which way it will go. Not safe IMO.
Redesign pending. Highest priority.

Didn't get a top speed due to carb issues, but got to high 20's. It was really fast enough already, so I'm happy with the speed, esp since there was no bow rise at all (not used to that).

The big PIA (such as it is) is that every fisherman that came to the ramp while I was there wanted to check it out. Got a bunch of compliments already. What a cross to bear 

Later guys . . .


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## PSG-1 (Mar 22, 2015)

CedarRiverScooter said:


> Shakedown cruise completed!
> 
> 
> Getting cavitation, but could work through it. Going to have to add spoon. Will be looking at PSG's posts for sure. Medium priority.



My boat doesn't have a spoon. With a 12 degree deadrise, I got lucky, with minimal cavitation. For building a spoon, Ranchero50 is the guy to consult for this.


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## amk (Mar 25, 2015)

IM actually thinking of going to a stick set up like yours and ditching my center console where did you get the chairs and stick controls from? 

Its probably unnatural to you now but you will get used to it. Airboats are all stick stear and they do alright its just a learning curve.


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## CedarRiverScooter (Mar 28, 2015)

Update:

Prototype spoon. Made from plywood pancaked up 3 levels, wrapped in vinyl & injected with canned foam. Deck screws holding it on with some goop adhesive. It just has to last for 15 minutes of testing.

Another change is that I reversed my stick steer action. I had it working opposite natural body english & I think that is why it was difficult to use. Now it steers the same way you turn to look. Hoping for the best there.


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## Ranchero50 (Mar 29, 2015)

Definitely needs seat belts.


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## amk (Mar 30, 2015)

https://www.ebay.com/bhp/seadoo-intake-grate

probably all you need I have one on my smaller jet boat works great there is an after market one for every ski made most likely


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## Tinny's Dad (Mar 30, 2015)

Love the boat, but also jealous of that workshop, looks great


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## Ranchero50 (Mar 30, 2015)

Did you change your pics?

I don't think the above spoon is going to perform very well? It's awfully abrupt at both ends and doesn't seem to have a smooth transition to the hull.

Here's a pic of mine for reference. I might be 1/2" deeper than the original keel strake. I welded a 1" x .125" length of angle from one end to the other on my keel and the spoon just barely covers it.


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## CedarRiverScooter (Mar 31, 2015)

Update - Tested the prototype spoon today, no good. Cavitates worse than before. I peeled off 2 of the 3 layers of plywood & not much improvement. 

Ranchero, I am sure you are right about the abrupt transitions. It is not easy to feather it in with the glue-on approach I used. If I had a Phd in hydrodynamics I would know the exact shape to cast in fiberglass, but the trial & error approach could be time consuming.

My new grate will be here next week, got my fingers crossed that it works.

In the meantime, I think I will need to change pop-off springs in the carbs. Not getting anywhere close to running right.

I think I need to have the impeller repitched too.

Got to do some bat-turns today, woohoo! It's going to be a blast once I work out the bugs.


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## dimecovers3 (Mar 31, 2015)

That is by far the most stone cold cool jet build ion planet Earth.


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## Flat_Bottum (Apr 2, 2015)

Well done on gettin her out on the water. You should be every bit of high 30's in your boat once you get it running right. If you dont mind me asking, what is the boat doing that you think the pop off is wrong? The new grate should help you out a lot, I know that it was horrid in my boat on its first run but once I added the little scoop to my grate, it helped out a lot. I lost a few MPH's but hook up was much much better. That was with my intake even with the boats bottom though, so I am hoping that by sinking it down (intake about 1.5-2" below boat bottom now) I can run a regular grate. Keep up the good work!


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## CedarRiverScooter (Apr 2, 2015)

The carb problem I have is that the throttle response is very flat (no response from throttle untill you hit 3/4 & then it jumps up). I have tried all combinations of lo/hi speed adjuster settings & I can't tune it out.

I have read different Greenhulk posts & also mikuni manual about this & decided to try different pop off springs (they are only $1.80 each from iboats). 

I am also going to repitch the impeller lower to handle the additional load, I think I'll do that first, maybe that is part of the problem.

Another upgrade I decided on is on the thru hull bearing. The one that came with the ski was the wrong part (maybe is a yammi or kaw) & the bushing was loose to boot. I got the repair kit but the bushing wouldn't fit. So I went to bearing store & got a bronze oilite bushing & turned it to fit. Locktited it & pressed it in, it is there to stay. Fast forward, I must have the clearance too tight because it is getting hot.

So instead of continuing on that path I bought this from watcon, should be good now:

https://www.watcon.com/jet-pump-drive-line/drive-shaft-bearing-carrier/drive-shaft-bearing-carrier-1


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## Flat_Bottum (Apr 2, 2015)

Well I hope you get it figured all out. Are you running a triple pump or the factory single? Also, what year of ski did your 750 come out of?


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## ColtonS (Apr 2, 2015)

If your carbs have not been rebuilt in the recent past I would HIGHLY suggest doing so and upgrading to a triple outlet fuel pump if you haven't already. Before you do all of that I would check each cylinders compression or pull the head caps off and take a visual look because the lack fuel to the front carb is known for burning the piston up in these motors.


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## CedarRiverScooter (Apr 2, 2015)

Hi Colton - on top of that. Have triple FP & just rebuilt carbs myself (that may be the problem). I have been monitoring piston wash after every run - it looks to be too rich. I have higher PSI pop off springs coming, which is supposed to lean it out at midrange.


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## ColtonS (Apr 3, 2015)

If you put all new parts (including pop-off springs) in the carbs when you rebuilt them it should not run poorly.

I would double check to make sure everything is within spec...fuel mixture/pop-off/return line pressure/jets/carb timing/needle and seat size.

Once I knew all of this is correct I would then question my reeds.


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## CedarRiverScooter (Apr 3, 2015)

Thanks for the advice Colton. I'll open thrm bwck up. Got to admit I didn't measure pop off pressure (but did put new black springs in). The return restrictor orifice is in place. One thing I have been wondering about - I have the gas tank open vented to atmosphere (out the stern). I didn't reuse the in-out check valves. I wouldn't think this is the problem, but you never know.

Are you running the stock impeller on yours? I am thinking I should repitch my impeller down. When it did hook up & go hard I wasn't getting anywhere close to 6800 RPM, more like 5800.

I have been readingf the how-to on DIY repitching, would like to give it a shot.


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## Ranchero50 (Apr 3, 2015)

I would run what you have for a season before playing with the pitch. You have no idea how it's going to run once you get some stuff dialed in and load the boat down with toys.

You also need to get the carbs working correctly before something dumb happens. #-o


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## Flat_Bottum (Apr 3, 2015)

Open venting your tank shouldn't matter, my tank is vented to atmosphere without any issues. One thing to check, and I know this sounds dumb, but check your spark plug wires to make sure they are all 3 securely connected to their boots. When I first dropped my 750 into my boat and fired it up, it would start and idle but just didn't seem to be right. Come to find out, the PTO cylinder plug wire had just barely come loose inside its boot. Once I screwed the boot back on better and put on a new tie, it was a night and day difference.


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## CedarRiverScooter (Apr 4, 2015)

Got the top loader grate today so just had to try it out (1 screw install).

1/2 way there. Got it to hook up well when going into the chop, but still cavitates going with the chop.

Engine is topping out at 5000 RPM, so I think I need the SLT impeller (lower pitch). With the good hookup I can nail the throttle so the mid range carb problem was not relevant. Piston wash says I'm rich so not melting any pistons.

I stared at the pump intake for a good 10 minutes & finally decided to try a 4 foot long spoon, made from treated 1x4. It will terminate at the junction of aluminum & ski hull (well before the intake). 1x4 will be on on each side of the center rib (tapered to fit tight) will be about same width as the intake. It will have the granddaddy of exit taper to match the flow of the hull. Hopefully this will direct the air outboard.

If I could start over I would plant the jet ski hull deeper & use it's keel as the spoon. It could have been so simple . . .


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## amk (Apr 4, 2015)

I don't know what size chop your hitting but your going to cavitate in chop due to skipping across waves. I cavitate in the bay with white caps and even rough rapids just due to going 40 mph in a 16ft boat with a 100 lb 90 hp jetski motor in it. If your running smooth in smooth water your good.


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## CedarRiverScooter (Apr 4, 2015)

Chop was 2 inch tall at most. You would probably have called it smooth water.


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## ColtonS (Apr 7, 2015)

CedarRiverScooter said:


> Thanks for the advice Colton. I'll open thrm bwck up. Got to admit I didn't measure pop off pressure (but did put new black springs in). The return restrictor orifice is in place. One thing I have been wondering about - I have the gas tank open vented to atmosphere (out the stern). I didn't reuse the in-out check valves. I wouldn't think this is the problem, but you never know.
> 
> Are you running the stock impeller on yours? I am thinking I should repitch my impeller down. When it did hook up & go hard I wasn't getting anywhere close to 6800 RPM, more like 5800.
> 
> I have been readingf the how-to on DIY repitching, would like to give it a shot.




Yes I have been running the stock impeller with absolutely zero issues, loads of bottom end and a smooth powerband through the mid and top end. 

I'm pretty sure the factory polaris impellers are variable pitch so re-pitching would probably be very difficult. I believe you would have to bend each blade instead of just cutting off and re-welding at a different angle like a traditional impeller or prop.

Hows the condition of your reeds?


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## Flat_Bottum (Apr 7, 2015)

I know that I could not stop cavitation in my boat with how I had it set up before (intake dead even flat with the bottom of the boat). I added a little scoop fin to the rear of my factory grate and it helped out a lot but still, like you, anything with a ripple in the water would cause cavitation. When I redid my boat for this build, I molded the boats hull to the contour of the ski hull and that sat the intake down almost 2" further into the water...which I am hoping will solve those issues. If you are set on adding a spoon, I would fab one from aluminum and weld it on. Anything else will end up peeling off the hull.


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## CedarRiverScooter (Apr 11, 2015)

Well, it's good to know that there will always be some cavitation in heavy chop. I may have set my bar too high.

I am on hull revision # 9 or ten, lost count.

Here is my latest concept (just a prototype).




More of a spork than a spoon.

The foam is just glued on with silicone seal. Believe it or not, it has survived 1/2 hour of running at high speed.

It works great in flat water, & pretty good in today's 2 inch chop. The worst of it is cavitation in hi speed port turns, but not so bad when turning starboard. Must have something to do with the rotation of the impeller.

I am going to proceed with a permanent version; will be treated lumber core with epoxy fiberglass skin. Glue & screw mounting. 

The other functions are pretty well fixed. She is running 6300 WOT, got the carb problem fixed (it was my bad on a pop off spring placement). 

Thanks guys, for all of your help on this.


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## Flat_Bottum (Apr 11, 2015)

Im glad you got her smoothed out. I have been thinking of getting a loader grate but not sure yet. Will see how my factory one works now that the ski hull is sank deeper into the boat. Keep up the good progress.


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## Ranchero50 (Apr 12, 2015)

CedarRiverScooter said:


> It works great in flat water, & pretty good in today's 2 inch chop. The worst of it is cavitation in hi speed port turns, but not so bad when turning starboard. *Must have something to do with the rotation of the impeller.*



Hull balance (or form) is the issue as the impeller doesn't induce torque like a prop does. Good to hear you are making progress.


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## CedarRiverScooter (Apr 12, 2015)

You nailed it! I am running solo on starboard side of boat, so port side would pick up air easier.


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## Ranchero50 (Apr 12, 2015)

That's one of the reasons I went for and aft seating. Figure the people are the heaviest items in the hull and have the most influence on it's balance. Besides, I can use the passenger as a bug catcher.


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## Flat_Bottum (Apr 15, 2015)

One of the reasons I have centered everything in my boat. I will be runnin solo most of the time so the 16g fuel cell, steering and engine are all dead center of the boat. Hows your spoon or spork commin?


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## CedarRiverScooter (Apr 16, 2015)

1st coat of glass went on today. I decided to build this in layers , as I normally won't be out in heavy chop, so may not need anymore height than this. This 1st coat is just to get the pcs stuck together. It will get 2 or 3 per side. Additionsl trimming & tapering is next. Leading edge is a bit crude but I will pack it well with glass. It was hard to miter 3 ways on same board with skil saw . . . I have the 2nd layer of boards cut, but hope not to need them.


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## Ranchero50 (Apr 17, 2015)

40 grit on a belt sander and then similar on a DA will make quick work of the rough form. Just try to avoid the metal as 40 will eat through the hull pretty darned quickly. Smooth transitions are the key. You want the water to follow the curve of the spoon, not break away which causes cavitation.

Oh yeah, when I did add on bodywork I'd use masking tape on the base material and then a layer of fiberglass before any structure. Pic is the prep work to fit the Boss 429 scoop to the hood on my F-350. The tape allowed the fiberglass to fit the hood exactly and made final sanding and fitment much easier vs. trimming a mass of glass to fit the hood. You can use this method to fit your spoon to the hull easier.


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## CedarRiverScooter (Apr 17, 2015)

Thanks for the tip but I'm working overhead. Just need to get it within .100 so the urethane sealant can fill the gaps.

Edit - I am using thin epoxy, so I think it would just run out of the gap. I'm pretty close already, it just has a high spot in the middle I need to grind down. 

I think it will work well, or at least good enough.

Enough boat building, I'm ready to do some fishing!


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## bdeemo (Apr 22, 2015)

Make sure you don't have pressure treated wood touching your bare aluminum, it will eat up the aluminum believe it or not.


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## CedarRiverScooter (Apr 22, 2015)

Yep, got that covered (in fiberglass)  

Still whitling pcs & glassing them, it's like a jigsaw puzzle.

Should be assembled this weekend, but will let the urethane sealant cure for a few days before tryout.

My reverse system parts are supposed to get here tomorrow, so that will be a good diversion while I wait out the cure time.


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## bdeemo (Apr 22, 2015)

CedarRiverScooter said:


> Yep, got that covered (in fiberglass)
> 
> Still whitling pcs & glassing them, it's like a jigsaw puzzle.
> 
> ...



I'm building a similar boat right now. I need to make a thread on here(didn't know there was so many jet jon conversions)
Getting lots of lessons from your guys' threads on here, (I think you gave me the idea of removing the stock bilges and feeding them to outside the hull) I'm doing a 2005 1448 grizzly tracker jon with a 95 seadoo xp 720 rotax in it. I'm near the end of my build. About ready to run it for the first time this weekend.


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## CedarRiverScooter (Apr 22, 2015)

Bd - Looking fwd to seeing pic of your ship. Post it here if you don't want to start a separate thread. Sounds like it will fly. I don't know how fast mine is going (speedo inop), but it was fast enough to feel uncomfortable! I don't have a console to hide behind.


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## CedarRiverScooter (May 1, 2015)

Status update:

Got the composite hydrofoil mounted & went for a couple rides.







Still no home run, getting more cavition than I would like. I am going to add another layer of wood on the leading edge. Will just screw that to the base layer, at least for now (had enough of the fiberglass routine for awhile!).

I took wifey & doggie out & it works much better with the extra weight. Reduced tendency to cavitate about 50%, I would guess.

Wifey liked driving it, was a good thing to hear.

An unintended consequence - Now I am getting much more vibration transmitted into the hull. I can feel engine vibration in floor & up thru the seat. At 1st I thought my engine was running rougher but I now think the wood panels are coupling the ski insert to the alum hull with much more rigidity. A good thing I guess. 

This is a long term hobby for sure!


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## amk (May 1, 2015)

I'm surprised your having this much trouble with it. Are you sure your prop is ok and pump housing all that is ok? I've done two of these boats and had zero cavitation issues and I didn't need a scoop or loader gate. Starting to think the drive train I selected has a lot to do with that.


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## CedarRiverScooter (May 1, 2015)

It hooks up great in flat water, so I don't think it is seals or pump. Factors such as chop, wakes, porposing, even just running straight into the wind will feed it air. I think part of the problem is I tried to keep the boat light, (reference that it works better with 2 people).

It is fine below 25 mph, it is at top end where it starts to burp. Holeshot is like a rocket.

I'll get it eventually!


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## CedarRiverScooter (May 1, 2015)

It hooks up great in flat water, so I don't think it is seals or pump. Factors such as chop, wakes, porposing, even just running straight into the wind will feed it air. I think part of the problem is I tried to keep the boat light, (reference that it works better with 2 people).

It is fine below 25 mph, it is at top end where it starts to burp. Holeshot is like a rocket.

I'll get it eventually! Maybe just add ballast.


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## Capt1972 (May 2, 2015)

Quick question....... Do you think if the ski hull was dropped deeper below the jon hull it would help? The reason I ask is I am starting my build and I am only doing the work once, hopefully. I always hunt big water and there is ALWAYS a wind chop. I think buy setting the ski hull a little deeper (like 2-3" below the boat hull) it will keep the pump hooked up. Thoughts anyone?


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## PSG-1 (May 2, 2015)

I think setting it lower would definitely help with cavitation. It will help to displace the air, and smooth the flow of water to the pump intake. 

Knowing what I know now.....if I were to build my boat again, I would do it with a delta-hull configuration.


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## CedarRiverScooter (May 2, 2015)

Yes, set intake as low as practical.

Today I cut the outing short because it was not any fun feathering the throttle constantly.

I sat all the way in front (while my son drove) & it helped only a little. I thought maybe holding the bow down would benefit.

If my next layer of spoon doesn't help, I am thinking of getting long pc of .125 alum sheet, forming a deadrise, & welding it on center. 

I have way too much time invested in this to not have it work well.

PSG, what is a delta hull?


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## PSG-1 (May 2, 2015)

The simplest explanation of a delta hull.....the design is similar to a tunnel hull modification, only, the exact opposite.

Instead of setting it up higher, you're dropping it lower.


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## CedarRiverScooter (May 2, 2015)

Thanks, that concept is where my thoughts were heading!

I suppose I've been bull headed about installing a welded alum spoon. Maybe that will be next
My buddy got a new welder, could be a case of beer project.


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## PSG-1 (May 2, 2015)

The delta hull is a bit different than a spoon. The delta hull involves cutting the center section out of the boat, for about 4 feet, dropping it about 3" at the transom, and tapering back to flush at the front end. The entire jet pump and engine is then set down into this.


When welding, use ER5356 wire, as it is marine grade. Do NOT use 4000-series (4043) as it will lead to corrosion. 4043 welds without all the soot and smoke, and makes a nice clean bead, but it will not last in water.

Immediately before welding, be sure you thoroughly clean all edges using a stainless steel wire brush.


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## CedarRiverScooter (May 3, 2015)

Sounds like that would be the ultimate solution. I remember having the 'most shallow' draft as my main goal when I started, doesn't seem very important now. Could you just leave the hull center solid, & weld the delta panel over it (except where the pump goes of course)? That is what I though of for a deadrise panel. Then taper at front to blend in with the bow.

A seconday benefit of a drop down center would be better tracking (I would think). The slide is kinda fun but have to slow down in tight corners.

I used 5356 in my welding because I have a old millermatic, no spool gun, & it wouldn't push 4043. 5356 would push MOST of the time!


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## PSG-1 (May 3, 2015)

Yep, 5356 is a harder wire, less prone to bird-nesting. Not much of an issue with spool guns, but with anything else, soft wire like 4043 can be a real b!+ch. 

As for the delta pad, you could probably just set the jet unit lower, provided that it will still line up with the engine. You can't run with any kind of offset. Even with a spartan jaw coupler, it still needs to be within .020" of square, or it will wear out bearings.

But if you do set it lower, you will definitely have better tracking, and will eliminate a large percentage of your cavitation.


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## Ranchero50 (May 3, 2015)

I still think 90% of your problem is your front edge of the spoon. I think you really need to smooth that transition out a lot flatter. In the pic it looks like the front and side edges have an abrupt wall. That wall will cause turbulence that I suspect the turbulence is capturing the air in the water column that feeds the pump vs. allow it to travel around the inlet. Your spoon is just a very smooth transition into clean water so the hydrodynamic forces will draw the clean water into the pump. I suspect your design is plowing through the water vs. riding through it. Hard to describe.

Just remember, abrupt edges cause shear which leads to cavitation. Cavitation will be amplified by any air in the water stream. I suspect on flat water you still have some cavitation, just not enough to unload the impeller.


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## CedarRiverScooter (May 3, 2015)

Ranchero - Thanks for that tip. My guess is that you have a fluidics degree from the school of hard knocks! 

Yeah, I was thinking I to separate the air to the sides, that is why I set the angle at 45 degrees (not to mention that is as far as my skil saw tilts). I'll don some PPE & grind her down to maybe a 10 degree leading edge. There is plenty of material there. Won't cost anything but some watts & a new grinding disk.


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## Ranchero50 (May 4, 2015)

Yeah, picked up some good books in a used book store over the years. 'Fluid dynamics' from the '50's is great for understanding stuff like this. I typically read deep enough to understand the subject without going through the math. I don't have a way to measure the forces involved so I need to understand the dynamics and results along with the data I can figure.

45mph is 66 feet per second. Water isn't compressible but air is. Cavitation happens during pressure drops when compressed air intrained in the pressurized fluid reduces pressure suddenly. Your abrupt wall is creating a pressure spike and the resulting reduction of pressure helps bring air bubbles into the water stream going into the impeller. You always want very smooth transitions


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## CedarRiverScooter (May 9, 2015)

Ranchero - you were right as rain. I feathered out the leading edge & cavitation is mostly gone. Wasn't very rough yesterday but it didn't burp at all when I crossed some wakes.

Finally remembered the GPS - topping out at 35 MPH with 2 people on board.

Thanks everyone for your advice - I could not have completed this project without the help!


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## Ranchero50 (May 9, 2015)

Cool. That looks really good. Now if you need to tune it you can just add some tape to build up the 'depth'. Was that speed with the scoop or without?


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## CedarRiverScooter (May 9, 2015)

The 35 MPH is with the spoon, going into 10 MPH headwind. I'm happy with the performance.

I have a small leak to chase down (only leaks during running, that makes it elusive) & then I am done boat building - till something breaks!

Time to go fishing.


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## badrider77 (Oct 14, 2016)

HI cedar, badrider77 here. You commented on my deep set hull and I want to report in. Unfortunately I'm disappointed at how much cavitation I get in the chop. I still plan on fiberglassing to smoothing out the bottom and help seal it up. Does have a slight leak that I think is coming from the main keel. Want to try a top loader but scared to death of sucking up a rock.

I also think using urethane may not have been the best choice to seal it in. It is very messy and thick. So I ran beads but should have knocked them down (didn't realise how much it raised the hull from the boat). But still was able to bolt it down.

The good however is on the smooth water it's run's excellent! My first time out was on a lake with my two boys. Had the whole family of 5 (me included) plus the dog on the river and it was a beast! Later had my oldest son and myself on the river. At WOT it did porpoise.

Do you think a top loader with some guarding might work? Would love to make a grate like psg.
Thanks!


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## SC Designs (Oct 14, 2016)

badrider77 said:


> HI cedar, badrider77 here. You commented on my deep set hull and I want to report in. Unfortunately I'm disappointed at how much cavitation I get in the chop. I still plan on fiberglassing to smoothing out the bottom and help seal it up. Does have a slight leak that I think is coming from the main keel. Want to try a top loader but scared to death of sucking up a rock.
> 
> I also think using urethane may not have been the best choice to seal it in. It is very messy and thick. So I ran beads but should have knocked them down (didn't realise how much it raised the hull from the boat). But still was able to bolt it down.
> 
> ...



Hi there badrider77, I have built three Jetjon's now and I may can help a little from my past experiences. The main way to get rid of cavitation is to build or form a spoon design in front of the pump. The problem with these flat bottom boats is that there is no "VEE" in the hull to separate the air out. As far as the Top Loader grate, they do help but will not eliminate the cavitation completely. I ran a Top loader grate on my first build. I was pleased with it but it does have its draw backs. The fact that you could suck up larger objects that could damage your impeller is possible. It really depends on where and what kind of waters you run in. It wasn't a big concern for me because where I boat is mostly a sandy bottom. I recommend doing the fiberglass. I did on all 3 builds, it does help on leaks but it really cuts down on the drag. Make sure to use a poly resin, It is best for metals. I have heard horror story's of people that used epoxy resin and it separating. Hope this info helps. 

SC


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## CedarRiverScooter (Oct 14, 2016)

+1 on SC's comments. I had a toploader last year & accidently rn up on a gravel bed, had rocks stuck between the impeller blades before I could shut down. I changed back to OEM grate, there is a reason they have the fins narrow on those!

See threads by Rancero50, he said to have really gentle angles on the spoon. Mine is about 4 ft long, essentially creates a semi vee when on plane.

I think it is common for jet jon builders to make a revision after 1st season, there is a lot of black art in the project!


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## badrider77 (Oct 15, 2016)

Thanks for the advice fellas. When I built mine it has somewhat of a spoon from where I hammered the bottom of the boat to fit hull. I was looking at it today and the spoon is lower than the leading edge of the intake. I think that will need some work. Plus my boat is a 2052 so I'm not sure if the length has an affect on the cavitation. I'll post some pics of the spoon and would be open to any feedback.


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## badrider77 (Oct 16, 2016)

Tonight I removed the screws I had to hold the edge of the boat when it was cut. They where right in front of the intake but they were somewhat in front of the grates of the intake. Used the urethane to seal the holes off and around the rest of the screw underneath. Gonna test tomorrow! This photo is before the screws were removed.


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## CedarRiverScooter (Oct 17, 2016)

You may have too much of a step between aluminum hull & jet ski insert. Maybe you could use some bondo to feather it out (just as a test). It looks like the center rib could be tapered out more gradually too.


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## badrider77 (Oct 18, 2016)

Tested yesterday and still no good. Used some more urethane to smooth out the step in front of the intake. Going to try and massage the bottom up some and then try to taper the the spoon to a better front curve. Probably going to remove about a foot of the main keel and smooth out the metal break in the center. You said your spoon was about 4' long right?


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## CedarRiverScooter (Oct 18, 2016)

There are pictures earlier in this thread which are a good example of what doesn't work :? 

Here is a photo of what I did to fix it. I used the dropout aluminum fron when I cut the tunnel hole. It is working very well, I have run in 3 inch chop at speed with no cavitation. Before this it would cavitate with ripples.

Good luck, let us know what you end up with.


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## badrider77 (Oct 18, 2016)

Nice work! That looks very nice. I'm a little embarrassed to post my pics...kinda feel like a hack; guess I am. Oh well, it's my first time! Tonight I removed more of the center section where the boat was broke at the factory. As you can see it opened it up (don't mind the hammer marks). Test tomorrow. I'll report in about this time.


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## badrider77 (Oct 18, 2016)

Not sure what happened to the upload


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## badrider77 (Oct 19, 2016)

Happy to report that it's heading in the right direction. Was able to run through the bumps with no problem. 3" chop At 1/2 throttle. Any bigger was 1/4 throttle or less. Was able to turn sharper without cavitation. 
Still considering the top loader and building up the front of the spoon. I asked my son to record a video for me...thought youth was good with technology. Didnt get anything but rocks!


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