Alumacraft Jet Jon Project

[amk]

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.

 

[CedarRiverScooter]

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

 

[ColtonS]

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?

 

[Flat_Bottum]

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.

 

[CedarRiverScooter]

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.

 

[Flat_Bottum]

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.

 

[Ranchero50]

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.

 

[CedarRiverScooter]

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

 

[Ranchero50]

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.

 

[Flat_Bottum]

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?

 

[CedarRiverScooter]

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.

 

[Ranchero50]

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.

 

[CedarRiverScooter]

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!

 

[bdeemo]

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

 

[CedarRiverScooter]

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.

 

[bdeemo]

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.

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.

 

[CedarRiverScooter]

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.

 

[CedarRiverScooter]

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!

 

[amk]

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.

 

[CedarRiverScooter]

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!