Alumacraft Jet Jon Project

[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! Maybe just add ballast.

 

[Capt1972]

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?

 

[PSG-1]

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.

 

[CedarRiverScooter]

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?

 

[PSG-1]

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.

 

[CedarRiverScooter]

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.

 

[PSG-1]

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.

 

[CedarRiverScooter]

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!

 

[PSG-1]

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.

 

[Ranchero50]

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.

 

[CedarRiverScooter]

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.

 

[Ranchero50]

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

 

[CedarRiverScooter]

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!

 

[Ranchero50]

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?

 

[CedarRiverScooter]

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.

 

[badrider77]

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!

 

[SC Designs]

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!

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

 

[CedarRiverScooter]

+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!

 

[badrider77]

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.

 

[badrider77]

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.