# Jetjon motor location?



## bnt5 (May 14, 2014)

For those that have experience with jetjons, I am trying to plan my motor/pump placement in the boat. Is it better to have the ride plate outside of the hull as far back as possible? I can build an aluminum tunnel that encapsulates the nozzle for protection and allows the ride plate to be bolted to the bottom flange of said tunnel. I have seen a few builds where they push the ride plate back as far as possible so maybe that helps with the ride..... :?: 

Oh, btw I am building the boat myself from some online boat plans in order to mimic the bottom contour of the jetski in the hopes this prevents cavitation and gives a better ride. We shall see, I'll probably end up wanting a 180hp 4-stroke motor by the time I'm done.LOL

Thanks
Bill


----------



## He Reigns (May 14, 2014)

Bill, I have the same question!!! I just don't know enough about fluid dynamics. I wanted to move my motor and pump as far back as possible for more room in the boat but was not sure if that would mess up the pump for cavitation, porpiosing or ? I'm all ears on this one.

I was going to make a video tonight showing how its coming together so far.


----------



## bnt5 (May 14, 2014)

The way I look at it, the jetski has the ride plate in the very back of the ski between two sponsons with a fiberglass deck over. The rear of the boat is never out of the water so I would think that is where we want to position the intake, as far back as possible. Since I am actually building my boat from scratch (wood with fiberglass) I can move the motor/intake/ride plate wherever it becomes the most responsive, to me that means as far back as possible. If you look at the jet boats they use for racing or even the alaskan skinny water boats, the pump unit is actually sticking way out like a turtles head...and those boats runs great. Maybe someone else can chime in here to shed some light on this.


----------



## He Reigns (May 14, 2014)

One of my limiting factors is If I decide to keep it stock, is my exhaust. It exits the hull on the side of the pump box housing. If I decide to redo the exhaust I can move the pump and motor back another 6 inches or so.


----------



## bnt5 (May 14, 2014)

what ski are you using for a donor? Mine has the waterbox and exhaust hose running out the aft end...I am not sure how to get rid of the muffler waterbox...or even if you can? I was planning on using the engine layout as a whole, considering Kawasaki had engineers design it and it seems to work correctly. I don't remember what PSG-1 and ranchero did on theirs but I don't recall seeing a waterbox, now that I mention it...... :shock:


----------



## He Reigns (May 14, 2014)

Mine is a 1994 Polaris SLT 750, I'll make a vid in a bit and post to YouTube for my build thread. Then you can see what I'm up against.


----------



## PSG-1 (May 15, 2014)

Mine has the factory Yamaha water box:




It's a tight fit, needless to say. When I first bought the engine to swap out for the old 2 stroke, I really had some concerns if the 4 stroke was even going to fit. Indeed, it was taller, so, we had to make a different cowling, which, honestly looks better than the old one anyhow. 
Also, because of the different exhaust configuration, I had to move the port side fuel tank forward, to make room for the water box to go in that area.

We barely had enough longitudinal clearance for the engine, in fact, I had to cut the tubing back on the pump housing virtually all the way back to the bearing area, as well as cutting the drive shaft shorter, to be able to set the FXHO engine in the same space once occupied by the 2 stroke engine.

Anyhow, as for placement of pump, generally, you want it so that the only part extending beyond the transom is the thrust nozzle, and the steering nozzle. You will need about 3-4 ft of longitudinal space for the engine and pump, maybe a little more if you use a 4 stroke.

If you have issues with porpoising, you can correct it with a negative degree thrust nozzle, like a -2 or -5. You can also weld a tab across the back edge of the ride plate with a slight downward angle. Or, you can make your own ride plate from 1/4" material like I did, and simply bend the last inch at a downward pitch, to achieve the same result as welding the tab.


----------



## He Reigns (May 15, 2014)

Thrust nozzle and steering nozzle behind the transom, pump output parallel to the bottom of hull. Awesome information!!! Anyone have input on the height of the pump? Does the pump need to be submerged at a static float?


----------



## PSG-1 (May 15, 2014)

For the most part, your pump will be completely submerged at static flow. The Yamaha XL1200 pump is about 8" in diameter, so, that should give you an idea for the inset needed. Remember that when you are on plane, virtually none of the pump is in the water, it is just skimming the surface.

However, if you are fabricating an aluminum tunnel for the pump to bolt into, I would recommend making your tunnel a little wider, and taller than your pump, to give you enough room for all your hoses and cables, as well as routing your exhaust into the tunnel. Routing the exhaust into the tunnel, and then using some rubber skirting trimmed to fit around the nozzle (take a look at a Yamaha 4 stroke wave runner to see an example) will tremendously cut down on the sound signature of your boat.


----------



## bnt5 (May 16, 2014)

PSG-1, since you have some experience in this department, what would be the ideal hull bottom design for a jetjon? Obviously a flat bottom has cavitation issues, a deep vee has shallow water limitations, so I am guessing somewhere in the middle? What would your ideal hull look like if your could do it all over again?


----------



## PSG-1 (May 17, 2014)

Ideally, I would want one of the newer johnboat hull designs (I think Xpress is who makes it) Anyhow, that design has a modified V, as well as a flat in the center, and that flat area is just about the perfect width for a pump. 

My boat is a 12 degree deadrise, so, it's not a completely flat bottom. For most of my average running, I do not have issues with cavitation. Only time I get into it really bad is when I am in heavy chop or crossing boat wakes. On inland rivers, I never cavitate unless the pump has sucked up some leaves or other debris. But here in the wider expanses of coastal waters that can get rough, sometimes that jet drive can have its limitations. 

You can minimize it somewhat by using a Wet Wolf AAT nozzle, and try experimenting with different size shuttles. I have one on my boat,it does help. Also, you can run a top loader intake grate, this will feed more water to the top radius of the intake duct/wear ring area, knocking out some of the air, which helps cut down on cavitation. 

Only problem with a top loader is that it's a wide open grate, which leaves the pump susceptible to ingesting grass, leaves, and even rocks. On a PWC, it's not a problem. But if you get something jammed in the intake on a boat, especially in the middle of winter, you are gonna be really pissed! I ran a top loader on my boat for a while, but then decided it was more important to be able to clear grass, etc from my intake, so, I went to a stomp grate, and did away with the top loader.

Another option is to fashion a "spoon" and install it forward of the intake. The spoon will knock out the aeration in the water going to the pump, and will eliminate cavitation. Rachero50 did this with his boat, as have a couple of others here.


----------

