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big jet boat
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Ranchero50
Well-known member
Honestly, I think you are going to have way too much power and complexity (duals) for a 16 footer.
I'm 16'-6" x 48" with a 85hp Seadoo 717 and I run 45 with near jet ski maneuverability. I had to build a spoon to get the pump to load correctly. Read my build thread, it explains a lot of why I did what I did. PSG-1's build is similar. These boats are not terribly difficult to build but take a good bit of thought to get to run correctly.
I think a shorter fatter boat with twin power is going to handle poorly and have some stability issues.
Good luck and keep us informed of your progress.
I'm 16'-6" x 48" with a 85hp Seadoo 717 and I run 45 with near jet ski maneuverability. I had to build a spoon to get the pump to load correctly. Read my build thread, it explains a lot of why I did what I did. PSG-1's build is similar. These boats are not terribly difficult to build but take a good bit of thought to get to run correctly.
I think a shorter fatter boat with twin power is going to handle poorly and have some stability issues.
Good luck and keep us informed of your progress.
PSG-1
Well-known member
Not necessarily, Ranchero.
My Sea Doo Speedster was 14 feet long, and it had a 7 ft beam (measured at the top) It had twin 717 Rotax Sea Doo engines, and ran over 50 MPH. It had good handling, but then again, it was a different type of hull design than a johnboat.
The issue I see with using 2 engines is that it's twice as much work! Also, depending on the existing hull design, it may require a seperate spoon for each intake, or possibly one big spoon across the entire hull, I'm not sure.
But if built properly, (and by properly, I mean, being able to feed water to both pumps without cavitation) this boat should perform as well as our jetboats. Steering will actually be MORE responsive, because, in addition to being able to turn the nozzles to steer, you can back off the throttle on one engine, and gun the other engine, to make it whip around faster....i.e. making a right turn, you back off the starboard throttle, and gun the port throttle, and vice-versa. (I used to love taking my Sea Doo through some of the serpentine channels in the marsh, yanking and banking like that)
My Sea Doo Speedster was 14 feet long, and it had a 7 ft beam (measured at the top) It had twin 717 Rotax Sea Doo engines, and ran over 50 MPH. It had good handling, but then again, it was a different type of hull design than a johnboat.
The issue I see with using 2 engines is that it's twice as much work! Also, depending on the existing hull design, it may require a seperate spoon for each intake, or possibly one big spoon across the entire hull, I'm not sure.
But if built properly, (and by properly, I mean, being able to feed water to both pumps without cavitation) this boat should perform as well as our jetboats. Steering will actually be MORE responsive, because, in addition to being able to turn the nozzles to steer, you can back off the throttle on one engine, and gun the other engine, to make it whip around faster....i.e. making a right turn, you back off the starboard throttle, and gun the port throttle, and vice-versa. (I used to love taking my Sea Doo through some of the serpentine channels in the marsh, yanking and banking like that)
PSG-1
Well-known member
That's a big boat!
IMHO, you need 2 engines to push that boat.
Not sure what your boat weighs, but just for reference, my twin engine sea-doo weighed right at 1500 lbs, and its combined HP was 170 ( 2 X 85HP 717cc engines)
IMHO, you need 2 engines to push that boat.
Not sure what your boat weighs, but just for reference, my twin engine sea-doo weighed right at 1500 lbs, and its combined HP was 170 ( 2 X 85HP 717cc engines)
bigwave
Well-known member
I say go for it, It intrigues me to watch what you jet boat guys come up with.
PSG-1
Well-known member
I concur with bigwave....go for it! And keep us updated.
Personally, I enjoy watching other people do these jet john projects, each one is unique in its own way, it's always neat to see what the next builder comes up with.
Personally, I enjoy watching other people do these jet john projects, each one is unique in its own way, it's always neat to see what the next builder comes up with.
So at this time I have jet ski's cut apart.......suggestions on mounting them in the boat? The ideas that I got for mounting was from you tube. Video was twin jet ski in a jon boat.....its actually a power point and shows the highlights of his install and how he cut boat and then fiberglassed it all in. If you watch video please let me know if its good way to install or should I go different route.
Ranchero50
Well-known member
Sorry, slept all day (work all night)...
I'd still try to just run one engine, especially if your weight is similar. I'm not a big fan of the cut and graft method either but if it's all you have then...
What's the intended use and duty? What is your expected lifetime for the hull once done? It's easier to give sound advice when we know what your plans are.
Where are you measuring the width? Compared to the seat, the bottom of the hull doesn't look 6'-6" wide. With a tall gunnel your overall width can get crazy with a 48" bottom.
I'd still try to just run one engine, especially if your weight is similar. I'm not a big fan of the cut and graft method either but if it's all you have then...
What's the intended use and duty? What is your expected lifetime for the hull once done? It's easier to give sound advice when we know what your plans are.
Where are you measuring the width? Compared to the seat, the bottom of the hull doesn't look 6'-6" wide. With a tall gunnel your overall width can get crazy with a 48" bottom.
54" at the bottom........77" at the widest part of the boat. Going to run this on the river and small lakes to fish.
and as far as similar weight, i have twice the surface area of the boat on the water so the drag is considerably more. Won't that affect the overall speed and power of the boat? I am only want 30-40mph if that.
and as far as similar weight, i have twice the surface area of the boat on the water so the drag is considerably more. Won't that affect the overall speed and power of the boat? I am only want 30-40mph if that.
Ranchero50
Well-known member
At speed I only have 4' of my hull in the water... What's the HP rating for your pump and motor? How much does your hull weigh? Are you going to stay with a forward console?
Ranchero50
Well-known member
I think running a single will do everything you want it to do. I don't see a huge benefit to running duals on that hull vs. the expenses to get them installed and working correctly, much less the fuel costs of running twin 2 strokes.
Post up some pics of you hull bottom from the rear and side, same for the ski.
What fab skills and tools do you have?
Post up some pics of you hull bottom from the rear and side, same for the ski.
What fab skills and tools do you have?
painlesstom
Well-known member
I would expect one motor to push you fairly well in that, two would definitely get you toward the 40 mph range. You might get some ideas from my build when it comes to grafting ski to aluminum hull. My Jon is a 1236 with the Seadoo 580 motor, 36 mph top speed. https://www.tinboats.net/forum/viewtopic.php?f=21&t=28350
I hope you do twin engines, would love to see that in action! Twice the work and planning though.... Wishing you luck. 8)
I hope you do twin engines, would love to see that in action! Twice the work and planning though.... Wishing you luck. 8)
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