I need advice

[debruin]

Hi guys after thinking a lot about it, I drew a basic 3 seat water scooter powered by a 60cc 10 hp engine I already have. I assume that the shaft on which the prop is mounted is supposed to be 1.5x as long as I have drawn it. And can a prop intended for a 5 hp Yamaha engine handle the 10 hp engine if I most of the time go 2/3 throttle? And what ratio should the gears be? Max rpm is 15000. If you see anything wrong on the drawing please tell me, I'd rather not build something so it can sink.

[Cavalier]

Originally Posted by debruin

Hi guys after thinking a lot about it, I drew a basic 3 seat water scooter powered by a 60cc 10 hp engine I already have. I assume that the shaft on which the prop is mounted is supposed to be 1.5x as long as I have drawn it. And can a prop intended for a 5 hp Yamaha engine handle the 10 hp engine if I most of the time go 2/3 throttle? And what ratio should the gears be? Max rpm is 15000. If you see anything wrong on the drawing please tell me, I'd rather not build something so it can sink.

I'm responding to this post on the basis that it's serious and not an attempt to troll the forum into debating a highly unusual design.

You're gonna struggle to keep those handlebars straight at any greater speed than 2 knots! If you're intent on having the steering on the front, a better design would be to have at least 2/3 of the surface-area of the rudder behind the shaft, rather than all of it in front as you've drawn.. as it stands, the center of pressure is in front of the center of gyration, so it'll be inherently unstable and not stay pointing straight!

Also, are you planning on having a second pedal on the port side of the scooter for the brakes?

[debruin]

Thanks for the advice, I'll change that.

[sailormed]

It would be much more steerable if you could attach the engine on a pivoting chassis, enabling steering like an outboard with a stick. Added benefit would be you are closer to the engine controls.

[debruin]

I'd rather have the steering in front because then the passengers sit behind the driver which makes visibility much better.

[2big2small]

You’d have to build it very wide to prevent capsizing. With passengers on top the center of gravity will be very high.

You should also make it much longer or it will sit too deep and be a dog to move. I would go with a catamaran design myself, with two long and narrow hulls, wide apart. That way you get stability in all directions and high performance for your hp.

10 hp isn’t going to make a short square hull plane with 3 people on board, but a cat with two long narrow hulls would slice through the water. (Assuming you do get the gear ratios and prop right to get the best out of your engine.)

[StuM]

As pointed out in a previous thread, you're probably looking at a max RPM of around 3,000 so you would need a 5:1 reduction.

Appropriate prop size depends on a lot more then HP. Other factors are planned top speed, vessel displacement, hull type. Without that data, it's hard to determine what prop will be suitable.
Google "boat prop sizing" and you will find a number of online calculators to help in determining an appropriate prop for your use case.

[2big2small]

Besides you’ll have to elongate the hull quite a bit just for trim since the pilot sits in a fixed position forward. With a short hull as drawn the bow would sink to deep and the stern would lift up too much if he were the only one on board.

[debruin]

Thanks guys, I totally forgot about center of gravity, I'll redesign it tomorrow, likely a 2 seater with seats inside the barrels. Also, a catamaran would be nice but I'll stick to a simple motorized raft made out of barrels and wood. I'll try to figure out what the right prop is when I get the engine running.

[Don C L]

Have you seen these engines/long shafts common on Amazon River and tropical shallow rivers?
https://www.beaverdammudrunners.com/about/whyCLP.php

[newhaul]

Originally Posted by Don C L

Have you seen these engines/long shafts common on Amazon River and tropical shallow rivers?
https://www.beaverdammudrunners.com/about/whyCLP.php

that would be a great addition to your boat to Don just hook it to this 10 horse motor.
https://www.ebay.com/itm/New-Sigma-E...4383.l4275.c10

[Don C L]

Originally Posted by newhaul

that would be a great addition to your boat to Don just hook it to this 10 horse motor.
https://www.ebay.com/itm/New-Sigma-E...4383.l4275.c10

Good God Man! You may be on to something there! As if my boat were not unique enough already!

[newhaul]

Originally Posted by Don C L

Good God Man! You may be on to something there! As if my boat were not unique enough already!

makes her a diesel

[lonesoldier0408]

Wider/more floatations forward, less aft. Think jet boat. What about keeping that ignition dry? Although re-invention is a way of exercising the mind, the forward facing rudder would require more effort than most would be willing to apply for any length of time. Just because you can, doesn't mean you should.
I like the Catamaran idea.

[a64pilot]

The 10 Hp 60cc two stroke, first I assume it must have an about three ft long expansion chamber to get that kind of Hp out of such a small motor?
Then as I said in your other thread, do not count on that 15,000 RPM and 10 Hp for more than a few seconds. Very high output air cooled two strokes like that will seize as there just isn’t enough cooling for high power for very long at all.
Then, unless I’m mistaken the noise will have to be heard to believe? What kind of exhaust system does it have?
Two stroke outboards I believe run in the neighborhood of five to six thousand RPM? I think, been a long time actually since I measured one.
My old Kawasaki Jet ski stock hit its rev limiter at about 5,000 RPM. I built a PJS dual carb reed motor with expansion chamber etc and I think it came onto the pipe at around 10,000 RPM then, maybe less, but you never ran it for more than maybe 10 sec at full tilt or it would break a piston, and it was water cooled.

In short, best thing you could do is sell that motor and buy some kind of four stroke lawnmower or pump motor, it’s just not suited for this use, in my opinion.