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Old 07-01-2018, 12:24   #1
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I need advice

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.
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Old 07-01-2018, 12:32   #2
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Re: I need advice

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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?
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Old 07-01-2018, 12:39   #3
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Re: I need advice

Thanks for the advice, I'll change that.
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Old 07-01-2018, 12:41   #4
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Re: I need advice

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.
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Old 07-01-2018, 13:07   #5
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Re: I need advice

I'd rather have the steering in front because then the passengers sit behind the driver which makes visibility much better.
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Old 07-01-2018, 14:04   #6
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Re: I need advice

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.)
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Old 07-01-2018, 14:10   #7
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Re: I need advice

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.
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Old 07-01-2018, 16:08   #8
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Re: I need advice

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.
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Old 07-01-2018, 16:12   #9
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Re: I need advice

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.
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Old 07-01-2018, 16:36   #10
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Re: I need advice

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.
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Old 07-01-2018, 16:50   #11
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Re: I need advice

Have you seen these engines/long shafts common on Amazon River and tropical shallow rivers?
https://www.beaverdammudrunners.com/about/whyCLP.php
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Old 07-01-2018, 17:20   #12
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Re: I need advice

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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
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Old 07-01-2018, 17:53   #13
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Re: I need advice

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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!
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Old 07-01-2018, 17:58   #14
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Re: I need advice

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Good God Man! You may be on to something there! As if my boat were not unique enough already!
makes her a diesel
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Old 07-01-2018, 17:59   #15
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Re: I need advice

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makes her a diesel
You're right, boring... have to run off methane from my goat farm, now that would be cool.
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