Newbie needs help

[debruin]

Hi guys, my name is Magnus and I want to use one of my engines for a boat. It will be similar to the little boat in the James Bond man with the golden gun movie. If I use the small engine it's going to be a small 1 seater.

I'm mostly concerned about the rpm, because the shaft and prop will be mounted to the crankshaft horizontal with no gears involved so the prop will have the same rpm as the engine. Here are the specs of the engines:

Chevrolet 250:
inline 6, water cooled 4 stroke
4093cc
110 hp
4000 rpm
200 kg

IAME Parilla:
standing single cylinder, air cooled, 2 stroke
60cc
10 hp
15000 rpm
10 kg

The Chevrolet engine can easily be tuned to get to 150 hp, but the IAME engine is at it's limit because 15000 rpm is very high. What would you guys choose because I have no idea how fast a prop should spin.
Also the engine will steer along but I don't really care how heavy the steering is.

[SV Bacchus]

Hi debruin and welcome aboard! I am no expert so others will chime in with much more knowledge than me. That being said my two cents worth is that I have never heard of a prop designed for boat use that would work at 15000 RPM and not churn the water into a bubbling mess (cavitation). Not saying there isn't one out there, just I've never heard of it.

[GordMay]

Greetings and welcome aboard the CF, Magnus.

[StuM]

Most high speed boat props will turn at a maximum of around 3000 RPM.

This document lists lots of engine WOT and gear ratios which should give you a good idea.

https://www19.corecommerce.com/~thep...PM%20Chart.pdf

[Fiveslide]

The small two stroke will probably be the easiest to adapt to a boat like that. And it can probably be run backwards so, with some ingenuity in modifying the starting mechanism, you have the option of reversing thrust. You don't really have to marinize it since it is air cooled.

[Fence Man]

That style of "motor" is a Thai Longtail. They are surprisingly effective with relatively low horsepower. The last one I "built" was a 13hp Honda on a Swamp Runner kit. I could get 14knots with two men and a cooler of beer going upstream in this 12' boat I built with a 60" wide bottom, and that was before jetting the carb and upgrading the intake. (They say 22 hp or so) A longtail is much more efficient on a narrow boat, as it's designed to run in the high water where the wakes come together. Once we put any kind of load in the boat, it was no longer possible to get on plane, so I ended up switching to a 50hp outboard. The silence of sails makes me smile, but so does sliding a 12' slick bottom Jon boat sideways at 50 knots. Good luck with your build, and keep us posted!

[Fence Man]

With a light load, we could jump beaver dams in the back waters. That was a fun little outfit.

[belizesailor]

Built a similar long tail 5 HP rig with a friend of mine at my place in Guatemala last year. Fun project.



https://www.dropbox.com/s/e93ccdlsti...03715.jpg?dl=0

https://www.dropbox.com/s/kdyz3rmc7n...01739.jpg?dl=0

https://www.dropbox.com/s/f8om0s6h5g...03725.mp4?dl=0

[a64pilot]

The very few single cylinder 60 cc two strokes that I have been around that will make 10 HP, won’t do so for very long at all. A saying we used to have racing years ago is never go full bore for longer than you can hold your breath, the little motors will seize if you do.
However a five to one gear ratio ought not be too hard to achieve with belts.

[Fence Man]

Originally Posted by belizesailor

Built a similar long tail 5 HP rig with a friend of mine at my place in Guatemala last year. Fun project.



https://www.dropbox.com/s/e93ccdlsti...03715.jpg?dl=0

https://www.dropbox.com/s/kdyz3rmc7n...01739.jpg?dl=0

https://www.dropbox.com/s/f8om0s6h5g...03725.mp4?dl=0

That looks like a fun rig!

[belizesailor]

Originally Posted by Fence Man

That looks like a fun rig!

It was a full little project. Friend of mine brought the shaft etc down in his luggage. One of my guys welded up the base (using some parts brought down). We bought a water pump w a 5hp motor, because it was available locally and it was cheaper than ordering just the motor.

A few hours later it was going for a test drive clamped to one of my guy's cayuco.

[Fence Man]

Originally Posted by belizesailor

It was a full little project. Friend of mine brought the shaft etc down in his luggage. One of my guys welded up the base (using some parts brought down). We bought a water pump w a 5hp motor, because it was available locally and it was cheaper than ordering just the motor.

A few hours later it was going for a test drive clamped to one of my guy's cayuco.

What kind of speed did you get out of it?

[belizesailor]

Originally Posted by Fence Man

What kind of speed did you get out of it?

Not much. A few knots. Small prop and small motor. But, it worked and it was way cheaper than an outboard. That was my friends objective, he wanted to show the locals how to build a cheap propulsion system. Systems like this are not used in Guatemala.

The loclas thought it was fun too...then after he left they put the pump head back on the motor, used it as a pump and went back to outboards for propulsion...old habits die hard! I havent told him yet. [emoji16]

[debruin]

Thanks for the help, I changed my plans a bit, I'll probably build a motorized raft built with two 55 gallon barrels and pieces of wood to become a simple jetski for 3 people, since I have no boat experience at all. Chain drive seems suitable for it. Do props usually spin clockwise or counter clockwise? And what kind of prop and size would you guys recommend me?

[Fence Man]

I hate to ever be the guy to shoot down a dream, but if you're going to go jumping sharks, this might be cheaper and easier.....

https://www.youtube.com/watch?v=aoCwxMnxOy8

We've built some two man NZ style river jet boats with jetski power plant transplants. Way easier than reinventing the wheel, IMHO.

If you must, I'd consider a belt drive ahead of chain drive.