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25cc 4 cycle boat

Old 12-05-2020, 10:29 AM
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meowy84
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Default 25cc 4 cycle boat

I just had a nearly new Cub Cadet SS470 grass trimmer fall in my lap. What's the big deal you say? Well, the trimmer has a 25cc 4 CYCLE engine which I think might be perfect for a 'scale' vintage wooden boat (see pictures for ideas). Ah, the beautiful 4 stroke sound. Anyways, at the moment I'm in the research, planning and feasibility stage. Obviously there's lots of info on here on 26cc 2 strokers and others but not a great deal on the 4 cycle engines. My biggest hurdle I feel is boat length/weight target and prop selection. Any ideas on those targets?

The engine specs are as follows:
-weight with oil/gas but without trimmer attachment: 10lbs (yes, pretty heavy!)
-idle: 3000rpm
-max rpm: 7000-7200rpm
-power: 0.7kW/1.0hp









Old 12-06-2020, 06:25 AM
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Hydro Junkie
 
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I can tell you that any of the high speed type hulls won't work due to lack of rpm. That would definitely include the third boat you pictured.
I would be looking at a larger scale cabin cruiser or deep vee sport hull for that engine. The biggest issue you will have will be cooling and, with that engine, you will need air flow over the cooling fins or will need to fabricate some sort of water cooling set up for it
Old 12-06-2020, 06:41 AM
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Tidnab
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If you need to water cool it,lookup Bob Decortin on Jim's R/C Boat Dock.That's his user name on there also. He used to have a site,RC Boat World and I believe he did a Honda four stroke trimmer engine and water cooled and modified it.
Old 12-15-2020, 01:26 PM
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meowy84
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Thank you for the info and comments gentlemen. Upon some reflection I see what you mean H.J. about a cabin cruiser or a 'sport hull' being more suited to the engine's power band than the hydro in the last picture. As for the cooling, yup, that's another hurdle although I was thinking that with a cabin cruiser design like an Aeronaut Diva for instance you may be able to leave the engine air-cooled and make a natural looking 'deck air scoop' of sorts by making an opening to channel air into the bowels of the hull where the engine will be buried. Then the hot air will exit through the cabin and out the cabin window openings. With the other more sporty hull designs water cooling would be a must.

There's also one other hull I've been thinking about: an unconventional catamaran design with the engine (left in it's original air-cooled state) suspended between the two hulls turning a single central prop with a 3 rudder setup (2 at the hulls and the 3rd just behind the prop). Again, just thinking out loud.

Anyways, back to the prop selection and prop size. Any ideas what prop range (and what maker/supplier?) and approximate hull length I should aim for? As an aside the idea of a 4 stroke gas boat is not that unusual. The Brits have been doing the 4 stroke model boat stuff for a long time, the Impex/Adcolantic 30cc engine being an example of a powerplant sometimes used.

Old 01-04-2021, 06:52 PM
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meowy84
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I checked out the leads you guys posted. Very helpful. thanks again.

Last edited by meowy84; 01-04-2021 at 07:17 PM.

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