An inboard propulsion 3.5 m planing boat is it possible??

[jx85]

Hi, I am new in this forum. I am currently designing the inboard propulsion system for a double chine 3.5 m boat for my academic subject. I was thinking of using a motorcycle engne as there will be clutch system included and also it is smaller in size. However, the performance curve of those engine is really hard to obtain. Anyone had any idea where to get this data or maybe some other alternatives? The calculated resistance suggest that 15hp engine is enough to reach the design speed of 20 knots. Because this type of boat is quite rare the other systems for a boat of this size is also hard to obtain.

Thanks in advance.

[robherc]

15hp isn't a motorcycle engine, it's a scooter/dirtbike engine. Motorcycle engines routinely top 100hp now. Unfortunately I don't know much about scooter/dirtbike sales, but I'd check the mfgr's websites first, and magazines second. For any motorcycle of respectable linneage, it's easy to find peak torque/hp & RPM data on the manufacturer's website, and full HP/torque plots on motorcycle magazines' websites...hopefully for you, dirt bikes & scooters are the same.

[robherc]

P.S. Some of the scooters will already have centrifugal clutches & CVT transmissions...that'll make your job marinizing them FAR easier.

[rasorinc]

Motorcycle engines require high RPM to produce power and torque. High RPMs
are no use in a boat. Also, air cooled does not work in a boat. Consider other options which will perform better and cost far less.

[robherc]

@rasorinc
There ARE water-cooled, high-torque/low(er) RPM motorcycle engines...BUT you're right in that they'd still be of no use to him here.
My Ducati's 2-cylinder, 998cc engine produces LOADS of torque from about 1000RPM up, but at 127HP, it'd turn his little boat into a jetski, and at about $9,000 it'd be MANY TIMES the price range I'm sure he's looking for!

[drmiller100]

bah, motorcycle should work fine for what you are after.

it is very light. it has gears, so you can take off in first gear then shift if you want.

I'd be inclined to run a big enough propeller. too small would really be bad, and if it is too big it will have extra drag.

and, like the other guy said, pretty easy to find motorcycles with over 20 horsepower.
shaft drive motorcycle engine???? or will you try to hook the chain up to something?

[robherc]

@drmiller

If you use a small, air-cooled motorcycle engine (I.E. EVERYTHING with anywhere near LITTLE enough power), you'll have to run a special ventilation system to keep wind blowing over it at 50-60MPH so it doesn't overheat.
As far as connecting a shaft-drive system, that's as easy as attaching your shaft-drive system to the shaft that normally would have the front (drive) cog attached to it. I doubt that will be the hardest problem he comes across.
Also, most of these smaller motorcycle engines were not made to be run for extended periods of time, so reliability could become a HUGE issue if he's going to be motoring for more than a few minutes to an hour, at a time.

[robherc]

jx85-

I think you'd be better off with a motor that's intended to be used in a stationary/low-speed application, like a lawn-mower motor, or a horizontal-shaft motor from a hardware store.

[liki]

For example Honda Power Equipment lists a model GX360 at least on the local importer's pages. 2cyl, 13hp @ 3600rpm, 45kg, horizontal shaft, and water cooled. But that model is not listed on the international pages, only air cooled models.

[robherc]

Here in the US, you could buy the air-cooled version (or at least a decent knock-off of it) at Harbor Freight, and it'd work fine for this application (as long as he mounted it abovedeck, or ran decent ventilation through the engine room). I don't know about the supply situation in Malaysia though, so I can't recommend any specific models.

[jx85]

I have look through those lawn mower and also some generator engine. The problem is that the size is quite big compared to the small motorcycle engine(of same power) although i understand that the bike engine revs considerably higher to achieve the same power. If i use those lawn mower engines, I would have to design the clutch system as well because normally the engine only comes with simple clutch that is start and neutral rite? Compact is one of the reasons i try to opt for bike engines. Do u guys know any website contains info about simple clutch system and gearbox? Probably a forward, neutral and reverse for the boat i am designing. Another thing is that I am considering gasoline 4 stroke engine as this is one of the requirements. It would have been much easier to find a two stroke from go-kart. Do correct me if i am wrong.

[ahmadfaisal]

In terms of size, I do agree with you regarding the usage of a small motorbike engine. By default, it is quite impossible to have a motorbike engine to have reverse gear, but easily you can do some configuration to achieve it. I have a mechanical engineering background, some easy gear mechanics might help you with your project.

[Tcubed]

There are a number of threads in the propulsion forum that are informative , if you haven't read them already.

The basic problem with motorcycle engines is not the air cooling as that can be arranged but that they are designed to operate at 25 % power. They develop their peak power but only when madly accelerating they're not meant to run continually like that. Therefore you end up with a fairly big engine for your needs if you want the engine to survive for any length of time.

The generator i think might be a more promising as they are much closer to the marine and aircraft engine which are designed to run at 75% continuously.

By the way, for effective air cooling there is no need to blast hurricane speed wind across the fins. Check out some wind chill charts to see there is not much point blowing air at more than 30 mph.

Personally i would be quite content to not bother with reverse on a small motorboat. I can dock sailboats under sail no problem that has no reverse, so it is just a question of learning how to maneuver without reverse. But that's just me.

[Village_Idiot]

I would not look at motorcycles specifically, but I would look in the same family - that is, ATVs and UTVs. There are many air-cooled as well as water-cooled versions, and they are designed to produce power at lower rpms relative to your typical motorcycle engine. They have much more weight to lug around at much lower speeds. Granted, a lot of it is in the gearing, but the end result is the same. Most also have incorporated transmissions with forward and reverse (most newer ones have "single-gear" automatic or CVT trannys). Most ATVs and UTVs probably run between 15-40hp. Given their popularity, they may be cheaper/easier to come by, at least in some countries, than motorcycle engines.