Light, compact and cheap (?) inboard idea

[BarnacleGrim]

I think I have already aired this idea, but having an old outboard in the garage I have been thinking more about it recently. Having an inboard has some great advantages, but for boats much smaller than mine it becomes nearly impossible to find a light enough water cooled inboard. So why not use an outboard power head?

It must be a two-stroke so it can run with a horizontal shaft. Put a 90° bend between the engine and the carb, a thrust bearing on the shaft, and you already have a water cooled block and exhaust to pipe out the transom. Could it be done?

[Paul Pless]

Why not just put the whole outboard in a well without modification?

Failing that rather than turning the outboard on it side just unbolt the foot and fit a ninety degree gearbox between the downward driveshaft and the propshaft in your boat?

[Hwyl]

http://www.saildrive280.com/ and lots of others

[Paul Pless]

http://www.saildrive280.com/ and lots of othersno doubt the ideal, but barnacle already has an outboard

[boylesboats]

http://www.saildrive280.com/ and lots of others

now thank dude...
That give me a reason to pull a water cooled 18 hp Kawasaki engine outta someone else John Deere 345....;) just kiddin'... It can be done however...

[Bob (oh, THAT Bob)]

I think I have already aired this idea, but having an old outboard in the garage I have been thinking more about it recently. Having an inboard has some great advantages, but for boats much smaller than mine it becomes nearly impossible to find a light enough water cooled inboard. So why not use an outboard power head?

It must be a two-stroke so it can run with a horizontal shaft. Put a 90° bend between the engine and the carb, a thrust bearing on the shaft, and you already have a water cooled block and exhaust to pipe out the transom. Could it be done?

Well, you are not taking the best advantage of each system.

Inboard/outdrives (I/O), have the durability of a larger engine, and mounted lower in the boat for lower center of gravity. But the outdrives are notoriously troublesome for maintenance (two 90 degree gear sets, U-joints, etc.), and the 2 gear sets cost 10% in effeciency, If I recall correctly

Outboards are compact and easy to mount. But they require a 90 degree gear box. That gear box hurts durability, and costs 5% of the power, if I recall correctly. If you get rid of that, you might as well just mount a small auto engine or smaller horizontal shaft engine outputting directly to a prop, with a flex coupling between, that is essential. Straight through transmissions are common, if you can package it. If the engine must be mounted far aft, you may need a V-drive, which is more expensive.

[Candyfloss]

I'm not familiar with that particular outboard, but you only need the 90d bend if your carburetor has a float. Unlikely on small, modern motors. If it is a diaphragm carb it will work any which way up. You only need to relocate the primer pump.
I can't see any reason your idea won't work, provided there are sufficient & strong mounting points for whatever kind of bracket(s) will be your engine mounts. And some way of adapting the powerheads' exhaust outlet into a water muffler & overboard.
I say go for it. Marine engines small enough to fit in a Folkboat are pretty much nonexistant to my knowledge.

[andrewe]

Nice if you like fiddling with mechanical solutions. But most people go inboard to get rid of petrol and ignition systems and to increase reliability and durability.
A

Having said that, I am fiddling with a 4-T air-cooled single driving an outboard leg as a sort of sail-drive.

[BarnacleGrim]

I already have a proper inboard in my Folkboat and no real plans for a second boat, so this is just a hypothetical discussion.

I don't feel comfortable having an updraught carburettor in a boat, any excess fuel would drip straight into the bilge and be a literal ticking time bomb. A proper marine sidedraught (or downdraught?) carb with flame arrestor would be necessary.

It's not so hard to find an purpose-made inboard from say, 6 hp and up, but for smaller pocket cruisers one may be tempted to use a well, which wastes space, creates venting problems, etc.

Another solution is to use a horizontal shaft air cooled stationary engine, like Donald Branscom did, but then you have cooling issues, noise, and a hot exhaust to worry about.

The outboard may still need a reduction gear, which could be a chain or a V-belt, while at the same time making a thrust bearing easier to install.

[Hwyl]

I completely misunderstood your first post, sorry

[andrewe]

Most 2-T outboards seem to run at 5,000.plus RPM, so a reduction would be essential. Back to the old reverse problem.. Honda actually make a 2cyl water cooled stationary engine which would be suitable for this, but would need a heat exchanger. About 18 hp and up.Your outboard conversion sounds possible, but would need loads of special (home made) bits. As you are not using the leg, a raw water pump would be first up. I hate to mention this, but Jet Skis have suitable doner motors, if you can find a small one. Most of the probs sorted, except for the reduction (and reverse).
This is not helping the old motor at the back of your garage though.
a

[Tylerdurden]

I was experimenting with that idea but as a hybrid drive. Using an electric motor and timing belt/pulleys to match output speed. In a sail configuration it could be regenerative but without ducting the prop it would also have a lot of loss.
It would work well but lack of access to machine tools killed it for now.

[Candyfloss]

The outboard may still need a reduction gear, which could be a chain or a V-belt, while at the same time making a thrust bearing easier to install.

This would place a side load on the bottom bearing for which it was not designed.

[frank pedersen]

I have continued to obsess about a similar problem: how to get a light-weight, low-power saildrive. The Saildrive 330 (update of the 280) is rated at 15 h.p. and drives a 12" folding prop - far more power than I need. I contemplated writing to the technical director of the Swedish company that converts outboards for the 330 to see whether they could fabricate a 5 h.p. one-off unit, but I could not find a small (less than 12") folding prop. So I gave up again.