I would look at small boats that already have pointy ends and see what they use

Lifeboats for example all used to use Lister diesels

I think it might be better to use an electric motor to power the boat, with an electronic sound unit – much easier to adjust power required for the size of the boat. You can also then 'tailor' the sounds to achieve what you want.

Although somewhat non-prototypical for your proposed boat I wonder if a smaller version of the Ricardo Dolphin motor might work. The originals were known for being able to run reliably at low speeds in fishing boat applications so at least the marine connection is there. No relation to the more modern two cylinder Dolphin two stroke marine motor tho'.

Allegedly its quite easy to convert a two stage piston compressor into a Dolphin style engine :- **LINK** . Quite easy is of course a somewhat elastic concept but that one looks very much shade tree engineering yet apparently runs quite well. It runs too fast for you but the layout provides considerable scope for throttling and the independence of induction cylinder compression and air delivery from the power cylinder compression ratio suggests that adding an air bleed or pressure relief valve might enhance low speed running.

Section for a proper Dolphin here :-  http://www-g.eng.cam.ac.uk/125/achievements/ricardo/page07.htm. Note automatic inlet valve. Possibly a proper cam operated one would enhance low speed running albeit at considerable extra complexity. I imagine that the compressor conversion merely has exhaust ports cut in the bottom of the low pressure (power) cylinder and a new, flat base cylinder head with bulb type combustion chamber on top to take the spark plug and inlet valve.

Gordons suggestion is by far the easiest way of course. A flat out cheat but whatever works!

Clive.

Edited By Clive Foster on 21/01/2018 13:44:42

Hi Mark,

First of all, what size of engine do you think you require?

for around 4 hp you could look at one of the Seagull outboards – 2 stroke, slow runner, can be mounted at virtually any angle as it doesnt have a sump.

A problem would be that although it has a gearbox that turns the drive through 90 degrees only the silver century model has a clutch unit and no reverse.

So, as Jason says, the Stuart engines are good, but getting very hard to obtain, and the very small ones (1.5hp) only have a centrifugal clutch.

Another option would be to couple up suitable engine to one of the ride on mower gearboxes via belt drive, (seagull or briggs would suffice) long stroke two stroke may sound more to your requirements if run a little rich and allowed to 'four stroke'

Regards

Lofty

Perko7 beat me to it.

I would try a small 4 stroke engine of around 100-200cc with a large flywheel and self regulating throttle so it can increase torque without increasing engine speed too much.

**LINK**

Although a small, relatively modern, engine with small carburettor and large flywheel can be coerced into running at a suitably low speed it won't give the "long stroke put-put-put" noise desired by the OP. This is characteristic of the low compression, hot bulb semi-diesel engines once popular when robust simplicity and long running reliability were more important than compact power.

Getting that sound out of modern style engine might be possible with careful exhaust tuning but I suspect anything suitable will inevitably run too fast. Using a four stroke will help but the exhaust will probably still have too much of a harsh bark and natural speed too high if developing useful power.

Semi-diesels are almost invariably very slow running, maybe 90 to 200 rpm in normal operation, near constant speed two stroke devices. They are also very, very large for their modest power output, maybe a horsepower per litre swept volume at best, usually less.

Decent running small and model size hot bulb engines have been made but small incarnations tend to be fuel fussy, unlike the original which will burn pretty much anything inflammable thin enough to be pumped, and usually require continuous heating of the bulb for reliable running.

The aforementioned Dolphin engine shares most of the running characteristics and sound of the semi-diesel but is much smaller for similar output. The single power cylinder ones used in the Newhaven fishing fleet boats were, I think, around 3 1/3 litres. In those days a semi-diesel engine would have been the preferred choice of motor but the boats were too small to carry one. Hence the use of the Dolphin which had much the same characteristics and robust simplicity in a far more compact package.

Spark ignition forces the use of petrol for fuel but will almost certainly make for much more reliable running.

Although there is no way to purchase a Dolphin type motor today the Vee twin compressor conversion of the style linked to in my original post doesn't look unreasonably difficult for someone with adequate workshop facilities.

Clive.

Edited By Clive Foster on 22/01/2018 11:48:01

I think Hatz made a quite tiny diesel a very long time ago (before the 7*/7*8 range). They rarely come up for sale, but might be worth watching out for – but they are not cheap even if a non-runner – and quite possibly they would not run slowly enough.

well the original Bill engine was designed for hulls about 18ft long so as the model engine is 1/3rd scale you are probably looking at a hull in the region of 6ft long so your bed size was about right. At least with the Bill engine you will not have to worry about a 90deg change of direction in the drive train, looking at images of the hull structure on line you can just mount the engine on the bearers and take the propshaft out the back.

As for new machines something like a SC4 lathe and SX2 or SX2.7mill from ARC should handle it easily.

I have one – had it for 58 years & it still runs great. Centrifugal clutch is a new one on me & we have a few in out sailing club. But if you want one then you go to—- "Saving old Seagulls" in Tillingham, Essex. It is where they go to die then get reincarnated by a really nice fellow who lives & breaths Seagull outboards.

If you mount it out of plumb for long you will never start it though.

Edited By Sam Longley 1 on 22/01/2018 22:10:57

Ah! Before my last post I missed that you had said bed size for your model – must be my age!

As far as machine tools delivered from U.K. to France is concerned I have had good experience with Arc Eurotrade and Ketan who posts here is very helpful. There are a number of sellers of Chinese machine tools here but the prices are higher than the UK even after transport. Optimachines are one popular one.

Russell

OK, If you are also considering 2 stroke air cooled engines, then the Villiers MkII 98cc Midget engine may also be worth considering seen here fitted to my 1958 Mower –

There is also a 79cc MkI version (angled fins). Both quite common and parts are still available.

edit- more pics in my album

Edited By Richard S2 on 23/01/2018 14:41:00