Redneck diesel outboard?

electrosys said:
it made precious little difference: sure the exhaust was quieter, but now I was being deafened by the engine noise itself, which is normally masked by the exhaust noise.

BTW - with a tidal stream of 7 knots on the river, you're going to be travelling backwards in a K20, no matter how many horses are on the transom.
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Hmm interesting info about the exhaust noise thanks.

I am well aware that hullspeed is around 6 knots. The poster was suggesting a 5hp would be adequate. My real world experience is that I get more speed from a 7.5hp than a 5hp (nearer to hull speed)
 
go for it....sounds like you enjoy making stuff in your shed just like me, the only thing i would suggest is maybe go for a more readily available engine, something like a yanmar L40 or L70 they go on eBay for not much, see here and are mega reliable, it does however add the problem of getting the drive round 90deg :-)

A 4 stroke outboard would of course be easier and do all you need....i dont think the fuel consumption is really relavent, but then you would still need another project for the shed....

good luck
 
Why don't you just get a nice modern 4stroke O/B as previously suggested. Reliable, quiet and if you get the right type it will charge your boat batteries too. I know what I'd rather be pressing the start button on when the going got tough. (or pulling the rope).
A modern 4 stroke sips fuel in comparision to older 2stroke types. There was a test in YM a month or two ago. The chinese types didn't come off well but the suzuki and mariner both did. The suzuki won the test if I remember.... that was on a 5hp type.
 
As others have mentioned, there were Italian diesel outboards, which unsurprisingly failed due to the massive weight.

On a 20' boat 4-5hp is plenty ( well it's taken me across the Channel Yarmouth - Guernsey in calms quite a few times, 22' boat ) but leaving the engine in all the time, as implied by the weight of the engines the OP mentions, is toe-curling, the drag & trim will kill any sailing performance stone dead, and the only thing accelerating will be the corrosion.

A good 2 - stroke 4 -5hp would be a much lighter option, relatively easily lifted, if one can be found; 4-strokes are smoother & quieter but much heavier.

I get 2.5 hours per gallon motoring at 4.75 - 5 knots from a 5hp 2-stroke Mariner, in this case in the well of a streamlined boat.

If I need more power, that usually means there's wind, so either motor - sail almost directly to windward if necessary, or just sail !
 
sam_uk said:
I have a 4hp backup motor. It gives me around 4 knots

The 7.5hp will give me 5.4 knots.

I sail in a area with considerable tidal streams. every knot counts.
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Yes, the tidal streams around the Channel Islands, Cherbourg etc are notorious for never really being strong enough when you need them...
 
This is how the Chinese use them. This is the 24hp version (3 in a row). They're deafening!
chinadiesel.jpg
 
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yep you're mad. but my kind of mad.
whats the point of having a boat if you dont experiment?
the only downside is the sort of money you are spending on this 'experiment' would buy a nice shiny new, clean, quiet 4 stroke.

In terms of reliability I should think modern petrols with electronic ignition are far less bothered by damp etc than older ones.

if you go for it i'd love to see how you get on. if its aircooled you can wrap copper pipe around the cylinderhead in a coil to provide water cooling. you'll need a pump either belt driven or electric. once the water has gone through the head you'll need an exhaust elbow welding up then inject the waste coolant into this. unless you bury the exhaust really far down I wouldnt worry about back pressure. only bummer will be getting the right amount of water so it doesnt run too hot or too cool. you wont have a thermostat so you'll just have to pitch it right at cruising rpm and accept its going to run cooler at idle...

good luck!
 
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