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Old 21 December 2006, 12:45   #1
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Long shaft engine on a little ickle boat

I have a Johnson 6hp 4 stroke aux, which is long shaft 'cos otherwise it wouldn't reach the water on my Humber

I've been looking at one of the little plastic boats like the Mac 270 or the Smartwave 2400, just for something to mess around with - they are only a few hundred quid to buy and it seems like fun on days when I can't be bothered to launch the RIB, it would also give me something I could confidently run up on a beach without having to worry about damaging it so I could tow it behind the Destroyer if I wanted to land on an island or somewhere where there was a rocky beach which wasn't RIB friendly. But (it doesn't say) I'm assuming from their dimensions that they will certainly be intended for a short shaft engine. Both are only rated to 5hp but I suppose a 6 should be ok if I don't give it full beans - the engine is the same unit/same weight as the 4 & 5 hp models.

Apart from drag and therefore a bit lower top speed, are there any other reasons against using a long shaft engine on a small boat? I suppose you have to be more careful in shallow water but the Johnson has a "shallow water drive" mode (tipped right up with the prop just in the water) that seems to work ok on the Destroyer so should get around that problem - and since the boat hulls only weigh 45kg, if you got beached you could hop out and push anyway

Would anybody hazard a guess at the likely speed penalty from hanging an extra five inches of engine into the oggin? I guess it can't really be more than 1 or 2 knots? I think I worked out the theoretical top speed of the 6hp engine (from gear ratio/estimated slip etc) to be about 15kt so I reckon I ought to get about 10kt anyway which is probably fast enough for something I can barely lay down in.

Ta
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Old 21 December 2006, 13:15   #2
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Why not make up some sort of bracket - would be easy enough - doesn't need to be much - you could almost hold one of those engines....

And looking at those boats - why not use a bath? It would be pretty tough and even has a drain hole built in. Ask Paul Tilley to make some tubes for it......
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Old 21 December 2006, 13:54   #3
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I use a long shaft Honda on a wee inflatable. It's been fine but you have to lift it smartish if coming ashore on a beach.
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Old 21 December 2006, 14:25   #4
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Quote:
Originally Posted by codprawn View Post
Why not make up some sort of bracket - would be easy enough - doesn't need to be much - you could almost hold one of those engines....

And looking at those boats - why not use a bath? It would be pretty tough and even has a drain hole built in. Ask Paul Tilley to make some tubes for it......
Now you are being silly - why would I want to put tubes on a bath? Baths do not leak anyway the plug is designed to keep water in, not water out!!

Could make a bracket but if I am already putting on an engine that is 20% over-rated, applying the load as an odd twisting moment might be pushing my luck a bit... its a concern I have with using a long shaft is that the twisting motion on the transom might be (must be) amplified by the extra shaft length, which might not be a good thing. Imagine a thrust force of say 200lb thrust at the prop, if it is on the end of a 20" shaft instead of a 15" shaft then the "torque" applied to the transom mount must be 33% more if my mind is working correctly. So a 6hp long shaft engine might exert 1.6x the torque on the transom (compared to the correct engine i.e. 5hp short shaft) which is quite a lot more (1.2x the power x 1.33 for the extra shaft length). All hypothetical but must have some basis in fact...

I have run my engine in a tank and judging by the amount of water it shifts around (and out of) the tank I very much doubt whether you would hold on to it at anything above idle
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Old 21 December 2006, 17:49   #5
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Rig it up like a longtail on a pivot - they do that with V8s no problems!!!
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Old 21 December 2006, 22:27   #6
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The Mac - Maybe. But that Smartwave? ... it looks like a cross between a schoolbus and a seacoffin. It's not enough to sleep with the fishes, they actually want you to attend their school.
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Old 22 December 2006, 05:24   #7
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The Mac - Maybe. But that Smartwave? ... it looks like a cross between a schoolbus and a seacoffin. It's not enough to sleep with the fishes, they actually want you to attend their school.
True, but its also 500 and unbreakable

I think I'd probably pay the extra and get the Mac anyway, but apparently the Smartwave is quite popular for a tender.
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