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.