How fast

I'm toying with the idea of getting a little boat something like one of the Suzumar (http://www.suzuki-marine.co.uk/suzumar/specs.php) range, that I could stick my new 6hp aux engine on to for messing around on where I can't (or can't be bothered to!) put the Destroyer in the water for whatever reason.

What sort of speed might I expect to get out of something of the 3m size range, 1 fat bloke and a 6hp on it - I'd guess somewhere between 10kt and 20kt - is this reasonable? Would there be any significant difference between, say, the Suzumar 290 SIB and 310 RIB? I have no idea how the "hull efficiency" would compare between a SIB and a RIB of the same size, I assume the RIB might be better but the ability to roll it up and stuff it in the back of the vehicle would be handy :)

The other thing is that it's a long shaft engine as that is what is needed for the Destroyer, while the little Suzumar's transom height looks like it would suit a short shaft engine. I guess that'll knock a bit off the speed with the extra length dragging in the water.

Working on theoretical figures, with a 7" pitch prop and 10% slip and the gear ratio listed on the Suzuki website (the 4/5/6 Johnson four stroke engines are made by Suzuki) it ought to do a whisker under 15kt but I don't know what sort of slip you should expect on a much smaller boat or with a small engine?

Any thoughts please? It is only a vague idea that it might be a bit more fun for not a lot more money and make better use of a shiny new engine :)

Fat bloke on a 2.65 suzie with a 5HP yamie 5 Knots, 11Knots on the plane, but he has to make out he is making love to it (thrusting his weight forward) to get it up on the plain!

I have a Quicksilver 310 with a wooden floor and an inflatable keel. It has a 15hp Suzuki 4 stroke and it flies along - almost 25mph. It also uses hardly any fuel and we have crossed 25 miles of open sea. Love it!!!

The wooden floor is a pain if you want to keep taking it apart but it dopes make it a lot more rigid. The same boat with an air floor is only rated to 10hp. Your 6hp should be fine with an air floor.

The long shaft could be a problem - maybe you could make up some sort of bracket?

Worth their weight in gold are the launching wheels we have fitted to the transom - they swing up out of the way when not in use. Makes launching so much easier. The ones with the inflatable wheels are the type to have.

QUICKSILVER 310 AIR DECK is rated to 15hp not 10hp!
The wheels are great.
:thumbs:

QUICKSILVER 310 AIR DECK is rated to 15hp not 10hp!
The wheels are great.
:thumbs:

That is interesting to note - they USED to be rated to 10hp for the airdeck and 15hp for the wood floor. I see the web info has now been changed but it also says it in my handbook. Either something has changed in the design or they aren't being as cautious as they were!!!

Fat bloke on a 2.65 suzie with a 5HP yamie 5 Knots, 11Knots on the plane, but he has to make out he is making love to it (thrusting his weight forward) to get it up on the plain!

Hmmm five knots is a bit dull... I was expecting about five knots out of the Humber with the aux on the back (based on what others have told me). 10 knots would be ok, though I suppose 5 knots sitting in a fender with a transom on the back feels more exciting than it does in a big boat? :)

I guess the answer would be to borrow something similar (if I can find one) and see what it feels like - I am not intending to buy another engine 15hp or 25hp or anything, just that little inflatables are not a bad price (sub £1000) and it might make better use of the aux rather than having it sitting around going rusty and (hopefully!) never getting used :)

I would agree with people here - my 3.9m boat whilst being a bit heavier than a SIB only does 21 knots maximum (flat calm - 2 people on board, some gear) with 20HP so I doubt you will get the speed you were hoping for.

Interested to hear that you think the length would make a big diff. My boat is sold as taking either a long or short. And I was sold the long as it was less likely to ventilate in tight turns - and was unlikely to make much difference to performance at this speed. I understand the theory why it might - but does anyone have practical experience of putting relatively small HP long/short engines on small boats. Would I get an extra knot or an extra 5 knots by swapping the engine (or moving it up).

How about 23mph with a 5 H.P.? You can get the plans for this thing (http://www.glenl.com/picboards/picboard5/pic347a.html) as a winter project, adding a set of Zodiac Futura mini-tubes to make it a RIB :D , I got the tubes (used) for 40€.

I would think you would get about 15knts with the 6hp with one person onboard, however with extra weight or a passenger the speed is greatly reduced and it would probably only just plane. I don't think there would be a great difference between the 3m SIB and 3m RIB speed, However the RIB would be much more comfortable to sit in because its deeper. (My 3m with a 3.3hp will do 9knts 20knts + with yam 9.9).

I tried a 5hp longshaft outboard on a old 2.7m inflatable with a shortshaft transom, I raised the transom by about 3 inches it was still too deep in the water but it went OK I probably lost about 1knt compared with a shortshaft. The outboard also looked very tall.

Thanks. I might see if I can scrounge something small from one of the local yachts and see how it goes with 6hp on it before I commit any cash to the exercise :)

I haven't tried it on the Humber yet - hopefully this weekend if the weather is fit.

Though having had it running in a bucket of water I have to say it is a lovely little engine, runs like a sewing machine at idle, sounds very like a nice 125cc four stroke bike engine. Not at all like the coughing farting bucket of sH&* in the middle of the transom, which doesn't even run on four cylinders till it gets to about 2000rpm...