Expected auxiliary engine performance

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Heland

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Hello All,

I'm a newbie to the forum and haven't taken full advantage of the search capabilities yet, but wondered if anyone had a rough idea on what speed I can expect from an auxiliary engine that I'm planning to buy? My boat is a RIBTEC 740 (7.25m) RIB. She weighs 2,300Kgs and reaches 42kts or so with the Mariner Optimax 200hp with only two crew and around 25kts fully laden with 6 or 7 divers and dive gear (+ 2 crew).

I know the rule of thumb for a Aux engine on a RIB is 1HP per meter, but due to the weight on board, I've overpowered the engine (18HP). Obviously, a Tohatsu 18hp 2-stroke auxiliary will not get her on the plane - but what speed would I expect at WOT? I'm looking to be as close to 9 or 10 knots as possible - Too optimistic?

Thoughts appreciated, and Kind regards, Andy
 
Far too optimistic. Your speed is limited to displacement mode and the practical limit is around 5.5 knots for that size boat. After that you need an enormous increase in power to overcome the hump to get onto the plane. Your auxiliary is just that - an emergency motor to keep you moving and under some sort of control. You will also find any sea will seriously affect your speed, particularly if fully loaded even with 18hp. It is not really the lack of HP that limits you in adverse conditions but the inability of the propeller to convert it into forward motion.
 
Ah okay thanks Tranona. Do you think that I'd be no worse off going for a 9.9HP then? More or less keeping to the 1`hp per meter guide.
 
Heland said:
Ah okay thanks Tranona. Do you think that I'd be no worse off going for a 9.9HP then? More or less keeping to the 1`hp per meter guide.
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Don't know where that rule of thumb came from, but it does not make sense as weight is far more important than length of hull. The latter only determines (limits) speed potential in displacement mode.

Don't have direct experience, but would think 9.9hp would not do much to shift nearly 3 tons at anything more than a couple of knots.

Why not ask the designer/builder of the boat?
 
I have a friend with a 7.5m rib and his 8hp aux pushes it along at over 6 knots.(WOT). as you said a 18hp wouldn't get it planning however my thinking is if 8hp gets you 6.5knots, 18hp would surely get you to the 9knots you desire... but that's a lot of extra weight for just a couple more knots although it would get you 7-8knots without wringing its neck....
 
Its a funny one as you could go for the smaller engine and overload it and trash the engine, best to go slightly higher with an engine rather than go slightly lower.
 
Being honest I’d think your expectations would be quite realistic as long as the bracket and or engine height are set up right. As in the cavitation plate of the aux engine being below the line of the hull and in unrestricted flow of water. Plenty of power. Clearly not going to plane, but will charge on. Certainly do 7/8/9 knots.
 
Firefly625 said:
I think I would go for one of these... and my rib would become a company asset and used for commercial purposes..

https://www.ebay.co.uk/p/Mercury-2-...Use-15hp/1628681794?iid=152689242465&thm=1000
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Amazing find! That's what I've got on my tender, but afaik it isn't produced anymore, because not emission compliant. If anyone is interested in something lighter and more reliable than any modern 4T engine, not to mention capable to eat them for breakfast, look no further! :encouragement:

Ref potential speed of the thing with an 18hp o/b, I believe Tranona is right when he says that it ain't enough to go over the hump, but that doesn't mean being restricted to D speed, because it ain't a matter of black or white.
I'm with QBhoy with his guess of high single digit speed.
Not very efficiently of course, and pushing around plenty of water, but that might not be a big concern for an auxiliary...?
 
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