# Engine selection for RIB

Discussion in 'Sterndrives' started by athvas, Aug 13, 2013.

1. Joined: Feb 2013
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### athvasSenior Member

Could anyone have any links to select engine to achieve the required speed for the rigid inflatable boat...
Boat details...
Length 7m
Beam 2.8
Draft 0.6
speed (full load) > 32 knots
GRP construction
Propulsion - Inboard diesel engine with stern drive

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### Mr EfficiencySenior Member

I'd be surprised if a 7 metre hull of any kind weighing 3.5 tons loaded will do 32 knots with a diesel, but happy to be shown otherwise.

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### athvasSenior Member

How to calculate required engine power for this. Is there any standards plz share (For small crafts less than 8mtr in length GRP construction).

Plz help .....

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### CDKretired engineer

Experience from several experiments: at least 220 hp.
Large adjustable trim flaps to ease getting over the hump. The weight/hull length ratio is far from ideal.

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### Mr EfficiencySenior Member

Draft 0.6m ? Hull only ? Not sure how that can be. 3.5 tons would have to be at least 2 tons more than dry hull weight less engine(s).

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### athvasSenior Member

Thanks Mr CDK...
As per my calc i'm getting around 226 HP from the below equation
Max recommended motor power rating is given by
P = 10xF(d) - 33
F (d) = l x b
P in KW

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### Olavnaval architect

Where does this come from?

Sounds more like the upper limit from a regulatory body than something to estimate the required power. You won't get around doing a proper resistance estimate and propulsion analysis.

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### Mr EfficiencySenior Member

Without including the weight or the bottom shape, that formula is next door to useless. The only way your boat will reach 32 knots with 225 hp will be if you drive it over a waterfall. Try this formula:

Waterline length (metres) X Waterline beam (metres) X Hull only draft (metres) X 50 = horsepower required.

It won't be far wide of the mark in defining how much power you need to reach your desired speed.

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### J FeenstraJunior Member

Do you want to go 32 kn with a 7 mtr RIB?
And why so heavy? perhaps you want to increase the lwl a bit and the beam.

Mr Efficiency where did you find that formula?

cheers.

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### Mr EfficiencySenior Member

Just a formula I worked out myself from observation and experience, and yes it does mix metric with imperial units, but it is pretty accurate for any boat that is likely to be trailered, for a cruise speed of 25-30 knots. And it will work for power cats as well, just use twice the waterline beam of the demihull, not the overall beam of the boat. I would like to hear how this RIB has 0.6 m of draft.

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### Olavnaval architect

I'm always very uncomfortable with rules of thumb like this. It may be ok if you derived it from a series of very similar hulls and use it as a first guesstimate for a similar new hull.

What makes me extremely doubtful about this one is the absence of the speed. This formula doesn't take into account whether you want to drift around at 0.5 knots or scream along with 30+ knots - it always gives the same power just based on hull volume and a magic constant (which includes a constant block coefficient).

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### Mr EfficiencySenior Member

Yep, I should have mentioned the speed, and I have specified that above as a cruise range of 25-30 knots. Key to making it reasonably accurate is the hull draft, which caters pretty well to variations in weight and deadrise. If his boat is to exceed 32 knots, 25 knots as a cruise speed sounds reasonable.

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### Olavnaval architect

OK, I see. But surely there are other limits such as the general hull shape (hard chine vs. round bilge) and L/B and B/T range.

If you are within these limits, such a formula can work well to make a first rough guess, but you have to know them.

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### Mr EfficiencySenior Member

To include the speed in the formula, I'd go this way.........

HP required = WLL (m) x WLB (m) x Hull draft (m) x (desired cruise speed) squared, then divide the result by 16. By that reckoning, a lightweight aluminium runabout with 4 metres WLL and 1.5m WLB, hull draft (with load on board, of course) of 0.25m, with a 15 knot cruise, will require 4 x 1.5 x 0.25 x 225 / 16 = 21 hp. A larger performance boat like a Formula 233 would need (roughly) 6.2 x 2.1 x 0.4 x 900 /16 = 293 hp for a 30 knot cruise.

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### Mr EfficiencySenior Member

I mean full planing forms, which of course what a 32 knot RIB is. Obviously any skegs are not included in the hull draft. I think it works pretty well over the spectrum of trailable boats, beyond that it may be less accurate.

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