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Old 17 March 2010, 10:45   #1
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Outboard horsepower

Looking for clarification on horsepower rating. It has been said that adding additional outboards does not necessarily double or triple your horsepower. Example: does twin 50ís make your boat twice as fast now that you have double the horsepower to 100 hp? Would twin outboards totaling say 200 hp be as fast as one outboard with a 200 hp rating under equal conditions? I am trying to determine speed in relation to horsepower rating.

Any thoughts would be appreciated.
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Old 17 March 2010, 11:22   #2
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Firstly, welcome to ribnet! This is a perennial discussion round here!

In summary: Maybe! The only definite thing I can say is that you will have the same rated horsepower at the prop(s)

There's a lot to consider -
Drag through the water goes up as a cube of the speed, so , for example the difference between 2x 100Hp Vs 1x 200 is going to be greater than, say 2x25 Hp instead of a single 50. The smaller Hp will be going slower, so genetrate less drag, so less difference. Also at the bigger sizes one "lump" can be sold as sometimes 3 or 4 horespower ratings, so twin whatevers can end you up with a very lardy / draggy oversise lump of plant on your transom.

The fact that most manufacturers will use the same physical lump of engine and tune (or more usually de-tune for the smaller horsepowers), means that the single / twin argument varies between makes and models for a given overall HP. Net result if you go up the horespower range & compare an Xhp with 2 off <X/2> HP, the heavier setup will usually alternate between the single & the twins as you creep up the overall power scale. And if you do the same with different manufacturers, you'll get a different spread of lighter setups. Same goes with leg size (drag), although some legs will cover 6 or 7 different engine sizes. (e.g the leg / gearbox on my old Yam 55 was the same part number as the one on the 90 - the powerhead changed roughly every 2 sizes as you crept up the HP scale)

Then of course you have two propellors, so in theory twice as much grip, so can reduce diameter a bit (less grip per prop), which reduces the torque required to turn the prop, so you can then load them back up (to prevent the engine over- revving) by upping the pitch = more speed...... But then you're into looking at torque curves for a given powerhead........


In summary it's not a simple answer! You need to do a LOT of research, and remember that some transoms are not designed for twins......


Your example of twin 50s Vs a single 50, of course you'll go faster - you have twice the horses to push you along. Doubling your speed is unlikely, simply because as you speed up, the water drg on the engine goes up as the cube of your speed, aerodynamic drag also does, but with less of a steep curve. Because of this non constant drag, double speed for double horsepower is very unlikely.

That was a long post for a one word summary! Hope it helped.
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Old 17 March 2010, 12:00   #3
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Quote:
Originally Posted by stayner View Post
Looking for clarification on horsepower rating. It has been said that adding additional outboards does not necessarily double or triple your horsepower. Example: does twin 50ís make your boat twice as fast now that you have double the horsepower to 100 hp? Would twin outboards totaling say 200 hp be as fast as one outboard with a 200 hp rating under equal conditions? I am trying to determine speed in relation to horsepower rating.

Any thoughts would be appreciated.
A 50 hp Honda on a 5.6 rib pushed it along at 12 knots but with the two both working @ wot 30 knots
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Old 17 March 2010, 13:01   #4
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I have always used the simple formula that a twin engine installation will give a similar performance to a single engine fit of 75% of the combined HP of the twin installation. e.g. 2xI50hp = 1x 225hp. Rough guide but seems to be about right based on my experience.
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Old 17 March 2010, 14:21   #5
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Thanks guys or should I say mates, for the great feedback and the warm nice welcome. Very helpful and as 9D280 suggests... no simple answer when considering the many variables.

I donít have a lot of boat experience, but acquired a retired, badly neglected search & rescue boat that stole my heart. I feel blessed, even if she wanted more than my heart and is presently trying to empty my wallet as well. But hey, can you really put a price on passion!!!

Thanks again
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Old 17 March 2010, 15:41   #6
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Thanks guys or should I say mates, for the great feedback and the warm nice welcome. Very helpful and as 9D280 suggests... no simple answer when considering the many variables.

I donít have a lot of boat experience, but acquired a retired, badly neglected search & rescue boat that stole my heart. I feel blessed, even if she wanted more than my heart and is presently trying to empty my wallet as well. But hey, can you really put a price on passion!!!

Thanks again
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Old 17 March 2010, 17:16   #7
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This thread just proves how informative and valueable this forum is.
I think the answers given, both rule of thumb and scientific models are great.
Keep up the good work.
Regards from a more informed and educated ribnet member
Cheers,
Gerry
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Old 17 March 2010, 20:25   #8
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Example: does twin 50’s make your boat twice as fast now that you have double the horsepower to 100 hp?
No, drag increases as the square so at double speed the drag is four times greater. There will be some other factors involved too but as an approximation, if you double the power you can expect half the speed increase eg. if 50hp gives you 20mph, 100hp will give you 30mph.
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Old 18 March 2010, 05:24   #9
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The other thing to remember apart from extra drag and weight is the running costs of a twin rig - especially if you're struggling to refill the rapidly emptying wallet! One motor is cheaper to service and more economical than two!

Sonar - not a fair test if the second motor was just trimmed out of the way - the weight of the outboard, the extra fuel and tanks were all present and it would have been pushing from one side
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Old 18 March 2010, 06:00   #10
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The other thing to remember apart from extra drag and weight is the running costs of a twin rig - especially if you're struggling to refill the rapidly emptying wallet! One motor is cheaper to service and more economical than two!
Not necessarily double if you can do your own servicing....., but yes wil lneed twice as many spares.


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Sonar - not a fair test if the second motor was just trimmed out of the way - the weight of the outboard, the extra fuel and tanks were all present and it would have been pushing from one side
And probably propped for twins being in action as well, so likely well over pitched.
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