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Old 18 May 2011, 12:00   #1
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What are the advantages of 2 40hp outboards than 1 90hp outboard.
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Old 18 May 2011, 12:01   #2
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What are the advantages of 2 40hp outboards than 1 90hp outboard.
Chris - its an age old topic - search for "twins" and you'll have a good days reading!
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Old 18 May 2011, 12:18   #3
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None for performance and cost.

Double redundancy the only real advantage, but only if you have two batteries, fuel tanks etc
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Old 18 May 2011, 12:20   #4
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None for performance and cost.

Double redundancy the only real advantage, but only if you have two batteries, fuel tanks etc
Yep. 2x40hp is only equivalent to about 60hp.

Looks cool though

Personally I wouldn't have twin engines unless I wanted more HP than can be supplied in one engine-and there's even caveats to that.
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Old 18 May 2011, 12:40   #5
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But worth considering if you lost one of the two you will have a seriously compromised performance compared to a single of the same HP as the one remaining working engine.
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Old 19 May 2011, 04:07   #6
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Double redundancy the only real advantage, but only if you have two batteries, fuel tanks etc
Twin 40s? I could pull start them both with no battery on board!

If you are looking engines that size, chances are you're gonna have a set of Hulks feeding them anyway....

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Yep. 2x40hp is only equivalent to about 60hp.
Not the blanket 20% quote again.....

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Looks cool though
Yep!

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if you lost one of the two you will have a seriously compromised performance compared to a single of the same HP as the one remaining working engine.
True, seriously compromised but still better than a 4 pushing you along at displacement speed. If one of that pair dies, you got a 40 Hp Aux!




In short at thhe small end of the HP scale the differences in drag are nothing like what you'll get trying to shift 2 150+ Hp sized gearboxes through the water at 45+ knots versus a far smaller lump(s) of metal at 25-30 Knots. Drag goes up as approx the cube of the cross section and the square of the speed. I'll leave you lot to do the maths......

Also the "twins are heavier" is mostly myth, and depends on which set of twins & single you choose. Usually heavier with the single anyway by the time you add an Aux.... Also at the big end of the scale the same hardware is de- tuned across a wider variation of HPs, so the weight differences can get quite spectacular.


Do the search as Pol says for details & worked examples!





Admin. Can I combine my various "small end of the scale" posts into a sticky for the FAQ?
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Old 19 May 2011, 17:58   #7
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Twin 40s? I could pull start them both with no battery on board!
Yep, true.
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If you are looking engines that size, chances are you're gonna have a set of Hulks feeding them anyway....
True. The only real disadvantage there is that it's hard to gauge the 1/3 out,2/3 back rule.


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Not the blanket 20% quote again.....
Guilty. It's just an easy quote to trot out.However, 2x40hp is still going to be less than the equivalent of 80hp and it's going to be using the same amount of fuel as an equivalent model 80hp.
You're less likely to have power trim on a pair of 40s as well,with the associated performance restrictions.

The weight thing is entirely dependant on what the motors are-for example a pair of 40hp Yam 2 strokes are going to weigh quite a bit less than an opti 90.
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True, seriously compromised but still better than a 4 pushing you along at displacement speed. If one of that pair dies, you got a 40 Hp Aux!
Worth finding out if it WILL plane on one engine 1st-a 40 correctly propped for twins is quite possibly not going to get you planing on a boat big enough for 2x40hp.
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Old 20 May 2011, 04:21   #8
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Absolutely. Like all things, it's a compromise.

Thing is, even if it won't plane, I would rather it was propped correctly for the twins, because that's how it will spend 99.9% of the time on the move, but if my main engine died halfway through somewhere like the Corryvrechan, I'd still rather have 40 horses to shove my displacement hull out the way than 4......
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Old 20 May 2011, 05:12   #9
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Guilty. It's just an easy quote to trot out.
Same here.

Obviously the maths change depending on the exact setup, but the OP asked what the advantages are and that to me suggests they are expecting to have something comparable to 90HP "in the water" with twins.

Factors to weigh up:

Purchase costs,
Maintenance costs,
Running costs,
Weights,
Operating speeds,
Perceived security vs actual security,
Likelihood of total and partial loss of drive.

Any more?
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Old 21 May 2011, 03:25   #10
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You can do some really nice maneuvering with twins if you get used to working them. Spinning in place, walking sideways, etc. Takes a bit of practice though.

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