One probably for GLUDY

DERF

DERF

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A boat has 1 x 300hp motor.

If one wanted to achieve the same performace but with 2 x smaller motors, what hp should the twin motors be? 150, 165, 175, 200?

I have asked a number of people in the marine industry with very different results.

the relionship of power/weight vs performance cannot be linear.

Does any one have any emperical evidence?
 
This might give you a clue (Sealine S29 performance figures)

1 x Volvo Diesel D6 310/DP @ 310 hp 36/38 knots
2 x Volvo Diesel D3 160/DP @ 160 hp 36/38 knots

Personally I don't completely believe these figures. The extra weight of the twin installation plus the additional drag from having an extra leg in the oggin would make the twin engined boat slower. Therefore my answer to your question would be 1*300 = 2*165.

Pete
 
What are you on about? Extra Weight?

Each D3-160 weighs only 330kg, total 660kg, whereas a D6-310 is 750kg.

So the drag of the extra outdrive probably compensates for the 90kg LIGHTER twin installation...

dv.
 
Well on a big RIB I have been let to believe that twin outboards provide 60% more performance than a single outboard of the same size.

So if y=given performance.

Single engine: 300=y

Twin engine: 2x300=1.6y

y=600/1.6=375

Therefore 2 x 187.5HP engines would give the same performance as 1 x 300HP

I think.
 
Yes, Mr. Magnum, we had better call for Gludy, I like your maths but you've successfully proven that 2 x 187.5 is 375 Hp which is 25% more power to start with over a 300Hp single. Surely if 2 engines are 60% more efficient the engines would have a combined Hp of less than 300 not more? Far more convincing if 2 x 100Hp outboards produced 320Hp i.e. 60% more! Don't you think? /forums/images/graemlins/confused.gif
 
I don't know the answer to the original question, but I think you've misunderstood magnum.
He's postulating that the two 187.5hp engines will provide a total of 1.6 times 187.5 hp i.e.300hp.
If his theory is right, then his sums are too.
 
Maybe I didn't make myself clear. What I was saying is that 2 x 300HP engines would give 60% more performance than 1 x 300HP engine.

I never stated 2 engines are more efficient, in fact my maths proves they are not with a doubling of horsepower only giving a 60% improvement in performance.
 
Its all boils down to leg pulling this thread - the question is if its ok to just pull one leg or two at a time .......

In terms of simple efficiency I have always preferred to pull one leg at a time. Pulling two legs at a time usually needs more than double the pull.
/forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif
 
For outboards the 2x165 or 2x175 would be about right. Need to be careful about flavors here, 2-stroke vs 4-stroke. For inboards (shafts not sterndrives) or jet drives it gets a lot more interesting. Dunno what the correct call would be for those cases.

Kelly Cook
 
Or maybe Engines are like resistors in Parallel...
Two engines of the same value equals half the value of one of them.

ie...1/R = 1/r1 + 1/r2

That could be transposed to read: 1/hp = 1/hp1 + 1/hp2

Mind you you could put them in series....ie one behind the other then you would have:

hp tot = hp1 + hp2.

But I must admit it would be a bit inconvenient climbing over a 200hp lump to get from the galley the the dinette has anyone tried this??


Steve
 
For an isosoles triangle, the sum of the square roots of the two shortest sides is equal to the square root of the longest side, therefore the answer is clearly 42.
 
60%

Mags,

Thats the whole problem...

I've been told 40%, 50%, 75%, from three different sources. Using your maths that's a diference between 2x 215hp or two 175hp

It does make a considerable differece to the weight, fuel burn and handling of a relatively light RIB.

Could be a costly mistake to make!
 
Re:ahoy electrickery person!

If they were the same as resistors thats exactly what you'd get in parallel...ie wired together side by side...actually for my old Perkins it's probably not far off.

ie 2X150hp side by side = probably 75hp true output.


However, seriously though...like all Power calculations Inverse square law probably applies....and if you want to Double your speed then you would have to quadruple your true power output...but then that relies on hull performance remaining constant throughout the power transfer curve...etc etc..
Oh bug*er it I think I feel an SD versus planing coming on. Where's GLUDY...???

Steve
 
Re: 60%

My guess is that in terms of efficiency you would be better increasing the power of a single engine, ie replacing what youve got with something bigger.
A 40% efficiency loss (but it's probably more when everything else is taken into cosideration), is quite a lot, and you would be burning proportionately more fuel than the efficiency loss becuase of the heat losses from another engine.
Dont ask me to work it out..it's just my gut engineering instinct.

Steve.
 
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