One against two

[chooky]

This question arose out of a pub debate as Dignity stated.

[stevej]

Mmmmm. What if I chose to run a 90hp on my boat instead of the 115 it has on it? I reckon it would still be able to cruise at 20 knots but the 90 would probably be working harder and using more fuel than the 115 at the same speed. Kinda flaws your theory a little.

my 90 is the same block as the 115 hp 1.7l v4
nothing would really change just less top end

[stevemid]

I've come up with a new saying: "All things being equal never are."


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[fisho64]

To be somewhat scientific about it, to propel a certain boat at a set speed, requires X HP, now whether that HP is produced by 1 engine or two makes little difference, it still takes X HP, and to produce that said HP takes the same amount of fuel.

thats way flawed Noel.

There are efficiencies in scale and two sets of gears transmitting 150hp each is less efficient than one with 300hp
Take it to extremes, how much do you reckon a containership would burn at 20 knots producing 50,000 hp?

However Chookys comparison of a poorly set up single doesnt give much to work on.

[chooky]

the plot has been lost somewhat. Hypothetically a boat has a single 250Hp but requires a 300hp. You move the 250hp over and strap on another 250hp (500hp). You would be down in the rev range to achieve the same cruising speed that you would attain with the single.
Would there be little difference in fuel burn as the single 250hp would be working a lot harder.

[Noelm]

We are not comparing one boat to a container ship, we are comparing the same boat cruising at a determined cruise speed, and whether 2 motors of equal HP to one big would use more or less fuel to maintain that cruise speed.

[scottar]

In a perfect world - where the massive extra weight didn't upset the applecart and there were no losses either within the engine or due to hydrodynamics they would be about the same I suspect. In the real world I think you would totally upset the performance of the hull and due to this and the fact that there are losses the twins would use more fuel.

[gofishin]

In a perfect world - where the massive extra weight didn't upset the applecart ....
....In the real world I think you would totally upset the performance of the hull and due to this and the fact that there are losses the twins would use more fuel.

x 2 for above!

Chooky, the problem with your 'hypothetical question' is that the 'twins' option is way above that supposedly required (over the top even), but the single is only just under.

In addition, these days a 300 sticker is the same weight as a 250 sticker (in one V6 colour) or marginally heavier in another V6 colour (but with ~double the actual extra HP from memory too). So being 50 'sticker HP' under may not really be that bad!

But hey, given perfect conditions, I know which one would win a race - on a dead-flat sea.

Maybe a more 'somewhat realistic' comparison would have been (if you are using the same motor choice) is:
Recommended power 200-250
Max power 300
Option 1 - 1 x 150
Option 2 - 2 x 150

Then the answer would be reversed. Funnily enough, on rare occasions you do see a boat which is way underpowered like above, and could have two of the donks which it has one of!

Getting back to reality, the best technical comparison of twins vs a single, that I can remember reading, was by F&B back in Aug/Sept 2008.

They did a detailed performance comparison on two identical 685 Explorers, one with an F250 and the other with 2 x F115's. A summary can be found here:

http://www.australianboatmags.com.au...VEY31_prev.pdf

I have the mag somewhere, but it has been that long since I have read it the details are a bit vague.

From memory:

• at cruise - similar performance, near identical fuel consumption
• WOT - single of course had more top end (less drag & slightly more HP)
• grip & 'zap' of the twins was understandably fantastic

At the time I was considering both options on my 685, but I went the single, and am very glad I did.

Cheers
Brendon







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[Lovey80]

the plot has been lost somewhat. Hypothetically a boat has a single 250Hp but requires a 300hp. You move the 250hp over and strap on another 250hp (500hp). You would be down in the rev range to achieve the same cruising speed that you would attain with the single.
Would there be little difference in fuel burn as the single 250hp would be working a lot harder.

In that exact situation, unless the boat was already set up for twins with twin tanks etc and they were absolutely necessary... the solution would be to sell the original 250HP and put it towards the money being spent on the additional 250HP and get a single 300HP.

[stevemid]

Brendon, the the F&B review only came with page 1 (or is it my setup?) Can you attach the whole article please? IMO this article comes closest to "all things being equal" in the One against two debate.
Steve

[gofishin]

Hi Steve,
Not your setup or problems your end, only the first page is available as a free download. Members can view whole file - which I no longer am.

Will dig the article out for you at home, will take a few days though. PM me your email.
Cheers
Brendon


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[LoungeLizard]

The simple answer is: "There is no simple answer"

Without further knowledge of the hull, motor characteristics, water conditions, RPMs @ cruise speed and so on it's simply not possible to say whether a single or twin setup will use more fuel.

From your example, when you say "boat has a single 250Hp but requires a 300hp" do you mean recommended HP? If so you could possibly still achieve cruise speed at (for argument's sake) 4500 rpm. With twins this might drop to 3000 rpm. If that's the case I would say the single will still be more fuel efficient.

However, if the boat is grossly underpowered and the single needs to revved to WOT (at which point it become VERY fuel inefficient) then the twins would win.

[Beatsworkin2]

Next boat will have twins reason because it looks good reason2 safety reason 3 as u get older u can afford it

[honda900]

worth a look.

http://www.youtube.com/watch?v=gG_-YmGN8SY

Regards
Honda.

[caravellerob]

Wow thats suprising, I would have thought the single would have been a bit quicker top end. Looks like the twins beat the single in every way except for fuel consumption.