Single vs Twins

[Guest]

The age old argument, I know. But here it comes with a new twist.
I've recently had it suggested to me that two drivelines are approximately 10% more efficient than one. Now this runs against all that I've ever heard, read, or understood. We are not talking about a counter-rotating system such as the duoprop, but two entirely separate drivelines - be they outboard, inboard or sterndrive.
Can anyone point me towards comparisons between otherwise identical boats, tested with both single and twin installations?

Will

 

[oldsaltoz]

G’day Will,
You don’t say where you got this information so it’s a bit harder to verify, however, Bruce Roberts used to have an excellent article on this. He is an International Designer and has no axe to grind. I will continue to search his extensive site and send you the URL. Then you can decide.

Andavagoodweekend Old Salt Oz……/forums/images/icons/cool.gif

 

[Guest]

Ah - do I detect a fellow Aussie by some chance?!!

The info is by no means from an official source - simply a suggestion by someone (from another forum in fact!). I'll be interested to see what you come up with from Mr. Roberts....

I'm sure that I've read inumerable comparisons in the past - I'm just not sure where to find them....

Will

 

[Deleted User YDKXO]

Normally, this is'nt true because the weight of a twin installation is far greater than a single with the same power output. It may be a reference to prop efficiency. I stand to be corrected but I believe a smaller prop is inherently more efficient than a larger one but, as I say, I think any efficiency gain is negated by the extra weight

 

[ccscott49]

Why do we have reduction gearboxes and bigger props, if smaller props are more efficient? It doesn't sound right, but a lot of things that are right don't sound it!! /forums/images/icons/smile.gif

 

[Geoffs]

For certain, not right. Big slow revving prop more efficient that small high revving prop.

Thinking about it again, probably only at displacement speeds, can't see a 50X50 prop doing 100 rpm pushing a boat along at 40 knots, even if it would fit.

Old Chinese proverb 'Man who sail boat into rice field, soon get into paddy'

 

[Deleted User YDKXO]

Well I did sort of say I could be talking bollocks. We need an expert

 

[mustard]

Hey, hey, I got pasted for using b****ks !! /forums/images/icons/wink.gif

 

[oldsaltoz]

<blockquote><font size=1>In reply to:</font><hr>


G’day Will,
And yes, you have detected a fellow Ausie. I had a good look around the Bruce Roberts site, but as it has over 1,000 pages, I gave up and e-mailed them, below is an extract from their response. And remember this bloke has no axe to grind and access to hundreds if not thousands of boats built to his design and fitted all manner of engines.

“Quote”
<font color=blue>
Basically, as a rule, a twin engine installation requires 25% more total hp and correspondingly more fuel consumption than a single engine to achieve the same performance.</font color=blue>

“End Quote”

I hope this answers your question.

Andavagoodweekend Old Salt Oz……/forums/images/icons/cool.gif

<hr></blockquote>

 

[Deleted User YDKXO]

I was using the word in its biblical sense. Anyway, its in the OED so it must be OK

 

[ccscott49]

Tell me who it was who gave you a pasting, mate! The swine, I'll kick him in the bollocks for you! /forums/images/icons/wink.gif

 

[mustard]

It was RayB! It was when I was just a learner. He made me astericks it, like v****o. I was so ashamed, and being PC and antipodean i apologised meeekly, and here you are, using it freely! The shame of it...

Next bloody time....

Kick his dogs' when you done his, will yuh! /forums/images/icons/wink.gif

 

[ccscott49]

There, there lad don't go on so, it'll be oright', we won't tell nobody back in Oz what yer' dun' apart from a few chosen mates I have down there on channel 7!! /forums/images/icons/laugh.gif

 

[Guest]

Re: b*******s

Well the Sex Pistols got away with out their album called "Never mind the bollocks we're the Sex Pistols"

"The Med is calling me"

 

[mustard]

Bloody insults, too, I have to suffer now tooo !!

Thats NZ not OZ...../forums/images/icons/mad.gif

 

[Guest]

In his argument, my colleague suggested that weight for the two systems must be the same. Possible, but not common, I agree.
I think you will stand corrected as far as propeller size goes. Most designers generally strive for the greatest diameter possible, but are constrained by things like draft, hull interference etc. That is one of the greatest advantages of using surface-piercing drives, where diameter is almost unlimited.

Will

 

[ccscott49]

Sorry! NOT!! didn't realise you were a K one W one! /forums/images/icons/wink.gif I thought you would be pleased to be called an Aussie, nobody would admit being a KIWI, surely? /forums/images/icons/smile.gif

 

[mustard]

Now look here, you, you (splutter, splutter) POMMY GRANITE, you RAGGIE, its no good taking out your America’s Cup inferiority complex on us humble Kiwi’s.../forums/images/icons/wink.gif

 

[ccscott49]

Careful laddie, pommy refers to Englishmen, I'm a Scot!! and a ragpotter to boot an' I don't give a stuff about the mercan' cup neither, however please, don't mention the rugby!! Pretty please? /forums/images/icons/wink.gif By the way Humble and kiwi should not be used in the same sentence!! /forums/images/icons/smile.gif

 

[mustard]

A Scot, eh! Thats all right then. Me Kiwi mum used to put me in a kilt and me bruv's played in a pipe band, and we all went to inglesides, and we are all emotive when we hear the pipes...

So truce, truce... and your've got both sails and an engine or two so that's OK, and both my brothers were All Black trialists.. (oops!) /forums/images/icons/smile.gif