Twin v Single fuel costs. Theory and reality

[Hugin]

....Nimbus make some nice single screw boats.

Agreed, but as far as efficiency goes, Marex seem to be ahead of the pack

Nimbus 365 Coupe and Marex 370 ACC have almost identical dimensions for both overall length, waterline length and beam. Displacement is also the same within 100kg, both just under 7 tons

They can both be delivered with a single Volvo D6-435 and a straight shaft. The Nimbus run out of steam around 28kts while the Marex manages 31kts.....
This should mean that Marex 370 will use around 20% less fuel than the Nimbus for any comparable cruising speed.

I understand some kind of "sleeve" around the spinning shaft contribute to Marex's reduced drag at speed, but not sure if this explains the entire difference. If yes, then that particular sleeve manufacturer should have golden days coming up

 

[Wavey]

Anything in the Broom range up to and including the new 430 is available with a single. Makes a considerable difference to the price.

When we were looking at buying a 42cl, one in Eire with a single made it to the short list.

 

[Sneds]

Our Mustang 2800 has two AD31P 150 HP diesels driving via VP duo prop out drives and we use around 30 LPH at 3000rpm cruising at around 20 knots.That 30LPH total, not each engine.
This fantastic economy is the main reason we are keeping hold off her.
But back to the OP's question, all other Mustang 2800s are fitted with only one engine (each) either a petrol or diesel unit. There are a few diesel ones in the UK, anyone reading this own one and want to compare usage?

 

[Sneds]

The down side of course Is two engines and drives to service and the associated costs. Would I prefer a single unit set up?
Yes.

 

[Viscount]

Our Mustang 2800 has two AD31P 150 HP diesels driving via VP duo prop out drives and we use around 30 LPH at 3000rpm cruising at around 20 knots.That 30LPH total, not each engine.
This fantastic economy is the main reason we are keeping hold off her.
But back to the OP's question, all other Mustang 2800s are fitted with only one engine (each) either a petrol or diesel unit. There are a few diesel ones in the UK, anyone reading this own one and want to compare usage?

Impressive fuel consumption; if my arithmetic is correct, that's about 3nm/gal. I have twin AD31s (130hp) and get about 2nm/gal. What is your displacement?

 

[MapisM]

we use around 30 LPH at 3000rpm cruising at around 20 knots.That 30LPH total, not each engine.

Mmm... I know close to nothing about both the AD31 and the Mustang 2800, but assuming that those engines burn 200 g/kW-h (and if I'm wrong, I bet that the mistake is rather on the optimistic side), you are implying that it only takes 160hp or so in total, for cruising at 20 kts.
That sounds like a helluva result, if she is - as I would guess - a 28 footer.
How big/heavy is the boat exactly?

 

[Beamishken]

Mmm... I know close to nothing about both the AD31 and the Mustang 2800, but assuming that those engines burn 200 g/kW-h (and if I'm wrong, I bet that the mistake is rather on the optimistic side), you are implying that it only takes 160hp or so in total, for cruising at 20 kts.
That sounds like a helluva result, if she is - as I would guess - a 28 footer.
How big/heavy is the boat exactly?

we ran a searay 290 sundancer fitted with 2x300es diesel mercruisers (4.2 efi diesels) and over 5 years of ownership we worked on 2.5-3mpg at a usuall cruise speed of 27 kts @2700rpm so im not surprised at sneds 3mpg with the smaller engines in the slightly smaller mustang

 

[MapisM]

Well, good for you.
I never came across a twin diesel boat in that size bracket capable to make more than 2 mpg at best, but I take your word for it.
Otoh, I just had a quick look on boattest, and found a few somewhat comparable boats, none of which can make even 2 mpg at cruise speed.
http://www.boattest.com/boats/boat_video.aspx?ID=1945#Test-Result
http://www.boattest.com/boats/boat_video.aspx?ID=349#Test-Result
http://www.boattest.com/boats/boat_video.aspx?ID=500#Test-Result
Not saying that your or Sneds boat can't be as economic as you are reporting, of course.
But hopefully explains why, while you are not surprised, I was a bit...