Mercruiser 1.7 Fuel efficiency

Paul M · 08 March 2004, 22:11

Does anyone know what sort of efficiency a mercruiser 1.7 should be running at in a 6.5m RIB?? I have a 21inch prop running at 4,000revs travelling at 27knots and is burning 32litres an hour with only 2 passengers - does this sound right to anyone?

Thanks
Paul

simmons0 · 08 March 2004, 22:48

That sounds awful!!!!!!

Be worried, Mercruiser claim 9 lph but you should be expecting 12-15 lph.

Is that 32lph when its Flat out or just general cruising speed?
Could be that flat out it will return that sort of fuel consumption.

Send a PM to Mike C or Andre who actually have these engines in a boat. Not me who has one in a box..

Andre · 09 March 2004, 12:51

Paul

I have or should I say I had a Parker 630 fitted with the Mercruiser 1.7 L and 21"LASER II prop(Just sold it and having the same again) Anyway back to the subject. At 4000 + revs you should be pushing over 30knots (31/32)

At 27 knots the revs on my engine were up to 3600/3800 from memory

as for the fuel consumption then there is for sure something wrong with your motor cos I always like to give as an example my cruise with Mike C to Cherbourg in June of last year. Nab Tower to Cherbourg 66Nm (top up from a jerry can at Nab Tower) ,cruising speed all the way 26/28knots and filled up in Cherbourg 46 liters.
Total passage time 3hours. 2 persons on board and the sea all the way was flat not a wave in sight.

MikeC has a Humber 630 with the same engine and his fuel consumption was the same

Andre

Garygee · 09 March 2004, 13:22

Hi Paul

Cant comment on RPM & speed without knowing load levels & any hull fouling.

However 32L/H for 27Kts, I am getting better than that from a 240HP Yanmar and a 7.8M heavy RIB so I would say their is somthing wrong.

Regards Gary

Mike C · 13 March 2004, 23:53

Hi Paul,

I'm a bit late joining in on this... Hardly been on Ribnet for ages... Too busy with work alas!

Anyway, 32 LPH is absolutely awful. I've done several checks on mine and it always comes out around 10 LPH for an average cruise.
I get 30 knots at 4,000 revs with a 23" prop and a full tank of fuel, and about 32knots at 4,200 revs as the fuel gets lower. Mine is a 6.3 Mtr boat weighing around 1.2 tonnes... (Those are maximum speeds... I usually cruise at about 23 knots)

So I would take a look at your setup.

Best of luck,

Mike C