Fuel economy

[gruntahunta]

I have a 4.75 metre Quintrex Coast Runner with an 85 2stroke Yamaha fitted with a 17iinch Pitch prop. At around 4500-5000 RPM, GPS reads around 50kmh.

20 Litres of fuel disappeared in about 20mins. Is this about right, I am thinking way too much fuel for such a little time. The boat gets out of the water with no more than 1/4 throttle instantly and I am not looking for fast top end speeds, should I try a 21 inch prop which will reduce revs considerably and help fuel economy?

I tried cruising around at 20 -30 Kmh yesterday and I am sure that has helped but really want to go a bit faster than that going out to the Blinker at Maroochdore or around Green and further etc.

The other problem I had was the boat pulled HARD to the right, so have adjusted the Trim Tab( which was not facing straight), and hope this fixes that problem next time I go out. Would this also contribute to the bad fuel economy?

Any advise would be greatly appreciated.

Cheers Pete

[Angla]

If you knock the throttle back to around 2/3 of the max rpm or say 3000 rpm then you should find a better economy. Another thing that sucks the juice is when you alter the throttle constantly.
If you can cruise out at one speed then you will get a better economy for sure.
Can you trim the motor at all? as this then reduces the friction of the hull length and will allow the motor to rev more freely, giving you more forward thrust. then reduce the revs back to that 3000 range and economy will also be better.

Enjoy your boating

Cheers
Chris

Ps the trim tab you speak of is the anti twist thingy to oppose the rotational torque of the propeller (or something like that). I reckon that should stay where it was. Not straight backwards

[gruntahunta]

Cheers Chris. I have to do something with the steering pull cause it is dangerous, take the hand off steering wheel and around you go. So I will need to adjust that, will take sokets out next time on water.
How much fuel should I expect to use in say 1 hour of cruising, trimmed out of course doing say 3000RPM,s?

The biggest motor I had b4 this was a 50Hp Johnson on a 4.5metre Savage. Fuel economy was exvellent.

[lucee81]

i have a 90 yammie on a 5.5mt and use about that much fuel if the canapy is up.
Not sure if you have a canopy but it makes a world of difference i found. also i run a 4 blade s/s 13 1/2 inch pitch on a four seasons bow rider.
Wouldnt using a bigger pitch make the motor work harder thus using more fuel?

[Tailortaker]

I've got a 50hp 2st Merc on a 4.6 side console, I'm sitting on about 50kph at 5000rpm and use nearly half the fuel that you are.
Cheers, TT

[Fed]

Balance the boat front to back & left to right as best as you can.
Move the trailing edge of your trim tab to the starboard side and trim your motor out.
If you don't have power trim move your trim limit pin out, I'd suggest 2 holes to start with.
Providing the steering is then OK (feels safe) give it a WOT run and check your RPM & speed, you should be getting min 5500 RPM & close to 70 KPH.

[Noelm]

OK, lets look at two problems, one at a time, the bigger proprellor to keep RPM down will NOT save you fuel, you need the right prop to save fuel, make sure your motor can acheive full max RPM with a lightish load, if it does, the prop is the rght one, too big and you will use a lot more fuel, and slowly kill your motor! now for the steering bit, the little trim tab needs to be adjusted to steer the MOTOR, it kind of works oposite to you think, you need to adjust it so at cruise speed, the steering is somewhat neutral, with cable steering, you will never get it truely "hands free" the steering will vary with trim and speed, so it needs to be right at normal cruise speed, your fuel use does sound to be a tiny bit high, at a litre per minute, not too sure what goes on there just yet, lets sort out one thing at a time.

[Malcolm W]

74542


60 litres per hour for the 3 cylinder 85 seems excessive to me. At 50km ph thats around .9 km to the litre. My 115 2 stroke is on a plate 5.6m and full with dive gear averages 1.2 1.5 km to the litre @ 3800 rpm. At 3000 rpm on your 475 quintrex I would expect the 20l to last the hour. Have a look at the link above its the 85 on a heavier boat than yours.

[Noelm]

I've got a 50hp 2st Merc on a 4.6 side console, I'm sitting on about 50kph at 5000rpm and use nearly half the fuel that you are.
Cheers, TT
yes, but at a lot less HP remember. Nearly half the HP = nearly half the fuel.

[Lachie1]

For what its worth, in my experience, generally the best fuel economy for a decent speed is usually obtained around 4000 rpm. At around 3000 rpm most boats are struggling to stay on the plane and are displacing a fair bit of water creating slower speeds and increasing fuel use..

Regards,

Lachie

[hard2catch]

Hi Gruntahunta,something definitely wrong there.My last motor was a 90hp merc saltwater,according to a review by Andrew Norton,the most fuel this engine could flow is 37litres/hour.My current boat is a signature 610/175 opti,which cruises at 3500 RPM at 50kmh using 27 L/ph.

[gruntahunta]

Hi Gruntahunta,something definitely wrong there.My last motor was a 90hp merc saltwater,according to a review by Andrew Norton,the most fuel this engine could flow is 37litres/hour.My current boat is a signature 610/175 opti,which cruises at 3500 RPM at 50kmh using 27 L/ph.

Yes, something is not right from those figures......motor seems to run fine & sounds great, starts instantly, so I am thinking it is not that it is out of tune or something like that.

[Noelm]

you need to start at the very basics, do your WOT test and see what RPM you can get, no use chasing anything untill you start with that first.

[Malcolm W]

Hi Gruntahunta,something definitely wrong there.My last motor was a 90hp merc saltwater,according to a review by Andrew Norton,the most fuel this engine could flow is 37litres/hour.My current boat is a signature 610/175 opti,which cruises at 3500 RPM at 50kmh using 27 L/ph.

That looks about the same as the figures quoted in the PDF I posted above. The maximum fuel usage for the 85 yamaha is no more than 35l per hour. 20l in 20mins is almost double, must be some un-burnt fuel going somewhere. You seem to be getting ok top revs as 4500-5000 is maximum quoted for this motor by yamaha.

[Tailortaker]

yes, but at a lot less HP remember. Nearly half the HP = nearly half the fuel.

Yes true, but my point was that with a boat only just smaller and nearly half the hp I'm achieving the same speed at the same revs. I realize that my boat is smaller but would 0.15m and a bit more weight really need another 35hp to push it at that speed with those fuel figures? I would think that with less hp like mine it would have to work harder to get the same speed therefore increasing fuel consumption.

[gruntahunta]

you need to start at the very basics, do your WOT test and see what RPM you can get, no use chasing anything untill you start with that first.

Yes Noelm...I indeed will be doing that next time out......bloody work interfering as usual....Thank you

[nigelr]

If you can get hold of a spare 13.5" prop, I'd be trying that, 17" seems a bit large to me for that set-up. Sit on about 3600 - 4000 and see how you go; my 2S 90 (on a 5.3 tinny) chews the juice at full stick (5100-5200) but is pleasantly frugal at about 36-4000, which is generally fast enough for my open water useage. If I want to go faster, I just accept the extra fuel usage, inherant with a 2S I guess.