Fuel burn

[79_banshee]

Took the boat out on the lake today and we were out for around 45 mins to an hour and it burned 20 litres (5 gal) does that seem like a lot or is that normal. We where crusing at around 5000 rpm and 45 mph. My sec question is should it kind of pull to the right it felt like if I let the wheel go it would have just shot to the right is that just the force of the engine pushing the boat

 

[tpenfield]

79_banshee. You will have to provide some information about your boat in order for forum members to give accurate responses. 20 litres for an hour at high speeds on even a small boat does not sound bad.

 

[Scott Danforth]

Most boats get well under 2 miles per gallon, some get a bit more (not getting into BSFC), 20 gallons in an hour at WOT is not bad. Infact it is really good. My guess is you are running a small aluminum boat with about a 115 hp outboard, or a small fiberglass inboard with a 3.0 or maybe a 4.3 based on the low fuel consumption rate

BTW, cruise is the speed and RPM that gets you the best fuel economy. normally around 3000 RPM and about 30mph. banging against the rev limiter at WOT will burn fuel fast.

If I cruise an hour at WOT, I would burn aproximately 20 gallons of fuel.

 

[Scott Danforth]

regarding pulling to the right, most likely your running an outboard or a 3.0 without power steering. normal prop side thrust.

 

[racerone]

Need to adjust the trim tab on lower unit of that outboard.

 

[79_banshee]

ok thanks guys sorry I forgot to mention it's a Fiberglass banshee with a 115 Mercury. I just didn't think it would burn that much cause it's a small light boat I under stand that at wot a person it going to burn more than at a lower rpm. Racerone thanks for the tip have to give that a try and get it out one more time before the season is over. It seems to grind going into gear forward or reverse that is probably not normal is it. Once it is in there is no grinding or anything everything seems normal but just when shifting from neutral into gear. Am I going to have some lower leg work to do

 

[jimmbo]

Does you engine have power trim? Are you trimming out as the boat gets on plane and speeds up? If the engine is trimmed in too much on plane there will be a pull to the right. Try trimming the engine out. If no power trim then making sure the engine tilt pin is adjusted properly. As mentioned before an adjustment of the trim tab may be needed if trimming the engine doesn't help
What is your Full throttle RPM? 1978 - 1987 merc 115 WOT range is 5000 - 5500 for the inline 6, the 1971 thru 1977, speced 4800 - 5300 rpm. I will assume your engine is the same year as the boat, so 5000 - 5500. I will also guess you are using a 19 inch prop.
5 mpg is very good for an outboard boat.
Regarding the grinding going into gear. You're not trying to gently ease it into gear are you? I hope not. These units are either in gear or not. Easing it in will only wear out the gear and the clutch dog, and make a bunch of noise. A quick swift into or out of gear is the correct way.

 

[79_banshee]

Yes it is a power trim I had it all the way down and was only getting about 40 mph and about 4500 rpm. I did trim it up a bit as we were playing with the boat and it started hitting 45mph and 5000rpm. It's a 17 pitch prop so I know it's not for speed it's for a better hole shot. The engine is 79 as well in line 6 yes. And yes I was easing it into gear not realizing that you should just through it in to gear. Learning somethin new every time I'm on here thanks a lot guys. Have to take it out in the next couple days and play some more.

 

[gm280]

79 B, I had a boat with the same size Mercury inline six engine and when I trimmed it out, I could control the steering pull via the trim. So play with the trim and see if you can releave some of that right-hand pull. Just remember, when you trim it out and then throttle back the boat will try to pull the opposite direction. Just something you will get use to. JMHO

 

[79_banshee]

K thanks for the heads up gm280

 

[Scott Danforth]

Hey, my BSFC calculation was right on for the power your using (didnt go digging to see what you were running)

115hp is 115hp, Most internal combustion spark ignited motors burn about 0.4# per gallon per hp per hour. diesels burn about 0.333# per hp per hour

so if your at WOT using all shes got, shes going to burn some fuel (at the rate you discovered). now at cruise she will burn less as your using less hp.

 

[79_banshee]

Thanks for everyone's input. Took the boat out today and played with the trim a bit more and as I trimmed up I could let go of the steering wheel just as jimmbo said. As I got the trim set I only got a couple more mph but the rpm was coming up to about 5800 so I throttled it back to 5500 got a little more than an hour out of my 20 litre tank too. As for the grinding going from neutral into gear it was a lot better when I just put it in vs trying to ease it in but the reverse is still kind of jumping/grinding. The forward seems kind of like the prop is slipping as well just before it gets on plane once it's up out of the water every thing seems fine.

 

[jimmbo]

more than an hour on 20 liters? That's a lot better than I ever got with my 115. At WOT I was burning about 50 liters/hour @ 5500 rpm. 50 liters is about 10 Imp Gallons or 12 US Gallons. Boat was going 49 mph at 10 gals per hour, I was getting almost 5 miles/gal. That's in Canada. in the US it would be getting just over 4 miles/gal.
If I backed off the throttle to about 4000 - 4500, it did burn a lot less, 50 liters lasted about 2 hours.

 

[79_banshee]

Ok cool thanks so I am getting fairly good fuel burn then. I was a little more concerned about the slipping in forward until I get up on plane and the jumping/grinding in reverse. I'm not easing it into gear like I did the first time I was just kind of throwing/slamming it in to gear giving it 30-45 seconds in neutral before going from forward to reverse and vice versa

 

[jimmbo]

You don't have to wait 30 - 40 seconds in neutral when shifting from forward to reverse or vice versa. Almost all shifting is done at idle speed, Of course if you are coming off plane, you shouldn't normally be slamming it into reverse until you are at idle and the boat is creeping along
The slipping could be a vented prop, which is designed to slip at low boat speeds

 

[79_banshee]

K there seemed to be a bit of a shudder the last time I shot out of the water then we heading back to the dock cause we were almost out of gas. I was at low speeds as I was shifting I just was many to sure how long to wait to shift from forward to reverse. It seems to grind and go into reverse then it kind of jumps. Makes me a little nervous. Maybe I'm just worrying about nothing just don't want to be out in the middle of a lake and be paddling back lol