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Help Me Figure Out Where MY MPGs Are Going In My Four Stroke

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So I have a 2002 Yamaha F115 4-stroke on a 17' Lund Fisherman. For the last several years, I have noticed it seems to get really poor gas mileage, but I never got around to GPSing my distance and doing the calculations to figure it out. Well this week in Canada I finally took the time to figure out some stuff. We ran the boat about 10 miles each day (took it across a public lake to a secret portage, left the boat tied up in a well-used channel, so we didn't leave anything in it, like my HDS-8, that could get stolen). I'm a horrible estimator of distances, but I would be shocked if the trip across the lake was more than 3-5 miles, and probably closer to 3. Then when we filled up the tank (it was full at the start of the trip) we had to add 7.5 gallons. So that means with a total distance traveled of 12-20 miles, I was getting at most less than three gallons per mile, and probably closer to two. Now from everything I've read I should expect anywhere from 4-6mpg from my motor, and closer to 5-6. However, there's an additional bit of information. My RPMs are seeming to max out at about 4400. The motor sounds normal, but the RPMs aren't revving near their max, and my speed at full throttle (minimal load) is in the low-mid 30s, I think it should be almost 40.

So my first thought is maybe it's a bad prop? We've gone through a couple, and the last one my dad had put on, so I don't know the specs of it. He knows next to nothing about this stuff; to him, a prop is a prop. I know a prop with the wrong pitch will affect your RPMs, which then would affect your speed. Would that affect your gas mileage (roughly cutting it in half) that drastically? I realize that I shouldn't expect my best mileage to be at full throttle, or even 4400rpm, but I can't imagine it would go down that drastically.

One or two years ago the motor was serviced and tuned up by a local marina, and pronounced it good to go. Since then it's hardly been used (maybe 20 hours, if that). The oil/gear lube is a couple years old, but again it's barely been used, I do plan on changing it this fall though, if not sooner.

Well that's all the information I can think to give you guys, does anyone have any advice/tips on how to figure out the problem? Thanks!

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I bet you have too much prop - meaning too high pitched. What is the pitch on the current prop? That could be the source of too low RPM and also the source of horrible fuel economy.

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If you did 20 miles and 7 gallons that is 3 miles per gallon, not 3 gallons per mile which would be even worse than my old Merc 115.

Which lake? Can you measure the distance on a map or google earth? I am really bad at distances on the water, maybe you are too?

Your fisherman seems very similar in size etc to my 1998 provee 1775. Sounds like you have too big a prop pitch on there. Mine would go 40 or so WOT with the 115. I would think maybe a 17 or even a 15 might be best. I think merc has some recommendations and boat tests that might give you an idea.

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Use the GPS or google earth to tell you how far you've gone. Google earth is only a reasonable option if you went a few straight lines, of course. Until you do that, you're just guessing.

I took my boat on a cruise from Carver boat ramp to the MSP airport and back, which was over 50 river miles, a couple years back and got roughly 4 mpg doing WoT about half the time and 80% or so the other half. This is a 2 stroke 125 hp on a 1850 sportfish, 1998 vintage. If you're getting less than that with a 4 stroke, something is wrong, and the first thing I'd suspect is the prop. Do some research to figure out what you have on there and what you should have. If unable to do the research yourself, might be easier to just call someone who deals in props. Soderbloom is a good option, I've found they have good prices and fast shipping on props (and repairs).

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When my Optimax 135 was not getting to full rpm's it had a bad injector. The motor was only running on 5 of 6 cyclinders. The motor was smart enough to run fine through out the RPM range, it just would not go that last 1000 RPM at WOT.

Just throwing that out there.

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Thanks for the tips guys, I'll pull the prop off sometime this weekend and see what the specs are on it. If I had an injector problem, wouldn't that have came up when my dad had the marina tune up he motor a year or two back? Thanks for the tip though.

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The spec's for the prop should be right on it, no need to remove it.

I had a 17' ProSport with a 90 honda carb, and with me and a half tank hit 39.1 max on a gps. That was with a stock 17 pitch aluminum prop.

To really get an accurate number, you have to run the gps distance. An estimate of "3-5 miles" just doesn't cut it. Many gps units have a trip calculator you can use to get exact distance. Simply "reset trip" before you start, then run your boat for X number of gps miles, then refill tank. That should give you a close MPG for it.

If you really are getting only 3 mpg, something is wrong. My boat averaged 6.5 overall. I spent a week on Rainy and traveled 126 miles, from idle to WOT and everywhere in between. When I refilled it took 19 gallons.

I would suspect your rig should be in that same ballpark. My 2c

PS: Or your tach is bad. I had to have mine replaced on my '12 Tyee 1950 last fall, was only showing 4100 rpm while running at 51.7 mph/gps with a 175 Verado gringrin

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I get 2.1 MPG at wide open in my boat. Wanna trade?

On the serious side it's your prop, but don't expect to get 10 miles per gallon it's not going to happen. It's a boat, it has tremendous drag going through water and at wide open throttle fuel economy is just not there. If you want to get maximum economy out of your motor the best thing you could do is, if your motor is capable of it, to network it to your sonar so you can monitor your fuel economy while under way.

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Sorry guys! Been a busy week. I checked and it looks like it's a 13x19. Now my next question is what should I get instead? I've seen some guys saying they run 19s and get good performance, others saying they had to move down to a 17. Same thing with the other number. Now obviously different boats will need different props, but I've tried to keep my research to 17-18 foot console deep-v's, and I've still gotten mixed results. I'd appreciate any advice you guys could give.

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Call Jay Soderbloom in Cromwell Mn. and he will tel ya what to get. He has done great work on my props and has the best prices. Im thinking a 17 stainless would do ya wonders.

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You really need to consult with that prop guy or work with a boat dealer willing to let you try different props. It's critical to have you motor propped to get in the recommended rpm range at full throttle with your boat carrying its normal load of fuel and people. I learned the hard way propping for mph not rpm and damaged 2 motors. My mechanic layed it on the line saying can you pull a fully loaded trailer from a dead start with manual tranny in a high gear? Yes by flooring it and over working the engine or you can put it in the proper gear and take of nice and easy and not over work the motor same principal . You are over working your outboard and using more fuel. Look in you manual and see what the recommended rpm rage is and try to find a prop to hit the high end of that and you get the most performance and fuel economy as the engines is working the way it was designed. In my case I had propped to 45 mph @ 4400 rpm blew rings after 2 years rebuilt engine. Mechanic re propped to run 37mph @ 5750 rpm motor ran 10 years without another issue.

Mwal

Mwal

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