fuel burn Merc 15hp 4 stroke??

[Lowrider1]

I know...this is a how long does it take to get from here to there question. I bought a new Merc 15 hp fuel injected four stroke last year and put it on a 12' Duroboat. It is broken in barely and it took about 3 gallons to run about 6 hours, most of which was idle to half throttle and back down again so I don't really have a good feel for what it will burn at 3/4 throttle or thereabouts. I'm thinking about putting a 6-gallon alum tank under the front seat for two reasons: 1st to put some weight up front to help balance the boat and 2nd to be able to fill the tank and do several days on big lakes without need to refuel. I'm heading out with the boat as soon as the water gets soft again...maybe April so I'd like to get a tank installed in the next month or so.
Question...anyone have a like motor/boat combo and can estimate fuel consumption?

Thank folks!!

 

[Pappy]

At the risk of sounding a bit abrupt. This is a common sense question.
Run the engine like you normally run it and log the fuel you burn. Simple as that.
Common sense again. If you need the weight up in the bow move weight up there that stays in place moreso than a fuel tank that loses weight.

I put a 6 gallon tank in my Gheenoe for a while to see if I needed it to run to a place I fished a lot. Found out I did not need it so replaced it with a 3-gallon. No biggie. That took 18# out of the stern in a narrow boat.
When I flew a lot more I calculated the fuel burn knowing that the FAA says I need to have a 45 minute reserve left in the tanks when I landed. They pretty much made this rule to force folks to use common sense when flying since nothing is ever absolute. Weather, available airports, etc.
This pertains to boats as well.

 

[MrGiggles]

Rule of thumb is 1gph for every 10hp at wide open. So your 15 should drink 1.5 gph or so.

Throttling back to ~75% can cut fuel consumption down by half.

 

[LDUBS]

I ran a four stroke merc 25 on my previous 15' side console boat. I never had a problem with having the 3 gallon tank, but then ran it at up on plane for probably no more than 20 to 30 minutes during a typical day fishing, and just about never on WOT. Rest of the time was trolling at idle which just sipped fuel. If I thought I needed to run longer I brought a 1 gallon gaso can just in case. BTW, a side benefit of the smaller 3 gallon tank is you turn the gasoline over enough so as not to have to worry about it going bad. Even with all of that, I always did want to have a 6 gallon tank though, just for the added peace of mind. Current boat has a 12 gallon portable tank. I'm happy with that.

Four strokes being heavier I think you are probably making a good decision about moving the tank up forward. Might want to consider going with a portable 6 gallon tank just to see how it works for you before taking the plunge.

 

[turbotodd]

the prop makes a huge difference in fuel burn rate.

If you are under-propped, it'll run against the limiter (and sometimes you never know it), using excessive fuel.

If you are over propped, lugging the engine, it'll use a TON of fuel.

4 stroke engines are much more sensitive to propping! The old 2 stroke stuff you could miss the pitch by 2-3" and get away with it (for a while anyway), but on a 4 stroke, one way you're lugging the engine and making oil, the other way you're burning it up, both ways your fuel burn will be excessive.

The manufacturers (well yamaha anyway) publishes "performance bulletins" that kinda guide you in how much fuel usage to expect, but bear in mind that they measure this with ideal prop and typical conditions that a boat/motor might see. IMO it's very very close to actual.

So you are gonna need a tachometer. Run it at a normal load, full throttle, across the water and see where your maximum rpm ends up. Then adjust your prop pitch from there. 1" makes a huge difference with a little motor, much more difference than it does with a 150+. The 300rpm per inch of pitch rule is null. My experience, with a 20hp, 1" of pitch = 500-600 rpm, and I suspect a 15 is gonna be about the same way.

I run 3 gal tank in mine (25hp) but I know exactly how much fuel it'll use, and know how long it'll take to get to a point and back. I have never found the need for more than 3 gallons. It's lighter than 6 gallons, and it doesn't go bad before it's used up. But where I use the boat at, usually isn't too terribly far from any gas station....maybe 20 min away from the ramp/campground (at the most). I realized everyone's got different experiences.

 

[RaisedByWolves]

Common sense again. If you need the weight up in the bow move weight up there that stays in place moreso than a fuel tank that loses weight.

Exactly, battery in the nose as far forward as possible.

 

[LDUBS]

Common sense again. If you need the weight up in the bow move weight up there that stays in place moreso than a fuel tank that loses weight.

Exactly, battery in the nose as far forward as possible.

Geez, sometimes the obvious is staring me right in the face and I still miss it. Thanks!

 

[RaisedByWolves]

Common sense again. If you need the weight up in the bow move weight up there that stays in place moreso than a fuel tank that loses weight.

Exactly, battery in the nose as far forward as possible.

Geez, sometimes the obvious is staring me right in the face and I still miss it. Thanks!

I’m 300+ and with me and a 120lb motor in the back these things become necessary to proper balance. With a passenger in the front it’s no big deal, but solo? Yeah.

I have the battery (80lbs) in an expanded foam cradle within 1’ of the bow, the trolling motor (50lbs) and another 25lbs of anchor and gear up there and the trim is perfect with the motor tucked in on the last pin hole. And that is solo or with a 150lbs passenger.

I may have to make adjustments when the floor is completed, but not much.

 

[user 26281]

.5 gallons per hour at 3500 rpms with Suzuki Df9.9 converted to DF20.

 

[Lowrider1]

Thanks Pappy...not to be abrupt but it's 12F outside at the moment...guess you didn't understand what hard water was...it's ice on the water. I grew up in Winter Park so I understand not knowing what ice on the water would be. I've been flying for 52 years, Commercial multiengine instructor with 4800 hours +/- and bout 2700 of that was in AK flying Part 135 in the bush and half of that was IFR so I kinda have a feel for airplane fuel requirements but thanks for the reminder/refresher.

For everyone else....Thanks for the well thought out info and I did learn about prop pitch on 4 strokes and that makes sense. Gal/hr/10 hp is great info for an estimate.

Thanks to everyone for your time!!!!!!!!!!!!!