Fuel burn question.

CHANCE1234 · #1 07-09-2013, 10:51 PM

How much fuel would you realistically expect to burn cruising at 23 knots for 32 miles and just puttering around while fishing and drifting? 1990 merc 115 on a light 20 SF. calm seas. 50 gal tank started at 3/4. Thanks

erebus · #2 07-10-2013, 07:28 AM

Might be of no help whatsoever but, my 18SF with a 1998 Merc 115 has so far burned 20 gallons (half tank), which includes:
A roundtrip run from Meetinghouse pond in Orleans to Chatham Light in Pleasant Bay.
A roundtrip run from Sesuit harbor in Dennis to Billingsgate Shoal and trolling for 3 hrs
And tubing on Long Pond for about an hour.
Had to fill up the oil tank once so far, but she's still burning up that first tank of gas.

Sorry for the fairly vague parameters...

CHANCE1234 · #3 07-10-2013, 08:06 AM

thanks Zak. Heres why i ask. i installed a brand new 49 gal fuel tank and sender this winter. left barnstable harbor 2 weeks ago with just over 3/4 tank and steamed to ptown to fish which is roughly 16 miles. once there we just idled and drifted for the most part. 16 miles home to barnstable. gauge read, about an 1/8th at the pier. yesterday, i wanted to fill it up again and was only able to put 17 gallons in before it came out the vent and burped out the fill twice.

got u/w last night and guage read between 1/2 and 3/4. 15 miles at cruising speed and again idiling around looking for fish. than back at the pier last night read 1/2 tank.

assuming that when i left the first time i had about 36 gallons. when i returned by gauge told me that i had about 6 gallons left. should i have burned 30 gallons during that ptown trip? or is it more likely i only burned 17 gallons which is all the tank allowed me to put in yesterday before my night trip? thanks for any help.

gofastsandman · #4 07-10-2013, 07:53 PM

My guage is way off at anything but displacement speed. Just under full on plane can read just under half. Trolling against the stream here, I can see negative mpg on some days. Crazy, I know.

CHANCE1234 · #5 07-10-2013, 08:36 PM

So is it safe to assume I have either a bad gauge or more likely a sender, despite the sender being brand new this year? Thanks

gofastsandman · #6 07-10-2013, 08:46 PM

No. At displacement speed, the gauge is most accurate. On plane the go go juice splashes around and reads much higher as the sender is at[normally] the rear of the tank
or close to it.

CHANCE1234 · #7 07-10-2013, 09:15 PM

The boat was filled on the trailer which sits pretty level. I was expecting to put in 40 gallons according to my gauge reading 1/8. I was then only able to put in 17 gallons before it came out the vent and burped out the fill twice. The gauge then read 1/2 but all my senses were saying the tank is full. The sender is at the very rear of the tank and the fill at the front. There must be something wrong. Right??? As for my first post, is it more likely I burned 30 gallons over 30 miles or 17 gallons over 30 miles?

McGillicuddy · #8 07-11-2013, 12:07 AM

Quote:

Originally Posted by CHANCE1234

The boat was filled on the trailer which sits pretty level. I was expecting to put in 40 gallons according to my gauge reading 1/8. I was then only able to put in 17 gallons before it came out the vent and burped out the fill twice. The gauge then read 1/2 but all my senses were saying the tank is full. The sender is at the very rear of the tank and the fill at the front. There must be something wrong. Right??? As for my first post, is it more likely I burned 30 gallons over 30 miles or 17 gallons over 30 miles?

I have an older Merc I-6 on a 20 Seafari. At the cruise speed you mention I am consistently getting 2.5 mpg w an all day fishing load and a full 22 gal bait tank in front of the splash-well- barely 1.5 mph at idle speeds. I would expect you would do 10 to 15% better with a 4 cyl.

Your meter readings are similar to something mine was doing. Is it possible that your fuel meter and sending unit are operating at different ohms? I found someone had put an automotive or euro fuel meter on a marine sender in mine and units were behaving like unfriendly neighbors. One operated at 0-90 Ohms, the other at 33-240 Ohms. My old sending unit had even been tweaked (ineffectively) to accommodate to ohm variance. I swapped it all out for a WEMA reed sending unit and meter and am very happy with the results.

Heres a simple explanation from Autometer:
http://www.autometer.com/tech_faq_an...x?sid=1&qid=36

gofastsandman · #9 07-10-2013, 10:18 PM

I always like to run with a full tank. You just never know. I do your runs almost to a T.
I can`t slow down and get 10+ mpg with a strong wind and stream. Not even close.

Cheers,
GFS
Pigs on the wing.

Bushwacker · #10 07-10-2013, 11:20 PM

Chance,

I made 6 trips to Abaco in the Bahamas back in the late 70's/early 80's with a 1975 115 Evinrude, back when they were rated at the powerhead instead of the prop, so it probably wasn't much over 100 hp at the prop. Running with a very heavy load (dive gear, 2 wks worth of food, etc.), and 65-80 gal of gas, (35 of which was even in the main tank!), I averaged 2.8 mpg pretty consistently, burning about 8 gal/hr and cruising about 20 kts. With a lighter load and more modern motor, you should do at least that well. Denny