Fuel Consumption Rate for 25-HP Two-cycle - Moderated Discussion Areas

  ContinuousWave
  Whaler
  Moderated Discussion Areas
  ContinuousWave: Whaler Repairs/Mods
  Fuel Consumption Rate for 25-HP Two-cycle

search | FAQ | profile | register | author help

Author Topic:   Fuel Consumption Rate for 25-HP Two-cycle

fittongr posted 04-01-2009 10:57 PM ET (US)
[What] is the fuel consumption in gallons-per-hour for a new Yamaha 25-HP two-cycle outboard? How many hours will a 1969 13-foot Boston Whaler run before it uses six gallons at three-quarter throttle? Thanks.
jimh posted 04-01-2009 11:19 PM ET (US)
Most classic two-cycle motors have a brake specific fuel consumption (BSFC) of around 0.55-lbs/HP-Hour. You can figure that three-quarter throttle is about 0.75 of full power. So for a 25-HP motor that implies
25 X 0.75 x 0.55 = 10.3-lbs/Hour
Gasoline has a density of about 6.1-lbs/gallon, thus
10.3-lbs/Hour x 1-gallon/6.1-lbs = 1.69-gallon/Hour
The time to use 6-gallons of fuel will be
6-gallons x 1-hour/1.69-gallons = 3.55 hours or 3 hours 33 minutes.
fittongr posted 04-01-2009 11:30 PM ET (US)
jimh, thanks for the formula. after plugging in the numbers, does 3 1/2 hours on 6 gallons seem about right?
jimh posted 04-01-2009 11:41 PM ET (US)
An estimate of 3.5-hours should be fairly accurate if the estimate of the horsepower (25 x 0.75 or 18.75-HP) was accurate.
Peter posted 04-02-2009 07:01 AM ET (US)
The Yamaha 25 2-stroke is not fuel efficient. At 3/4 throttle (4500 RPM), Yamaha's performance reports show a fuel consumption rate of 2.1 GPH. At WOT (6000 RPM), Yamaha's data shows a consumption rate of 3.6 GPH.
Compare that to the consumption of the E-TEC 25, 1.4 and 2.5 GPH, respectively or the Yamaha F25, 1.6 and 2.5 GPH, respectively.
jimh posted 04-02-2009 07:59 AM ET (US)
That is a thirsty motor!
If it burns 3.6-GPH at full throttle, and we assume that is 25-HP, that implies the brake specific fuel consumption for the motor will be:
3.6-gallons/1-hour X 6.1-lbs/1-gallon x 1/25-HP= 0.88-lbs/HP-Hour
Wow--That is off the chart in terms of a BSFC. Are those figures accurate?
Peter posted 04-02-2009 08:22 AM ET (US)
Don't know about the accuracy but Yamaha reports similar numbers in two reports, the only two reports that they have published.
Peter posted 04-02-2009 08:30 AM ET (US)
As a comparison, the Yamaha 40 HP 2-stroke burns about 5 GPH at WOT, which seems a bit high.
5 gallons/1-Hour x 6.1 lbs/gallon x 1/40 HP = 0.76 lbs/HP-Hour
fittongr posted 04-02-2009 12:16 PM ET (US)
thanks for the info. next week i will take the boat out and burn 6 gallons and see how long it takes. we will know for sure and i will post the results on this string. anybody is welcome to take a guess.
fittongr posted 04-02-2009 12:28 PM ET (US)
belay that last post. because the motor only has 4 hrs. on it, it will be a while before i hold it at a steady speed for long periods. but i will post the results.
Tohsgib posted 04-02-2009 01:46 PM ET (US)
Those numbers are for steady cruising. I doubt you will be able to cruise steady for 3+hours in a 13 without having to stretch or do Yoga. My 35 OMC would burn on an average day about 1gph per tank with cruising, slow speed, WOT all mixed in. Your 25 should be pretty close. My 40 2 smoke would do about 2GPh per tank with a bit more cruising involved. My 40 4 stroke now does less than half of the 2 smoke.

Hop to:

Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.

Author	Topic: Fuel Consumption Rate for 25-HP Two-cycle
fittongr	posted 04-01-2009 10:57 PM ET (US) [What] is the fuel consumption in gallons-per-hour for a new Yamaha 25-HP two-cycle outboard? How many hours will a 1969 13-foot Boston Whaler run before it uses six gallons at three-quarter throttle? Thanks.
jimh	posted 04-01-2009 11:19 PM ET (US) Most classic two-cycle motors have a brake specific fuel consumption (BSFC) of around 0.55-lbs/HP-Hour. You can figure that three-quarter throttle is about 0.75 of full power. So for a 25-HP motor that implies 25 X 0.75 x 0.55 = 10.3-lbs/Hour Gasoline has a density of about 6.1-lbs/gallon, thus 10.3-lbs/Hour x 1-gallon/6.1-lbs = 1.69-gallon/Hour The time to use 6-gallons of fuel will be 6-gallons x 1-hour/1.69-gallons = 3.55 hours or 3 hours 33 minutes.
fittongr	posted 04-01-2009 11:30 PM ET (US) jimh, thanks for the formula. after plugging in the numbers, does 3 1/2 hours on 6 gallons seem about right?
jimh	posted 04-01-2009 11:41 PM ET (US) An estimate of 3.5-hours should be fairly accurate if the estimate of the horsepower (25 x 0.75 or 18.75-HP) was accurate.
Peter	posted 04-02-2009 07:01 AM ET (US) The Yamaha 25 2-stroke is not fuel efficient. At 3/4 throttle (4500 RPM), Yamaha's performance reports show a fuel consumption rate of 2.1 GPH. At WOT (6000 RPM), Yamaha's data shows a consumption rate of 3.6 GPH. Compare that to the consumption of the E-TEC 25, 1.4 and 2.5 GPH, respectively or the Yamaha F25, 1.6 and 2.5 GPH, respectively.
jimh	posted 04-02-2009 07:59 AM ET (US) That is a thirsty motor! If it burns 3.6-GPH at full throttle, and we assume that is 25-HP, that implies the brake specific fuel consumption for the motor will be: 3.6-gallons/1-hour X 6.1-lbs/1-gallon x 1/25-HP= 0.88-lbs/HP-Hour Wow--That is off the chart in terms of a BSFC. Are those figures accurate?
Peter	posted 04-02-2009 08:22 AM ET (US) Don't know about the accuracy but Yamaha reports similar numbers in two reports, the only two reports that they have published.
Peter	posted 04-02-2009 08:30 AM ET (US) As a comparison, the Yamaha 40 HP 2-stroke burns about 5 GPH at WOT, which seems a bit high. 5 gallons/1-Hour x 6.1 lbs/gallon x 1/40 HP = 0.76 lbs/HP-Hour
fittongr	posted 04-02-2009 12:16 PM ET (US) thanks for the info. next week i will take the boat out and burn 6 gallons and see how long it takes. we will know for sure and i will post the results on this string. anybody is welcome to take a guess.
fittongr	posted 04-02-2009 12:28 PM ET (US) belay that last post. because the motor only has 4 hrs. on it, it will be a while before i hold it at a steady speed for long periods. but i will post the results.
Tohsgib	posted 04-02-2009 01:46 PM ET (US) Those numbers are for steady cruising. I doubt you will be able to cruise steady for 3+hours in a 13 without having to stretch or do Yoga. My 35 OMC would burn on an average day about 1gph per tank with cruising, slow speed, WOT all mixed in. Your 25 should be pretty close. My 40 2 smoke would do about 2GPh per tank with a bit more cruising involved. My 40 4 stroke now does less than half of the 2 smoke.