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Author Topic:   30 on a 13 -- Fuel Consumption

blacksmithdog posted 11-02-2010 07:43 AM ET (US)
This is my fifth Whaler, it's a 1978 13' Standard with a 1991 30 hp Evinrude with tiller steering. Right now I only have one 6 gallon tank, and it kinda makes me nervous, cause I have no idea what kind of fuel consumption I'm going to experience. What should I expect (gph) at cruising speed (3000-3500 rpms)?
jimh posted 11-02-2010 07:53 AM ET (US)
A fairly accurate estimate of fuel consumption for classic two-cycle motors is as follows:
At rated horsepower, they will burn the rated horsepower divided by ten gallons per hour.
For a 30-HP engine, this implies 3-GPH. At lower throttle settings, the rate will be proportionally lower. At a nominal cruising speed, we could estimate the engine power will be about two-thirds maximum, and the fuel consumption rate will be in proportion; for your engine that would be about 2-GPH.
The rate of fuel burn is based on the Brake Specific Fuel Consumption (BSFC) of the engine. For a classic two-cycle engine we use a BSFC of 0.6-lbs/HP-hour. For gasoline fuel at 6.25-lbs/gallon, this implies a fuel rate of
0.6-lbs/HP-hour x 1-gallon/6.25-lbs = 0.096-lbs/HP-hour
For 20-HP (cruising speed) this means a fuel rate of
0.6-lbs/HP-hour x 1-gallon/6.25-lbs x 30-HP = 1.92-gallon/hour
blacksmithdog posted 11-02-2010 08:32 AM ET (US)
Thanks Jim, sounds like I need another tank.
Tohsgib posted 11-02-2010 10:00 AM ET (US)
I had the same setup and she burns roughly 1.5gph at cruise. It is nice to have 2 tanks so you can run one dry and then mix it proportionately instead of guessing. Use a pint to 6 gals of fuel(50:1 no matter what the engine says on it). If you are not cruising long distances, a 6 gal tank should be fine. I was in the Keys with my 13 and a 40 4 stroke and she burned 3.7 gals in 5.1 hours. Most of your time in a 13 is not going to be at cruise unless you live on a long calm river.
jimh posted 11-02-2010 07:44 PM ET (US)
For a four-cycle engine at cruise you can figure the BSFC will be about 0.4 at best. For 20-HP (or a 30-HP engine at two-thirds throttle), you could estimate fuel flow as
0.4-lbs/HP-hour x 1-gallon/6.25-lbs x 20-HP = 1.3-GPH
AZdave posted 11-03-2010 01:18 AM ET (US)
I had the same hull and horsepower for a number of years. I never got through a six gallon tank in a day at the lake. This involved towing kids in tubes, and general messing around. I notice that you have access to the tidal lower end of a river and the intercoastal waterway. You might have motivation for longer runs, and a second tank would be in order.

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Author	Topic: 30 on a 13 -- Fuel Consumption
blacksmithdog	posted 11-02-2010 07:43 AM ET (US) This is my fifth Whaler, it's a 1978 13' Standard with a 1991 30 hp Evinrude with tiller steering. Right now I only have one 6 gallon tank, and it kinda makes me nervous, cause I have no idea what kind of fuel consumption I'm going to experience. What should I expect (gph) at cruising speed (3000-3500 rpms)?
jimh	posted 11-02-2010 07:53 AM ET (US) A fairly accurate estimate of fuel consumption for classic two-cycle motors is as follows: At rated horsepower, they will burn the rated horsepower divided by ten gallons per hour. For a 30-HP engine, this implies 3-GPH. At lower throttle settings, the rate will be proportionally lower. At a nominal cruising speed, we could estimate the engine power will be about two-thirds maximum, and the fuel consumption rate will be in proportion; for your engine that would be about 2-GPH. The rate of fuel burn is based on the Brake Specific Fuel Consumption (BSFC) of the engine. For a classic two-cycle engine we use a BSFC of 0.6-lbs/HP-hour. For gasoline fuel at 6.25-lbs/gallon, this implies a fuel rate of 0.6-lbs/HP-hour x 1-gallon/6.25-lbs = 0.096-lbs/HP-hour For 20-HP (cruising speed) this means a fuel rate of 0.6-lbs/HP-hour x 1-gallon/6.25-lbs x 30-HP = 1.92-gallon/hour
blacksmithdog	posted 11-02-2010 08:32 AM ET (US) Thanks Jim, sounds like I need another tank.
Tohsgib	posted 11-02-2010 10:00 AM ET (US) I had the same setup and she burns roughly 1.5gph at cruise. It is nice to have 2 tanks so you can run one dry and then mix it proportionately instead of guessing. Use a pint to 6 gals of fuel(50:1 no matter what the engine says on it). If you are not cruising long distances, a 6 gal tank should be fine. I was in the Keys with my 13 and a 40 4 stroke and she burned 3.7 gals in 5.1 hours. Most of your time in a 13 is not going to be at cruise unless you live on a long calm river.
jimh	posted 11-02-2010 07:44 PM ET (US) For a four-cycle engine at cruise you can figure the BSFC will be about 0.4 at best. For 20-HP (or a 30-HP engine at two-thirds throttle), you could estimate fuel flow as 0.4-lbs/HP-hour x 1-gallon/6.25-lbs x 20-HP = 1.3-GPH
AZdave	posted 11-03-2010 01:18 AM ET (US) I had the same hull and horsepower for a number of years. I never got through a six gallon tank in a day at the lake. This involved towing kids in tubes, and general messing around. I notice that you have access to the tidal lower end of a river and the intercoastal waterway. You might have motivation for longer runs, and a second tank would be in order.