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Author Topic:   Fuel Economy Estimates

alfred posted 10-11-2006 01:57 AM ET (US)
I know it will depend on the actual motor, sea conditions, load on the boat, speed, etc, but how many nautical miles should I be able to cover with a full tank of gas [--it also depends on how large the tank of gas happens to be--jimh] on a lightly loaded [Boston Whaler boat of] 18 [feet in length powered by a 1986 Evinrude 150-HP V6 outboard] if I were traveling at about 25-knots constant? I am just trying to get ballpark figures and will take all estimates with a pinch of salt!
Peter posted 10-11-2006 07:36 AM ET (US)
At 29-MPH (25 Knots), the 150 will be loafing along at about 3400 RPM. Figure on fuel consumption being somewhere between 7 to 8 GPH yielding about 3.5 MPG, worst case. Using the 90 percent rule for range estimates at given speed, you would be able to travel about 200 miles (~175 nautical miles).
Tom W Clark posted 10-11-2006 08:53 AM ET (US)
Having owned, for many years, an Outrage 18 with a 90 degree OMC 150 outboard and a FloScan fuel gauge I can tell you that at 25 mph (a more typical cruising speed) the motor consumes about 6-7 gallons per hour.
At 29 MPH the motor is not loafing but beginning to work a little harder. Fuel consumption of 8-9 gallons per hour is typical at this speed.
jimh posted 10-11-2006 09:22 AM ET (US)
A general rule for estimating fuel consumption of a two-stroke outboard motor is to take the rated horsepower and divide it by ten. This will be the number of gallons per hour consumed at that horsepower. You can figure that horsepower is proportional to engine speed. Let us see how this works out in this case:
According to the rule of thumb I propose above, a 150-HP engine will burn 15-gallons per hour when producing 150-HP. This will be at an engine speed of 5,500-RPM. If you operate the engine at 3,500-RPM (as suggested above) this should produce about
(3500/5500) X 15 = 9.5 gallons per hour
This estimate is consistent with the data mentioned above obtained from actual measurement by others.
How far you can go at 9.5-GPH depends on how much fuel is aboard and the boat speed.
If you allow for your 18-foot Boston Whaler to be an OUTRAGE with a 63-gallon fuel tank, you ought to have about 57-gallons of useable fuel. This implies you can operate for
57-gallons / 9.5 gallons/hour = 6-hours
How far you can travel in 6 hours depends on many factors, and not the least of those is how the boat is set up for a propeller. There can be 10 to 15-percent changes in fuel consumption just based on choice of propeller. And, as you mention, sea conditions will also be very important. In rough seas in an 18-foot boat you will be forced to come off plane and travel at slower speeds. Fuel consumption will be very much higher in those conditions. It is not unusual for fuel consumption to double when trying to maintain 8- to 10-MPH headway into rough seas compared to planing at 20-MPH over smooth seas.
jimh posted 10-11-2006 09:31 AM ET (US)
Taking the 6-hour endurance calculated above and using your figure of 25-knots (or 29-MPH), this implies that you ought to be able to cover
6 X 29 = 174 statute miles
with your 63-gallon fuel tank full. This implies a fuel economy of about 3-MPG. In my experience, that is a reasonable number for a 18-footer with a 150-HP two-stroke operated at an optimum speed. However, as a practical matter, I would never embark on a 174-mile run with only 63-gallons of fuel aboard.
alfred posted 10-11-2006 09:47 AM ET (US)
Thanks for the estimates and the "how to calculate" formulas.
Next question. Do you folks leave the motor running all the time when you drift fish?
The grounds are about 30nm out, so getting there and back is easy, but from what you folks are saying, if I let the motor run all the time, I am burning gas fast. The trip I am planning will be about nine hours on the water. Not all of it with the motor on.
Peter posted 10-11-2006 10:02 AM ET (US)
That motor will gobble up fuel at a rate of about 1.5 GPH at idle. compare that to a modern DFI 2-stroke at 0.2 GPH!
Fuel consumption is not exactly linear with engine speed. 9.5 GPH at 3500 RPM, even on the old 150 cross-flow, will be on the high side.
Tonym posted 10-11-2006 10:14 AM ET (US)
Alfred,
When I drift I turn my engine off if it's not too rough and if the engine is running good. My only concern would then be the battery draw down from the electronics I have running. I do have a kicker though which can be pull started easily and will charge the batteries in that event.
Good luck, Tonym
Peter posted 10-11-2006 10:34 AM ET (US)
A 1980s era 150 Evinrude V6 can be pull started fairly easily in the event of a dead battery.
bsmotril posted 10-11-2006 12:43 PM ET (US)
A 150 hp V^ from that era on a medium weight boat prop'd correctly will get you about 2-2.5 mpg. On a light flat hulled bassboat, you might see 3.5 tops. Those were thirsty motors, as were their little sister v4s. A 1984 100HP V4 on a Montauk gets about 2.5-maybe 3mpg on a calm day with no excursions above 4000 rpm. BillS
Buckda posted 10-11-2006 01:08 PM ET (US)
A good option on the 18' Outrage, when fishing "offshore fishing grounds" beyond 30 miles out, is a portable Pate Plastics 27 gallon tank. That provides you with a more comfortable capacity cushion.
Burn the 27 gallons in the portable tank first, which should easily get you offshore to the fishing grounds. You will then have about 170 mile range in the belly tank to use as you choose.
If you're worried about a dead battery, carrying a spare (even from the tow vehicle) to use in an emergency is not a big deal.
I'd normally recommend drift fishing with the motor OFF, however, if offshore 30 miles with no other boats or other shelter around, I'd be very hesitant to shut down a 20 year old motor.
The best practice is to keep another boat within 10 miles or so who can check in with you on the radio. That way if you DO have problems, someone can come to your rescue.
Also - if regularly going that far offshore, you should consider the appropriate safety gear as standard equipment - EPIRB perhaps (they're down to around $500 and a great safety feature) and of course an inflatable or a survival suit.
I know it's a fishing trip, but even when I'm not planning to fish, if I'm going offshore (even in the Great Lakes) more than 10 miles out, I bring some fishing gear as a general rule. Better to be stranded with the means to catch some dinner than to be without and wishing you had it.
Incidentally, last summer, I ran offshore about 60 miles in Lake Superior to Isle Royale running aroudn 25 MPH on GPS in "average" conditions with a 150 Merc (1986) and burned approximately 24 gallons of fuel in the crossing.
Good luck.
Dave
WT posted 10-11-2006 04:06 PM ET (US)
alfred:
Let us know you fuel consumption for the trip. My guess is that you will burn fuel at 2 mpg. If you leave your motor on for 9 straight hours while drifting you will get less than 2 mpg.
First of all, I am envious of you if you can average 25 knots on a 30 mile run offshore in a small boat. Those days are rare along the Northern California waters of the Pacific ocean.
I average 4 mpg in my 2004 Montauk with a 90 hp 4 stroke on a 60 mile round trip while drift fishing with my motor on for 9 straight hours. Swells would average 4-6 feet, leave the ramp at 6 am and back at 3 pm.
Have fun on your trip and let us know your real life fuel consumption. Just remember to reset your GPS trip calulator before you leave.
Warren
Sal DiMercurio posted 10-11-2006 08:02 PM ET (US)
I'm sorry but I don't buy 3.5 mpg in an old 6 cylinder 2 stroke with 6 carburators.
I really do think you guys are dreaming because those engine were thirsty gas guzzlers & even 2.5 mpg might be stretching it at cruise.
Hell my 1980 - 15 hp kicker uses 4 times as much fuel as my 2000 - 200 hp DFI weather it's trolling or running at 2/3 throttle.
Sal
alfred posted 10-11-2006 08:46 PM ET (US)
Thanks for the advise folks.
I will have all the safety gear, Eprib, parachute and handheld flares, jackets, VHF etc on board. It's a requirement here in Perth, Western Australia where the water is usually pretty rough, but there are days at times where it is flat and I am hoping to make the trip out to the FADS to fish for mahi mahi.
I am keeping watch for a 2 day break in the weather and will head out early and get back before the typical afternoon breeze picks up.
Have never been out that far here before, but when I lived in Hawaii, I went out often about 20nm in a Montauk! Had 6 gas cans on board, but with the 70hp, never needed to use them all.
I will post when I make the trip.
WT posted 10-11-2006 09:11 PM ET (US)
We ran Strike 3, our 19 foot Revenge with a 140 Suzuki 4 stroke EFI from Bodega Bay to the Farallons 2 weeks ago in FLAT water. Matt drove down at probably 4200 rpm and I drove back up at 4000 rpm. The 80 mile round trip used 23 gallons which equates to 3.48 mpg.
Using the same boat the week before I used 11 Gallon to get to Fort Ross from Bodega Bay. That's a 17 mile run using 11 gallons or 1.5 mpg. I was going uphill and could not go faster than 12 mph at around 3500 rpm.
Your mileage may vary. :-)
Warren
Sal DiMercurio posted 10-11-2006 10:30 PM ET (US)
Ft Ross to Bodega Bay is down hill with the swell , not uphill.
Sal
Sal DiMercurio posted 10-11-2006 10:48 PM ET (US)
Sorry, missed the "to" Fort Ross,...yep it's uphill.
Sal
Perry posted 10-11-2006 11:13 PM ET (US)
alfred, I didn't know they had FADS in Australia. I always figured there were exclusive to the waters here in Hawaii.
Good luck and let us know the MPG you achieve on the long trips.
alfred posted 10-12-2006 07:02 AM ET (US)
The FADs are not permanent. Every year they are towed out and anchored in the same spot, in the first week of November and then towed back in at the end of March, so that they don't get lost with the winter storms. Having said that, we have the most number of flat days during winter, only problem is that it is usually very cold to be out on a boat at that time.
I normally would not complain about the cold, as I remember sailing in Chicago and had to shake snow off the sails, but as my bones get older and my movement slows and the body aches, the cold could be bothersome!
Love catching mahi, that is why I am planning the trip out.

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Author	Topic: Fuel Economy Estimates
alfred	posted 10-11-2006 01:57 AM ET (US) I know it will depend on the actual motor, sea conditions, load on the boat, speed, etc, but how many nautical miles should I be able to cover with a full tank of gas [--it also depends on how large the tank of gas happens to be--jimh] on a lightly loaded [Boston Whaler boat of] 18 [feet in length powered by a 1986 Evinrude 150-HP V6 outboard] if I were traveling at about 25-knots constant? I am just trying to get ballpark figures and will take all estimates with a pinch of salt!
Peter	posted 10-11-2006 07:36 AM ET (US) At 29-MPH (25 Knots), the 150 will be loafing along at about 3400 RPM. Figure on fuel consumption being somewhere between 7 to 8 GPH yielding about 3.5 MPG, worst case. Using the 90 percent rule for range estimates at given speed, you would be able to travel about 200 miles (~175 nautical miles).
Tom W Clark	posted 10-11-2006 08:53 AM ET (US) Having owned, for many years, an Outrage 18 with a 90 degree OMC 150 outboard and a FloScan fuel gauge I can tell you that at 25 mph (a more typical cruising speed) the motor consumes about 6-7 gallons per hour. At 29 MPH the motor is not loafing but beginning to work a little harder. Fuel consumption of 8-9 gallons per hour is typical at this speed.
jimh	posted 10-11-2006 09:22 AM ET (US) A general rule for estimating fuel consumption of a two-stroke outboard motor is to take the rated horsepower and divide it by ten. This will be the number of gallons per hour consumed at that horsepower. You can figure that horsepower is proportional to engine speed. Let us see how this works out in this case: According to the rule of thumb I propose above, a 150-HP engine will burn 15-gallons per hour when producing 150-HP. This will be at an engine speed of 5,500-RPM. If you operate the engine at 3,500-RPM (as suggested above) this should produce about (3500/5500) X 15 = 9.5 gallons per hour This estimate is consistent with the data mentioned above obtained from actual measurement by others. How far you can go at 9.5-GPH depends on how much fuel is aboard and the boat speed. If you allow for your 18-foot Boston Whaler to be an OUTRAGE with a 63-gallon fuel tank, you ought to have about 57-gallons of useable fuel. This implies you can operate for 57-gallons / 9.5 gallons/hour = 6-hours How far you can travel in 6 hours depends on many factors, and not the least of those is how the boat is set up for a propeller. There can be 10 to 15-percent changes in fuel consumption just based on choice of propeller. And, as you mention, sea conditions will also be very important. In rough seas in an 18-foot boat you will be forced to come off plane and travel at slower speeds. Fuel consumption will be very much higher in those conditions. It is not unusual for fuel consumption to double when trying to maintain 8- to 10-MPH headway into rough seas compared to planing at 20-MPH over smooth seas.
jimh	posted 10-11-2006 09:31 AM ET (US) Taking the 6-hour endurance calculated above and using your figure of 25-knots (or 29-MPH), this implies that you ought to be able to cover 6 X 29 = 174 statute miles with your 63-gallon fuel tank full. This implies a fuel economy of about 3-MPG. In my experience, that is a reasonable number for a 18-footer with a 150-HP two-stroke operated at an optimum speed. However, as a practical matter, I would never embark on a 174-mile run with only 63-gallons of fuel aboard.
alfred	posted 10-11-2006 09:47 AM ET (US) Thanks for the estimates and the "how to calculate" formulas. Next question. Do you folks leave the motor running all the time when you drift fish? The grounds are about 30nm out, so getting there and back is easy, but from what you folks are saying, if I let the motor run all the time, I am burning gas fast. The trip I am planning will be about nine hours on the water. Not all of it with the motor on.
Peter	posted 10-11-2006 10:02 AM ET (US) That motor will gobble up fuel at a rate of about 1.5 GPH at idle. compare that to a modern DFI 2-stroke at 0.2 GPH! Fuel consumption is not exactly linear with engine speed. 9.5 GPH at 3500 RPM, even on the old 150 cross-flow, will be on the high side.
Tonym	posted 10-11-2006 10:14 AM ET (US) Alfred, When I drift I turn my engine off if it's not too rough and if the engine is running good. My only concern would then be the battery draw down from the electronics I have running. I do have a kicker though which can be pull started easily and will charge the batteries in that event. Good luck, Tonym
Peter	posted 10-11-2006 10:34 AM ET (US) A 1980s era 150 Evinrude V6 can be pull started fairly easily in the event of a dead battery.
bsmotril	posted 10-11-2006 12:43 PM ET (US) A 150 hp V^ from that era on a medium weight boat prop'd correctly will get you about 2-2.5 mpg. On a light flat hulled bassboat, you might see 3.5 tops. Those were thirsty motors, as were their little sister v4s. A 1984 100HP V4 on a Montauk gets about 2.5-maybe 3mpg on a calm day with no excursions above 4000 rpm. BillS
Buckda	posted 10-11-2006 01:08 PM ET (US) A good option on the 18' Outrage, when fishing "offshore fishing grounds" beyond 30 miles out, is a portable Pate Plastics 27 gallon tank. That provides you with a more comfortable capacity cushion. Burn the 27 gallons in the portable tank first, which should easily get you offshore to the fishing grounds. You will then have about 170 mile range in the belly tank to use as you choose. If you're worried about a dead battery, carrying a spare (even from the tow vehicle) to use in an emergency is not a big deal. I'd normally recommend drift fishing with the motor OFF, however, if offshore 30 miles with no other boats or other shelter around, I'd be very hesitant to shut down a 20 year old motor. The best practice is to keep another boat within 10 miles or so who can check in with you on the radio. That way if you DO have problems, someone can come to your rescue. Also - if regularly going that far offshore, you should consider the appropriate safety gear as standard equipment - EPIRB perhaps (they're down to around $500 and a great safety feature) and of course an inflatable or a survival suit. I know it's a fishing trip, but even when I'm not planning to fish, if I'm going offshore (even in the Great Lakes) more than 10 miles out, I bring some fishing gear as a general rule. Better to be stranded with the means to catch some dinner than to be without and wishing you had it. Incidentally, last summer, I ran offshore about 60 miles in Lake Superior to Isle Royale running aroudn 25 MPH on GPS in "average" conditions with a 150 Merc (1986) and burned approximately 24 gallons of fuel in the crossing. Good luck. Dave
WT	posted 10-11-2006 04:06 PM ET (US) alfred: Let us know you fuel consumption for the trip. My guess is that you will burn fuel at 2 mpg. If you leave your motor on for 9 straight hours while drifting you will get less than 2 mpg. First of all, I am envious of you if you can average 25 knots on a 30 mile run offshore in a small boat. Those days are rare along the Northern California waters of the Pacific ocean. I average 4 mpg in my 2004 Montauk with a 90 hp 4 stroke on a 60 mile round trip while drift fishing with my motor on for 9 straight hours. Swells would average 4-6 feet, leave the ramp at 6 am and back at 3 pm. Have fun on your trip and let us know your real life fuel consumption. Just remember to reset your GPS trip calulator before you leave. Warren
Sal DiMercurio	posted 10-11-2006 08:02 PM ET (US) I'm sorry but I don't buy 3.5 mpg in an old 6 cylinder 2 stroke with 6 carburators. I really do think you guys are dreaming because those engine were thirsty gas guzzlers & even 2.5 mpg might be stretching it at cruise. Hell my 1980 - 15 hp kicker uses 4 times as much fuel as my 2000 - 200 hp DFI weather it's trolling or running at 2/3 throttle. Sal
alfred	posted 10-11-2006 08:46 PM ET (US) Thanks for the advise folks. I will have all the safety gear, Eprib, parachute and handheld flares, jackets, VHF etc on board. It's a requirement here in Perth, Western Australia where the water is usually pretty rough, but there are days at times where it is flat and I am hoping to make the trip out to the FADS to fish for mahi mahi. I am keeping watch for a 2 day break in the weather and will head out early and get back before the typical afternoon breeze picks up. Have never been out that far here before, but when I lived in Hawaii, I went out often about 20nm in a Montauk! Had 6 gas cans on board, but with the 70hp, never needed to use them all. I will post when I make the trip.
WT	posted 10-11-2006 09:11 PM ET (US) We ran Strike 3, our 19 foot Revenge with a 140 Suzuki 4 stroke EFI from Bodega Bay to the Farallons 2 weeks ago in FLAT water. Matt drove down at probably 4200 rpm and I drove back up at 4000 rpm. The 80 mile round trip used 23 gallons which equates to 3.48 mpg. Using the same boat the week before I used 11 Gallon to get to Fort Ross from Bodega Bay. That's a 17 mile run using 11 gallons or 1.5 mpg. I was going uphill and could not go faster than 12 mph at around 3500 rpm. Your mileage may vary. :-) Warren
Sal DiMercurio	posted 10-11-2006 10:30 PM ET (US) Ft Ross to Bodega Bay is down hill with the swell , not uphill. Sal
Sal DiMercurio	posted 10-11-2006 10:48 PM ET (US) Sorry, missed the "to" Fort Ross,...yep it's uphill. Sal
Perry	posted 10-11-2006 11:13 PM ET (US) alfred, I didn't know they had FADS in Australia. I always figured there were exclusive to the waters here in Hawaii. Good luck and let us know the MPG you achieve on the long trips.
alfred	posted 10-12-2006 07:02 AM ET (US) The FADs are not permanent. Every year they are towed out and anchored in the same spot, in the first week of November and then towed back in at the end of March, so that they don't get lost with the winter storms. Having said that, we have the most number of flat days during winter, only problem is that it is usually very cold to be out on a boat at that time. I normally would not complain about the cold, as I remember sailing in Chicago and had to shake snow off the sails, but as my bones get older and my movement slows and the body aches, the cold could be bothersome! Love catching mahi, that is why I am planning the trip out.