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ContinuousWave: Whaler Performance
Formula for fuel consumption?
|Author||Topic: Formula for fuel consumption?|
posted 07-11-2003 07:46 PM ET (US)
I am taking my maiden saltwater voyage on my Montauk with my 2 stroke 90 hp merc. I am wondering if there is a fuel consumption formula (or even a good guess at this point) for my boat.
Obviously when the tank is at half head back, any other info?
posted 07-11-2003 07:58 PM ET (US)
1/3 there, 1/3 back and 1/3 reserve...
It's a good rule...
posted 07-11-2003 09:20 PM ET (US)
Jarhead's rule *is* a good one...
You should be able to bet on 2 to 2-1/2 miles per gallon at cruising speeds.
posted 07-11-2003 09:45 PM ET (US)
I have never gone more than 5 miles out with my own boat. Now that I have the Whaler, I dont want to get stupid, but I'd like to go further. 2 to 2.5 mpg tells me much. Thanks.
Also the 1/3 rule will be in the memory bank forever. Thanks men. Wish me luck leaving in 10 hours for the coast. Maiden saltwater voyage in my Whaler. I can't wait.
My tank is something more than 24 maybe 28 that gets me out where I want to be.
posted 07-11-2003 10:08 PM ET (US)
Have fun, be safe and let us know how it went..:)
posted 07-11-2003 10:20 PM ET (US)
As well, and good luck - keep us posted.
posted 07-11-2003 10:48 PM ET (US)
Another rule of thumb is that your 90 hp motor will burn about 9 gal/hr at full power.
posted 07-12-2003 10:03 AM ET (US)
You will get closer to 5mpg than 2-2.5.
posted 07-12-2003 11:23 AM ET (US)
I got about 80 miles on 20 gallons of gas last weekend in my 17 Guardian powered by an '87 Evinrude 88sp. Most of the miles were at 4000 rpms on a calm surface although the last 16 miles were a bit slower against a three foot chop.
If I'm not mistaken the 88sp. has a reputation for being a bit of a gas guzzler.
posted 07-12-2003 11:49 AM ET (US)
My comment about 2 to 2-1/2 mpg was intended to be conservative, and was intended to be stated that way. I would still bet on those numbers, but maybe I should have said, "at a minimum". I would not bet on 5 mpg, and have *never* been able to get 5 mpg, with a Montauk and 90 HP Evinrude, a Montauk and 100 HP Mercury, or an Outrage 22 and 225 HP Evinrude.
On your first outings, and until you are familiar with the particular performance of your own boat, you obviously are going to have to be the judge regarding what *you* want to bet on.
posted 07-12-2003 09:45 PM ET (US)
I agree with Bigshot. Although mpg is a bad way to gauge fuel consumption, I am seeing numbers like he is. With my johnson 90, I am consistently seeing 4 1/2 mpg with mileage travelled measured by my GPS. Water conditions don't seem to make a difference nor does speed.
posted 07-12-2003 11:04 PM ET (US)
A two-stroke gasoline engine consumes about 0.5 pounds of fuel per hour per horsepower.
Assume your 90-HP engine outputs about 60-HP when running the boat on hydroplane at moderate speeds, say 25 MPH. At a 60-HP setting you will consume 30-pounds of gasoline in an hour. Gasoline weights about 6-lbs per gallon, so you will consume about 5 gallons. This will give you a fuel consumption rate of about 5 MPG.
The fundamental assumption, the amount of fuel consumed per hour per horsepower varies with the particular engine.
posted 07-13-2003 09:19 PM ET (US)
You could also top it off, run it for an hour under normal conditions, top it off again and figure what it actually burns per hour.
posted 07-14-2003 10:41 AM ET (US)
My Newport with an 88spl burned exactly 6GPh at 4k and about 28-30mph so about 4.5-5mpg. My 70 4 stroke burns less than 3gph....god I love that thing.
posted 07-14-2003 04:31 PM ET (US)
On my 1986 90 hp 2 stoke Evinrude which is attached to a 1986 Montauk with bottom paint and a 30 gallon tank I average 5 imperial gallons an hour in ocean conditions.
posted 07-14-2003 11:52 PM ET (US)
The formula I mentioned (which was 0.5 pounds of fuel per horsepower per hour) is probably fairly accurate for 2-stroke gasoline engines, at least plus or minus 10-percent.
The problem comes into the "fuel economy" equation in two other places.
First, how much horsepower are you using? You have to estimate this from throttle position. If you have a 200-HP engine you are not running it wide-open all the time.
Second, if you want to translate the fuel consumption per hour into a miles-per-gallon figure, you must consider the type of boat and load. I might have a 70-HP on a little hydroplace and at 70-HP it goes 100-MPH. You might have a 70-HP on a barge with a 20-foot beam and at 70-HP you go 2.5-MPH. We are going to burn the same amount of fuel each hour, but I am going to get much better MPG figures.
Even on similar hulls, the presence of bottom paint, the choice of propeller, and the weight of the gear and people aboard will affect the MPG numbers.
posted 07-15-2003 08:46 AM ET (US)
A rough fuel consumption formula for diesel engines is one gallon per hp per day. Two stroke outboards seem to be closer to 1.5 gal/hp/day.
posted 07-16-2003 12:16 PM ET (US)
The 1/3, 1/3, 1/3 works if you're not burning any fuel when you get wherever "there" might be. If "there" is trolling for extended periods, you're biting into your reserve.
posted 07-16-2003 05:20 PM ET (US)
Obviously the 1/3 there includes whatever fuel you use before heading back...
posted 07-17-2003 09:31 AM ET (US)
I disagree Jarhead. It isn't obvious, particularly to folks who haven't had to think about fuel planning very much. However, the 1/3 rule is a rational and conservative approach to the issue -- provided one adheres to the rule. I parse my fuel planning a little differently. Since I do alotta offshore fishing with extended periods of trolling I allow a max of 1/4 to get there (for my boat that's 40 gallons), 1/4 to troll/transit the fishing area, 1/4 to get home with and 1/4 in reserve. It is a less conservative approach than the 1/3 rule since it leaves you with less reserve. It so happens I've never come close to using 40 gallons to "get there", but I have used 40 gallons trolling, and have nearly doubled my return fuel consumption when the wind shifted contrary to expectations. The less certain a boater is with the environment (for whatever reason), the more conservative one should be with fuel planning. My biggest recommendation is that whatever "rule" one chooses, NEVER bust the allowances under any circumstances.
posted 07-17-2003 10:32 AM ET (US)
What type-hp. engine[s] do you run and how long does it take you to burn 40 gal. trolling?
posted 07-17-2003 01:00 PM ET (US)
Twin 150s -- 8-10 hours at 4-5 gph.
posted 07-17-2003 04:24 PM ET (US)
I have been running a spread sheet on my 2000 Montauk with a 2001 90 hp Johnson 2 stroke since I put the engine on and an engine hour meter.
I have averaged just under 4 gallons per hour and just under 4 nautical miles per gallon with about 100 hours of engine operation. Distance is based on GPS readings.
I typically run at 25-30 knots in the Barnegat Bay, and run around 18-22 knots in the Ocean.
Hope this is helpful.
posted 07-17-2003 04:28 PM ET (US)
Just an observation but I have concluded that you may be new to boating, based on your reaction to the 1/3 rule.
That is one of the basics that is taught in the USCG Aux course, so I have surmised that you have not taken that course.
I strongly suggest that you and any other family member who regularly goes out with you sign up and take the course. It is a reasonably inexpensive educational experience that just might save your life someday.
If you are an old salt, please accept my impertinence.
posted 07-20-2003 08:03 PM ET (US)
Thanks for all of the input and discussion. I ran no performance statistics from my trip but I may have averaged close to the 4 mpg range. I dont think much more.
Tightpenny thanks for the suggestion I'm sure that course would be a good one even for a seasoned boater. I am in my 19th year of boat ownership, just new to my Whaler. I am not an "old salt" however as I am landlocked and boat primarily freshwater where fuel comsumption is just not really an issue. You just cant get but so far from the dock in a freshwater lake in SC.
My Montauk outperformed all of my expectations, I told my wife if I had had any idea I would have bought one ten years sooner. What a difference. I do need to make a prop change, but overall I am blown away with my boat.
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