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Author Topic:   Formula for fuel consumption?
Aldog posted 07-11-2003 07:46 PM ET (US)   Profile for Aldog   Send Email to Aldog  
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?

Jarhead posted 07-11-2003 07:58 PM ET (US)     Profile for Jarhead  Send Email to Jarhead     
1/3 there, 1/3 back and 1/3 reserve...

It's a good rule...

kingfish posted 07-11-2003 09:20 PM ET (US)     Profile for kingfish  Send Email to kingfish     
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.


Aldog posted 07-11-2003 09:45 PM ET (US)     Profile for Aldog  Send Email to Aldog     
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.

Jarhead posted 07-11-2003 10:08 PM ET (US)     Profile for Jarhead  Send Email to Jarhead     
Have fun, be safe and let us know how it went..:)
kingfish posted 07-11-2003 10:20 PM ET (US)     Profile for kingfish  Send Email to kingfish     
As well, and good luck - keep us posted.
Richard Quinlivan posted 07-11-2003 10:48 PM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     
Another rule of thumb is that your 90 hp motor will burn about 9 gal/hr at full power.
Bigshot posted 07-12-2003 10:03 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
You will get closer to 5mpg than 2-2.5.

gnr posted 07-12-2003 11:23 AM ET (US)     Profile for gnr  Send Email to gnr     
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.

kingfish posted 07-12-2003 11:49 AM ET (US)     Profile for kingfish  Send Email to kingfish     
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.

whalerron posted 07-12-2003 09:45 PM ET (US)     Profile for whalerron  Send Email to whalerron     
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.
jimh posted 07-12-2003 11:04 PM ET (US)     Profile for jimh  Send Email to jimh     
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.

tomroe posted 07-13-2003 09:19 PM ET (US)     Profile for tomroe  Send Email to tomroe     
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.
Bigshot posted 07-14-2003 10:41 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
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.
andiamo posted 07-14-2003 04:31 PM ET (US)     Profile for andiamo  Send Email to andiamo     
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.
jimh posted 07-14-2003 11:52 PM ET (US)     Profile for jimh  Send Email to jimh     
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.

frostbite posted 07-15-2003 08:46 AM ET (US)     Profile for frostbite  Send Email to frostbite     
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.
VMG posted 07-16-2003 12:16 PM ET (US)     Profile for VMG  Send Email to VMG     
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.
Jarhead posted 07-16-2003 05:20 PM ET (US)     Profile for Jarhead  Send Email to Jarhead     
Obviously the 1/3 there includes whatever fuel you use before heading back...
VMG posted 07-17-2003 09:31 AM ET (US)     Profile for VMG  Send Email to VMG     
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.
Jarhead posted 07-17-2003 10:32 AM ET (US)     Profile for Jarhead  Send Email to Jarhead     

What type-hp. engine[s] do you run and how long does it take you to burn 40 gal. trolling?

VMG posted 07-17-2003 01:00 PM ET (US)     Profile for VMG  Send Email to VMG     
Twin 150s -- 8-10 hours at 4-5 gph.
TightPenny posted 07-17-2003 04:24 PM ET (US)     Profile for TightPenny  Send Email to TightPenny     
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.

TightPenny posted 07-17-2003 04:28 PM ET (US)     Profile for TightPenny  Send Email to TightPenny     

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.

Aldog posted 07-20-2003 08:03 PM ET (US)     Profile for Aldog  Send Email to Aldog     
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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