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Is Fuel Consumption a Secret?

3810 Views 21 Replies 16 Participants Last post by Tallswede, May 17, 2014

wannabsailor

Discussion Starter · May 14, 2014

Is fuel consumption a secret? I’m having a hard time finding the numbers and it seems as if the manufacturers don’t want to give out the information. I want to put a 9.9 hp. outboard on a 26ft. Macgregor, the older models
(1990-1995) not the newer power sailboat type. I’m trying to find out what my fuel consumption would be to average 5 kts. Example: @ 4,000 rpm you should average 5kts and burn .6 gallons of fuel an hour…etc.
Anyone know of any sites with such info? Thanks for any help.

1 - 20 of 22 Posts

Multihullgirl

· #2 · May 14, 2014

too many variables. What is the sea/wind state? Currents? How heavily is your boat loaded? What is the state of your bottom? Windage from rigging, sail bags, etc?

Reactions: 1

Alex W

· #3 · May 14, 2014

Also depends heavily on the prop. Outboards of that size normally have at least two prop options, one will be aimed at powerboats and have a higher pitch, the other will be aimed at sailboats and have a lower pitch but provide more bite.

Coquina

· #4 · May 14, 2014

Rough guess is HP x 0.1 = GPH. So a 5 hp engine running flat out might consume 0.5 gph. It can vary - some engine are more efficient than others, 2 strokes generally use a bit more, and without a tach and power curve from the engine builder you don't know anything about less-than-full-throttle.
Keep in mind GPH<>MPG. You can use a lot of fuel to go exactly noplace into 50 knots of wind and rough seas for one example.

christian.hess

· #5 · May 14, 2014

outboards are not like inboard engines as like said there are way to many variables not the least of which is hull type

planing or not

in your case I would look for info of said motor with those using the same motor on their sailboats maybe just maybe you get lucky and get on a boat specific model forum where people can give real life details

honestly you are going to have a VERY hard time getting any manufacturer specs for consumption especially for use on sailboats

my rough guess is .75gph for half throttle to 3/4 throttle on your boat which is normal cruising speeds

Tallswede

· #6 · May 14, 2014

I have a Hunter 23.5 with a Tohatsu 8hp 4-stroke. Under power alone with smooth conditions at 6 mph I burn about 0.3 - 0.4 gph. This can go up quite a bit if I try to keep speed up into a chop and wind. This can raise consumption to .75gph. Current can really mess up your mph numbers as you will be cruising at your normal rpms and your speed instruments showing normal speed but your speed over the ground is much lower or higher depending on which way the current is going. GPS helps out a lot in calculating your MPG.

Lots of acronyms, I know, LOL.

Kevin

Omatako

· #7 · May 14, 2014

The only fair and equitable measure of fuel consumption that can be quoted by a manufacturer would be based on a dynamometer test in the factory because that is the only environment in which there are no variables.

And when a dynamometer test is done, the engine is loaded up and run at max horsepower and at wide-open-throttle (WOT). Hence fuel consumption figures when they are quoted (also very rare these days) are quoted at WOT.

And these are quoted in grams per kilowatt hour (or whatever un-metricated measure is used in the US). So this is in any case of limited use to the outboard motor operator who has almost an infinite number of variables and wants to know gallons/hour.

So it’s not a secret, it just isn’t generally published in any format we would find useful.

Minnewaska

· #8 · May 15, 2014

It also depends on what power setting you run at. It's not uncommon for the last 20% of power to increase fuel consumption by as much as 50%.

capta

· #9 · May 15, 2014

Put a gallon in a fuel tank. Run the boat for 15 minutes in a straight line at X rpm, and measure how much you've used and multiply by four. Repeat for other rpms. Then repeat in variable conditions.
Or just carry a couple of 6 gallon tanks for your outboard and estimate your general usage over several months and I'm sure you'll be just as close.

chucklesR

· #10 · May 15, 2014 (Edited)

No such thing as miles per gallon on a boat, it's no secret at all.

It's all about fuel consumption per RPM for your engine. How the heck would a boat manufacturer know which engine you were using, with what prop, and how clean your bottom was.
There are however rules of thumb you can go by as a fair approximation. Try 1 gallon per hour per 10 hp at cruising rpm (2/3 throttle).
Better yet get a 1 gallon tank and a 2 gallon tank, run the 2 gallon tank for a hour at 2/3 throttle and pour the remainder in the 1 gallon tank. Measure speed/distance if you want, but the next day it will change. What that will tell you in on that day, at that time it consumed that much, because humidity and temperature also affect consumption.

If you want real fun, I've got a 44hp diesel and 2 30 gallon tanks that are an odd shape and defy measurement (they don't even have an access for a stick to measure with).
I monitor hours run, guess 1.0 gal a hour ( you have to adjust for rpm) and hope I don't run dry

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Reactions: 1

Shinook

· #11 · May 15, 2014

Measure consumption in gallons per hour at certain power settings, the rest depends on a lot of other factors.

As a prior post indicated, you can measure the consumption yourself by timing your usage against a known amount of fuel in the tank.

davidpm

· #12 · May 15, 2014

You have to pick a number, right?

A gallon an hour
Figure 4 miles per hour.

If you burn 1/3 of your fuel not including the spare get home 5 gal tank and you are not 1/2 of the way there then turn around.
Your not having a good time anyway.

There all done.

Reactions: 2

Minnewaska

· #13 · May 15, 2014

You don't have to put a measured amount of fuel in a tank to run a consumption test. Top it off and go run the motor over varying conditions for a while. Choose either a constant rpm or a constant boat speed (which may require differing rpms for differing conditions). Keep a log of the time it takes to burn a good portion of the tank. The more time you spend or fuel you burn, the merrier. Then top off the tank again and the pump will tell you exactly what you burned. Do the math to determine your average fuel consumption for whichever of the above (rpm or speed) interests you.

Reactions: 1

Group9

· #14 · May 15, 2014 (Edited)

chucklesR said:
If you want real fun, I've got a 44hp diesel and 2 30 gallon tanks that are an odd shape and defy measurement (they don't even have an access for a stick to measure with).
I monitor hours run, guess 1.0 gal a hour ( you have to adjust for rpm) and hope I don't run dry

I've been trying to guess how much fuel capacity I have for four years now.

(I guess I am going to have to break down and just measure the darn tank and calculate it).

According to the manufacturer, my tankage is somewhere between 38 and 46 gallons of diesel, depending on what day of the week they made my boat.

I have run it down past empty several times, to the point I am getting nervous, before filling up and I have never been able to put more than 25 gallons of diesel in.

But I have actually figured the diesel mileage on mine a few times when I was motoring down the ICW and I was amazed to find I have done as well as 15-17 nautical miles per gallon at my preferred cruising speed of about 5.5 knots. Not bad compared to the twin inboard gasoline boat I used to have, that I calculated my mileage in a similar fashion at about 1 nautical mile per gallon.

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CarbonSink62

· #15 · May 15, 2014

I have (had) a 9.9 Honda on a Catalina 25 (4500lbs). My notes from Cruzapalooza 2013 show 5 gallons and 51 miles on the first day. The notes don't say, but I recall that was 10 hours of motoring at cruising rpm.

So, you could likely run your engine at .5 gph; how far that gets you depends on fast you go. That's something only trial and error will give you a definitive answer on.

Ken

tommays

· #16 · May 15, 2014 (Edited)

I have NEMA 2000 Fuel flow on my A4 and all it takes is a minor bit of weather or foul current to triple the fuel burn over a given distance or needing to 6 knots instead of 5

Reactions: 1

CarbonSink62

· #17 · May 15, 2014

tommays said:
I have NEMA 2000 Fuel flow on my A4...

That's going on my wish list.

tommays

· #18 · May 15, 2014

I got the package 8 years ago and moved it from my 19' powerboat to the Cal 29 as it was a 50th birthday present and FWIW the 3500C was and still is a nice unit despite all the new fangled stuff

The fuel flow was also very useful on the powerboat as it was very easy to go form 5 MPG to 1 MGP when having to run slow in rough seas

chucklesR

· #19 · May 17, 2014 (Edited)

Fuel Flow doesn't work on a diesel engine unless it takes into account the fuel being returned to the tank via the return line. Fuel returning to a tank is typically 3 times that consumed and varies with temperature, injector pump pulsation and other factors.
That's why sensors for a gas engine are under 200 bucks, and for diesel 2 times that, and you have to buy 2 sensors that work on differential flow.
My low(er) cost solution so far is to carry a 5 gallon tank of fuel for when I run dry

The A4 is a gasoline engine

I've been researching this for a while to solve my 'problem'.

Panbo: The Marine Electronics Hub: Maretron FFM100, positive displacement fuel flow metering

DrivinSteve

· #20 · May 17, 2014

wannabsailor said:
Is fuel consumption a secret? I'm having a hard time finding the numbers and it seems as if the manufacturers don't want to give out the information. I want to put a 9.9 hp. outboard on a 26ft. Macgregor, the older models
(1990-1995) not the newer power sailboat type. I'm trying to find out what my fuel consumption would be to average 5 kts. Example: @ 4,000 rpm you should average 5kts and burn .6 gallons of fuel an hour…etc.
Anyone know of any sites with such info? Thanks for any help.

Average consumption is a nice to know; but what you really NEED to know is, in a pinch, what "performance" can I get per hour at the low end (max endurance/range) or, if I need to run fast for weather/tide/daylight/injury, do I have the fuel to do that. For instance, how fast can I make a Closest Point of Approach" to get/give help. I may need to adjust my plan if the fuel available prevents me from fighting a foul tide to make my destination. At what throttle setting do I get my "hull speed" and in what sea state does adding throttle just waste fuel for no incremental gain.

If you don't buy the safety aspect, think about a plan to buy fuel in the least expensive places by giving up half a knot (500RPM) to get there with some reserve.

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