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Old 13-08-2020, 06:17   #1
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4 stroke outboard fuel consumption???

I just purchased a gulf 29 pilothouse. A shop screwed up an oil change on the Universal m18 and replaced it with a transom mount and a new Mercury 15hp prokicker. I cant wait to get a diesel back in it but I want to get it home, Vancouver to Prince Rupert, 500+km with the Merc outboard. The boat has 23.5 feet in the water and 7500lbs displacement. Does anybody have a good idea what I can expect for fuel consumption?? There are concerns of first nations fueling centers not being open for business due to covid.

Thank You in advance
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Old 13-08-2020, 06:29   #2
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Re: 4 stroke outboard fuel consumption???

About 1.4 GPH at WOT. Assuming about 6 knots, 4-5 mpg. But I'm guessing on the speed and I doubt you will run quite WOT. Yeah, you'll need a lot of cans.
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Old 13-08-2020, 06:42   #3
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Re: 4 stroke outboard fuel consumption???

A pretty good rule for outboards that works from 9.9 to 300+ HP is that each 10 HP will burn one gallon an hour. Yeah that means the big boys with quad 400’s if they open it up are burning 160 gallons an hour.

However I’d expect you to not use more than 3/4 throttle, some four strokes run lean at partial throttle and get real stingy, but close to WOT they go real rich cause rich mixture makes more power and runs cooler at the expense of higher fuel burn.
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Old 13-08-2020, 07:10   #4
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Re: 4 stroke outboard fuel consumption???

Not sure about the 15 HP 4 stroke but my 2011 Mercury 5 hp 4 Stroke Outboard uses about 1.2-1.5 gallons or so to cross 18 miles of Chesapeake Bay which can take around 4 hours at times depending on tide etc.

The boat it's pushing (Bristol 27) has a displacement of around 6600 lbs.

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Old 13-08-2020, 07:16   #5
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Re: 4 stroke outboard fuel consumption???

Being an injected 4 stroke I'd expect you to use less than 1 gallon per hour if correctly propped as A64 said.
Man thats a horrible thought to go that distance under outboard power. Hope you get fair winds.
Curious about how they screwed up the oil change if you care to tell the story.
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Old 13-08-2020, 08:34   #6
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Re: 4 stroke outboard fuel consumption???

All present-day manufacturers of 4-stroke outboards show fuel economy of 9.5-10.5 horsepower-hours per gallon, and this matches my personal experience. (In other words if you run at 10 horsepower for an hour you will burn about one gallon of gasoline)


Stock high-thrust motors are usually propped for around 8 knots at WOT. You're unlikely to achieve that so you're probably a little bit overpropped and won't get the full 15 HP even at WOT.


I would guess, for planning purposes, you'll get around 5-6 knots. 500 km = 270 nm = around 50 hours


I would expect you'll burn around 50-75 gallons. Headwinds or heavy seas would make that worse.



You would want to confirm the burn rate and speed, obviously, before embarking on a voyage where you depend on that figure being correct.


You can get 12 gallon portable fuel tanks for outboard motors. It would be reasonable to carry two of those or maybe even three. Unlike jerry cans you would not have to pour fuel while under way.
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Old 13-08-2020, 08:43   #7
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Re: 4 stroke outboard fuel consumption???

A shaker siphon is the bet way to transfer underway. Less chance of spill, you don't have to hold the can, if there is something higher to sit it on.
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Old 13-08-2020, 08:48   #8
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Re: 4 stroke outboard fuel consumption???

If your diesel tank is empty (or just empty it) you could fill with gas and use a transfer pump to fill jerry's or buy a length of fuel hose and run direct. Make sure your tank setup is safe for gas.
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Old 13-08-2020, 10:19   #9
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Re: 4 stroke outboard fuel consumption???

Needless to say You guys are awesome.
That is exactly the type of math i was hoping for. The story is oil change gone wrong but i wont pry until I meet the prev owner in person. And the shop has agreed to let me take away what remains of the Universal m-18. I hope the gear box, prop and shaft are still there.
So Jammer's 50 to 75 gallons with 19 gallon tank on board and my 12 gallon jerry is 1.6 to 2.4 fills.
Leaving loaded that means 1 or 2 fills on route, fuel stations permitting, should do it.
I have this idea displacement boats wont mind an additional 12 gallon jerry for safety. Do you think this is overkill.
My mother and father in law, the sailors responsible for our safety, are considering bringing a backup 20hp four stroke but it is a short shaft. What would you guys do? I want to trust the nearly new Merc and use sails as backup.
Does anybody else want to come, the boats not quite full yet??
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Old 13-08-2020, 10:53   #10
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Re: 4 stroke outboard fuel consumption???

I only use my motor when there is little to no wind.

If the wind is right, sail as far as possible or motor sail.

You may not use much fuel at all if you plan and the wind co-operates. ( or you may use half of what you predict to motor the whole way.

Of course here, I have a 20-30 mile wide bay to work with.......with usually pretty decent wind and spots to anchor.
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Old 13-08-2020, 11:17   #11
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Re: 4 stroke outboard fuel consumption???

a) you should factor in adverse current and wind condition....where your actual speed over ground may be half what your speedo sez...and carry plenty of " reserve" fuel....
b) in order to keep going without stopping between tanks, you need to have a foolproof way of transferring gas to the tank the engine is hooked up too...while maintaining steerage way, etc...did I say foolproof..it bears repeating...think carefully how you plan to do this.....
c) there is always the chance the outboard will quit, at any time at any location, so you need to have a plan "B" ready at all times....anchors, lights, etc...
d) nobody here has to tell you that gasoline is highly flammable...swapping tanks, etc...could cause spillage or other fuel related problems....and you don't want this near a hot engine....
e) without question, have a fire extinguisher or two, handy nearby...
f) having a spare engine on stand by....even a short shaft..as long as you can get the prop under the surface of the water will likely enable you to get out of a jam.

on a separate issue...I once helped a sailboat travel over 50 miles, by tying my fishing boat to his stern quarter, like a push boat, it worked fine, except for when passing boats threw out a big wake....then it was a jousting match for a few minutes until things settled down again...
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Old 13-08-2020, 11:24   #12
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Re: 4 stroke outboard fuel consumption???

My Tohatsu 9.8hp burned 1 gallon per hour pushing a Catalina 25 at roughly 6 knots (around 5.5 in a strong headwind). This was over a 30 hour period so I got a pretty good feel for the burn rate. This was also at about 80% throttle.
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Old 13-08-2020, 11:52   #13
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Re: 4 stroke outboard fuel consumption???

Quote:
Originally Posted by muskeg View Post
There are concerns of first nations fueling centers not being open for business due to covid.

Shearwater is still open for fuel: https://www.instagram.com/p/CC3xE3Mhnp1/
And so is Dawson's Landing: Home


But you probably won't find much else between Port Hardy and Prince Rupert.
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Old 13-08-2020, 12:04   #14
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Re: 4 stroke outboard fuel consumption???

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Originally Posted by n5ama View Post
My Tohatsu 9.8hp burned 1 gallon per hour pushing a Catalina 25 at roughly 6 knots (around 5.5 in a strong headwind). This was over a 30 hour period so I got a pretty good feel for the burn rate. This was also at about 80% throttle.
That's very surprising that a 9.8 hp outboard if it's a 4 stroke would burn that much gas.

I rarely have more than 1/2 tank (3 gallon tank) of gas and regularly motor home 4 hours or so back across the lower Chesapeake Bay. I've been doing this the past 8 years .......

My engine is a 5 hp 4 stroke but it's pushing a 6600 lb Disp Bristol 27. The Catalina 25 only displaces 4550.

I rarely go above 1/2 - 2/3's though on the throttle
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Old 13-08-2020, 13:58   #15
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Re: 4 stroke outboard fuel consumption???

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Originally Posted by thomm225 View Post
That's very surprising that a 9.8 hp outboard if it's a 4 stroke would burn that much gas.

I rarely have more than 1/2 tank (3 gallon tank) of gas and regularly motor home 4 hours or so back across the lower Chesapeake Bay. I've been doing this the past 8 years .......

My engine is a 5 hp 4 stroke but it's pushing a 6600 lb Disp Bristol 27. The Catalina 25 only displaces 4550.

I rarely go above 1/2 - 2/3's though on the throttle

Hey Thomm,


You will find small adjustments to speed will make big adjustments to fuel consumption.


Going 10% slower, saves more than 10% of the fuel. It is a non-linear relationship.



Playing at 5 vs 5.5 vs 6 knot will show big fuel savings, biggest (per mile sailed) as you slow down.



How happy are you with 5 hp? Money no object, would you use a more powerful unit, if so, what hp would you elect?


And do you have a long, or an extra long leg?


Thank you in advance! Our boat also displaces about 6-7t and given the age of our inboard I often mentally rehearse moving to an outboard.


Cheers,
barnakiel
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