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Old 14-07-2010, 14:41   #1
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Gallons per Hour . . .

I have a Pearson Invicta Mk2 powered by a Yanmar 3GM30 with a 3Blade Prop.
I will find this out first hand...but for preparations sake, what kind of fuel consumption is the norm w/ a clean hull/prop, cruising up the ICW?

I have a 25gal fuel tank, also have the ability to lash additional jerry cans on deck to have a bit more fuel on hand.
I just don't know and was hopping someone out there on the Cruisers Forum could shed some light on this for me. I know there are more variables that will influence answers. That aside, what do you think?
Thank you in advance, David
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Old 14-07-2010, 15:01   #2
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About a half gallon per hour. Lots of places to get fuel on the ICW but a couple of jugs on deck will be a good idea.
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Old 14-07-2010, 16:24   #3
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I think the rule of thumb is 1/2 gallon per hour per 10hp. So 100hp diesel would use 5 gal per hour. From Vigor's little book of boat facts as I recall.
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Old 14-07-2010, 16:28   #4
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I think the rule of thumb is 1/2 gallon per hour per 10hp. So 100hp diesel would use 5 gal per hour. From Vigor's little book of boat facts as I recall.
that seems like a lot to me. i've got a 33hp engine, and i'm definitely not burning 1.5 gph - closer to about 0.75 gph.
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Old 14-07-2010, 16:34   #5
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I have a heavy steel boat with a 73HP Perkins and at revs for 5-6knts would use (average) 1gallon per hour.

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Old 14-07-2010, 16:45   #6
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Our Perkins 4-236 (85 horse power) pushing a 46 footer at a tick over which keeps us moving at 5-6 knots we burn about gallon per hour.
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Old 14-07-2010, 16:51   #7
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Our 40 HP 3jh3CE's burn about .7 GPH at 85%. I would expect you would burn a little less.
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Old 14-07-2010, 17:00   #8
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The 1/2gal/10hp is not what you HAVE but what you are using. So that will account for some of the difference.

So if you have a 40hp engine, but are running it at a 20hp rate your usage would be about 1 gal/hour.

That is the rule of thumb, so your usage should be thumbing like that.
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Old 14-07-2010, 17:05   #9
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So if you have a 40hp engine, but are running it at a 20hp rate your usage would be about 1 gal/hour.
wow, really? are we all getting that small a return on the published HP specs for our engines? (not calling you a liar - it's an honest question)

Quote:
That is the rule of thumb, so your usage should be thumbing like that
i'm stealing that one!
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Old 14-07-2010, 17:10   #10
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we have a 50 hp Volvo and burn about .6 gls. per hour about 2200 rpms at 5-6 knots boat is a GulfStar 37 wt at 20k pounds North East Coast U.S. average about 1 fill up per season (45) gls fuel
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Old 14-07-2010, 17:53   #11
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Fuel consumption has little to do with the HP rating of the engine and everything to do with displacement, speed, prop, weather and hull cleanliness.

a half-gallon per hour at around 5 knots is a rough guide for typical small cruisers.
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Old 14-07-2010, 18:07   #12
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Originally Posted by daddle View Post
Fuel consumption has little to do with the HP rating of the engine and everything to do with displacement, speed, prop, weather and hull cleanliness.
.
That's not actually correct. On my boat, for example, at 2,800 rpms I'll burn .75 gph regardless of whether the hull is dirty, the water is choppy, or the boat is hauling extra weight. While all of those factors will effect my speed through the water, none of them effect the burn rate. At a given RPM, the engine will burn a given rate.

At 2,800 RPMS, with a clean bottom and flat water, I'll do eight knots. Dirty up the bottom and throw some chop at me and I'll quickly lose a knot of that speed. My distance per hour changes, but not the burn rate. The only thing that changes the burn rate is to change the RPMs. If I throttle up to 3,200 rpms to compensate for the dirty bottom and get my speed back up to eight knots, then I'm going to burn 1.25 GPH. But that won't happen if I keep the RPMs the same.
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Old 14-07-2010, 18:25   #13
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kb79,
You'll notice that horsepower ratings are tied to rpms of the engine. If you run your engine at half the rpms then you'll get about half the hp. Most folks I know never run their engines consistently over 2000 rpm and rarely do I do more than 1800. If I were shopping for an engine I'd take that into consideration. My little Mercedes is rated at about the same as a Perkins 4-107 or 4-108. If I run it at about 1800 rpm I can expect to get about 25 hp out of the engine. That'll push the 42 footer (34 ft waterline) just fine at 5-6 knots. Hull speed is nearly 8 but I'd have to really crank the engine up to well over half gallon an hour to make that speed in flat water.
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Old 14-07-2010, 18:30   #14
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That's not actually correct. On my boat, for example, at 2,800 rpms I'll burn .75 gph regardles
You are agreeing with me. I said "HP rating". Most small diesels whether 15 or 55 HP will burn about the same per hour for any given HP. And a half-gallon per hour is a good rule-of-thumb for casual log distance cruising in a typical small boat. Yes, one will be slowed down in bad weather.

My 50 foot 42 HP boat burns about .6 per hour. My 36 foot 20 HP boat about .5 per hour. Both traveling around 5 knots in calm conditions. How far I go per hour is the variable.
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Old 14-07-2010, 18:33   #15
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Originally Posted by Bash View Post
That's not actually correct. On my boat, for example, at 2,800 rpms I'll burn .75 gph regardless of whether the hull is dirty, the water is choppy, or the boat is hauling extra weight. While all of those factors will effect my speed through the water, none of them effect the burn rate. At a given RPM, the engine will burn a given rate.

At 2,800 RPMS, with a clean bottom and flat water, I'll do eight knots. Dirty up the bottom and throw some chop at me and I'll quickly lose a knot of that speed. My distance per hour changes, but not the burn rate. The only thing that changes the burn rate is to change the RPMs. If I throttle up to 3,200 rpms to compensate for the dirty bottom and get my speed back up to eight knots, then I'm going to burn 1.25 GPH. But that won't happen if I keep the RPMs the same.
Our marine diesels are constant RPM engines, right? So fuel burn is really a product of load and RPM? So, disregarding prop slip for a minute, 2800 Rpm should give you a constant speed, regardless of a clean or dirty bottom, but a dirty bottom would burn more fuel as the engine governor supplies more fuel to maintain the 2800 RPM due to the extra load caused by the resistance?
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