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Old 28-10-2011, 15:11   #1
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Diesel Usage

I have been on multiple day trips on a Cal 39, Bennatue 37, & a Catalina 32, and compiled probably 15 hours each trip of motorsailing. It seems on each boat during refueling it averages about 1 gal of diesel to 1 hour of motoring. Is that a "ballpark" standard of consumption?
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Old 28-10-2011, 15:35   #2
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Re: Diesel usage

Depends on the type of engine, size of engine, how fast you are motoring, the weight of the boat, the type of underbody (more or less drag?) and the prop.

On my prior boat, 13,500 lbs, full keel, Yanmar 40, motoring at the "sweet spot" (2200 rpm) with a Maxi prop, I would average about 3/4 of a gallon per hour. In my current boat, a 19,000 lb Bristol 38.8 with a Westerbeke 46, fin keel and a fixed prop, I average about .9 gallons per hour at 2150 rpm with the centerboard up.

When you try to go above the engine's "sweet spot" the fuel efficiency decreases dramatically. A boat is no different than a car in that respect.
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Old 28-10-2011, 15:47   #3
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Re: Diesel usage

Quote:
Originally Posted by rpoint16 View Post
I have been on multiple day trips on a Cal 39, Bennatue 37, & a Catalina 32, and compiled probably 15 hours each trip of motorsailing. It seems on each boat during refueling it averages about 1 gal of diesel to 1 hour of motoring. Is that a "ballpark" standard of consumption?
Seems high to me for those boats. I would expect the 32 to be no more than 5/8 gal/hr. The 39 maybe 3/4. My Heavy 44 footer with a Perkins 4-108 and Max prop averaged about .65 gal/hr over 3 years use. (I kept a log of all fillups and hours, silly me) However, seems to me this was exceptional. Boat had a 39 waterline though.
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Old 28-10-2011, 15:48   #4
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We use about 3/4 gallon per hour running 1800-2000 rpms. Our boat is a hefty 38000 pounds, old 46foot Morgan. If we run a little above idle, say 1300-1500 rpm's, it's only about 1/2 gallon per hour.
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Old 28-10-2011, 17:36   #5
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Re: Diesel usage

!0,500 lb. fin keeled boat with a Yanmar 3gmd (20HP) running @ 2800 rpm = about .6 gph
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Old 28-10-2011, 17:57   #6
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Quote:
Originally Posted by terra57
We use about 3/4 gallon per hour running 1800-2000 rpms. Our boat is a hefty 38000 pounds, old 46foot Morgan. If we run a little above idle, say 1300-1500 rpm's, it's only about 1/2 gallon per hour.
After a 3 week cruise and a lot of different running Rpms I averaged . 7 gph cummins 65 horse boat weight 31000 lbs
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Old 28-10-2011, 18:00   #7
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Re: Diesel usage

If you want to get really technical with this then look up your engines specific fuel consumption which basically tells you how much horsepower is generated at a specific fuel flow. Then look up the size of your propeller and then go to tables which tell you how much horsepower your propeller requires to spin at a given RPM.

Or just ask someone with a similar boat and engine.
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Old 28-10-2011, 18:04   #8
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Re: Diesel usage

Great input. THX

I'm new at this and currently planning a charter. Just want to get an idea of what to expect.
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Old 28-10-2011, 21:53   #9
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Re: Diesel Usage

Quote:
Originally Posted by rpoint16 View Post
I have been on multiple day trips on a Cal 39, Bennatue 37, & a Catalina 32, and compiled probably 15 hours each trip of motorsailing. It seems on each boat during refueling it averages about 1 gal of diesel to 1 hour of motoring. Is that a "ballpark" standard of consumption?
Using Gerr's formulas, assuming 5kt boat speed, assuming a 3000lb load on the boat and no sails up

The Cal should get about 7.0 nm/gal depending on model
The Beneteau should get 7.5-7.8nm/gal depending on model
The Catalina should get 8.2 nm/gal.

Given that you were using significantly more fuel than this with sails up for all these boats I suspect:
A) You are maintaining a high throttle setting,
B) the engines all have problems,
C) the bottoms are all significantly fouled,
D) some combination of the above,
E) something is affecting all these boats that I haven't thought of.

Here is the link to the calculator that I figured out all the formulas from.

Boat Speed Calculator
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