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Old 28-12-2013, 06:38   #1
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MPH Motoring a 47' Sailboat?

Does anyone have any empirical data on mph for a 47' sailboat, 30K pounds (a Cardinal 46, not unlike the old Swan 47)?

I'm most interested in extending the range in calm weather. At 1600 rpms, I get five knots which my crew often finds maddeningly slow and burns .5 gals per hours.

If I get her up to six knots at 2,000 rpms, am I likely to get further? Or maybe higher rpms?

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Old 28-12-2013, 06:43   #2
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Re: MPH motoring a 47' sailboat?

Your probably pretty close to optimum with your 1600rpm and 5 knots as your fuel burn will go up exponentially as you increase power. That's about the way I operate if I am motoring offshore....which is not very often.
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Old 28-12-2013, 07:13   #3
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Re: MPH motoring a 47' sailboat?

If you are trying strictly for the maximum range then faster will certainly give you less. However, with a theoretical hull speed of 7.5-8 kts bumping your speed up to 6 kts wouldn't reduce the total range that much.
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Old 28-12-2013, 07:27   #4
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What "empirical data" specifically are you looking for? You give actual (empirical by definition) RPM and speed you get now...is it no just a matter of doing some basic math using that empirocal data?
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Old 28-12-2013, 07:37   #5
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Re: MPH motoring a 47' sailboat?

My only empirical data is that 1600 rps burns .5 gals per hour and give 5 knots.
Increasing the rpms will increase the speed but also increase the gallons per hour.
The question is: what's the best trade off to get maximum range?
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Old 28-12-2013, 07:49   #6
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pirate Re: MPH motoring a 47' sailboat?

The only way you'll increase range is to strap an extra 150 litres on deck and fill as you go...
Its a bit like being on reserve in your car... and accelerating to get to the garage quicker..
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Old 28-12-2013, 07:52   #7
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Re: MPH motoring a 47' sailboat?

Quote:
Originally Posted by ssmith View Post
My only empirical data is that 1600 rps burns .5 gals per hour and give 5 knots.
Increasing the rpms will increase the speed but also increase the gallons per hour.
The question is: what's the best trade off to get maximum range?
In general the fuel burn increases at a greater rate than the speed so the fast you go the fewer miles per gallon you get.

But, the relationship isn't linear and at very low speed/rpm that relationship could reverse. In the range of practical use say 3-6 kts the relationship will be close to linear but one you get over 6 kts then the fuel consumption will starting climbing a lot for each additional half or quarter kt of speed. To get from 7.5 to 8 might double the fuel consumption.

These are of course very rough generalizations and as they say on the internet YMMV. Best way to determine exact numbers is to run a fuel burn test on your own boat. The quickest way is to get an accurately calibrated 5 gallon jerry jug and time how long it take to suck out a gallon at different RPMs.
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Old 28-12-2013, 08:03   #8
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Re: MPH motoring a 47' sailboat?

I've read that 80% of max engine RPM, assuming the engine is propped correctly, is a good tradeoff between speed and fuel consumption and properly loads the diesel. This may be what was rec'd for my boat. But seems a good start.
The only way you're going to get better consumption per mile will be to only run in the same direction as the current.
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Old 28-12-2013, 08:07   #9
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Re: MPH motoring a 47' sailboat?

Your way off with our 80% number
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Old 28-12-2013, 08:44   #10
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Re: MPH motoring a 47' sailboat?

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Originally Posted by robert sailor View Post
Your way off with our 80% number
Well, I have no problem being corrected....and it seems you know the secret number...please do tell.

Below is a cut/paste from my Op Manual for my engine. Although it doesn't speak to peak efficiency since the prop and hull are unknowns to them, 80% does fall safely in the range they expect for cruising RPMs. Running below 80% for extending periods runs the risk of glazing.

The JH4 series engines are designed to be
operated at maximum throttle
(3000 - 3200 rpm) for less than 5% of total
engine time (30 minutes out of every 10
hours) and cruising speed (2800 rpm or
less) for less than 90% of total engine time
(9 hours out of every 10 hours).
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Old 28-12-2013, 08:52   #11
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Re: MPH motoring a 47' sailboat?

I won't debate for a minute that you can run at 80% day in and day out without hurting your engine however 80% will not be anywhere near efficient when it comes to fuel burn on offshore passages. Once an engine is broken in its very unlikely that you will glaze cylinders running at reduced power for limited periods of time.
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Old 28-12-2013, 08:52   #12
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Re: MPH motoring a 47' sailboat?

I never heard the 80% rule, but it seems to match my experience.

I found the "sweet spot" of speed versus fuel consumption to be about 2450 rpm on my Yanmar 4JH3BE. That would give me about 6.5 knots and burn about .65 gal/hour. So about 10 NM per gallon. This was for a 12.5 ton boat.
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Old 28-12-2013, 09:21   #13
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Re: MPH motoring a 47' sailboat?

I've been thinking about the fuel consumption on our Yanmar 3GM30F. Searching the internet, folks claim diesels burn 1/2 gallon per 10 HP. From the Yanmar propeller HP curve, 2600 RPM is 11 HP, 3400 RPM is 24 HP and the curve is pretty linear. This equates to .55 gal/hr at 2600 RPM, and 1.2 gal/hr at 3400 RPM. In practice, I can get about 5.9 knots boat speed at 2600 RPM and 6.8 knots at 3400 RPM. Putting this together, at 2600 RPM, I can do 10.73 km/gal and at 3400 RPM, I get 5.67 km/gal.

For an additional 1 knot boat speed, I am more than doubling the fuel consumption. In practice, I plan on .66 gal/hr, at 2800 RPM, boat speed around 6 knots. This is consistent with what I've seen on this and other forums.

There is no best spot - it all depends on how far you plan on travelling and how much fuel you have onboard!.
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Old 28-12-2013, 09:41   #14
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Re: MPH motoring a 47' sailboat?

I simply offered 80% as a point of departure from something I had read. Way too many variables involved here for definitive empirical answers unless someone with the same boat/engine/prop weighs in. If the "real" answer is wanted, OP will need to do the Skipmac experiments...sounds like a fun way to spend a few weekends...running out of fuel and measuring the time/distance, then repeat!

I googled a tiny bit to see if I could find where I read "80%"...couldn't find it. However, the marine mechanic on the site below has similar guidelines.
Yanmar Marine Engine Help
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Old 28-12-2013, 10:15   #15
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Quote:
Originally Posted by ssmith View Post
My only empirical data is that 1600 rps burns .5 gals per hour and give 5 knots.
Increasing the rpms will increase the speed but also increase the gallons per hour.
The question is: what's the best trade off to get maximum range?
Since you actually have the boat (?, I infer from you post anyway), why not just to some test runs and do the math? This will far more acurrate than conjectures, or data from other similar boats, that you might get here.

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