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Old 28-12-2013, 10:56   #16
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Re: MPH motoring a 47' sailboat?

Having a fuel flow meter would solve all the questions but I don't have one.
Maybe there's someone with a similar boat with the ubiquitous Yanmar 4JHAE who might have some useful experience. And it's December and the boat in 3,000 miles away!
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Old 28-12-2013, 11:17   #17
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Re: MPH motoring a 47' sailboat?

I would expect raising the rpm to 2000, and the speed to 6 kts, to almost double the fuel consumption. Maybe 1 gal/hr?

I'm not clear whether you are trying to optimise range, or to get more speed?

Going slower is almost certain to extend your range.

However, most of the time my range is not an issue and I'd rather go at the extra knot. So I cruise at 6 kts, 2000rpm, and use 0.8 gal/hr.

If I was going at 5 kts the range would be much longer, I'm sure, so I would do that on a long trip.

One issue is that my engine runs too cool at 5 kts. 150F is too cool. At 6 kts it runs at a much healthier 175F. Much faster and it gets too hot.

Cruising throttle is usually a balance between impatience, noise, vibration, engine temperature, and fuel economy.

6 kts in my case also coincides with a (relatively) vibration-free engine rpm.

Only you can figure out the optimum. Sounds like you need to do a trip at higher rpm, figure out the fuel economy trade-off.
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Old 28-12-2013, 11:33   #18
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Re: MPH motoring a 47' sailboat?

Hereís the dilemma in narrative form:
We were heading from Tortola to Bermuda about 1,000 miles away.
The weather forecasts said it was likely there could light winds all the way.
Scarlet carries 65 gallons of fuel (I donít like having jerry cans on deck) which at 1600 rpms gives 4.5-5 knots, consuming .5 gallons per hour. (I assume this is a pretty efficient speed, but I donít know).So the cruising range is about 650 miles, 350 miles short of the total distance.
1. Should we motor faster and burn more fuel in hopes that the breeze will come up? (some of the crew has planes to catch)
2. Should we motor at what we think is the most fuel efficient speed, as far as we can leaving some fuel for harbor work?
3. Drift first and motor later?
4. Motor first and maybe drift later?

We choose No. 2 and arrived with about 12 gallons in the tank after a very slow passage. The departing crew missed all their flights.
So thatís why Iíd like to know to get the most most fuel efficient mpg.
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Old 28-12-2013, 11:41   #19
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Re: MPH motoring a 47' sailboat?

Here is the fuel curve. See page 2
http://mastry.com/mec/new_products/i...E_TechData.pdf
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Old 28-12-2013, 11:53   #20
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Re: MPH motoring a 47' sailboat?

Thanks. That seems to be a different model: mine is a naturally aspirated 4JHAE of about 55 hp. I'll check online to see if I can find that chart for my engine.
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Old 28-12-2013, 12:15   #21
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Re: MPH motoring a 47' sailboat?

Quote:
Originally Posted by ssmith View Post
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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In general, the slower you go the more efficient it is with regards to the water resistance. When propelled by a diesel engine, then you'd want to run it in its peak efficiency rpm/load. Looking at the charts, the peak torque is at 2000 rpm, the fuel curve ramps up after that.

With gasoline cars, the max efficiency is achieved by 'pulse and glide', whereby one runs the engine at max efficiency for a short time, and then shut it off and coast, then repeat. Thus the engine only runs at peak efficiency and the car moves at a lower average speed. Might work for a boat, might not.
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Old 28-12-2013, 12:27   #22
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Re: MPH motoring a 47' sailboat?

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Originally Posted by nimblemotors View Post
In general, the slower you go the more efficient it is with regards to the water resistance. When propelled by a diesel engine, then you'd want to run it in its peak efficiency rpm/load. Looking at the charts, the peak torque is at 2000 rpm, the fuel curve ramps up after that.

With gasoline cars, the max efficiency is achieved by 'pulse and glide', whereby one runs the engine at max efficiency for a short time, and then shut it off and coast, then repeat. Thus the engine only runs at peak efficiency and the car moves at a lower average speed. Might work for a boat, might not.
My understanding is that a diesel is most efficient at light throttle openings, dropping as the throttle is opened. A gasoline engine does rather the opposite. Therefore pulse and glide will not work with a diesel.

I drive a TDI Passat - the best economy is cruising at light throttle, keeping the rpm down.

I can get excellent fuel economy in traffic, by dropping back and keeping the car moving at light throttle (while everyone else follows 6 ft behind, slamming on the brakes and then accelerating).
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Old 28-12-2013, 15:36   #23
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Re: MPH motoring a 47' sailboat?

I didn't dissect the methodology here, but the fella in the link below plotted his boat speed vs RPM on a Yanmar fuel consumption chart. He determined the best RPM by looking at where the change in speed vs change in gph dropped off. He worked it out to be 2800 RPM on his 4JH3E...about 76% of the 3650 max continuous rated RPM.
Power Curve
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Old 28-12-2013, 16:46   #24
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MPH motoring a 47' sailboat?

From yanmar site 85 percent seems to be reccomended for some models.
"For prolonged running, a pleasure boat engine a good operating speed is around 85% of maximum continuous rated RPM (see the engine data plate)".
http://www.yanmarhelp.com/operate.htm
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Old 06-01-2014, 21:47   #25
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Re: MPH Motoring a 47' Sailboat?

I have a 47foot Wauquiez. The boat has a turbo diesel that is specified to run up to 4k2. I run at 2k8 rpm which gives me 7.2knts and .75gal an hour. I have another node at 2k2 with the same range. That give me just over 1000miles on a full tank.

The fuel consumption is off a fuel flow meter.

I found that the engine curves were fairly accurate.

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Old 06-01-2014, 22:10   #26
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Re: MPH Motoring a 47' Sailboat?

Here's another tidbit to chew on:
A general rule is Diesel engines consume about 1 gallon per hour for every 18 hp used. You can estimate the number of gallons consumed per hour by multiplying horsepower used by 0.055. The key is the word "used". If you don't use full power you will burn less fuel.
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Old 06-01-2014, 22:29   #27
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Re: MPH Motoring a 47' Sailboat?

Several issues. Best for engine life, versus best fuel economy, versus best fuel economy for conditions.

I have the described yanmar in a 40 footer and 2000 to 2200 rpm seems to be the sweet spot in most conditions for fuel economy which I think is a combo of prop, hull, and sea conditions.


Having said that I rarely need to as I have big tanks and tend to run at 80% of max as that is best for engine longevity all else equal with a dose of varying rpm's regularly too.
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Old 06-01-2014, 22:47   #28
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Re: MPH Motoring a 47' Sailboat?

Quote:
Originally Posted by SailFastTri View Post
Here's another tidbit to chew on:
A general rule is Diesel engines consume about 1 gallon per hour for every 18 hp used. You can estimate the number of gallons consumed per hour by multiplying horsepower used by 0.055. The key is the word "used". If you don't use full power you will burn less fuel.
And HP "used" can be approximated as Max HP times the cube root of the ratio of rpm to max rpm.

The OP mentioned the Yanmar 4JHAE (presumably 4JH4AE).

Max: 54HP @ 3000RPM so using the calculations above:

RPM===HP===GPH
1500----6.8---0.4
1750---10.7---0.6
2000---16.0---0.9
2250---22.8---1.3
2500---31.3---1.7
2750---41.6---2.3

Now just run sea trials to get the speed at the various RPMs and you can work out ranges and times.
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Old 06-01-2014, 23:02   #29
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Re: MPH Motoring a 47' Sailboat?

I have a Saga 48. with a 19 inch Maxprop. Purchased it in May 2013 the bottom was done during the haul out and the prop greased. It has a 43 feet waterline and is supposed to be able to cruise comfortably at 7 knots while motoring.
It's powered by a Westerbeeke 85hp. The previous owner assured me that he could reach 9 knots at full throttle . At max rpm of 3800 I do 7.2 knots and at 2000 rpm 5.2 knots. Engine seems to run fine and i have never hit anything with the prop. Does anyone have any idea why I go so slow?
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Old 06-01-2014, 23:02   #30
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Re: MPH Motoring a 47' Sailboat?

There are so many variables (gear ratio, prop {design, number of blades, diameter, pitch}, bottom condition, actual weight, alternator and load, phases of the moon ect.) that the only way to get real numbers for your boat is test it yourself. Even a sistership with the same engine may lead you astray with a different prop, alternator, refer units, and cruising displacement.
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