Fuel Economy Question

[GalaxyGirl]

Hello all,
I have narrowed my trawler search to a 66' Cheoy Lee or a 70' Widebody Hatteras, no one boat in particular as I am still in my planning stages and trying to decide whether to sell my house and take the leap this Spring or next year. But, I was wondering if anyone knows if it is possible to get a general idea of the fuel economy of a vessel by dividing the range by the fuel capacity?

[bobconnie]

Well it's a matter of what engines, and fuel capacity! A choy lee trawler will probly have smaller engines then the Hatt, and if the choy is a long range cruiser,(a model in the Line of boats they made) It will also have a greater fuel capacity then the Hatt. Most of the older Hatts had Detroit diesels, starting with 6V53 up to 1271s. with mostly what I consider small tanks for the engine sizes. Choy Lee trawlers are mostly run a lower RPMS and will get some better fuel mileage. Now again this is all just my experince talking this is really hard to judge without knowing engines and size of fuel tanks, ect but should either help ya or maybe not !! shoot me some info of what your looking at and I will try to help ya more ! Have fun Looking

[djmarchand]

Here are some rules of thumb to help you:

A full displacement trawler takes 15 hp per 10,000 lbs displacement to make hull speed. You probably won't go that fast and reducing your speed to 1 * sqrt(lwl) will cut that figure in half.

Old Detroits make 15 hp per gph of fuel burned.

So figure hp based on displacement and speed and divide by 15 for fuel consumption.This will get you close.

Designated range is a marketing number, not to be believed for real world use.

David

[Brian in Az]

Originally Posted by GalaxyGirl

Hello all,.... But, I was wondering if anyone knows if it is possible to get a general idea of the fuel economy of a vessel by dividing the range by the fuel capacity?

The answer to your question is YES.

The other 2 posts are a prime example of how people can complicate a discussion without addressing the question at hand.

[atoll]

Originally Posted by Brian in Az

The answer to your question is YES.

The other 2 posts are a prime example of how people can complicate a discussion without addressing the question at hand.

as is your response!

[markpierce]

Golly gee. If one doesn't know the fuel capacity and the nature of the engines, etcetera, who can tell. Galaxy, you need to inquire more deeply.

[rebel heart]

For whatever it's worth:

I have a 52 horsepower Yanmar diesel.
At 2.2k rpm, I burn 0.5 gallons of fuel an hour and make anywhere from 4.5-6 knots, depending on currents and chop.
At 3.0k rpm, I burn .075 gallon of fuel an hour and make 5.5-7 knots, depending on currents and chop.

Great for fuel economy:
- low chop
- following current
- low rpm's / low speed

Bad fuel economy:
- choppy gross wind waves or swells
- contrary current
- trying to go fast

[CaptForce]

Originally Posted by rebel heart

For whatever it's worth:
I have a 52 horsepower Yanmar diesel....................

Hey Rebel Heart, I share your economy, engine size and speed, but Galaxy Girl is planning to move those huge motor yachts that are the size of the houses that many of us have left behind on shore. Our fuel economy is insignificant to the math used to figure what's needed to move these behemoths.

[sailorchic34]

A 60' Ocean Alexander trawler gets 1.2 gallons per mile at 10 knots and 2 gallons per mile at 14 knots on flat water. You mileage for a 66 or 70' boat would be a bit higher.

Plus factor in running the genset for 120V power while underway too at .5 gph

[CaptForce]

Originally Posted by sailorchic34

A 60' Ocean Alexander trawler gets 1.2 gallons per mile at 10 knots and 2 gallons per mile at 14 knots on flat water. You mileage for a 66 or 70' boat would be a bit higher.

Plus factor in running the genset for 120V power while underway too at .5 gph

This data supports my thought that there is no similarity between
Rebel Heart's economy and these big motor yachts.

Take the middle speed for the 60' Ocean Alexander at 12 kts. 12kts will complete 13.8 miles (statute) in one hour. At 1.5 gallons per mile, from the middle of Sailordhic's data, that's 27.6 Gallons which equals 0.5 miles/gallon.....or......just take the empirical data 1.2 gallons per mile equals 0.8 miles/gal & 2 gallons/mile equals 0.5 miles/gallon.

Take the middle speed of Rebel Heart's 36' sailboat motoring at 6.5kts. 6.5 kts = 7.48 statue miles in one hour. At 0.75 gallons per hour this comes to 9.6 miles/gallon.

Therefore, the motor yacht in this example, at it's best 0.8m/g, is using twelve times the fuel of the powered sailboat for the same distance traveled. Now, I do know that there are some very efficient long range cruising motor vessels, but it is not the characteristic for these big motor yachts like the Choey Lee, Hatteras or the Ocean Alexander. You have to be in love with the luxury and space, but you can't pass them off as fuel efficient.

[JanetGroene]

Don't overlook the difference in fuel use between a slick hull and a fouled hull. Fuel economy may begin to go down between haul-outs.

[ranger42c]

Originally Posted by GalaxyGirl

Hello all,
I have narrowed my trawler search to a 66' Cheoy Lee or a 70' Widebody Hatteras, no one boat in particular as I am still in my planning stages and trying to decide whether to sell my house and take the leap this Spring or next year. But, I was wondering if anyone knows if it is possible to get a general idea of the fuel economy of a vessel by dividing the range by the fuel capacity?

If you know engine models, you can often find published performance curves... which would include fuel flow at given RPMs, etc.

For example, the curves for our engines indicate (just a few from the list): 2 GPH at 1000 RPMs, 3 GPH at 1200 RPMs, 10.4 GPH at 2000 RPMs, 13.4 GPH at 2200 RPMs, and so forth. Published data will often include power output and torque curves, too. If you have multiple engines, as I do, multiply

Mating that info with advertised cruise speeds (especially if given associated RPMs) might get you closer to being able to compare boats.

In our case, I can relate fuel flow at given RPMs to known average speeds. For example, at normal cruise speeds: 13.4 GPH at 2200 RPMs (x2) at 2200 RPMs (approx 20 kts) to derive MPG (20 kts ÷ 26.8 = ~.75 MPG). Or at just slightly above hull speed: approx 8.5 kts average ÷ 6 GPH (total) at 1200 RPMs = ~1.42 MPG.

And so forth. And all theoretical numbers, of course, and also affected by sea states, tides/currents, wind, bottom condition, etc.

Both of those boats will more likely be run close to hull speed, and the installed engines are very likely optimized for that... so both may get decent (enough, in the grand scheme of things) fuel economy.

I've not been able to derive anything useful from knowing advertised range (which is all over the map) and fuel capacity. Range was not fiigure that the manufacturer plublished at the time; range was published by various boating magazines in their reviews of the boat... but the ranges were simply calculated at various speeds.

-Chris