Fuel usage 135hp natural Perkins or Lehman vs.

Pick the best condition boat, displacement, single (many of em) or small twins..like a twin.Hatteras LRC you can find. Do NOT obsess over small GPH burn. The fuel, even at $4 will likely be the LEAST of your worries. Notice that i am not referring to 2000hp sport fisher go fast. I have a single 135hp FL that runs 5-6 knots. On a single look for or be ready to install a thruster, bow or stern.
Hull condition (no blisters), engine condition, genset, HVAC, paint and/or gel coat are all impt. IE: slip mate was quoted $750 per foot for paint from rub rail up, not even the hull.

Major engine overhaul can easily approach $10k. Full suite of prime electronics will will also get Ya in that $10k neighborhood. Hull peel, re glass! Barrier coat...ouch... New tanks you do not even want to think about. $90 an hour loaded labor yard rate, parts with marine written on them sold at the retail troy oz gold standard all add up in a hurry.

Even if you are skilled very few can do it all and then YOUR not cruising time is worth something...think about that too.

In the grand scheme of things the fuel burn is a pretty dim bulb in what could be a very dark room. Worry about all the other and then factor in fuel burn. Even a couple of gallons an hour works out to be small potatoes, so do not over sweat the small stuff.

Lest any forget, this is almost 2015 with common rail effectively a federal mandate (worldwide) and ObamaCare a reality. The refrain of turbos are no good and the Lehman 135 is better than gold need not permeate every engine thread. Not all have (or egads even like) KK42s and some prefer the redundancy twins offer. For the those who enjoy minutiae, the cost of fuel is a small % of the boating budget for some of us, much as the relevancy of the last 1/1000 of a BSFC number.

Oh well.

I have the Lehman 135 and get 1.75 GPH, including generator time. I probably run at higher rpms than John (Golden Dawn) and probably have been using my genny a bit more. I've heard 1.5-2.0 GPH, for FL135 equips KK42's,depending on how people have been cruising.

This past spring, a good friend just replaced his FL170 hp, which is a turbocharged engine. This was done by a long term, highly regarded local mechanic. Sadly the engine has just broken down and needs to be replaced. I'm not sure of the circumstances or cause of this. However, I would avoid boats that are equipped with turbo charged Lehman's if at all possible. Note that fuel consumption of the turbo equipped Lehman is similar to the "natural" engine, of you don't push it into Turbo mode, according to my friend who has this engine.


Jim
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Mules comments are spot on WRT the grand scheme of things...

...fuel economy has a very minor impact in the cost of owning and operating a vessel. The difference between 2 GPH and 3 GPH over 200 hours per year is $600/year at $3/gallon for diesel. However it is even less, if you factor in distance traveled, if you get to your destination quicker. If you travel a knot faster at 2 GPH, than at 1.5 GPH, it turns out you are only really saving about $129 over 1200 nm, which might represent a pretty average cruising season.

For 1200 nm:

1200nm/6.5GPH*1.5GPH*$3/G=$831
1200nm/7.5Kts*2.0GPH*$3/G=$960

Perkins and Lehmanns are both highly respected engines. When buying a vessel, the devil is in the details of how these engines have been maintained by their owners.


Jim
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Steve: There are many myth in the boating world.


Don't bother with the 1.34 hull speed baloney. That will give you a very inefficient operation speed. Square root of the water line length is real world guess for efficient speed, if the words efficient and boat should even be in the same sentence.. Just minimize the wake and keep the bow fully down.


BSFC differences are rarely shown on the prop curve just the max power curve. , The difference between those curves is worth understanding. BSFC varies on the max power curve usually only in the second decimal place. So what??


We seem to go nuts when contemplating fuel use between boats that all are very inefficient ways to travel . Is there really a big difference between 2,4 or 4 NMPG?? Boat size and weight and operating speed are really all that matters.




IMO if people started thinking in terms of Mpg there would be a much better understanding of how much fuel it takes to move through the water.

I agree, MPG is the best way to compare boats of similar size. GPH means nothing to me, or anyone except the operator. My 52 foot Krogen, with twin 120 hp DD 4-53s. gets about 3 mpg at 8 to 9 mph. It is way over propped and will not make the rated 2800 rpm. And it never gets close to that, to noisy. It will make 1900 rpm at 10 mph, which is faster than it likes and on the pins. 7E55 injectors so she dont smoke, but just makes more noise. Economy goes way down. At 8 mph its nice, smooth and much quieter. My concern for fuel usage is because I can only carry 1100 gallons. 3000 miles across the big blue at 3 mpg doesnt leave much margin.

I screwed up the units in my last post the correction is:

For 1200 nm:

1200nm/6.5Kts*1.5GPH*$3/G=$831
1200nm/7.5Kts*2.0GPH*$3/G=$960

Jim

kulas44 said:

I agree, MPG is the best way to compare boats of similar size. GPH means nothing to me, or anyone except the operator. My 52 foot Krogen, with twin 120 hp DD 4-53s. gets about 3 mpg at 8 to 9 mph. It is way over propped and will not make the rated 2800 rpm. And it never gets close to that, to noisy. It will make 1900 rpm at 10 mph, which is faster than it likes and on the pins. 7E55 injectors so she dont smoke, but just makes more noise. Economy goes way down. At 8 mph its nice, smooth and much quieter. My concern for fuel usage is because I can only carry 1100 gallons. 3000 miles across the big blue at 3 mpg doesnt leave much margin.

Click to expand...

Why do you use mph and not nautical miles per hr? Are your charts in statute miles?

Just curious.

You're numbers looked great till I realised they were sm, but they are still good.

...on the other hand, kts/gal, is a variable statistic, whereas GPH is a more stable. GPH isn't effected by weather and tides, and can be more easily calculated by your fill divided by engine hours. It's something that can be compared from year to year more easily. In the PNW, tides are a big influence on distance traveled per gallon. Plus you have to accurately log total distance traveled between fills, unless you have realtime consumption, not something I have on my boat. It's nice to go back and forth with both statistics.


Jim
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JDCAVE said:

...on the other hand, kts/gal, is a variable statistic, whereas GPH is a more stable. GPH isn't effected by weather and tides, and can be more easily calculated by your fill divided by engine hours. It's something that can be compared from year to year more easily. In the PNW, tides are a big influence on distance traveled per gallon. Plus you have to accurately log total distance traveled between fills, unless you have realtime consumption, not something I have on my boat. It's nice to go back and forth with both statistics.


Jim
Sent from my iPad using Trawler Forum

Click to expand...

Actually real time consumption is the way I often set my cruise speed. I set my rpm where I burn a total of 5 gallons per hour. This is not my most economical speed but is a compromise related to various comfort factors. Since this is done in the PNW the GPS speed may vary between 5 and 14 knots while the speed through the water is usually pegged at about 9.2K which is about my calculated hull speed. Attempts to go faster cause the fuel burn curve to rise steeply.

Actually I use nmpg. But that is hairsplitting . We could use gallons per furlong as well .My point is that people are far more familiar with mpg than gph and discussions in mpg of either sort are more meaningful to many.

MPG, NMPG, GPH - wow what a conundrum. Now that I am thoroughly confused I'll top my tanks when they get to between 25 to 30% fuel remaining. Unless of course I am at an el cheapo place that beats the price 200 miles or so up the line.

Or better yet, $ per mile which then gives one the opportunity to travel less if you're really a penny pincher. AKA a dock queen. They're everywhere. Notice how most of your slip neighbors seem to never go anywhere, do they have this conundrum figured out?

Data

I understand the advice on all the other costs involved in boating and I am still analyzing it. thought provoking, thanks I am a numbers guy, so thanks for all the data. It helps in deciding, so I don't buy too big or too much HP. I am surprised that a 39,000# full displacement can get better mileage (at the same speed) than a 22,000# semi. displacement. I guess I will stay away from semi planing or planning hulls. Thanks Steve.

SteveandZoila said:

I understand the advice on all the other costs involved in boating and I am still analyzing it. thought provoking, thanks I am a numbers guy, so thanks for all the data. It helps in deciding, so I don't buy too big or too much HP. I am surprised that a 39,000# full displacement can get better mileage (at the same speed) than a 22,000# semi. displacement. I guess I will stay away from semi planing or planning hulls. Thanks Steve.

Click to expand...

Just to throw a little more wood on the fire. The type of hull when making comparisons between semi and full displacement boats of similar length is not always the major factor in fuel burn. The engine size and stern gear- the waterline length -overall weight -freeboard and wind age-beam and nature of the hull entry fineness also have to be taken into account. Two 50 footers one semi and the other full displacement could have identical fuel burns at seven or eight knots or either could be better depending on factors other than hull type. Often the choices made in setting up the semi include the option of speeds above hull speed and the use of big twins thus a loss in economy, but nothing says a SD cant be set up for good fuel burn . So a lighter modest size single engine semi-displacement boat with a set up for good fuel burn can out-shine a heavier FD boat both traveling one knot below hull speed.

eyschulman said:

Actually real time consumption is the way I often set my cruise speed. I set my rpm where I burn a total of 5 gallons per hour. This is not my most economical speed but is a compromise related to various comfort factors. Since this is done in the PNW the GPS speed may vary between 5 and 14 knots while the speed through the water is usually pegged at about 9.2K which is about my calculated hull speed. Attempts to go faster cause the fuel burn curve to rise steeply.

Click to expand...

Maybe this is a stupid question, but isn't that normal, to set rpms?