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Old 11-27-2014, 02:38 PM   #21
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The GM engines are two stroke, and burn a bit more fuel for the hp they create. That Hatt is a BIG boat- I've worked on a few of the 53's. The GM's are very reliable if they start out in decent shape.

The Cats at 275 are very nice engines. Fairly good economy at hull speed, and are quieter than the GM's.

Regarding hp per gph, the 20 figure is about as high as it gets for our size engines. My Cummins will get 20 at cruise of 1800rpm on the prop load curve, making about 165hp and a little over 8gph, about 17kts. My boat happens to load the engine almost exactly along the 2.7exponent prop load curve from around 1800 to 2100 where I have good fuel burn data. That makes it easy to calc.

But at slow cruise of 7.7kts at 950rpm, it is doing much worse, probably about 12-14hp per gph, but I have no hard data to calculate that. The reason is the engine uses some fuel to just spin itself and pump fluids and air. At lower power settings, this becomes a larger percentage. Called "parasitic" hp loss. edit- see below.

It is probably fair to say that the parasitic losses are close to proportional to engine displacement. At least in our hull speed context.

Just hard to find the right boat that has the right engine loaded to the right spot on the BSFC map.

Edit: Just being bored I looked at the Cummins fuel map for my engine and it does go down to 1000 and 800rpm on the prop load curve. At 1000 it is making 34hp at 1.9gph, which nets 17.9hp/gph. At 800, it is making 18.6hp at 1.1gph, for 16.9hp/gph. Not too shabby for running hull speed. At 2000 it is 21.3hp/gph.

The Cat 275 is probably going to be similar.
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Old 11-27-2014, 02:54 PM   #22
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Bsfc

Good info, although I will have to learn what is BSFC and how you cal.it. Also mentioned in another post 1.34 factor or multipler, I have some nautical math to learn. Steve
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Old 11-27-2014, 03:23 PM   #23
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As noted above two stroke Detroits use more fuel than four stroke Cats, so what?
Heavier boats use more fuel than light boats.


I suggest rereading the above in reference to planing hulls.
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Old 11-28-2014, 06:34 PM   #24
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does anybody know if common rail engines do better fuel wise at lower loading than standard injection. It is my foggy understanding that a big part of the common rail SX is that it can meter the amount of fuel needed much more accurately based on load factors and will not over fuel at lower loads( thus no smoke and bad emissions). Would that also lead to better efficiency at the lower loads?? It sounds that way to me if you are not over fueling would you not see a better hp/fuel ratio??
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Old 11-28-2014, 09:06 PM   #25
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Common Rail vs Mechanical

Forum member Ski in NC in a moment of boredom did an analysis of common rail vs mechanical engine (QSB 380 vs 6BTA 370) fuel consumption and posted it on boatdiesel. Both engines have similar turbos, intercoolers and boost levels, so the main difference is the fuel injection.

Perhaps we could prevail on him to repost it here because it was interesting. As I recall there was little difference between common rail and mechanical diesel fuel economy at low or high power loads.

And it is not an over fueling issue at low loads. Over fueling can happen at high load where there isn't enough air to burn all of the fuel being delivered, particularly with mechanical injection which doesn't have the precise fuel mapping of common rail. At low load there is plenty of air to burn the little diesel being injected.

Diesels whether mechanical or common rail produce very little CO and hydrocarbon emissions which tells you that the fuel is all getting burned efficiently meaning that the fuel consumption should be the same. They can produce lots of NOx and that is what electronic common rail is good for- precisely timed pulses and in many case multiple pulses will keep the flame temperature down and limit NOx production.

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Old 11-28-2014, 10:01 PM   #26
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All of the following is probably old news to many of the trawler fans here but no one has laid out the data in this forum for this potential new diesel operator.

Most engine manufactures provide BSFC curves or at least fuel used per hour and the horsepower delivered. From the latter you can calculate BSFC. The interesting thing about this is how much the BSFC varies based on engine RPM.

For example on a Cat TA (435 HP) the following are representative numbers:

1500 RPM 67 HP .381 pounds diesel per bhp per hour (BSFC)
2000 RPM 159 HP .340
2200 RPM 211 HP .339
2600 RPM 348 HP .357

You can see that at max efficiency the 3208 TA is 12% more efficient that at a low trawler speed of 1500 RPM.

The Lugger 6108 is another example where you can see very detailed BSFC numbers and their significant variation based on RPM. The continuous power curve (300 HP) shows the following BSFC numbers:

1400 RPM 220 HP .379 (less than 80 prop HP)
1800 RPM 280 HP .360 (120 prop HP)
2000 RPM 292 HP .358 (160 prop HP)
2300 RPM 300 HP .368 (295 prop HP)

But, as shown for the Lugger Ė the prop curve is really where you need to be looking at the power and BSFC. So the Lugger is 5.8% less fuel efficient at low RPM than it is at 2000 RPM which is the minimum BSFC.

A little research will show you similar data for almost any available marine diesel.
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Old 11-29-2014, 06:09 PM   #27
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We have a Kadey Krogen 42' with Lehman 135. This summer we did 1,039 n mi, 183 hr on the Lehman and averaged 1.55 gal/hr. Between 6 & 7 knots most of the time. The fuel consumption includes the generator use too.
Yep, that's about right for me too.
I think I've posted numbers for every 100 rpm from 1400 to 2000 rpms.

And though people say it, I've yet to see a twin setup that is as efficient as a single, or a 200 hp motor that is as efficient as a 135 on the 6-8 kt range.

I'd stay away from ANY old turbo engine. To much opportunity for abuse.
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Old 11-29-2014, 07:03 PM   #28
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good numbers

What the weight of your boat? Thanks for the info, I am going to modify my search. I was just looking at an twin 375hp 47 aft and cockpit, but I am going to target singles or twins no bigger than 135hp. Thanks, Steve
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Old 11-29-2014, 07:31 PM   #29
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Kadey Krogen 42s are designed at 39,000 lb displacement. Likely heavier when fully loaded. We usually run between 1600 & 1700 rpm on the Lehman 135.
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Old 12-02-2014, 06:57 AM   #30
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The hassle is the modern electric 4 squirt injector may be more efficient at minor loads BUT

A mechanical injector like for a DD is a $40-$50 rebuild Cat about $65 locally.

The electric injector are about a grand each.
And a spare set should be aboard in case you ever get near a thunderstorm with lightning.

A lot of hassle and bother for 5% or even 10% better fuel burn on a pleasure boat,
if you run 2000 hours a year , thats different.
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Old 12-02-2014, 09:33 AM   #31
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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.
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Old 12-02-2014, 09:47 AM   #32
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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.
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Old 12-02-2014, 01:16 PM   #33
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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.


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Old 12-02-2014, 01:44 PM   #34
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Fuel usage 135hp natural Perkins or Lehman vs.

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.


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Old 12-02-2014, 05:30 PM   #35
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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.
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Old 12-02-2014, 09:34 PM   #36
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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.
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Old 12-03-2014, 12:44 PM   #37
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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

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Old 12-03-2014, 10:16 PM   #38
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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.
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.
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Old 12-03-2014, 11:25 PM   #39
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...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.


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Old 12-04-2014, 01:34 AM   #40
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...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.


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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.
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