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Old 12-05-2017, 02:44 PM   #41
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If you are needing 300hp, I would want the 14 liter.

If running some lower hp, smaller is probably more efficient.

My 8 liter is most efficient at 200 hp/2000 rpm. it is rated at 450hp.
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Old 12-05-2017, 02:59 PM   #42
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I have seen precious few engines that deal with engine oil coolers effectively. Never seen a thermostatically controlled oil cooler on a boat, though my sample size is fairly small. .
Many marine diesels are built with coolant/oil heat exchangers controlling the oil temps. On mine the oil operating range is about 185 - 190 F with the in-engine 180 degree coolant thermoset.
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Old 12-05-2017, 03:25 PM   #43
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My 8 liter is most efficient at 200 hp/2000 rpm. it is rated at 450hp.
Ski, can you hum a few bars about how you arrive at that? I have the Cummins 6CTA performance curves (Rated Power Output, Full Load Torque, and Fuel Consumption - Prop) but I dunno so much about how to interpret the data.

The table accompanying the Full Load Torque Curve suggests peak torque at 1800 RPMs (1395 NM/1029 lb-ft), while the graph suggest peak would be somewhere between 1600-1700 RPMs. The other two curves don't seem to present an obvious "best."

I'm assuming there's maybe some other data, not presented in their Marine Performance Curve document, or some other calculation that's not obvious (at least to me).

Can you instruct?

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Old 12-05-2017, 03:39 PM   #44
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Bored and a bit lazy, so time for fun with numbers...

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Old 12-05-2017, 03:50 PM   #45
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If you are needing 300hp, I would want the 14 liter.

If running some lower hp, smaller is probably more efficient.

My 8 liter is most efficient at 200 hp/2000 rpm. it is rated at 450hp.

Does Cummins publish or supply a fuel vs hp curve for those engines?
Those curves are very telling I have them for a few engines.
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Old 12-05-2017, 05:02 PM   #46
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You have to do a little math. Cummins does publish full load data, but also publishes on many engines the prop load data. It gives rpm and gph. At the bottom of the page they state that this load curve is along a 2.7 exponent. To find the hp at some lower rpm, calc ((target rpm/rated rpm)**2.7) rated hp. That gives you the hp at that rpm, and the chart gives you the burn rate.

Some later engines it gives rpm, hp and burn along the prop curve. Makes it easier. Some Cat engine lit give similar data.

So on the 6CTA8.3 450 at 2000rpm on prop curve, it gives the burn rate at 10.4gph.

2000/2600 at 2.7 exp is 0.492. 0.492x430hp is 212hp.

212hp/10.4gph is 20.4hp/gph. Can be done for any of the tabulated rpms.

Appologize, don't have proper math notation on this computer.

No guarantees that the engine theoretical prop curve is going to match the boat load curve. In fact most will stray badly. In my case the two important data points (hull speed, 950-1000rpm) and planing (1800-2000rpm) my gph numbers are very close to the prop curve numbers. Makes it easy there.
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Old 12-05-2017, 05:04 PM   #47
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I saw what you did there....
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Old 12-05-2017, 05:22 PM   #48
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If you are needing 300hp, I would want the 14 liter.

If running some lower hp, smaller is probably more efficient.

My 8 liter is most efficient at 200 hp/2000 rpm. it is rated at 450hp.
So Ski how much extra weight would the 14 liter bring to the boat?

Gas engines were common in the 50’s or before because all diesel engines were just too heavy. Weight of the engines counted then and it does now. DD’s were from that era and were then considered a lightweight. We’re all talk’in bout engine performance but that’s usless until the engine poweres a boat. A lighter engine at times will be more important than engine efficiency because it will make the BOAT more efficient.
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Old 12-05-2017, 05:51 PM   #49
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A lighter engine at times will be more important than engine efficiency because it will make the BOAT more efficient.
Depends on the boat.
I can honestly say I notice no noticeable difference in performance with near empty fuel and water tanks vs our now near full fuel and full water.
6500kg (7 us ton) difference, 7knots @ 1100 rpm and 8.5knots @ 1250 rpm
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Old 12-05-2017, 06:14 PM   #50
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If you have choices in engines for slow speed cruising, I would look for a boat with natural engines - no turbos. They use less fuel and last longer. Turbos came about to get more hp out of a given block size or to put an engine in a smaller space, And they're needed in ships, tugs and other high speed boating. But running near peak hp on a turbo engine causes excessive wear in the cylinder components (rings, valves cylinder walls) from high exhaust gas temperatures. In heavy duty, continuous duty engines, turbo versions usually get overhauled 2-3 times more often than non-turbo versions. Yacht engines are not heavy duty. Few are even medium duty. If you look at yacht engine ratings, most are listed as light or recreational duty. The fine print usually says something like - only run at hp rating 2 hours in 12.
The next issue is reliability. If most of your cruising is on the ocean, you need engines that don't fail. Especially if you choose to have a single. Talk to people that run many hours. Not 50 hours a year but a thousand. If you can, stay away from electronic controlled engines. The marine environment is hard on anything electric or electronic.
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Old 12-05-2017, 06:28 PM   #51
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A 14 liter is probably 1500lbm heavier than an 8 liter (about 1800-2000lb). On a heavy boat, it makes little difference. 1500lb sounds like a lot until you get into big boats and percentages. If boat is 100,000lb, a 1500lb difference is 1.5%. Trivial.
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Old 12-06-2017, 12:34 AM   #52
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My experience is with small diesels on sailboats and my current Cummins QSB 5.9L 380hp engine in my 43’ North Pacific.

I was very concerned about fuel economy and was asking the very same questions you are now. Here are my relatively ignorant thoughts based on listening to folks more knowledgeable than I and my own experience over the last 1 1/2 years of operation.

1. In our boats, throttle position is the single most important factor in fuel economy. Every hull is different, but there is a point where adding speed really drives up the fuel burn.

2. Larger engines will generally be less efficient than small engines. The difference is relatively small all things being equal. However, all things are NEVER equal. So the difference is more theoretical than practical.

3. Fuel cost for most of us is a very small percentage of our overall boating expense.

4. All boats are different. My North Pacific 43 has the 380hp version of the 5.9L QSB. The standard engine that was being put into those boats at the time was the 330hp version of the engine. The original buyer wanted more power. I don’t think I am being helped or hurt by that extra 50hp as it is the same block.

5. While Cummins doesn’t care if your engines are run for long periods at a low rpm, most experienced Cummins mechanics that I have spoken to or read, do recommend a short period of running at a higher rpm after hours of low rpm running. This is a very vague recommendation. I typically run at 1450 rpm. I have been bringing the rpm up to 1800+ for the last 15 minutes before I enter my harbor. It then takes me 20 minutes of running at a very low rpm through my no wake zone harbor to get to my slip so there is plenty of time for the engine to cool down. Does it help? No idea.

As others have said, if you are buying used pick the boat you want, not the engine. You are looking at buying a boat for what it can do for you. For most of us, any engine would do as long as it was put in the right boat.
Hi, The Owner's Manual does not mention that it's good to raise rmp for some time to burn the machine clean coal or other. The 1450 rpm does not yet lift the compression pressure CM qsb 5.9 380hp on a engines than the max 0.1 bar and this still does not help burning purely.

If you want to believe in your professional guys, then you will have to load the 2100-2200 rpm so that the turbo pressures rise and return more cleanly, but your Cummins does not need it because it burns cleanly + 800-2700 rmp. It knows how to dispose of fuel optimally as older generation machines do not know, here's a big difference. See, then, your turbo pressure gauge rises to over 1 bar (14,5psi), as I remember 1,7 bar max, If you look at your engine need this ...

NBs
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Old 12-06-2017, 01:25 AM   #53
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My 34 has 6.7 cummins 425 hp. 1 gph at 6 kts. At 8 kts 3 gph. At 20 kts 20 gph. No need to blow it out very often, it is a common rail diesel.
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Old 12-06-2017, 05:56 AM   #54
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It is so easy (and fun, if you are engineer or simply technically inclined) to go through calculating how fast your boat is, how many GPH (LPH) engine/boat uses, etc, especially when everything else on the boat is "fixed" and there are no no new tasks currently.

But, let's sum up some things which members have already said in this conversation:

1! If you are recreational user, fuel consumption is almost irrelevant compared to costs of boat slip, maintenance, and other boat involved costs.

2. If you are fishing, and have a big engine, it will probably be too fast even at low RPM for trawling (prop is most often optimized for correct top RPM).

3. With big engine going slow, you are paying for taking big weight of that engine all the time (and huge amount of usable space is taken by the engine). Oh, and working around the engine is "crampier", too.

4. The bigger will last longer before needing to overhaul (probably questionable if you are recreational user since at that duty cycle all engines can outlast you if used regularly or will corrode much before overhaul period).

I will also add this two:
4. If you have a big engine used for slow speeds, how much will you pay for higher amounts of oil, antifreeze, more expensive filters, spares in routine maintenance and taxes (this last one is especially considerable in Europe)? How much fuel could have been bought instead?

5. If you use the engine until it is time to overhaul/re-power, how much does it cost and how fast can it be done for big engines compared to small engines?

Although, I obviously vote for smaller engines (not too small though), mind that I do have a SD boat and big engine which enables me to go at higher speeds if I need to, but I have never needed to, I have only wanted to, and when I travel further, higher speeds become irrational due to fuel costs. Oh well, once on the river I was bored to wait for the array of barges to pass and then I went to planning speed to to go around behind them and I have reached on the other side at almost the same time as the guy in FD boat who has waited for the barges to pass, well that was really efficient.

And then, there is that famous sentence "for running out from storm". The only ones I know that have run out from the storm were the ones that did not came out on the water because of the weather forecast, everybody else on the water had to deal with the storm.

So, according to my experience, the most efficient selection of your new boat comes from the early decision at which one and only cruising speed you plan to travel and to have an "adequate" engine for that. And adequate is the one with which the boat is not underpowered (to deal with the storm, not to run out from the storm). Overpowered seems just to add up the costs and give that smile to the captain when reaching higher speed. You decide what is the worth of that smile.
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Old 12-06-2017, 06:50 AM   #55
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It is so easy (and fun, if you are engineer or simply technically inclined) to go through calculating how fast your boat is, how many GPH (LPH) engine/boat uses, etc, especially when everything else on the boat is "fixed" and there are no no new tasks currently.

But, let's sum up some things which members have already said in this conversation:

1! If you are recreational user, fuel consumption is almost irrelevant compared to costs of boat slip, maintenance, and other boat involved costs.

2. If you are fishing, and have a big engine, it will probably be too fast even at low RPM for trawling (prop is most often optimized for correct top RPM).

3. With big engine going slow, you are paying for taking big weight of that engine all the time (and huge amount of usable space is taken by the engine). Oh, and working around the engine is "crampier", too.

4. The bigger will last longer before needing to overhaul (probably questionable if you are recreational user since at that duty cycle all engines can outlast you if used regularly or will corrode much before overhaul period).

I will also add this two:
4. If you have a big engine used for slow speeds, how much will you pay for higher amounts of oil, antifreeze, more expensive filters, spares in routine maintenance and taxes (this last one is especially considerable in Europe)? How much fuel could have been bought instead?

5. If you use the engine until it is time to overhaul/re-power, how much does it cost and how fast can it be done for big engines compared to small engines?

Although, I obviously vote for smaller engines (not too small though), mind that I do have a SD boat and big engine which enables me to go at higher speeds if I need to, but I have never needed to, I have only wanted to, and when I travel further, higher speeds become irrational due to fuel costs. Oh well, once on the river I was bored to wait for the array of barges to pass and then I went to planning speed to to go around behind them and I have reached on the other side at almost the same time as the guy in FD boat who has waited for the barges to pass, well that was really efficient.

And then, there is that famous sentence "for running out from storm". The only ones I know that have run out from the storm were the ones that did not came out on the water because of the weather forecast, everybody else on the water had to deal with the storm.

So, according to my experience, the most efficient selection of your new boat comes from the early decision at which one and only cruising speed you plan to travel and to have an "adequate" engine for that. And adequate is the one with which the boat is not underpowered (to deal with the storm, not to run out from the storm). Overpowered seems just to add up the costs and give that smile to the captain when reaching higher speed. You decide what is the worth of that smile.
Well said
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Old 12-06-2017, 06:56 AM   #56
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I like using hp hr/gal as it is nice easy number to work with. Most modern engines are around 20hp hr/gal, or put easier, 20hp/gph. Know your hp going down the shaft, you can estimate the burn. Know the burn, you can estimate hp.

I went through some numbers on the Cummins 6CTA 450 on my boat to try to see how badly efficiency falls at super light load. Fortunately they publish 2.7 exp dyno load data down to 800rpm, which is nice to see.

These are the two rpm ranges I run. Lowest near hull speed, higher range is planing. The hp I calc'd from the 2.7exp load curve, gph and rpm from Cummins tabular data. I used 430hp (SAE) vs the metric 450 as to not be accused of brand favoritism. The 1000 and 2000 are my fave rpms to run and my dipstick reading are very close to the table data.

800rpm 1.2gph 18.0hp 14.9hp/gph
1000rpm 2.0gph 32.6hp 16.3hp/gph
1200rpm 3.0gph 53.3hp 17.8hp/gph

1800rpm 8.2gph 159hp 19.4hp/gph
2000rpm 10.4gph 212hp 20.4hp/gph
2200rpm 13.6gph 274hp 20.1hp/gph

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You have to do a little math. Cummins does publish full load data, but also publishes on many engines the prop load data. It gives rpm and gph. At the bottom of the page they state that this load curve is along a 2.7 exponent. To find the hp at some lower rpm, calc ((target rpm/rated rpm)**2.7) rated hp. That gives you the hp at that rpm, and the chart gives you the burn rate.

So on the 6CTA8.3 450 at 2000rpm on prop curve, it gives the burn rate at 10.4gph.

2000/2600 at 2.7 exp is 0.492. 0.492x430hp is 212hp.

212hp/10.4gph is 20.4hp/gph. Can be done for any of the tabulated rpms.

Appologize, don't have proper math notation on this computer.
Thanks, Ski, I'm getting the concept. I don't understand the notation for the highlighted part (2000/2600 at 2.7 is .492), and even though I knew about the 2.7 exponent on the Cummins prop demand curve... I've had no clue what to do with that tidbit.

Anyway, I do follow your conclusion, and I see why, especially from putting this together with your previous table, 2000 would be better than 2200 (20.4hp/gph is higher than 20.1hp/gph).

Appreciate the insight.

-Chris
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Old 12-06-2017, 07:42 AM   #57
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So, according to my experience, the most efficient selection of your new boat comes from the early decision at which one and only cruising speed you plan to travel and to have an "adequate" engine for that. And adequate is the one with which the boat is not underpowered (to deal with the storm, not to run out from the storm). Overpowered seems just to add up the costs and give that smile to the captain when reaching higher speed. You decide what is the worth of that smile.
Nidza

All the way from Serbia we get common sense. Good summary.

One thing often forgotten - most of the time we are buying used boats so engine selection cannot be optimized. Then, as was my case, we buy a spec boat where the dealer has chosen the engine option.

But, when buying a new boat you pay a huge premium for that new boat smell and perfect (to you) engine. This huge premium negates any financial gains for having the "perfect" engine. Plus in this day and age the new boat will not have the much sought after mechanical NA engine.
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Old 12-06-2017, 08:23 AM   #58
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Diesels produce about 20 HP per gallon of fuel per hour. It doesn't matter much what their max HP rating is or who made the engine.

You decide how much HP you want to use and therefore the amount of fuel consumed.

Hull shape weight makes a big difference in max speed.

It's very simple. Going faster uses more fuel.

Most boats in the size range you are looking at will get 2-3 NMpg at 7 kts and less than 1 NPG at 22 kts if they can plane.

Personally I spend most time going slow but like the ability to get on plane for open crossings, weather etc.

My advice is don't worry about engines and fuel, they are fairly consistant. First decide if ability to go fast appeals to you then get the living space you need to enjoy spending time on the boat.
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Old 12-06-2017, 08:52 AM   #59
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One thing often forgotten - most of the time we are buying used boats so engine selection cannot be optimized. Then, as was my case, we buy a spec boat where the dealer has chosen the engine option.

When we eventually arrived out our short list of boats with the features we wanted...

Once that happened, I had a preference for engines that could be rebuilt in frame versus not (e.g., Cummins C vs. B), or as second choice, a set-up where an engine (e.g, a 6BT of some persuasion) could be easily removed for a rebuild if necessary.

In our case, that gave us a choice of about 6 engines that I remember: Cummins C, CE, and B, Volvo 480 (TAMD74?), Cat something, Yanmar 440s... all generally available from the boat manufacturer (over various boat model years) and probably chosen as acceptable for the application (as well as any deals the boat maker had with the engine makers/dealers).

Might be that other engines extant at the time could also have been perfectly acceptable, maybe even "better" for one reason or another... and evolution happens, so there are some additional choices that would be available today...

But it was what it was... so we picked what we considered to be a viable candidate... and then went boating.

-Chris
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Old 12-06-2017, 09:07 AM   #60
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Nidza

All the way from Serbia we get common sense. Good summary.

One thing often forgotten - most of the time we are buying used boats so engine selection cannot be optimized. Then, as was my case, we buy a spec boat where the dealer has chosen the engine option.

But, when buying a new boat you pay a huge premium for that new boat smell and perfect (to you) engine. This huge premium negates any financial gains for having the "perfect" engine. Plus in this day and age the new boat will not have the much sought after mechanical NA engine.
I confess the mistake in expression, when I said "your new boat", I meant "boat new to you", not brand new. Well, since the financial power of nautical society in my country, everybody do buy used smaller boats and repower with even smaller used engines, buying new is out of the question. How practical is that with bigger trawlers I am not sure, but cheaper than new, definitively. So it depends if you just want to start the ride immediately (as most everybody wants) or you want to do the modifications (which is here, mostly, done for the purpose of reselling the boats). In the latter case, most often, the market dictates to integrate the most popular engine.

By the way, I am also in the group using mechanical NA engines from old era, but it is not that I am against new modern engines, the problem can be easily sought through all industries - quantity vs quality; replace vs recycle; Problem will persists since the prices go down due to quantity and the quality is seldom revised compared to older technology. "Dying" electronics (the most criticized part) does not mean that electronics are bad choice, it means that it is not being designed for the conditions in which it is being used, or it was not properly integrated for specified use. We still have decades old satellites in extreme conditions full of electronics and they still work, but they were designed for those conditions. Let's not even mention the deadlines in which something has to be designed today.

About the fun part, as everybody agrees, only correct way to exactly know your load, whichever engine, is to measure fuel consumption, although not always very practical way. And only then you can compare with diagrams from the manufacturer, as some of you are playing currently with exponents and calculations (been there, too; have fun ).
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