Big HP vs Small HP

[MacPhid]

NOT NOT NOT my theory! I read this in research from "experts" so asked a question.

Sorry Don, I was looking at the other part of your post "But the other part of me says ............BS bigger engines use more fuel PERIOD!"
Should have read more carefully to see you were questioning which was true.

James

 

[SteveK]

Interesting read of good guesses. Unless you line up the various boats/engines and have them travel at the same speed, over the same course, same current/windage and then measure the fuel consumed these answers will only be hypothetical.

I do remember when we used to pull the two barrel stock carbs to install holly 4 BBL to wake up the engine. That came with increased fuel consumption. We learned to drive these cars like a trawler running on only 2 of 4.

 

[jleonard]

By the way, concerning the QSB series, I have seen it said, but cannot vouch, that its easy to upgrade a 250 to a 380 after its installed. A tech comes aboard and downloads different code into the governor board, and cuts you a bill. It would interesting to know if that's true.

It is. A friend did that. went from the initial 250 to a higher HP rating. I don't think he went all the way, but to something like 330 ish.

 

[Ox76]

The simple fact is this: A 400hp engine, although maybe only needing 50hp in a certain scenario, will burn more fuel than a 200hp engine only needing 50hp in the same scenario simply because the moving members of the engine are larger, with more surface area (friction) and mass (weight), and requires more energy to keep spinning, i.e. more fuel needing to be burned. This is it in a nutshell. If you could magically make the revolving mass and friction of an engine capable of double the hp of a certain engine the fuel burn would be identical but magic isn't in the cards!

 

[rslifkin]

The simple fact is this: A 400hp engine, although maybe only needing 50hp in a certain scenario, will burn more fuel than a 200hp engine only needing 50hp in the same scenario simply because the moving members of the engine are larger, with more surface area (friction) and mass (weight), and requires more energy to keep spinning, i.e. more fuel needing to be burned. This is it in a nutshell. If you could magically make the revolving mass and friction of an engine capable of double the hp of a certain engine the fuel burn would be identical but magic isn't in the cards!

If the engines are physically different, that's likely to be the case, but it's not always a particularly large difference (especially if the big engine is turning less rpm to produce the 50hp). But if you're talking about a case of the same engine at 2 different ratings, the bigger one will often consume the same or less fuel at low power (depending on differences in operating RPM).

Gas engines are a whole different world due to throttling losses. That kills efficiency at light load, so higher powered gas engines burn more fuel at lower power outputs.

 

[ranger58sb]

7 knots seems to be the sweet spot for cruising speed.

Even that also depends on boat, especially on LWL.

The latest Beebe suggests a multiple of 1.1 or 1.2 -- versus the theoretical max multiple of 1.34 -- greatly increases fuel economy, and that appears to be the case in our situation.

But then whatever speed that delivers varies by boat length. In our case, with a theoretical (probably close) LWL of 50', then a factor of 1.1 would give us approx 7.7 kts, and 1.2 would give us about 8.5 kts.

All that then modified further by wind and/or current.

-Chris

 

[FWT]

It is. A friend did that. went from the initial 250 to a higher HP rating. I don't think he went all the way, but to something like 330 ish.

Thanks. That is really good to know.

 

[Don L]

I don't think the delta is that much in a modern common rail. Just guessing, no data to support it.

People keep writing this "this modern common rail" comment. In my case I would most likely be talking 1980s engnes.

In my case I am probably looking at a 40-45 1980s trawler and am trying to get a handle on whether if I kept the speed down to around 6-7 whether it makes any practical difference from a fuel burn rate whether it is a single 150 HP engine, twin 135 HP or even twin 350 HP engines.

Yes there some issue of running big engines slow a long time. That is a different subject.

 

[MacPhid]

I think in the case of single engine 40’ish trawlers the fact most have the prop behind a keel swinging a bigger prop (25+”) with a lower ratio (~2.5-1) gear box puts the engine closer to its torque curve compared to a twin such as mine with relatively smaller (20”) propellers turning faster because a higher gear ratio (2:04-1). It should be more efficient, how much is hard to say with so many variables.

James

 

[danderer]

Over seven summers cruising the Inside Passage, 4674 engine hours, and more than 26000 nm, almost always at 7.25 knots, we consistently average 3.7-3.9 nmpg. I'm told we could do considerably better yet if we reduced our speed by a knot.

I read this as saying: We get 3.7-3.9 MPG at 7.25 knots.

I don't see the numbers supporting that. 26000/4600 works out to about 5.7 knots average. Sure that's a rough figure but more plausible. All of our boats do much better at <6 rather than >7.

What was your total fuel burn for those miles and hours?

 

[RCook]

I read this as saying: We get 3.7-3.9 MPG at 7.25 knots.

I don't see the numbers supporting that. 26000/4600 works out to about 5.7 knots average. Sure that's a rough figure but more plausible. All of our boats do much better at <6 rather than >7.

What was your total fuel burn for those miles and hours?

The reason that calculation works out to 5.7 knots is that we spend a lot of time at idle speed or in neutral, fishing, critter-watching, sightseeing etc. At those times we cover very little distance. When we are traveling, we're almost always at 7.25 knots or very close to that.

Total NM: 26,236
Total gallons: 7,122
NMPG: 3.68

 

[Fletcher500]

People keep writing this "this modern common rail" comment. In my case I would most likely be talking 1980s engnes.

In my case I am probably looking at a 40-45 1980s trawler and am trying to get a handle on whether if I kept the speed down to around 6-7 whether it makes any practical difference from a fuel burn rate whether it is a single 150 HP engine, twin 135 HP or even twin 350 HP engines.

Yes there some issue of running big engines slow a long time. That is a different subject.

Cool, happy hunting.

 

[MacPhid]

The reason that calculation works out to 5.7 knots is that we spend a lot of time at idle speed or in neutral, fishing, critter-watching, sightseeing etc. At those times we cover very little distance. When we are traveling, we're almost always at 7.25 knots or very close to that.

Total NM: 26,236
Total gallons: 7,122
NMPG: 3.68

I think we underestimate the amount of time we spend idling, no wake zones docking etc. I was surprised when we did our loop that our average speed was 6.6 knots when we always travelled at around 7.5.
In the case quoted it’s more likely that at cruising speed you were under way for only 3600 hours, the rest idling, stationary, docking etc. what really counts as you’ve shown is how far we go and how much fuel did we burn. It’s still remarkable efficiency for a boat under way. Thanks for the detailed data!

James

 

[Simi 60]

Near enough to 2nm/ gallon @ 7.5 knots for us
1nm/0.52g

60 ft, 60 tonne
Single nta855m Cummins 350hp (14 litres)
Running @ 1150rpm producing around 100hp swinging a 50 inch prop in nozzle.

 

[PierreR]

When you get into older engines, especially Detroit's, larger engines at the same speed, water line length and weight burn 10-20% more at displacement speeds.

 

[mvweebles]

People keep writing this "this modern common rail" comment. In my case I would most likely be talking 1980s engnes.

In my case I am probably looking at a 40-45 1980s trawler and am trying to get a handle on whether if I kept the speed down to around 6-7 whether it makes any practical difference from a fuel burn rate whether it is a single 150 HP engine, twin 135 HP or even twin 350 HP engines.

Yes there some issue of running big engines slow a long time. That is a different subject.

Let's do this - take a Grand Banks 42. They were available early with a single 120HP Ford Lehman; twin 120's; and later twin >300hp Cat 3208s (1980's); and finally, a more recent GB42 with twin 375hp Cummins B-Series (Tier 3/Common Rail - T/A) engines.

At 7.5 kts, I will guess the following as 'directionally correct' for continuous running in open water (+/- 0.3 gph):

• Single 120HP - 2.25 gph
• Twin 120HP - 2.75 gph
• Twin 3208's - 3.0 gph
• Twin Common Rail - 2.50 gph

As comparison, I was aboard a friend's Willard 40 with a John Deere 4045TA (Tier 2 at the time - 2001 build) from Long Beach to La Paz MX, about 1000nms and burned under 1.7gph in open ocean conditions.

I'm sure there will plenty of feedback on the above guesses, but I think you're looking for relative comparison.

Peter

 

[dannc]

Even that also depends on boat, especially on LWL.

The latest Beebe suggests a multiple of 1.1 or 1.2 -- versus the theoretical max multiple of 1.34 -- greatly increases fuel economy, and that appears to be the case in our situation.

But then whatever speed that delivers varies by boat length. In our case, with a theoretical (probably close) LWL of 50', then a factor of 1.1 would give us approx 7.7 kts, and 1.2 would give us about 8.5 kts.

All that then modified further by wind and/or current.

-Chris

I have a spreadsheet that I started a decade ago that I use to compare and track various boat values and items. The spreadsheet started as a way to figure out how builders where figuring out HP and fuel burn. Over the years, I have come across a couple of ways to calculate HP requirements, including Beebe.

Here are some HP numbers for a boat using Beebe's HP method.

S/L is the speed length ratio with 1.34 being the theoretical max speed for a displacement hull. Gerr has an equation that determines the "real" max S/L for a given hull, which in this case is 2.1. Whatever the equations say is the "real" hull speed, the needed HP, and thus fuel burn, is the real limit.

-----------------------------SL-----HP----NMPH
BeBee's HP F1*F2 for S/L 1.0......20....8.1
BeBee's HP F1*F2 for S/L 1.1......28....8.9
BeBee's HP F1*F2 for S/L 1.2......43....9.7
BeBee's HP F1*F2 for S/L 1.30....64...10.5
BeBee's HP F1*F2 for S/L 1.34....70...10.8
BeBee's HP F1*F2 for S/L 1.4......90...11.3
BeBee's HP F1*F2 for S/L 1.5....116...12.1
BeBee's HP F1*F2 for S/L 1.6....142...12.9
BeBee's HP F1*F2 for S/L 1.7....169...13.8
BeBee's HP F1*F2 for S/L 1.8.....195..14.6
BeBee's HP F1*F2 for S/L 1.9.....222..15.4
BeBee's HP F1*F2 for S/L 2.0.....248..16.2
BeBee's HP with Gerr's Max S/L/Hull Speed 274 at 17.1 NMPH
Hull Speed(HS)/S/L Per Gerr 2.1

Later,
Dan

 

[O C Diver]

Near enough to 2nm/ gallon @ 7.5 knots for us
1nm/0.52g

60 ft, 60 tonne
Single nta855m Cummins 350hp (14 litres)
Running @ 1150rpm producing around 100hp swinging a 50 inch prop in nozzle.

I assume you were measuring fuel consumption instead of HP. Generating 100 HP requires atleast 5 GPH. If you're getting 7.5 knots at just under 4 GPH, that's about 80 HP.

Ted

 

[Simi 60]

I assume you were measuring fuel consumption instead of HP. Generating 100 HP requires atleast 5 GPH. If you're getting 7.5 knots at just under 4 GPH, that's about 80 HP.

Ted

I am measuring fuel burn

We have sight tubes so slide electrical ties around at start of fill and completed fill
Note hours at start and burn the fuel.
Pretty consistent 15lph including minimal Genset.

But looking at Cummins prop curve they seem to read 1150rpm puts out closer to 100hp but also says we should burn a bit more than we do.

Checked tacho admittedly a few years back and it was reading correctly.

 

[catalinajack]

Suspect there is a difference between best mileage and average mileage over a season. I'm amazed at planing or semi displacement boats that are getting 3.5 nmpg!

Our Bayliner 3888 has twin Hino 6.4l naturally aspirated diesels. When we did the loop we averaged 6.6 knots over 792 engine hours burning 2960 us gallons. We travelled 5237 nautical miles for a total average of 1.769 nautical miles per gallon. Thats over the full year with currents, against currents, idling, occasionally over 8.5 knots etc.

James

Hmmm, in doing the Loop in a 56,000# DeFever 44 with twin Lehman 120s, we burned 3,300 gallons driving 5,500 miles. That included about 200 hours of generator time.

 

[Hippocampus]

The simple fact is this: A 400hp engine, although maybe only needing 50hp in a certain scenario, will burn more fuel than a 200hp engine only needing 50hp in the same scenario simply because the moving members of the engine are larger, with more surface area (friction) and mass (weight), and requires more energy to keep spinning, i.e. more fuel needing to be burned. This is it in a nutshell. If you could magically make the revolving mass and friction of an engine capable of double the hp of a certain engine the fuel burn would be identical but magic isn't in the cards!

Think physics would support this. Same as one v two engines bearing the same load. More internal parts moving more friction. More friction more energy lost to heat. More heat more need for cooling. More cooling more energy required to run that system.

Always been curious about gph as it pertains to M rating. Understand rating relates to time of WOT allowed. But you lose energy to running the cooling system and various pumps, bearings and other factors not directly contributing to shaft HP. One would when thinking about the same block set up to run at a M2 rating v a M4 rating the M2 engine would be more efficient. Is it true comparing a M2 engine to a M4 engine of the same HP the M2 would be more efficient?

For the same block comparison would this remain true with the M2 engine running at half its rated HP the the M4 at a quarter even if both are tier3 common rail. If true would there be benefit to pick the best M rated engine that produces adequate HP for your boat? I wonder if this is true. Understand the drive to put in the largest HP engine at the lowest weight in SD and planing hulls but at what point does continue to make sense? Understand to get the same HP you would need a heavier engine but with modern engines the increased weight isn’t that much and HP requirements are based on total displacement with engine weight being just a fraction. Is my logic incorrect and why?

When I did the engine survey at purchase the load,, rpm, HP, fuel draw, temperature, turbo load, oil pressure and the usual parameters were measured. It was evident the knee in the gph curve didn’t match up with theoretical hull speed. This suggested to me there were other factors involved. Was this just once being on full plane energy demands become more linear? Or that higher M rated engines efficiency changes as they approach full load at WOT.

 

[ksanders]

My Cherubini gets 3.5 MPG (nautical miles per gallon at 7 knots with a single John Deere 4045 132 HP engine.

With new electronically controlled diesels, many have fairly accurate fuel consumption displays built in. My warmed up out of gear idle with the house bank fully charged is .3 to .4 GPH. When I bought the boat it had a Cummins 6CTA 450 HP engine. Fuel consumption was under 2 MPG. Some of the reasons included a mechanical injected engine that when running 7 knots was way out of its efficiency zone (peak torque area). It was also twice the liter displacement, requiring more fuel just to idle.

Ted

Thanks Ted!

I think people missed the point that you have numbers from your exact boat with both a large and small engine.

 

[Action]

This has been an interesting thread for me. Eepecially the recent post #46.
And as ignorant of diesels that I am, I do have a question if I can take this thread just a lil off course.

How do I know if an engine is modern?
Or in a given tier?
(Which is related to the vessel Don L asks about in post 38)

I think Lehmans, Perkins and Detroits would not be modern. However at least the first two are usually smaller displacement engines. Relating this to the automotive world, I would think smaller would consume less. But I get that crusing at 7 to 8 kts that speed is not using all of the available power anyway.

 

[mvweebles]

How do I know if an engine is modern?
Or in a given tier?
(Which is related to the vessel Don L asks about in post 38).

The Tiered standards started affecting the marine market in the mid/late 1990s, but didn't start taking effect until the early 2000s depending on horsepower.

https://dieselnet.com/standards/us/nonroad.php#tier3

Early in the 2000s, Tier 2 could be met with turbo and after coolers. When Tier 3 started, really required electronic controls to be added ---- and the common rail technology that is pretty dang efficient and quiet. It's why when you're stopped at a light next to an Audi, you may not know it's a diesel in the same way you knew a Mercedes 240D was a diesel 30 years ago. Especially when all you see are tail lights when the Audi Diesel floors it at the green light.

In today's parlance, under about 75 hp diesel can still be old school mechanical engines, which is part of the allure of the current Nordhavn 41 that is equipped with a pair of Beta 75hp engines that meet current EPA requirements.

How to tell if your engine is "modern?" You can look at the above link, or ask this forum. But if it has any electrical connections, chances are its electronically controlled.

BTW, the reason the venerable Lugger engines are no longer available is they found it top expensive to comply with Tier 3 standards and ceases propulsion manufacturing in the mid 2000s to focus on Northern Lights generators.

Peter

 

[Action]

Thanks for the answer and the link.

OP is looking at 1980s 40 to 45 foot vessels. (I am too however late 80s and 90s is likely more accurate) I would assume all of those vessels will have tier 1 diesels, correct?

And did not know that EPA has any control on marine engines. Unless (flash of light in brain) said marine engine was taken from the medium & heavy truck world. (Duh that makes sense. And I think I could have a conversation with myself and be entertained) If my statement above is accurate, do EPA standards get into medium and heavy trucks?

 

[O C Diver]

But looking at Cummins prop curve they seem to read 1150rpm puts out closer to 100hp but also says we should burn a bit more than we do.

Ok, this is a simple misunderstanding. The Cummins prop curve is a theoretical number. With all different shapes and sizes of hulls, this is their guess. Your a little below the curve. Think of it this way, take an inch off the pitch of your prop and you will still turn the 1,150 RPM, but it will require even less HP (with a slightly slower speed). Add an inch to the pitch of the prop and you will turn 1,150 RPM but your speed will be a little faster with more HP (and fuel consumed). Not advocating you change anything, just that their number is a guess, average, or approximation.

Ted

 

[Action]

Never mind. I just had to read the material in the link you provided.

 

[O C Diver]

Let's do this - take a Grand Banks 42. They were available early with a single 120HP Ford Lehman; twin 120's; and later twin >300hp Cat 3208s (1980's); and finally, a more recent GB42 with twin 375hp Cummins B-Series (Tier 3/Common Rail - T/A) engines.

At 7.5 kts, I will guess the following as 'directionally correct' for continuous running in open water (+/- 0.3 gph):

• Single 120HP - 2.25 gph
• Twin 120HP - 2.75 gph
• Twin 3208's - 3.0 gph
• Twin Common Rail - 2.50 gph

As comparison, I was aboard a friend's Willard 40 with a John Deere 4045TA (Tier 2 at the time - 2001 build) from Long Beach to La Paz MX, about 1000nms and burned under 1.7gph in open ocean conditions.

I'm sure there will plenty of feedback on the above guesses, but I think you're looking for relative comparison.

Peter

I'm assuming that boat was Aloha. I went down to La Paz to consider buying the boat and did make an offer on it. We went cruising with it for a couple of days.

I would think the Willard was quite a bit lighter, a more streamlined sailboat type hull, and a better matched drive train. Probably not a fair comparison to the GB 42.

The GB 42s with the exception of the single Lehman were way over poweed, IF you wanted to cruise at 7.5 knots. As an example the twin 3208s would each be producing <30 HP, not remotely close to what they were designed to do.

At the time Aloha was built, JD still also made the 4045 as naturally aspired with 80 HP. It would probably have been an even better choice (no turbo) as cruising at 7.5 knots 1.7 GPH, is equal to about 34 HP.

Ted

 

[mvweebles]

I'm assuming that boat was Aloha. I went down to La Paz to consider buying the boat and did make an offer on it. We went cruising with it for a couple of days.



I would think the Willard was quite a bit lighter, a more streamlined sailboat type hull, and a better matched drive train. Probably not a fair comparison to the GB 42.



The GB 42s with the exception of the single Lehman were way over poweed, IF you wanted to cruise at 7.5 knots. As an example the twin 3208s would each be producing <30 HP, not remotely close to what they were designed to do.



At the time Aloha was built, JD still also made the 4045 as naturally aspired with 80 HP. It would probably have been an even better choice (no turbo) as cruising at 7.5 knots 1.7 GPH, is equal to about 34 HP.



Ted

It was indeed Aloha. Much different boat than a GB42 - probably similar in weight, but a much smaller boat. I only included it as a reference point for the OP as he seems to be trying to get his bearings on fuel usage.

The original owner of Aloha worked for Willard. Deere was just entering the Marine Propulsion market at that time meaning engines they marinized vs another builder (Lugger used Deere engines in some of their builds). I remember Willard went back and forth on whether to use the natural 80hp or the turbo 120hp (I think that's what it specd at). Obviously, they went with the turbo version. Was a strong engine on my run, but had it been me, I would have gone with the natural.

Peter

 

[Pascall]

Here is a nice practical example,
The first engine in our ship was a 6 cylinder 60 hp Nanni.
The consumption was 4 liters per hour at a speed of 6 knots at 1400 prm
After 15 years, a new engine has been installed.
The second engine a 4 cylinder 100 hp Nanni
The consumption now is 4 liters per hour at a speed of 6.5 knots at 1400 prm.
The ship is a real displacer so the length of the boat ultimately determines the speed.
The 40 hp more power compared to the first engine results in a slightly higher top speed of 0.4 knots.