Single Diesel or Twin Gas engines on trawler/cruiser?

[Tony B]

OK, I decided to go ahead with the idea of buying a boat. Now the choice comes in for "single engine diesel or twin engine gasoline models. The price for the diesels obviously are more expensive.
Having logged in thousands of miles on my Mainship 36 Aft Cabin model running only 1 engine at a time and averaging 5.5 to 6 MPH I was averaging 2 miles/gal. fuel consumption. If I bought a 36' Albin trawler with a single diesel engine and ran it at about 5.5 to 6 MPH approximately what would be my fuel consumption?

 

[mvweebles]

OK, I decided to go ahead with the idea of buying a boat. Now the choice comes in for "single engine diesel or twin engine gasoline models. The price for the diesels obviously are more expensive.
Having logged in thousands of miles on my Mainship 36 Aft Cabin model running only 1 engine at a time and averaging 5.5 to 6 MPH I was averaging 2 miles/gal. fuel consumption. If I bought a 36' Albin trawler with a single diesel engine and ran it at about 5.5 to 6 MPH approximately what would be my fuel consumption?

My Willard 36 with a Perkins 75hp 4.236 diesel burns around 1-1/2 gph at around 7-kts (8-mph). My prior boat (Willard 30) was a 6-knot boat and felt really slow. Going 4-3/4 kts (5.5mph) sounds like punishment to me, but to each their own.

Honestly, except as a range extender when crossing an ocean, I just don't understand people with two engines who only run one at a time. Sounds like they bought the wrong boat.

Peter

 

[rslifkin]

Honestly, except as a range extender when crossing an ocean, I just don't understand people with two engines who only run one at a time. Sounds like they bought the wrong boat.

Seconded. Handling is significantly compromised on most twins with one shut down and with diesels where efficiency doesn't fall off as badly with light load as gas engines do there's often not much fuel savings either.

Realistically, at that speed with most diesel 36 foot boats that aren't a super efficient full displacement hull I'd expect 3 - 4 NM / gal. Speeding up to 6 - 6.5 kts (depending on waterline length) would get you 2.5 - 3 in most cases and would be a reasonable cruise speed.

 

[O C Diver]

A great deal depends on the diesel engine. You won't get the economy out of a 450 HP engine that you will out of a 132 HP engine. The boat (45') in my avatar got about 2 MPG at 7 knots with the 450 HP engine. When I swapped it for the 132 HP engine, fuel economy went to 3.5 MPG at 7 knots. Slowing down to 6 knots yielded 5 MPG.

With diesel engines (and probably gas engines) you need to operate the engine on the upper part of the torque curve. That's where the fuel efficiency comes into play. Running my 450 HP engine to get 40 HP (that was needed to push the boat at 7 knots) was an exercise in futility.

All this having been said, for most people, swapping engines (unless you can do it yourself) isn't cost effective. Just understand going in, if you buy a planing hull or a semi planning hull with a big engine, slowing down will only gain you so much.

Ted

 

[SteveK]

Ted, did you operate the 450 in the upper part of the curve.
What was the result?

 

[rslifkin]

Engines do lose efficiency when loaded very lightly as Ted found. The rate and severity of efficiency drop off varies between engines though with gas engines being at the worse and and modern common rail diesels usually being at the better end.

 

[READY]

Also many transmissions will be damaged by a spinning prop without the engine. You can lock it down but now you have increased resistance.

On the diesel at 5-6 mph I would guess 3.5-4.5 nm/gal. How many miles or hours do you expect to run each year?

Personally I just prefer diesels and a single is fine with a bow thuster.
In many 36ft boats fitting twins can really make servicing difficult. I would priority first finding the best fit boat for how I would use it most of the time. Then engines would be a factor in the final selection.
Ken

 

[O C Diver]

Ted, did you operate the 450 in the upper part of the curve.
What was the result?

So the engine I had was a Cummins 6CTA at 450 HP. The upper part of the curve was likely 1,600 to 2,000 RPM with WOT at 2,500. At 1,600 RPM the boat would go about 8 knots at around 1 GPM. Yes, I could have flattened the pitch of the propeller to get 7 knots around 1,600 RPM, but I would still be trying to get 40 HP out of a 450 HP engine.

A number of years ago a stout 50+' cruising sailboat past me going down the AICW. They're optimized to sail and can can be reasonable under power. It had less than 100 HP. Efficiency in displacement comes from optimizing the drive train. Speed on plane comes from lots of available HP, not efficiency.

Ted

 

[TBill36]

Fuel consumption is usually never the highest cost of owning a boat. If you want a single because it takes up less engine room, you have less maintenance than twins, and diesel boats don’t blow up then go for it. But if economy of fuel consumption with the numbers you are talking about is a factor then you can’t afford the boat to begin with.

 

[psneeld]

OK, I decided to go ahead with the idea of buying a boat. Now the choice comes in for "single engine diesel or twin engine gasoline models. The price for the diesels obviously are more expensive.
Having logged in thousands of miles on my Mainship 36 Aft Cabin model running only 1 engine at a time and averaging 5.5 to 6 MPH I was averaging 2 miles/gal. fuel consumption. If I bought a 36' Albin trawler with a single diesel engine and ran it at about 5.5 to 6 MPH approximately what would be my fuel consumption?

My memory is getting fuzzy on details but pretty sure my Albin 40/120 Lehman got 3.2NMPG average over 20,000 miles at 1600-1800rpm and averaged around 6.4 knots.

Other Albin reports are somewhat better. May Lehman 120 (and most are similar) burned around 2GPH at 1650+/-rpm. Some Albin 36s I think reported about 7 or better knots at that rpm. Never made much sense as the shorter waterline, but couldn't really argue as I never rode one. That would have bumped their NMPG up close to 4NMPG.

There are lots of past posts that should echo this, I remember even photos of fuel consumption gauges along side tachs to prove at least one posters claims.

Ultimately your choice of speed/rpm will determine consumption and ultimately NMPG.

At 5.5-6mph, my guess an Albin 36 would get around 4NMPG.

 

[ranger58sb]

Fuel consumption is usually never the highest cost of owning a boat. If you want a single because it takes up less engine room, you have less maintenance than twins, and diesel boats don’t blow up then go for it. But if economy of fuel consumption with the numbers you are talking about is a factor then you can’t afford the boat to begin with.

This. Especially when twin installations use smaller engines than what's used for a single installation.

Seconded. Handling is significantly compromised on most twins with one shut down and with diesels where efficiency doesn't fall off as badly with light load as gas engines do there's often not much fuel savings either.

And this, especially if the inactive trans isn't locked down in some cases... or at least approved for freewheeling...

Tony, there are ~2,376 threads here about single-vs-twin pros, cons, differences, etc. Ditto the diesel-vs-gas questions. Recommend you read some/all of those...

Most boil down to 1) fuel isn't the biggest deal, 2) other factors (like engine room access) are more important, 3) gas is cost effective for short trips, 4) diesel is nice for longer travels, etc etc etc.

-Chris

 

[Tony B]

The reason I asked is because I am not familiar with trawlers or diesel engines.
My last trip I left Kemah, Tx on my older Mainship 36 with Gas engines. Before I left, It was either on here or on CruisersForum that I asked about fuel efficiency because I was making a long trip. I received a reply from someone that did the Great Loop with the same exact boat as mine. He has gas flow meters and recorded everything on his trip. He traveled at 6MPH on one engine at a time. After logging in over 7000 miles he averaged 2MPG. So is did exactly what he did and I too was getting 6MPG. I'm sure I saved a thousand gallons or more on that trip.
I am not familiar with Diesel engines or trawlers and that is why I am asking.
Keep in mind that half that trip was going up river.

 

[mvweebles]

After logging in over 7000 miles he averaged 2MPG. So is did exactly what he did and I too was getting 6MPG.

Is there a typo here? 2mpg seems pretty low; and 6mpg seems very high. Most folks seem to report 3mpg when the dust settles. That said, unless a user runs long hours at their cruising speed, I don't think they have a good handle on their fuel efficiency.

Regardless, the savings came from going slow and may be too slow for peak fuel efficiency.

I'm sure I saved a thousand gallons or more on that trip.
I am not familiar with Diesel engines or trawlers and that is why I am asking.
Keep in mind that half that trip was going up river.

Well, the loop is 6000 miles and at 4mpg means 1500 gals fuel. I actually think 3mpg is probably closer to the number so they'd use 2000g fuel. Maybe $2k over the course of a 2-year trip. And then you'd have to back-out costs for increased oil changes and such. And going slow means the trip is longer - more marina stops, etc. (hopefully I got this right in statute miles).

Sounds like you're fine at trawler speeds. The 'exam question' is single diesel or twin gassers. That's easy - single diesel. More reliable, less service, easier to service, better economy, safer fuel, better resale. Only thing it doesn't do well is go fast.

Good luck with whatever you decide.

Peter

 

[PierreR]

My best estimate would be just under 3 mpg at around 8.5 mph. You will not go 5.5-6 mph. You need more heat for the engine than these speeds provide, you have the capacity to carry a lot of fuel so you will bump up the throttles and not care.
My last boat was a super efficient 26' boat driven by a Sabb H2 and contollable pitch propeller. I got over 14mpg at 6 mph. My parents looped with it on less than $700 in fuel.
Going from my last boat to my present boat was a hard decision as fuel consumption and seaworthiness seemed top priorities. Well, I bought a very efficient full displacement comfortable trawler. At first I got 2.8 mpg at around 6.8mph. It did not take long before the huge amount of fuel that I was carrying seemed limitless and burning more was not yet hitting the wallet. I am now at 8.5 mph at a fuel burn of 2.2 mpg. After restoring the boat the fuel costs seem paltry. I want the engines happy, the stabilizers to work, the refrigerator to work and don't give a rats ass about the fuel burn.
Just my experience. In two years I have put a little over 5,000 miles on the boat. My costs are $1,000 a month if it just sits.

 

[SteveK]

I cannot see how MPG is calculated to represent a clear picture, at least not where I boat with currents and wind changing during a trip.

The GPH I think is more accurate as it does not care about milage covered over ground, but rather milage/distance over/through water. You could calculate fuel burn going up river yet not moving over ground.

 

[High Wire]

3.0 nautical miles per gallon at 6.7-6.9 knots at 1700 RPM for me. Single 135 Perkins. If I slowed down to 6.1 or so it would bump up to 4.0 nmpg. My boat is very similar to an Albin 36/Grand Banks 36/Marine Trader 36/Present 36. Of course, this data is based on long trip averages.

 

[rslifkin]

I cannot see how MPG is calculated to represent a clear picture, at least not where I boat with currents and wind changing during a trip.

The GPH I think is more accurate as it does not care about milage covered over ground, but rather milage/distance over/through water. You could calculate fuel burn going up river yet not moving over ground.

In my mind, still water MPG is a useful point of reference. But as you mentioned, you do need to know GPH as well for better estimating burn in various situations.

 

[DDW]

A great deal depends on the diesel engine. You won't get the economy out of a 450 HP engine that you will out of a 132 HP engine. The boat (45') in my avatar got about 2 MPG at 7 knots with the 450 HP engine. When I swapped it for the 132 HP engine, fuel economy went to 3.5 MPG at 7 knots.

Hard to know for sure, but I'd guess a big part of that is a generational upgrade in diesel technology, and a small part a reduction in HP. Modern common rail diesels have a very flat efficiency map, older mechanically injected engines can have a pretty peaky map, especially in higher output versions.

 

[SteveK]

I do my calculations when I fill both tanks full and main engine hours since last full fill. (any generator use is included).
My last full fill took 1373.9L after 75 hours of operation. That is 18.32LPH or 4.84AGPH (American Gallons) for twin 250HP turbo engines run at 7.5-9 knots. WOT RPM is 3000, I run 1750 or 2150.
At 2500 12-13 knots but you can almost see the fuel gauge move.
At a cost of C$31.38PH I don't worry about saving a few bucks to go slower. We like to look at the scenery.

 

[Mac2]

Diesel engine for safety factor alone. It’s the fumes that blow up, not the fluid. I routinely end up with diesel fuel in the bilge-the last was from my generator’s fuel pump seal. On boats the fumes tend to gather in spaces not open to the outside air. One spark and that’s it. My coworker was exiting the marina when his antique woody blew up almost killing him. No thanks.

 

[O C Diver]

Hard to know for sure, but I'd guess a big part of that is a generational upgrade in diesel technology, and a small part a reduction in HP. Modern common rail diesels have a very flat efficiency map, older mechanically injected engines can have a pretty peaky map, especially in higher output versions.

While efficiency of electronic engines may be more stable over 50% of the RPM range, you can't take a 1,000 HP engine and expect it to be as efficient as a 40 HP engine when producing 20 HP.

If you factor in all the additional friction within the engine, pumps moving exponentially larger amounts of fluids, the corresponding transmission with all its additional friction factors, and then add the shaft and propeller, it's just not close.

A good test to prove the point would be engine idle fuel consumption, after being warmed up.

Below are the old and new engines.

The Cummins is 8.3 liters and the John Deere is 4.5 liters.

Ted

 

[DDW]

While efficiency of electronic engines may be more stable over 50% of the RPM range, you can't take a 1,000 HP engine and expect it to be as efficient as a 40 HP engine when producing 20 HP.

While that is true, a rather extreme example. In the real world, my 380hp engine idles at about 0.6g/h, underway at 7 knots (producing around 40 hp) it consumes 1.7 g/h. So 1.1g/h to provide motion. An engine of half of that displacement could idle at 0.3g/h (optimistic, as these things do not scale linearly) and would burn 1.4g/h at 7 knot cruise. That is an improvement from 4.1 to 5.0 mpg, a nice improvement but no where near as dramatic as you experienced. A 20 year fast forward in engine technology will do more.

The specific fuel consumption of common rail diesels does not vary much over about 90% of the map. A QSB6.7 for example produces 18.9 hp/g at 1400 rpm and 19.5 hp/g at 2900 rpm, plotted against propellor demand, only an 8% change over the useful range. It drops to 14.5 hp/g at 1000, just above idle.

 

[O C Diver]

While that is true, a rather extreme example. In the real world, my 380hp engine idles at about 0.6g/h, underway at 7 knots (producing around 40 hp) it consumes 1.7 g/h. So 1.1g/h to provide motion. An engine of half of that displacement could idle at 0.3g/h (optimistic, as these things do not scale linearly) and would burn 1.4g/h at 7 knot cruise. That is an improvement from 4.1 to 5.0 mpg, a nice improvement but no where near as dramatic as you experienced. A 20 year fast forward in engine technology will do more.

The specific fuel consumption of common rail diesels does not vary much over about 90% of the map. A QSB6.7 for example produces 18.9 hp/g at 1400 rpm and 19.5 hp/g at 2900 rpm, plotted against propellor demand, only an 8% change over the useful range. It drops to 14.5 hp/g at 1000, just above idle.

How do you know the engine is producing 40 HP at 7 knots? Your engine isn't producing any HP (for propulsion) at idle (.6 GPH). It then takes an additional .5 GPH to "provide motion".

My John Deere is consuming .3 to .4 GPH at idle. At 5 knots (idle in gear) consumption is .7 GPH in total.

Ted

 

[DDW]

How do you know the engine is producing 40 HP at 7 knots? Your engine isn't producing any HP (for propulsion) at idle (.6 GPH). It then takes an additional .5 GPH to "provide motion".

My John Deere is consuming .3 to .4 GPH at idle. At 5 knots (idle in gear) consumption is .7 GPH in total.

Ted

How do you know? From the propellor absorption curves. Being an electronic engine it also reports torque and fuel consumption in real time.

The point still stands: comparing a mechanically injected, 3V engine from 30 years ago to an electronically controlled common rail injected 4V engine, and attributing the differences only to displacement is misleading. Overpowering a boat these days typically costs much more in capital expense than additional fuel expense over its life. The smaller engine saves money, but not so much from burning less fuel.

At low power settings, a difference in displacement has far greater influence on a gas engine which has pumping losses.

 

[O C Diver]

How do you know? From the propellor absorption curves. Being an electronic engine it also reports torque and fuel consumption in real time.

The point still stands: comparing a mechanically injected, 3V engine from 30 years ago to an electronically controlled common rail injected 4V engine, and attributing the differences only to displacement is misleading. Overpowering a boat these days typically costs much more in capital expense than additional fuel expense over its life. The smaller engine saves money, but not so much from burning less fuel.

At low power settings, a difference in displacement has far greater influence on a gas engine which has pumping losses.

Unfortunately, the propeller curve is theoretical. It doesn't take into account your boat, propeller, or anything else. As an example, if you decreased the propeller pitch 2", the increased several hundred RPM, wouldn't mean your boat was requiring more HP for the same 7 knot speed. Further, if you increased the propeller pitch 2", you wouldn't be using less HP.

Ted

 

[Fletcher500]

SBMAR has performance curves for various Cummins engines from the older normally aspirated, non electronic up to the newest electronic Turbo versions. The Smart craft on on my QSB displays percent load, fuel flow, boost, etc. It’s all academic at this point, but it appears to me that DDW has made his case fairly clearly when one compares these curves. Yes, Prop curves are theortical and not boat specific but changing the pitch in either direction does not change the noted comparison between an older, larger engine, vs a newer common rail.

 

[O C Diver]

SBMAR has performance curves for various Cummins engines from the older normally aspirated, non electronic up to the newest electronic Turbo versions. The Smart craft on on my QSB displays percent load, fuel flow, boost, etc. It’s all academic at this point, but it appears to me that DDW has made his case fairly clearly when one compares these curves. Yes, Prop curves are theortical and not boat specific but changing the pitch in either direction does not change the noted comparison between an older, larger engine, vs a newer common rail.

First, almost every Cummins performance curve (haven't seen one that isn't) is for a planing hull. So for displacement speeds, the curves are worthless.

Secondly, most modern diesels, mechanical or electronic, produce about 20 HP in the upper part of the torque curve. Electronic engines do a bit better at lower RPM, but the electronics don't make up for low torque.

Third, if you have an electronic engine with built in fuel consumption, you can measure fuel burn through the entire RPM range with the transmission in neutral. As alluded to in post #22 and #23, that "cost" doesn't go away, and there's a negligible difference in "cost" between a 6BTA and its electronic equivalent.

Fourth, post #23 shows that the larger the engine the higher fuel burn (cost) out of gear. In post #22 at "Provide Motion" of the 1.1 GPH, more than half is the "cost" of running the engine, not HP propulsion.

As stated previously the bigger the engine, the higher the "cost" to run the engine, regardless of mechanical or electronic.

Ted

 

[Jeff F]

If I bought a 36' Albin trawler with a single diesel engine and ran it at about 5.5 to 6 MPH approximately what would be my fuel consumption?

That's very slow. I'm confident that you could run that speed at < 1 gph with a single smaller (< 200 HP) diesel.

 

[DDW]

First, almost every Cummins performance curve (haven't seen one that isn't) is for a planing hull. So for displacement speeds, the curves are worthless.

The propellor absorption curve for a specific prop is theoretical, but also very close to reality. The performance curve is the performance curve from dyno data, it doesn't know or care what boat it's in.

Secondly, most modern diesels, mechanical or electronic, produce about 20 HP in the upper part of the torque curve. Electronic engines do a bit better at lower RPM, but the electronics don't make up for low torque.

No idea what that even means.

Third, if you have an electronic engine with built in fuel consumption, you can measure fuel burn through the entire RPM range with the transmission in neutral. As alluded to in post #22 and #23, that "cost" doesn't go away, and there's a negligible difference in "cost" between a 6BTA and its electronic equivalent.

I will agree with that, but also the parasitic loss doesn't go away with a smaller engine, it is just a little lower. Not proportionally lower, as some losses are fixed.

Fourth, post #23 shows that the larger the engine the higher fuel burn (cost) out of gear. In post #22 at "Provide Motion" of the 1.1 GPH, more than half is the "cost" of running the engine, not HP propulsion.

You misread that. 1.7g/h at 7 knots, 0.6 at idle out of gear, nearly 2/3 of the burn is to provide motion, and 1/3 to turn the engine. This effect doesn't go away with a small engine, just changes the numbers a little bit.

As stated previously the bigger the engine, the higher the "cost" to run the engine, regardless of mechanical or electronic.

Ted

Yes - only if run at fractional power, and the difference is not that great even in old mechanical engines. At higher power settings generally the larger the engine the more efficient it is.

I'll agree with you that a smaller engine will be a little more efficient than a large one, at the same low power setting. Where we part company is the magnitude of that difference. Your experience was a 75% improvement in efficiency, and implied that was typical. In the same generation of engine, that is a fantasy, except in extreme cases like your 1000HP -> 40HP example. For realistic cases, I'd be surprised if it was as much as 10%, and there is sea trial data from builders that offer two engine options to back that up.

 

[O C Diver]

The propellor absorption curve for a specific prop is theoretical, but also very close to reality. The performance curve is the performance curve from dyno data, it doesn't know or care what boat it's in.

No idea what that even means.

I will agree with that, but also the parasitic loss doesn't go away with a smaller engine, it is just a little lower. Not proportionally lower, as some losses are fixed.

You misread that. 1.7g/h at 7 knots, 0.6 at idle out of gear, nearly 2/3 of the burn is to provide motion, and 1/3 to turn the engine. This effect doesn't go away with a small engine, just changes the numbers a little bit.

Yes - only if run at fractional power, and the difference is not that great even in old mechanical engines. At higher power settings generally the larger the engine the more efficient it is.

I'll agree with you that a smaller engine will be a little more efficient than a large one, at the same low power setting. Where we part company is the magnitude of that difference. Your experience was a 75% improvement in efficiency, and implied that was typical. In the same generation of engine, that is a fantasy, except in extreme cases like your 1000HP -> 40HP example. For realistic cases, I'd be surprised if it was as much as 10%, and there is sea trial data from builders that offer two engine options to back that up.

Performance curves for propellers in displacement mode versus planing are hugely different. But then you probably have a planing hull so you think the curve is accurate.

Parasitic friction losses are most definitely proportional. That's why my John Deere has about a 60% idling consumption of your Cummins idling consumption.

I didn't misread your 7 knot cruise. I was commenting on your in gear at idle consumption. You wrote that your in gear at idle consumption was 1.1 GPH. You wrote that your out of gear idle consumption was .6 GPH. Hence, more that half of your in gear at idle consumption is Parasitic Friction.

Further, your 7 knot cruise (1.7 GPH) is about 35% Parasitic friction. The remaining 1.1 GPH equals roughly 22 HP. In most modern engines, after parasitic loss, a gallon of fuel generates 20 HP.

I stand by what I wrote. Parasitic friction loss is most definitely proportional to engine size. A 1,000 HP engine will likely have 20 times greater fuel consumption at idle than a 40 HP engine. The 1,000 HP engine will likely burn more fuel at idle than a 40 HP engine pushing your boat at 7 knots.

Ted