Running on 1 engine for fuel savings?

[gc9773]

This subject comes up fairly often. More than a decade ago forum member Timjet and I ran some real world experiments. My procedure was based on engine prop charts, which track almost perfectly to well established twin engine fuel burn over long runs at constant power. I believe Timjet used fuel flow meters.

Test proccedure. Ocean Alexander 44. Calm day, no wind. current, or waves (unusual day on Lake Michigan). Set GPS speed at 8.2 knots, twin engine. Record engine rpm. Shut down one engine (prop freewheeling). Increase power on remaining engine to achieve same GPS speed of 8.2 knots...steady track. Record rpm and rudder angle. Using the same speed eliminates one variable.

Look up fuel burn for both twin and single engine rpm as recorded (twin engine times two). Compare. I found about 5% savings running on one engine. I don't recall the rudder angle, but the Will Hamm autopilot did fine and appeared to have plenty of excess capability had the ambient conditions been more demanding. The OA has relatively large rudders.

Timjet saw larger fuel savings on his 36 Carver. I do not recall if his test procedures were the same.

For the Ocean Alexander 44 (48' LOA) and a loaded displacement of 30,000 lbs (13.64 tons) the theoretical hull speed is 8.5 knots, with a spec'd speed of 12/16 knots (not sure what the "12" is, single shaft?):

Brake horsepower required to achieve hull speed: 27.3

A total of 500 brake horsepower installed yields 36.7 horsepower/ton...

That's fairly significant... And now my head is starting to hurt and I need lunch...

 

[gc9773]

This subject comes up fairly often. More than a decade ago forum member Timjet and I ran some real world experiments. My procedure was based on engine prop charts, which track almost perfectly to well established twin engine fuel burn over long runs at constant power. I believe Timjet used fuel flow meters.

Test proccedure. Ocean Alexander 44. Calm day, no wind. current, or waves (unusual day on Lake Michigan). Set GPS speed at 8.2 knots, twin engine. Record engine rpm. Shut down one engine (prop freewheeling). Increase power on remaining engine to achieve same GPS speed of 8.2 knots...steady track. Record rpm and rudder angle. Using the same speed eliminates one variable.

Look up fuel burn for both twin and single engine rpm as recorded (twin engine times two). Compare. I found about 5% savings running on one engine. I don't recall the rudder angle, but the Will Hamm autopilot did fine and appeared to have plenty of excess capability had the ambient conditions been more demanding. The OA has relatively large rudders.

Timjet saw larger fuel savings on his 36 Carver. I do not recall if his test procedures were the same.

Rufus, I just re-read the fuel curves for the Arleigh Burke (and my deference to Insequent as well), the typical fuel savings to maintain the SAME speed on trail shaft appear to be 8.9%, consistent with your findings. I was pulling from memory "Full Speed" (flank, both engines) compared to "Standard" (15 knots, trail shaft).

So it would appear that trawlers DO appear to scale well with ships, at least THAT ship...

Just happy to be a part of the miracle!

Garry

 

[Rufus]

An additional note....the prop charts typically show horsepower required and fuel burn (gph) for that specific engine. So no reason to estimate power required.

As an aside, the universal rule of thumb for maximizing range in a twin engine turbine helicopter is to shut one down. I had to do this once in a Coast Guard helicopter when we got diverted from a fisheries patrol to a far offshore rescue mission. The recovery of the victim took more hover time than expected, so we shut one down to ensure that we'd make landfall.

It's less cut and dried for turboprop fixed wing aircraft because even drag from a feathered propeller has an impact. That said, Navy P3 patrol aircraft routinely shut down two of the four engines to extend range. For a diesel boat, the savings is from moving the operating engine to a more efficient place on the operating curve. (Assuming rudder drag and trailing prop/driveline drag are less than the improved fuel specifics.

 

[rslifkin]

It's less cut and dried for turboprop fixed wing aircraft because even drag from a feathered propeller has an impact. That said, Navy P3 patrol aircraft routinely shut down two of the four engines to extend range. For a diesel boat, the savings is from moving the operating engine to a more efficient place on the operating curve. (Assuming rudder drag and trailing prop/driveline drag are less than the improved fuel specifics.

With the P3, I thought the shutdown was to improve endurance, not range. In other words, you could fly slow-ish for longer, but it didn't necessarily mean you could fly further.

 

[Rufus]

Both at low altitude.

 

[sunchaser]

I'd be very interested to see "real world" tests here from folks similar to the post by Rufus .

IMHO thread 28 is worth a re-read on this subject, especially the chart. No one else has posted data from "well instrumented" engines for boats represented on TF.

 

[tiltrider1]

Turbine Engines use more than half the HP just to run the compressor turbine. It’s a lot more economical to fly one engine at 60% than to fly 2 at 30% each. This is why the 4 engine aircraft are being replaced with twin engine aircraft. With helicopters it is rare to find a twin that can take off and land with one engine but many can maintain altitude in forward flight on one engine. This is what leaves an opportunity for fuel savings by shutting down one engine.

 

[sunchaser]

IMHO thread 28 is worth a re-read on this subject, especially the chart. No one else has posted data from "well instrumented" engines for boats represented on TF.

Edit
See Post #20 not 28

 

[Hippocampus]

I’m a newbie but my impression is that for internal combustion engines there’s an optimal load producing best efficiency ( also longest service life). I thought that was the justification for variable pitch props. In other words if you follow exhaust temperatures and adjust pitch accordingly so that the best efficiency was achieved at a given desired SOG you got the lowest fuel burn and greatest range for that SOG. Now I would have thought even with one or two fixed props the boats NA and manufacturer would have spec’d the boat to have total shaft HP spec’d to be close to the most efficient for the average SOG the operator would likely want.
So would think running on one engine would mean that engine would be more heavily loaded than usual so less likely to be running at its optimal load. ? Does that come into this discussion? Or it that such a minor effect as to not be relevant? Also the wheels are smaller on the twin v single version of the same boat. I thought a larger prop was more efficient than two smaller ones or did I misunderstand David Gerr? Does that have any relevance?

 

[ghost]

50% is apples to oranges and is simply not a valid measurement of single vs twin. The difference between one prop or two at the same lesser speed would be a more fair comparison and is going to be much less of a difference.

 

[gc9773]

Edit
See Post #20 not 28

Insequent showed that a 50% fuel savings could be achieved with just a reduction in power on BOTH engines, highlights a potential pitfall when addressing fuel consumption issues with trawlers (and boats in general), particularly boats discussed here.

I ran the numbers for cargo ships, boats discussed here, as well quite a few other naval ships of different classes (and different eras).

If the power necessary to drive a boat to hull speed is somewhere on the order of 2/hp to 3/hp per ton of displacement (minimum, not optimal), then the argument that many boats here are overpowered is probably correct.

The average I found for naval ships, particularly warships was 10hp/ton. THIS IS FOR A WARSHIP. Commercial trawlers (fishing vessels, etc)? Average of less than half that... Many cargo ships were averaging 5/hp - 6hp per ton, and the larger they became, the lower the number went. And these were diesel vessels...

Not being a ship designer, I suspect there is some relationship between hull length and efficiency/power required. For the largest vessels, >100,000 tons displacement/>1,000 feet LWL, I found figures as low as 1hp/ton...

There is probably some "middle ground" with any boat or ship where additional installed horsepower only yields higher maintenance and fuel costs given a certain displacement and installed power. I guess like all things, keep it balanced!

With the "SeaPiper 35", possessing a waterline length of 33', displacement of 17,000 lbs, and installed plant of 85 horsepower we find: 7.7 knots max hull speed and 11hp/ton, which seems very reasonable to me. This is a single screw boat, however...

8.9% is not much, but, on a trip from say, Seattle, WA to San Diego, CA, could be significant. And one would HALVE the run time on the engines.

At any rate, this was an interesting exercise!

 

[rslifkin]

Keep in mind, many of the boats on here sound more overpowered than they are. Most are not full displacement hulls, so the extra power isn't wasted, even if it's only used occasionally.

Assuming long tons, my boat (26k lbs loaded, 680 total hp) comes in at a whopping 58.6 hp/ton. However, while it's incredibly overpowered for running at or below hull speed, the hull is designed to plane, so it can take advantage of the extra power when desired. In my case, having gas engines, light load efficiency is awful. So I probably can stand to save more like 20 - 30% by shutting down an engine. However, I tend not to due to maneuverability and being very speed limited (6 - 6.5 kts is max comfortable cruise on one engine, especially without abusing the freewheeling trans). My typical 2 engine slow cruise is around 6.7 kts in calm water.

 

[Rufus]

I played around with a design layout for converting a twin engine boat to a single driving both props via a combining gearbox. In fact there was a gearbox based system being marketed a decade ago. The Ramsey chain company also has experience with manufacturing combining silent chain drive systems for a Navy fast patrol boat competition...four Yanmar diesels driving two props. One engine driving two props is basically variation on the theme. https://ramseychain.com/download/case_studies/High-Speed-Boat.pdf

Our 44' semi-planing boat with twin 250s for example, could still manage to get over the hump and maintain about 13 kts with a single 300. Good enough to punch through moderate chop or make better time between port calls. But at the moment, the cost for the chain drive combining box system would probably exceed the cost of a second (used rebuilt) engine. That said, the cost of fuel is poised to skyrocket when the next administration buys into the Paris environmental accords. Re-engining or reconfiguring a slobbering twin 400ish equipped boat might begin to look attractive.

 

[gc9773]

Keep in mind, many of the boats on here sound more overpowered than they are. Most are not full displacement hulls, so the extra power isn't wasted, even if it's only used occasionally.

Assuming long tons, my boat (26k lbs loaded, 680 total hp) comes in at a whopping 58.6 hp/ton. However, while it's incredibly overpowered for running at or below hull speed, the hull is designed to plane, so it can take advantage of the extra power when desired. In my case, having gas engines, light load efficiency is awful. So I probably can stand to save more like 20 - 30% by shutting down an engine. However, I tend not to due to maneuverability and being very speed limited (6 - 6.5 kts is max comfortable cruise on one engine, especially without abusing the freewheeling trans). My typical 2 engine slow cruise is around 6.7 kts in calm water.

You bring up a great point, if a hull is semi-displacement or designed to plane, then the additional power proves useful given that greater than hull speed can be achieved/maintained.

Another item is you are using gasoline engines, which provide a lower torque than a diesel would. In your case, it would seem the additional power compensates for this...

Much of this is academic, as for recreational use, most folks here probably do not put the hours on their engines that a fishing trawler would, for example, so fuel savings/plant hours are much less a consideration.

Garry

 

[gc9773]

Just did the numbers for a run from say, Seattle, WA to San Diego, CA, using 1,150 nautical miles in a "trail shaft" configuration, obtaining 2.5 nMPG, and diesel coating $2.90/gallon...

I used info from the following article..

https://www.google.com/amp/s/www.passagemaker.com/.amp/technical/feel-the-burn

Total fuel required with both engines: 460 gallons

Assuming a nominal 7.5% savings in "trail shaft", one would save 34.5 gallons, or $100.05...

I'm no expert here, but in light of the other costs associated with maintaining and operating a trawler, this is small potatoes...

 

[rslifkin]

I'm no expert here, but in light of the other costs associated with maintaining and operating a trawler, this is small potatoes...

Exactly. In many cases, maneuverability and other factors out-weigh the savings. There's some potential maintenance savings on top of the fuel savings, but it's not necessarily large.

 

[Rufus]

Another item is you are using gasoline engines, which provide a lower torque than a diesel would. In your case, it would seem the additional power compensates for this...

Garry

No, the engines in our boat are diesel. I don't believe Ocean Alexander ever build a gasoline powered boat.

 

[gc9773]

No, the engines in our boat are diesel. I don't believe Ocean Alexander ever build a gasoline powered boat.

That is what I thought as well (although I considered it likely there were gasoline trawlers out there)... Consider the boat owned by rslifkin...

Keep in mind, many of the boats on here sound more overpowered than they are. Most are not full displacement hulls, so the extra power isn't wasted, even if it's only used occasionally.

Assuming long tons, my boat (26k lbs loaded, 680 total hp) comes in at a whopping 58.6 hp/ton. However, while it's incredibly overpowered for running at or below hull speed, the hull is designed to plane, so it can take advantage of the extra power when desired. In my case, having gas engines, light load efficiency is awful. So I probably can stand to save more like 20 - 30% by shutting down an engine. However, I tend not to due to maneuverability and being very speed limited (6 - 6.5 kts is max comfortable cruise on one engine, especially without abusing the freewheeling trans). My typical 2 engine slow cruise is around 6.7 kts in calm water.

It's funny, but it would not surprise me if there were a gas turbine trawler or two out there...

Garry