Single engine cruising

[REBrueckner]

I posted the following on the Trawlers and rawlering message board. The second paragraph provides fuel comparisons. The remainder provides operating details.

Ok,ok,ok!!, I admit it!!!: This summer in the NE, with high fuel prices, I WAS jealous of you pokey trawler guys.
So I tried to become one! It was a lot more work and some worry than I figured! Thanks to listees who shared their "one engine operation" results online last winter. That was what motivated me to try this.
The following relates to my 1972, 48 ft Hatteras yachtfish, hardchine aft, 45,000 lbs, hardly the latest bottom design. Props are four blade, 28" x 32".

I ran on one 8V71TI, (rated 435 SHP), at various times when conditions permitted. (more on this later) I don't have flow meters, but measured fuel usage when refilling tanks for approximate confirmation. Instead of running twins at 1525 RPM and about 11 knots which I normally do, I ran one engine or the other at about 1425 RPM and got about 8.7 knots. Fuel consumption at 11 knots (two engines) is about 17 GPH, on one engine at 8.6 knots about 9GPH. So NMPG is about 11/17 or 0.65 and 8.6/9 or 0.95 respectively. NMPG change is 1- .65/.95 or about 32%.
So I achieved about a one third fuel saving running at 8.7 knots on one engine instead of 11 knots on twins, just a touch less, I recall, than others posted here last winter.

I suspect I am still running substantially over "hull speed" on one engine at this RPM. (If LWL is 46 ft, 1.2 x 46 is 5.5kts, approximate)

I tried to check speed with one prop freewheeling vs locked while having the "off engine" running in neutral to provide transmission lubrication. I could maybe two tenths of a knot difference, but sometimes it was more, sometimes less, depending on local currents I guess, and I was too lazy to make two way trips, so I got no data.

Surprises:
(1) The sound of a single exhaust instead of resonating twins (under my Whaler across the stern) really threw me...I checked the engine room repeatedly during the first hour running on each engine individually; it sounded so completely different!!! I opened and closed the aft soft window enclosure on the flybridge to hear better...all was well. Then a few days later I did the same thing again when running on the other. I chose relatively calm days for one engine running.

(2) I first tried to block off the non working transmission from turning with a one inch hardwood dowel braced against an engine stringer... I thought for sure this would work just fine....instead, the end just got crushed and it fell out... three times and the transmission turned. But I found this quickly, so no harm.
Of course each "check" involved leaving the helm and crawling the length of the engine room. Then I found that copper pipe caps fit over the dowel end just perfectly...A solution?? NO WAY!!! They would not stay in place..which really surprised me. Next I found some of that superhard South American wood (now used for home decks) under my aft cockpit floor...turns out cutting one in half lengthwise fit just perfectly between the transmission bolts and engine housing so it couldn't fall out due to vibration/pressure...but what about an emergency?? So I test "reversed" one engine, crawled under to see and for three times that bumped the stick out and it fell to the bilge...Success!!! but I freted about it repeatedly every time I had to start up a second engine for maneuvering.

(3) After the first run on one engine I realized, oh "darn" (you know what I mean) I forgot to turn on the parallel switch to permit charging of the other battery banks. I normally split my charging between each engine. In my concern about the "strange" exhaust sound, and this new method of running, I never gave it a thought.
(4) Next I began using the autopilot...How well would it work on one engine?? Well, if I manually turned the wheel appropriately to get the boat going straight ahead first before engaging the autopilot, the autopilot kept it going on course just fine. But if I did not provide the initial orientation, I swung all over until the autopilot "learned" the new routine. I did not expect that!
(5) I got so (darned) bored at the slower speed. I almost went nuts. How do you slow pokes handle that?? SO I began reading a newspaper when I had one and some physics books I had brought along ("The Fabric of the Cosmos, Brian Greene; Parallel Worlds, Michio Kaku,etc,etc I got to read, highlight, and reread again). Wow, it was great...until I got into eastern Mass and especially Maine with all those lobster floats...no more reading and in some places with high concentrations of floats, I gave up running on one engine to better maneuver.
(6) "Better" becamse relative, however. In tight lobster float situations, again running on two engines, I found I had become a slow poke trawler guy!! I initially misjudged, turning too late because of the extra speed. That's when I decided to run at a lower RPM on twins to ease around the congestion.

(7) Then I check the aft toilet and find it's overflowing on the floor!!! How can that be??...why does running on one engine cause this? Good grief!!! Well, the good news is it was just the intake ball shutoff which is supposed to close off via spring that let in CLEAN salt water; the bad news was that I had to unfasten the toilet base, remove the wet rug, rinse it off with fresh water,and let it dry outside, and reinstall a few days later. It appears this has been leaking all along, but at 11 knots or so I guess there is enough suction on the discharge side of the potty to let it leak out...so I thought. Then a few weeks later the discharge got clogged?? Was this the cause?? So I removed the hose from the seacock, cleared the blockage at the nipple, and a few weeks later, when overboard, cleared some barnacles from the seacock thruhull discharge side. This was the second year for this bottom paint; maybe the barnacles caused the clog. I suspect the yard never put fresh paint up inside the discharge line.

(8) I had the starboard engine aligned shortly after purchase (7 yrs ago) and that eliminated 90% of apparant vibration at that time. There has always been just a bit, I notice it sometimes, others don't. Running on one engine for long periods detected the remaining minor vibration: the port engine machinery seems to cause a bit of vibration not apparant from the starboard machinery. So this winter I'll check the alignment, I suspect that's the cause, not propeller nor shaft. If not, I'll have the prop rebalanced at spring haulout.

All in all it was a lot more than I anticipated...I should know better after all these years boating. I guess I'm a slow learner...

 

[Genesis]

Try running at ~9 kts on BOTH engines, around 1100 rpm or so.

They'll stay warm and I bet you get the same fuel economy - I get right near 1.0nmpg that way and yet I have immediate capability to maneuver using both if I need it.....

 

[Nonchalant1]

Just run slower with both engines if you want more fuel economy. Here's what I got on 2-way test runs for my 1978 53MY with 435HP 8V71TIs and 9.1 knots hull speed, using flowscans and GPS speeds with full fuel and water.

At 640 RPMs I get nearly 3 NM per gal. However, I've never gone a whole day at 6.3 knots because it drives me crazy. I can't keep my hand off that red knob. I actually average between 1.5 and 2.0 G/NM on long cruises.

RPM =Kts =GPH Gal/NM
0640 06.3 01.8 0.3
1060 08.7 08.0 0.9
1100 09.1 09.0 1.0
1200 09.3 10.4 1.1
1400 10.0 16.2 1.6
1600 10.8 23.4 2.2
1700 10.9 27.0 2.5
1920 15.5 34.2 2.2
1980 16.2 36.0 2.2
2200 18.3 44.4 2.4

Doug Shuman

 

[Genesis]

You cannot run Detroits (or most other high-performance diesels) at idle for long periods of time - they will load up and be damaged.

At around 1100 rpm you're ok from that perspective most of the time, as temperatures and piston speeds are both high enough to be ok for extended periods of time.

I can get 0.9nmpg at 9 kts, or 0.4nmpg at 18kts. My choice - I'm beginning to really LIKE 9 knots with the halving of my operating costs in terms of fuel....

 

[Pascal]

"I suspect I am still running substantially over "hull speed" on one engine at this RPM. (If LWL is 46 ft, 1.2 x 46 is 5.5kts, approximate)"

hull speed is the sqrt of LWL x 1.3 (or 1.3something...), so for 46' LWL hull speed is gonig to be around 8.7 kts. not 5.5 !!

you can't compare fuel economy between 11 and 8.7 kts... as soon as you go above hull speed, fuel flow increases much faster than speed and your 0.65 mpg at 11kts reflects that...

instead, try comparing single engine vs twin at 8.5 kts... i doubt the difference is worth the hassle.

i never run my 53 above 9kts... it just doens't make anysense. 1500rpm gives me 9.4 kts in 12'+ depths (or just 8.6/8.9 kts in shallower waters)... that's enough to keep the engine warm and still gets me around 0.9 mpg.

 

[SKYCHENEY]

Remember, if running on one engine, you have not only the possibility of damaging a transmission, but also your shaft log if it is of the dripless variety. I don't think I would ever run on one unless it was absolutely necessary, ie engine or prop damage. Even if you think that the shaft is locked in place, it could still break loose and you might not catch it until the damage is done. As stated earlier, I think that the difference in fuel consumption will prove to be minimal.

 

[REBrueckner]

Good suggestion regarding dual engine 1100 RPM cruising instead of one engine. I had wondered about that, but was concerned about the relatively low RPM. I'll try that this coming season. I do know that even at 900 RPM my engines maintain proper water temperature.

 

[Freestyle]

Since I may be the cheapest guy on the forum and I do run over 400 hours a year, the $3 per gallon diesel prompted me to experiment over the last few months.

Without giving the boring data and unscientific approach used to gathering it, a 43 DCMY with 6/71N's got about 6.5 gph running at 1500 rpms with both engines. This rpm translated to 8.2 knots more often than knot. Faster speeds were observed in deeper waters and of course when the tides pulled.

On days over my two week experiment where I alternated engines every hour at about 1850 rpms which generated the same 8.2ish speeds, I calculated about 5.6 gph. In my experience, the boat handled like a pig and the autopilot proved worthless. So I decided to look for a $3 per hour savings somewhere else in my life and have come to enjoy the taste of Skyy vodka every bit as much as Grey Goose.

 

[Capten]

REBrueckner:

Don't mean to hijack the thread but......

You mentioned lobster pots. What size shafts do you have? I have a set of Spurs Line Cutters for 2 1/2" shafts that work great for that type of thing if you are interested.

 

[jim rosenthal]

This is a great thread, very much of interest to me. I usually run my boat at about 1800-2000 revs, which is planing but less fuel used than 22-25 kts. A few questions:
-on planing hulls such as ours, there is a point, I think it is usually described as "getting up out of the hole", where the load on the engine is maximal- this is the point in the planing curve that you try to transition through as quickly as possible. If you don't have FloScans and EGT and boost gauges, how do you figure out where this is? Just curious.
-how slow can I run my CAT 3116s without "loading-up" problems? Clearly idle is not a good place to them to be run for longer than absolutely necessary. I am not sure where the turbos start to spool up and make boost. I suspect that I could run them at 1000-1200 and as long as the boat is not coming up on plane I would be avoiding the "out of the hole" point and still running at displacement speeds.

 

[Nonchalant1]

Karl,

There are sometimes long stretches in the river where you have to run below 1000 RPM for hours. I always run em up for 30 min per day or so to heat them up around 180 degrees for 15 min. If you do that, is it OK to go 2 or 3 hours at 650? I'm not sure I see any way around it.

Doug

You cannot run Detroits (or most other high-performance diesels) at idle for long periods of time - they will load up and be damaged.

At around 1100 rpm you're ok from that perspective most of the time, as temperatures and piston speeds are both high enough to be ok for extended periods of time.

I can get 0.9nmpg at 9 kts, or 0.4nmpg at 18kts. My choice - I'm beginning to really LIKE 9 knots with the halving of my operating costs in terms of fuel....

 

[Nonchalant1]

I suspect I am still running substantially over "hull speed" on one engine at this RPM. (If LWL is 46 ft, 1.2 x 46 is 5.5kts, approximate)

I think hull speed, in knots, is 1.2 to 1.4 times the square root of LWL. Therefore, for a 46' LWL boat, hull speed is 8.1 to 9.1 knots depending on hull shape and wetted area. I used 1.3 for my 53MY which has a LWL of 49' and get 9.1 knotts hull speed. When I exceed that, I'm trying to push 30 tons uphill on the bow wake, which shows in the NMPG chart. I didn't include 1800 RPM because that's where it falls off plane coming down or won't go up on plane yet coming up, and it's about 3.0 gal/NM.....so 1800 RPM is my worst fuel use speed!

Doug Shuman

 

[Fanfare]

In June of 1971 I contacted Detroit Diesel Allison to ascertain the optimal rpms for my 8-71N diesels, 350 hp each. Mr. C. J. Saurer, Supervisor, Sales Applications, replied: "Any speed between 1200 and 2000 rpm crusing is perfectly all right. ...between 1800 and 2000 rpm crusing are good because of reduced noise and fuel consumption."

"The only problem with sustained idling is that the engine is generally at no-load under these conditions and tends to cool down. This results in reduced combustion efficiency, which may in time cause deposits to form in the engine. Opportunity for this to happen in your application is remote. It takes some time for these effects to be noticed. Should you have a condition where you would be required to operate the engine at no-load for sustained periods of time, it might be wise to operate at somewhere near 1200 rpm. This will improve the performance of the battery charger generator (note: it may have been a generator then, as compared to today's alternator) and also help to maintain somewhat higher cylinder temperatures. The fuel consumption difference would be negligible."

In 1995 I repowered with DD 6-92TAs, 550 hp. and asked the same questions. In a telephone conversation I was told that the new engines did not fare as well at low rpm, and that they should be run above 1300 rpm, where the turbos began to kick in. Further, prolonged running at idle would "put a glaze on the cylinder walls" which, while not harmful, would cause increased lube oil consumption. This glaze could be removed by a short period of full-throttle running which would clear it. In practice, I have run long hours on the ICW at idle, since anything faster results in an unacceptable wake. I have never noticed any increased lube oil consumption. (Beyond the usual slurping, that is.)

Jim Grove, Fanfare 1966 50MY

 

[Trojan]

My DD book for My 6-71N reads 900 RPM is the least you can run for long periods and be safe. That also includes warm ups. I would never run on one engine unless it was the only thing that I could do. Bill

 

[Genesis]

Yep.

The issue with Detroits is that there's a big hunk of iron there and they WILL cool down if you try to run at idle for long periods of time. They just can't regulate their own heat levels well enough. This is true for MOST diesel engines - they don't like to run cold at all, produce deposits in the combustion chambers, and just generally don't get along well that way.

I troll from time to time, but if I do, I pick it up every couple of hours and run a good 15 minutes on plane to blow 'em out. You have to - otherwise they will load up and there'll be trouble.

You're ok at 1100-1200 RPM provided you can keep coolant temps over 160F. On a cold start at the dock I idle long enough to come off the temp gauge pegs, then take 'em to 1100 if I am going to be idle at the dock for any length of time.

I can get 3-4nmpg at 600 RPM - the problem is that I can't run there without trashing the motors for any length of time.

The solution to this, by the way, is diesel-electric propulsion with a littlle "wing engine" that can run under load for slow speeds. THAT will produce some amazing fuel economy when run slow and be perfectly safe (and QUIET!) However, you won't like what setting a boat up for that which wasn't designed for it originally will cost!

There was a place - FEYS - that was claiming to be doing retrofits of this. Their web is still there but they no longer answer inquiries and have had no updates on their web page for the last few years. I have to wonder if they're dead and gone.....

 

[REBrueckner]

Where to run a 3116 CAT, or any engine, will get you all sorts of answers. A critical criteria is that any engine be run fast enough to maintain normal coolant operating temperature. Some will suggest ramping up RPM every ten or twenty hours for a few minutes to "clean them out". If you do that and any smoke is light and clears quickly, say in 10 or 15 seconds, my GUESS is that all is well. Once any smoke clears you should be ready for another run at moderate RPM. I use this method to get some positive, visible feedback.

For "permanent" lower RPM running and more fuel efficient cruising you can also consider having smaller injectors installed, but check CAT for recommendations. This will of course reduce output HP and reduce top speed. Anyother option is to utilize AIRSEPS or RACOR CCV's(the latter preferred on this forum) to help keep turbo equipped engines cleaner. I don't believe RACOR's offer much in benefits for NA engines.

 

[Trojan]

Diesel - Electric ! What a way to go. I don't think our smaller boats could handle the wieght. Because of the torque of the electric motor. Would we still need 400HP electric motors and could we run 1 diesel and 1 generator? Now thats a different new concept for small boats. What about hydraulic motors? Everything ends up BIG and heavy. Maybe some new light weight technology is due here. Bill

 

[Genesis]

Diesel-electric should require less total horsepower (but not by a lot) and be able to be done in a single instead of two engines (the reason for the "less" is the reduction in weight - a single 800hp engine doesn't weigh as much as 2 400HP ones)

A drive motor is either equal in mass or lighter than a marine gear.

The control electronics weigh something, but not a huge amount.

Done "right" the engine(s) can also produce your 120V "house" (genset) power as well as propulsion, although there's no weight savings there as you need the second low-HP engine for low-speed cruising.

FEYS had an interesting idea but I challenged their "savings" in terms of engine sizing, and they were unwilling to put up when challenged to (they claimed they could produce equal performance to my 1000 shp with a 700HP main) My challenge to them was that if the performance they claimed would be there failed to appear they ATE the conversion cost. They declined. Now they appear to be gone. I didn't believe their 30% HP reduction (the reduction in mass is singificant, but its not THAT large!) however the economy argument of running a smaller engine fully-loaded for low-speed cruising cannot be debated - its a fact.

 

[Avenger]

Trojan,

Don't forget expensive. Add the cost of generators and motors, plus packaging, meaning where do you put the extra hardware. Electric motors make peak torque at zero RPM, great for a locomotive, I don't know how good that would work on a boat. Extra complexity, and weight not to mention efficiency losses from converting one form of energy to another then back again... Hydraulic motors are notoriously loud, and they make a lot of heat, so, more heat exchangers. There's reasons why this isn't done.

Karl's suggestion works best as he described; a dual operating mode where you can optimize both methods for their own parameters. Your regular engines for high speed, and a small diesel-electric setup at maximum efficiency for low speeds. We did some work on the NYC fireboat "Firefighter". It had two Cleveland 278A's driving three DC generators each. It allowed them to use the big generators for propulsion, then switch them to the pumps and use the midsize gen for stationkeeping. The small genny was for shipboard power. Really cool setup, Frankenstien electrical panel and all, 1938 vintage. But their dual operating mode justified the setup since gears and shafting to drive everything would have become prohibitive.

I'm going to stick to what I've got. I have enough trouble keeping up with maintenance and repairs on the three engines I have now. Trolling motor anyone?

 

[Freestyle]

Speaking of 6/71N temperatures...

When I bought the boat I was instructed never to put it into gear until warmed to 130 degrees. I have blindly followed this advice but thought I would check on it. I drive people crazy at the dock banging these loud engines for 10 minutes until they reach 130.

Is this necessary, advisable, or a complete waste of time?

Thanks

Bruce