disappointed with twin screw fuel economy

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seattleboatguy

Senior Member
Joined
Nov 2, 2013
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327
Location
USA
Vessel Name
Slow Bells
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Marine Trader 38
I was very interested to read the feedback from my earlier post "2 trawlers, same engine, different fuel consumption???". The great posts that were contributed on fuel consumption (many thanks to all who contributed) left me very surprised and disappointed with the performance of twin screw trawlers. I come from a sailing background, and as I start looking at a trawler for my next boat, I like the great maneuverability of twin screws, and the idea that I can keep on going if I loose an engine far from a SeaTow base. However, I incorrectly assumed that, with twice as much thrust, I could cut way back on the throttles and still cruise at the same speed as a single screw trawler. Not so. The best example is the data from Moonstruck and SomeSailor.

Moonstruck
120 hp Lehman, single screw
7 knots, 1.5 gph

SomeSailor
120 hp Lehman, twin screw
8 knots, 4 gph total, or 2 gph per engine

So, SomeSailor has twice as many horses in the engine room, drinking twice as much diesel, but cruises at about the same speed as Moonstruck. Why don't trawler skippers just shut down one of their engines, once they are clear of the marina?
 
Subscribed.:thumb:
I have some thoughts on the matter based on some time with twins in the guard, but I want to see what the pros have to say:thumb:

OD
 
A single engine is simply more economical than twins. My first trawler was 38 feet went 7 knots on one 135 HP volvo using about 3 gallons per hour. My present trawler is 40 feet goes 7 knots on two 165 HP volvos and uses about 6 gallons per hour.
If I shut down or loose one engine I drop to about 5-6 knots and cut fuel consumption almost in half.

I enjoyed the economy of a single engine in my first trawler and once had to use the come-home system powered by the generator when my main engine starter crapped out.

If I am so lucky as to get another mo-better trawler I think I will go with single engine and a come home system.
 
seattleboatguy said:
So, SomeSailor has twice as many horses in the engine room, drinking twice as much diesel, but cruises at about the same speed as Moonstruck. Why don't trawler skippers just shut down one of their engines, once they are clear of the marina?
Click to expand...

It's still about the economics of it. My neighbor has a very similar sized single engine trawler. He runs well below his hull speed (best mathematical efficiency) because he's pushing his single engine harder than would be efficient. With twins, I can operate less efficiently (two screws, two engines running) and still make my hull speed in a range that is most efficient.

If he and I were to run side-by-side AT hull speed (most efficient for the hull) I'd probably burn less fuel. He has to push his single to get there is all.

At some point, the displacement numbers become important variables in terms of efficiency. I don't know where that number is though? 40', 50' 60' LOA? It would be a fairly simple model to build though.
 
obthomas said:
I enjoyed the economy of a single engine in my first trawler and once had to use the come-home system powered by the generator when my main engine starter crapped out.
Click to expand...

Hi obthomas. Where should I look to get smart about "come-home" systems?
 
seattleboatguy said:
Moonstruck
120 hp Lehman, single screw
7 knots, 1.5 gph

SomeSailor
120 hp Lehman, twin screw
8 knots, 4 gph total, or 2 gph per engine

?
Click to expand...

Both at the same speed on identical vessels is the question. You are comparing apples and oranges. On a vessel with a 36' waterline 1 knot of speed can be a very large fuel burn differential.

I have seen detailed boat and engine manufacturers data for a NT Tug 52 and KK 52, both with twins vs singles at the same approximate installed HP. At most there was a 10% betterment for the single at various 0.9 to 1.3 various near cruise speeds. Art DeFever found about the same so 30 years ago he stuck with twins for his displacement designs. For some that 10% is worth dock talk, for others - trivial.

Don't forget that once you exceed 40' on the used boat market, twins are common and singles a rarity - unless you want to pay a hefty premium for a single such as KK, Selene, NT or Nordhavn.

But to make it simple and ease your mind, buy a single. Leave us with twins to suffer in our ignorance.
 
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There is also the matter of, I believe in most boats, having to keep the "off" engine's transmission from freewheeling which can cause damage. Probably calls for a trip to the engine room and messing around in the machinery to lock and then unlock the shaft for docking maneuvering. I read of one guy putting a big stillson wrench on the shaft coupling, if I did that I'd surely forget it was there and crank up the engine causing major damage. Sounds like one of those things best left for ocean crossings.

I am happy with the single and bow thruster, but the thought of the engine conking out while I am in heavy Mississippi River traffic at New Orleans has crossed my mind, so far the Cummins has performed fine.
 
sunchaser said:
On a vessel with a 36' waterline 1 knot of speed can be a very large fuel burn differential.
Click to expand...

I agree. On my 34 Californian with twin 85 hp Perkins, 8 Kts costs 4 gph (total) of diesel at 2350 RPM. Slow 2000 RPM and I see 7.4 Kts and 2.75 GPH total. BIG difference in fuel for a small difference in speed. Cut one engine and run at 2000 RPM at closer to 6 Kts.
 
I troll with one motor lit off. The reason is I'm trying to slow down as much as possible. With one screw dragging I can go as slow as about 1.5 - 2 kts. I think I'm more efficient with both running at lower RPM than one being pushed harder to get and hold hull speed.

Much less than 40 feet and I'd guess this is moot.
 
seattleboatguy said:
I was very interested to read the feedback from my earlier post "2 trawlers, same engine, different fuel consumption???". The great posts that were contributed on fuel consumption (many thanks to all who contributed) left me very surprised and disappointed with the performance of twin screw trawlers. I come from a sailing background, and as I start looking at a trawler for my next boat, I like the great maneuverability of twin screws, and the idea that I can keep on going if I loose an engine far from a SeaTow base. However, I incorrectly assumed that, with twice as much thrust, I could cut way back on the throttles and still cruise at the same speed as a single screw trawler. Not so. The best example is the data from Moonstruck and SomeSailor.

Moonstruck
120 hp Lehman, single screw
7 knots, 1.5 gph

SomeSailor
120 hp Lehman, twin screw
8 knots, 4 gph total, or 2 gph per engine

So, SomeSailor has twice as many horses in the engine room, drinking twice as much diesel, but cruises at about the same speed as Moonstruck. Why don't trawler skippers just shut down one of their engines, once they are clear of the marina?
Click to expand...

Whoooaaaa!!!! 2 boats is not enough data to write anything in stone...better get a couple dozen more for even a ROUGH guess what you can expect.

My last winter's trip of 2000 miles I averaged 6.3 knots, and maybe 1.9 gal/hr = 3.3NMPG in a 36 foot waterline, single trawler. The hours times fuel burn worked out to be similar numbers too as a double check. I run at around 1650RPM with a Lehman 120.
 
Steve said:
There is also the matter of, I believe in most boats, having to keep the "off" engine's transmission from freewheeling which can cause damage. Probably calls for a trip to the engine room and messing around in the machinery to lock and then unlock the shaft for docking maneuvering. I read of one guy putting a big stillson wrench on the shaft coupling, if I did that I'd surely forget it was there and crank up the engine causing major damage. Sounds like one of those things best left for ocean crossings.

I am happy with the single and bow thruster, but the thought of the engine conking out while I am in heavy Mississippi River traffic at New Orleans has crossed my mind, so far the Cummins has performed fine.
Click to expand...

Maybe I didn't read this right, but wouldn't locking a propshaft and running on one engine cause a lot of drag? I would think that the propshaft would have to freewheel or it would drag, and pull the boat to one side?
 
Some transmissions require the shaft be locked down to prevent freewheeling. My Borg Warner Velvet Drives can free wheel without damage. Another issue for some is keeping the dripless shaft log from overheating. I added a crossover line so both logs get water from each engine.

Locked or not, the drag caused by the non-turning prop requires rudder to offset the turning tendency, which in turn causes even more drag.
 
SomeTexan said:
Maybe I didn't read this right, but wouldn't locking a propshaft and running on one engine cause a lot of drag? I would think that the propshaft would have to freewheel or it would drag, and pull the boat to one side?
Click to expand...
There's conflicting data out there on whether a freewheeling or locked prop creates more drag...a lot of "ifs" have to be answered before it tips the scales absolutely I believe.
 
Over 3 seasons with a total of 328 hours my twin 175 hp Hinos have averaged 3.1 gph, this includes the gen time. If I deduct .5 gph for the generator hours it comes down to 2.7 gph for the Hinos alone, I normally run 1000 to 1700 rpm with occasionally runs over 2000 & some wot to make sure everything is working as it should. I haven't done anything with this seasons numbers yet, the hours I'am sure are way down from my normal use because of being in Mn. working on a friends boat, plus I haven't winterized yet & I'am still planning on being out a few more times.
 
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but cruises at about the same speed as Moonstruck.


NO NO NO , depending on the WL length of the boat that 1K can double the thrust required !

1 MPH in your car or pedaling your bike is nothing , 1 K or even lubber MPH is a BIG DEAL on a short waterline.

Look in any book on boat design .

At some speed point the HP required curve starts to be a Moon shot.
 
A moon shot until a planing hull planes:D
 
With all the single v. twin discussion, I think the OP misses the central point. It takes the same energy to push a boat at any given speed. In theory, it should take exactly the same amount of diesel fuel burnt to produce that set amount of energy, whether the energy is supplied by a single powerplant or twins. In practice of course, as several posters have noted, there is a difference in fuel consumption between a single and twins although an almost negligible difference.

One thing to note-from my experience, many twin displacement/semi-displacement hulls with twins are overpowered. This seems to be the case a bit more with older boats. I know Steve D'Antonio is big on this theory, perhaps to an extreme. Many newer displacement boats will carry almost identical HP whether in a single or twins.
 
I do not know who made the get home I had. I looked like a truck power steering unit mounted on the front of my 7.5 KW two cylinder Onan generator with two fan belts; with hoses to a hydraulic motor on a metal platform bolted to the top/back of my borg warner velvet drive transmission. The hydraulic motor had a small sprocket. I had to install a size 60 motorcycle chain around the motor sprocket and another larger sprocket sandwiched between the tansmission and shaft couplings when I needed to use the get home. The pump on the generator had an electric clutch like an automobile air conditioner compressor. I flipped the switch with the transmission in neutral and went home at about 4 knots.

One time about 40 miles out in the Gulf of Mexico I got water in my main engine thru the exhaust while anchored. I burned up my starter and had to use the get home. I went through two sets of fan belts because the adjustment feature was limited. Changed that shortly afterwards.:D
 
THD said:
One thing to note-from my experience, many twin displacement/semi-displacement hulls with twins are overpowered.

I believe this. My twin engine boat would perform very nicely with about half the HP in 2- 4 cylinder engines vrs the 2- 6 cylinder engines I have. :D
Click to expand...
 
seattleboatguy said:
Very cool. What do they cost, assuming you already have hydraulics on the boat?
Click to expand...
I haven't got that far yet so the answer is "I don't know." I've learned, however, that I'd have to add a 12KW genny or larger to run the hydraulics and my little boat doesn't have near enough room for that. (I think SomeSailor's remark about "more than a second engine" is probably correct.) :blush:
 
This example was interesting on what I assume would be similar style hulls.

54 ft Defever
These 4 cylinder diesels (Cat D 330T x 2) are running between 1400-1500 rpm at a cruise speed of 7-8 knots. I have never been able to figure out the fuel burn exactly, but I estimate at less than 5 gallons per hour total.
So 19 l/h
http://www.trawlerforum.com/forums/s9/54defever-offshore-pilot-house-trawler-11400.html#post187590
Click to expand...


Vs

60 ft trawler
This vessel cruises effortlessly at 8knots with a fuel burn of about 10 liters per hour with a single Gardiner 8cyl 200 hp
Maurice Drent Australasia: Boats for Sale
Click to expand...
 
SeaHorse II said:
I haven't got that far yet so the answer is "I don't know." I've learned, however, that I'd have to add a 12KW genny or larger to run the hydraulics and my little boat doesn't have near enough room for that. (I think SomeSailor's remark about "more than a second engine" is probably correct.) :blush:
Click to expand...

WESMAR offers the APU in two sizes. The 200 series has a maximum rating of 25 hp and can take up to a 2.38-inch-diameter prop shaft. Pricing starts at $10,695. The 300 series maxes out at 100 hp, takes a prop shaft of up to 3.38-inch diameter, and lists for $12,400. Prices do not include installation. Expensive? Compared to the cost of a second engine and the required running gear—not to mention the additional mechanical complexity and hydrodynamic drag—it’s a good deal. And, as they say in those MasterCard commercials, being a hero in an emergency is priceless. :D
 
Vessel Assist and an adequate anchoring system could provide the get home protection for singles in many locations in this country. Of course, the more remote areas may not have this option.

But weren't we talking about single-and twin-engine efficiencies?
 
obthomas said:
The 200 series has a maximum rating of 25 hp and can take up to a 2.38-inch-diameter prop shaft.
Click to expand...


Two questions:
  1. How do you propose to provide the 25 HP and at what cost?
  2. What if the reason the boat isn't moving is a broken shaft, strut, prop or shaft through hull?
The answers to these two questions are why twins or separate get home engines are desired by some.
 
[QUOTE
Moonstruck
120 hp Lehman, single screw
7 knots, 1.5 gph

SomeSailor
120 hp Lehman, twin screw
8 knots, 4 gph total, or 2 gph per engine

So, SomeSailor has twice as many horses in the engine room, drinking twice as much diesel, but cruises at about the same speed as Moonstruck. Why don't trawler skippers just shut down one of their engines, once they are clear of the marina?[/QUOTE]

Big fan of single screw here but you are comparing 2 different boats at 2 different speeds which are near their most critical speeds as far as fuel consumption is concerned. You need better information b/4 you can make a decision correctly. I will add that the partial redundancy provided by twins is, for me, outweighed by the exposed running gear, added complication, noise, fuel burn, maintenance, & cost on twins.
 
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In my opinion on a trawler the ideal set up is a single screw with a thruster (bow or stern or both). For ocean and remote cruising a hydraulic drive off the generator would be a good thing. With the reliability of a well maintained diesel engine and towing services for a back up, I think that twins are unnecessary in most cases.
 
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