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Old 11-05-2013, 09:49 AM   #1
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disappointed with twin screw fuel economy

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?
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Old 11-05-2013, 10:11 AM   #2
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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

OD
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Old 11-05-2013, 10:24 AM   #3
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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.
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Old 11-05-2013, 11:18 AM   #4
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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?
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.
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Old 11-05-2013, 11:50 AM   #5
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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.
Hi obthomas. Where should I look to get smart about "come-home" systems?
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Old 11-05-2013, 12:16 PM   #6
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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

?
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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Old 11-05-2013, 12:17 PM   #7
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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.
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Old 11-05-2013, 12:38 PM   #8
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On a vessel with a 36' waterline 1 knot of speed can be a very large fuel burn differential.
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.
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Old 11-05-2013, 12:40 PM   #9
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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.
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Old 11-05-2013, 01:00 PM   #10
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Originally Posted by seattleboatguy View Post
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?
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.
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Old 11-05-2013, 01:05 PM   #11
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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.
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?
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Old 11-05-2013, 01:15 PM   #12
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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.
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Old 11-05-2013, 01:15 PM   #13
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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?
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.
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Old 11-05-2013, 01:24 PM   #14
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Hi obthomas. Where should I look to get smart about "come-home" systems?
Read this article about get home systems. I had a hydraulic one on my first single engine trawler. It was powered by the generator and drove the propeller shaft at about half speed.

http://www.passagemaker.com/articles...re-the-options
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Old 11-05-2013, 01:51 PM   #15
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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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Old 11-05-2013, 02:12 PM   #16
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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.
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Old 11-05-2013, 02:18 PM   #17
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A moon shot until a planing hull planes
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Old 11-05-2013, 02:45 PM   #18
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But to make it simple and ease your mind, buy a single. Leave us with twins to suffer in our ignorance.
That's what I do but am thinking about making a slight alteration to the ER.

http://www.wesmar.com/pdf/APU/apu_brochure_web.pdf
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Old 11-05-2013, 02:54 PM   #19
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That's what I do but am thinking about making a slight alteration to the ER.

http://www.wesmar.com/pdf/APU/apu_brochure_web.pdf
Very cool. What do they cost, assuming you already have hydraulics on the boat?
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Old 11-05-2013, 03:23 PM   #20
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Very cool. What do they cost, assuming you already have hydraulics on the boat?
More than a second engine.
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