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Old 05-06-2011, 11:48 PM   #1
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Running single on a twin to save fuel

Remember some time ago I started a thread on Boat Design to help us out w this question? Well, they started it up again and are up to response # 20. You can join free but you don't need to be a member to browse. Search Boatdesign.net. Click on forums, Propulsion and Diesel Engines. You may not need Propulsion. The thread is on "Diesel Engines". The name of the thread is the same as here. If you join you can take part in the discussion and if you do please refer to me as "Easy Rider". FF is there too and please use his username also. One of my favorite guys there talks about removing one prop and running to Ketchikan. That could be an idea for someone here.
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Old 05-07-2011, 09:00 AM   #2
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RE: Running single on a twin to save fuel

Checked it out.* Sounds like one prop too many.
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Old 05-07-2011, 10:34 PM   #3
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RE: Running single on a twin to save fuel

Mark,

When your engine quits under the bridge on a strong ebb tide you'll drop your bias on twin engines fast.

boatk9,

You're one of the guys here on TF that should be on Boat Design. Did you read the thread? There were some great responses and experience posted there. There was or is (?) a single prop twin engine geared system featured on PMM and it was said to have greater efficiency. I'd rather just have twin engines of the right size. There was a GB 36 in Yacht World that had two 55hp Yanmars. Somebody else obviously shared my view. Twin 18-20hp engines for my Willy would mean running 2 cylinder engines and I'm sure I would'nt like the vibes especially the harmonics when out of sync.

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Old 05-08-2011, 04:43 AM   #4
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RE: Running single on a twin to save fuel

There was or is (?) a single prop twin engine geared system featured on PMM and it was said to have greater efficiency.

These are OTS and cheap.

Used on LST and similar Navy boats , (since WWII) usually (2) 6-71 and a single shaft.

Last I saw in Boats and Harbors a USN rebuilt package was $6,000 for the set. But NOT LIGHT! about 6,000 lbs

I have always thought a rebuilt 6-71 could easily be swapped for a 2-71 or 3-71 and a great offshore package created.

Rated about 30 HP per cylinder the 3-71 injected for 60-70 hp,could take care of long range cruising on a larger boat with the 6-71 reserved for coastal work, faster , but with fuel stops handy.

The parts (exhaust manifold, starter) and many others are identical on the 3 or 6.

The biggest problem to "go modern" with more efficient engines than the DD would be today's modern engines do not use a SAE 1 sized bell housing.
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Old 05-08-2011, 07:22 AM   #5
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RE: Running single on a twin to save fuel

"When your engine quits under the bridge on a strong ebb tide you'll drop your bias on twin engines fast."

That actually happened to me and I am still a single engine fan. We never practised for this but we were prepared* and skilled enough to take care of business.

*
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Old 05-08-2011, 07:44 PM   #6
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RE: Running single on a twin to save fuel

A few observatons on the single vs twin ad infinitum.

1)* Over the years I've spent considerable time in*Nordhavn ERs. It is amazing how many of these vaunted singles have a wing engine.

2)* It seems that once you get over a trawler size of 45',* the question is not about saving 10 to 15% in fuel $$, it is*increased range for long distance blue water cruising that a single normally provides.

3)* There are many who like the "insurance" twins allow.* I met a boater last year who was running from Wrangell to Anacortes (700 nm or so) on one engine since his other had a crankshaft failure.

4)* Some boats are very nearly the same fuel burn with twins as a single. Nordic Tug compared (PMM article late 2005 or early 2006) a 52 with single vs twins, about a 5% difference.

5)* I burn about 4.5 gph with my twins at "hull speed." Shutting down one engine with the risk it entails, to save about $400 - 500 max per year, is not justifiable IMHO.
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Old 05-08-2011, 08:50 PM   #7
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RE: Running single on a twin to save fuel

But Eric, a*single engine is twice as easy to maintain and half as expensive as two, and is much more likely to be keel-protected.* And I feel much more safe than in a single-engine plane.

*
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Old 05-08-2011, 09:51 PM   #8
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Running single on a twin to save fuel

Tom, Mark,

You say " Some boats are very nearly the same fuel burn with twins as a single. Nordic Tug compared (PMM article late 2005 or early 2006) a 52 with single vs twins, about a 5% difference." If you're pushing the same boat at the same speed you're using the same amount of power. The difference is that w the twin there is much more area (in most old boats) exposed to combustion whereas the heat from our heat engine dissipates more readily and becomes a loss. Heat loss in a heat engine. Same as running a SD boat at 30% load. Could be done w an engine half the size at 70%. Less heat loss (and mechanical friction too). Also twins don't cost twice as much to maintain at all. On a proper twin engines are half the size. That means fuel and oil filters will cost close to half as much, 2 small head gaskets is about the same as one large, belts are about the same and even an oil change will be about the same. There is some advantage to propeller protection but a twin could be built that would have engines and propellers close inboard so prop protection would be close to the same. Twins are better*** ....but like most better things**** ...more expensive.


-- Edited by nomadwilly on Sunday 8th of May 2011 09:51:41 PM
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Old 05-08-2011, 10:10 PM   #9
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RE: Running single on a twin to save fuel

Interesting discussion...

Prop protection is a real advantage of singles. I wish had the same level of protection on my twin.

It raises the question, can prop protection be effectively improved on twins by placing a partial shallow keel ahead of each prop, possibly surrounding the shafts? I suppose the parasitic drag would increase slightly, not unlike trim tabs, but the benefits would seem to outweigh the costs.

Do any twin manufacturers do this?

Sorry for the hijack...
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Old 05-09-2011, 12:15 AM   #10
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RE: Running single on a twin to save fuel

Quote:
nomadwilly wrote:
Tom, Mark,

You say " Some boats are very nearly the same fuel burn with twins as a single. Nordic Tug compared (PMM article late 2005 or early 2006) a 52 with single vs twins, about a 5% difference." If you're pushing the same boat at the same speed you're using the same amount of power. The difference is that w the twin there is much more area (in most old boats) exposed to combustion whereas the heat from our heat engine dissipates more readily and becomes a loss. Heat loss in a heat engine. Same as running a SD boat at 30% load. Could be done w an engine half the size at 70%. Less heat loss (and mechanical friction too). Also twins don't cost twice as much to maintain at all. On a proper twin engines are half the size. That means fuel and oil filters will cost close to half as much, 2 small head gaskets is about the same as one large, belts are about the same and even an oil change will be about the same. There is some advantage to propeller protection but a twin could be built that would have engines and propellers close inboard so prop protection would be close to the same. Twins are better*** ....but like most better things**** ...more expensive.
*I wasn't aware that twin-engined boats under 40-feet-in-length typically had*insufficient power to exceed displacement speed.* So I'd presume that twin-engined boats had much more power than a single-engined, displacement-engined boat.* I wouldn't presume two powerplants each with fuel and air filters etcetera would cost the same to maintain with a single engine with half those parts.* What mid-sized trawlers with two engines/propellers are offered with keels protecting each?* I'm no expert.* Just tell me where I'm ignorant.
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Old 05-09-2011, 12:44 AM   #11
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RE: Running single on a twin to save fuel

Nordhavn 55 and up do.

gordon.
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Old 05-09-2011, 12:47 AM   #12
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RE: Running single on a twin to save fuel

Is there a twin-engined, displacement-speed boat produced*under 40 feet in length?* If not, why not*since dual engines are supposedly superior to single-engined boats?
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Old 05-09-2011, 12:53 AM   #13
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Running single on a twin to save fuel

Quote:
flashwillie wrote:
Nordhavn 55 and up do.

gordon.
I wasn't thinking of super-large yachts.* Nevertheless,*are they twin-keeled, protecting each propeller and shaft?* I doubt it.


-- Edited by markpierce on Monday 9th of May 2011 12:57:29 AM
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Old 05-09-2011, 02:17 AM   #14
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RE: Running single on a twin to save fuel

i found a couple of pictures you may like to see.they are of n55 new paige.before and after.
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Old 05-09-2011, 07:41 AM   #15
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RE: Running single on a twin to save fuel

The Great Harbour N37 boasts small efficient twins for redundancy and long range. With keels on both sides, if the boat is grounded it will sit safely upright until the next high tide.*
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Old 05-09-2011, 09:31 AM   #16
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RE: Running single on a twin to save fuel

Flash,

Thanks. I wonder what that hole is aft of the rudder? My guess is a thruster port.

Mark,

"Just tell me where I'm ignorant." Not at all Mark. Most trawler manufacturers in the 70s put the same engine in twins and singles. That means singles had 120hp and twins had 240. Remember when we discussed the engine in your boat and if you had the right amount of power? There is the "more is better" philosophy and twins would certainly seem better to most at the time. After all they paid for it. If you bought a 1960 Plymouth in 1960 you'd get the V8 if you wanted more power or the 6cyl if you were inclined toward economy. The V8s power can be used on the road and the 6cyl will have better economy. But in a full disp boat double the power is stupid. Consider 160hp in your Coot??? Most all trawlers are semi-disp boats and double the power is at best half stupid. It would have made more sense for the GB 36 to have had 140hp standard with an option of 180hp. Or 90hp w an opt of 150hp. Ideally you have both amounts of power available in either twin or single. As it was you had to buy an over powered boat to get twins or an under powered boat to get a single. With a F Disp boat the correct amount of power is within a very narrow range * ....like 60 to 85hp (approx) on your Coot. Engine choices in the 70s trawlers made more sense at the time when fuel was so cheap but now almost everyone w a twin runs very underloaded and only uses about 25% of available power so almost all would be better off now w a single.*

As to the maint issue a twin or a single engined boat should have the same amount of power. The power level being a wise and sage choice by the designer. Then your boat would be offered w perhaps one 80hp engine or two 40hp engines. So look at our two engines maint cost wise. I'll bet belts, filters, oil, gaskets, injectors, pistons, gearboxes and all other things taken together are very close to half as much for my engine and twice as much for yours. It's going to take twice as much time to pump twice as much oil out of your twice as big engine as my little guy. In a comparison of twin v/s single one must compare twin engine w single engine when both are the same amount of total power. So a boat with a single engine and another with twin engines ALL OTHER THINGS BEING EQUAL (and they should) there is very little (hardly any) difference in maint costs.
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Old 05-09-2011, 12:31 PM   #17
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RE: Running single on a twin to save fuel

There have been a number of discussion and for the vast majority a twin engine boats running on one engine uses about the same fuel as running on two.** Sure there are a few boats you can find but no many.** If you ask engine mechanics they will tell you in general single engines are better maintains then twin engines and* the way two engines are better than one is if the engines are completely separate; air, battery, fuel, filters and the boat can steer/make head way with one engine.*
*
A lot has to do with the grade reliability of the engine.* There are some band/reliability of engine you need two or more. Also the HP required and sizing the engine accordingly.* Most twin engine boats are over powered, the exception are long range trawler like Krogan, Norhavn, Selen and a few custom Defevers/Cho Lee.* Many/Most long range trawlers have get home engine/means.* Most use the generator to power a hydraulic package.* So the we have 1 1/2 engines.*
*
I would prefer having two engines to power one prop like some the commercial trawlers, where they can power up one engine while working on the other and/or use the spare engine for parts.*** *
*
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Old 05-09-2011, 12:59 PM   #18
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RE: Running single on a twin to save fuel

Quote:
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I would prefer having two engines to power one prop like some the commercial trawlers, where they can power up one engine while working on the other and/or use the spare engine for parts.*** *
*
*Me too, in a perfect world ....
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Old 05-18-2011, 05:02 PM   #19
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RE: Running single on a twin to save fuel

when i first looked at boats, especially the boat we bought, I thought it was strange so many have twin engine/ propulsion.
not so much due to the HP but as we all know, the power-train on a boat is typically the most expensive to repair and maintain.
redundancy? ok sure, but most of us are coastal cruisers and dont really need that redundancy.
steering-wise there is a definite advantage to twins (sans thrusters) and for powerboats it may be difficult to "pack in" so much HP in one powertrain.
sometimes you see very fast fishing type boats with 3 or 4 o/b engines, guessing its for a similar reason.
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Old 05-19-2011, 04:33 AM   #20
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RE: Running single on a twin to save fuel

" I would prefer having two engines to power one prop like some the commercial trawlers, where they can power up one engine while working on the other and/or use the spare engine for parts.



Me too, in a perfect world ...."

OTS , used on landing craft for at least 60 years , made by Twin Disc , so they are genuine marine trannys.
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