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Old 17-08-2010, 18:49   #1
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Using Only One Engine ?

On multihull sailboats, when motoring, one engine only is often used to save fuel. Is the same done on powerboats (ie Grand Banks trawlers) with twin engines?
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Old 17-08-2010, 18:58   #2
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On multihull sailboats, when motoring, one engine only is often used to save fuel. Is the same done on powerboats (ie Grand Banks trawlers) with twin engines?
Delivered a Monk 42 with twin Cummings 6's across the Trent-Severn system a few years back.
Can't remember the speeds now, but do remember that at 1800rpm we were only a 1/2 knot faster on two engines than on one.
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Old 21-08-2010, 15:45   #3
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I'm surprised to hear that's so common on Multihull Sailboats - considering they are likely saving what, maybe 1/2 a gallon per hour on a larger boat? Even less on a smaller one? Do people really do that? The only time I use one engine on a powerboat is in fairways in marinas or in idle speed/no wake zones when the boat I'm running creates a wake at idle. This is never the case in trawlers. At idle they are slow. at cruise, they aren't much faster but I love 'em. And the beauty is, if you shut one down, you're still only saving maybe 2gph... The speed lost will diminish the savings even more. The time you really see fuel burn is at the higher RPMs, when you'd lose even more efficiency by fighting one screw with the rudders...or, if it's a planing boat, you couldn't do it even if you wanted to. I don't see the benefit.
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Old 22-08-2010, 10:43   #4
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I'm surprised to hear that's so common on Multihull Sailboats - considering they are likely saving what, maybe 1/2 a gallon per hour on a larger boat? Even less on a smaller one? Do people really do that? ....
Yes they do, especially with folding props. In addition to the fuel savings engine run time is reduced 50%.
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Old 22-08-2010, 12:45   #5
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Hm. Interesting.
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Old 22-08-2010, 13:36   #6
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1700 rpm-2 eng 7.2 kts . 1 eng 1700rpm 5.3 kts. Fuel burn 120 lehmans 2 gal /hr per eng.
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Old 22-08-2010, 13:57   #7
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Perhaps these one engine cats are actually boosting Sailing speed in light airs, not serious propulsion, just running closer to natural hull speed by using one engine at low power setting. Plus the bonus off battery charging for those that haven't got around to solar cells yet.
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Old 22-08-2010, 14:09   #8
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I run only 1 engine 90% of the time when Im motoring - I save about 1 gph in fuel ,along with wear and tear, and maintenance, this adds up a lot when your paying $10 a gal for diesel and $10 a quart for oil and the longevity of the engines themselves- With 1 engine I cruise at 7.5 knots (or less) and with 2 engines 11 knots or less-
That 10% of the time I use both engines is usually going into a strong head wind
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Old 25-08-2010, 08:14   #9
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Wow.. yeah, if I was paying $10/gal for diesel or the same for a quart of oil, I guess I'd be running with one engine too. I suppose it's safe to assume the hourly fuel burn does not get cut directly in half by cutting one engine, as you're creating a certain amount of inefficiency by running an offset engine - then you have the speed loss, in which case it obviously takes you longer/more fuel to get somewhere - however this does not get cut directly in half either - so there is some savings/benefit. I'm curious what the net benefit is. Ram - what do you normally burn per hour with both running? I'm assuming it's more than 2gph?
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Old 25-08-2010, 08:31   #10
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Wow.. yeah, if I was paying $10/gal for diesel or the same for a quart of oil, I guess I'd be running with one engine too. I suppose it's safe to assume the hourly fuel burn does not get cut directly in half by cutting one engine, as you're creating a certain amount of inefficiency by running an offset engine - then you have the speed loss, in which case it obviously takes you longer/more fuel to get somewhere - however this does not get cut directly in half either - so there is some savings/benefit. I'm curious what the net benefit is. Ram - what do you normally burn per hour with both running? I'm assuming it's more than 2gph?
Thats hard to say in real life as the amount of fuel burned is different with changing wind/sea conditions- I would say about 40-50% less fuel is used- in that area- Can do 7.5 knots at 90% power on 1 engine and with 2 engines about 10-11 knots- 1gph each engine at those speeds- I normally run 1 engine at around 5/8 speed and get about gal per hour-speed about 5.5 knots- this is on flat water-
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Old 09-11-2010, 17:31   #11
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Before you shut down one engine and run on the other, make sure the transmission can handle the freewheeling without being damaged. Some are designed to do that but others are not.
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Old 09-11-2010, 18:12   #12
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Yes they do, especially with folding props.
Unfortunately cost of efficient folding prop Vs fixed, on a powered vessel, could negate any savings made
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Old 09-11-2010, 18:16   #13
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Before you shut down one engine and run on the other, make sure the transmission can handle the freewheeling without being damaged. Some are designed to do that but others are not.
ZF reckon mine are OK to do that (ZF25) but I wonder about the shaft bearings without forced water lubrication?
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Old 10-11-2010, 10:34   #14
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Unfortunately cost of efficient folding prop Vs fixed, on a powered vessel, could negate any savings made
Go back and read post #4 - my response was to a question about sailing cats.

However, I actually am considering installing Autoprops on a power cat. Overall efficiency/performance improvement may prove to be worthwhile.
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Old 10-11-2010, 18:10   #15
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Simple answer to the original question, NO!

A multihull sailboat maybe able to cope with only one engine running but on a monohull motorboat the rudders are MUCH smaller and so the helm would need to be almost hard over to maintain a straight line.

I personaly own a 57ft flybridge motoryacht fitted with twin Detroit Diesels and have tried running her on one engine just to see how she would handle should one fail at sea. Let me tell you that keeping on your desired course like that is almost impossible and you can pretty much only make a course correction in one dirrection, ie, to the opposite direction of the running engine.

These types of boats are not designed to be opperated in this way and usually have very small rudders that are only really effective at a reasonable speed. For example on mine, anything below 4-6 knots, the helm is pretty much useless and you can only steer by useing directional thrust from each engine in turn.
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