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Old 07-17-2018, 07:22 AM   #1
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Alternating Engines while Underway

Iím new to a twin-engine trawler but have a friend who owns a catamaran sailboat giving me USA (un solicited advice). He only powers up both diesels when departing or arriving or when he anticipates needing extra maneuverability. When in open water, he swears by using only one of his screws. He claims it halves the number of engine hours, lets the one engine operate at a proper load, and significantly reduces his fuel consumption.

Are there counterpoints that arenít apparent to me? Is it a common practice in the trawler community?

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Old 07-17-2018, 07:48 AM   #2
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The only thing I would agree with is it halves the number of engine hours. I do not see how it would let the one engine operate at proper load when now that engine is doing the work of two engines. As far as fuel consumption, you might save a little, but your not going to save half of it. Another thing you could also be damaging the transmissions. Some transmissions require you to lock the shaft if that engine is out.
I don't think this is common practice with any twin engine boat.
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Old 07-17-2018, 08:19 AM   #3
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It does depend on how you would run the engines overall anyway.

Definitely lots of variables on speed, fuel consumption, where in the power curve(s) you are....

Usually one piece of advice with many variables not exactly like your situation is just that....one piece of advice.
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Old 07-17-2018, 08:21 AM   #4
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His cat may have folding props, and folding props don't free-wheel the shaft while underway. That might provide an advantage for one engine underway.
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Old 07-17-2018, 08:25 AM   #5
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The cat has a sail. It's likely also got saildrive type folding propellers. This is key, otherwise the prop still spins, causing drag. A boat like that is designed to operate without an engine running at all. So, yeah, if the winds aren't sufficient it would make sense to only add as much engine as is desired for the conditions.

Be wary of sailors and their tales of savings. Cheap f'ers, will claim all kinds of nonsense, and jump through all sorts of stupid hoops in the name of 'savings'. I joke, a ragbagger can squeeze a nickel so hard you can hear Jefferson scream.

But I suppose, poking along at such slow speeds leaves them with too much free time to dwell on it.

I'm with Ken, powerboats with twins are generally designed to operate with both of them running. Steps may well need to be taken to avoid damaging the transmission and/or shaft bearings by leaving a prop spinning without the engine powering it.
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Old 07-17-2018, 08:30 AM   #6
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We used to do it a bit and for us it did save a lot of fuel. With both engines cruising at 1200 RPM we got 7 knots and a fuel burn of 12 LPH. With one engine at 1200 RPM we got 6 knots and fuel burn of 7 LPH. She didn't handle as well with only one engine operating and in any sort of sea the autopilot struggled a bit because of the asymmetric thrust.
The first thing to do though is make sure your gearboxes are safe to operate freewheeling with one shut down. Some are not rated to freewheel or will have a time limit on them. Our ZF80's where fine but if you have to lock a shaft due to a lubrication or heat issue the drag would be huge.
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Old 07-17-2018, 08:57 AM   #7
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At the same speed through water there is typically little to no advantage.
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Old 07-17-2018, 09:50 AM   #8
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Quote:
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Be wary of sailors and their tales of savings. Cheap f'ers, will claim all kinds of nonsense, and jump through all sorts of stupid hoops in the name of 'savings'. I joke, a ragbagger can squeeze a nickel so hard you can hear Jefferson scream.

But I suppose, poking along at such slow speeds leaves them with too much free time to dwell on it.
Tongue in cheek I suppose, but a very broad brush. I have a sailboat and a trawler. The sailboat cost way more than the powerboat, the sails cost more/mile than diesel for the powerboat, and the sailboat is faster under typical passage conditions.

A catamaran sailboat under power isn't really the same animal as a twin motorboat, unless that motorboat is also a catamaran.
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Old 07-17-2018, 09:56 AM   #9
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Tongue in cheek I suppose, but a very broad brush.
Yes, all in jest.
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Old 07-17-2018, 10:16 AM   #10
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Most but not all Twin Disc gears will tolerate free wheeling and their website has a list. Some gears, like Allison M20, can't be free wheeled at all and the shaft must be locked down.
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Old 07-17-2018, 06:19 PM   #11
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I donít want this to sound mean, but just use the boat the way is was intended. There is little advantage using a twin screw powerboat to save a few percent on fuel and stand to do more damage AND not have full thrust in the direction you need in case of an emergency.
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Old 07-17-2018, 06:46 PM   #12
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like everything in life there is a time to run on one engine and a time to run on two. Normally I run two engines at 10 knots and burn 1.1 gpm. If I want to max my gpm then i slow the boat down to 6.5 knots. Problem now is that I am out of safe operation parameters for my Cat 3208t engines. If I operate just one engine I can maintain proper rpms and temperatures while only going 6.5 knots there by halving my gpm. The cost in this case is time. It’s rare that fuel is more important to me then time. However if I was poor and wanted to get from Seattle to Ketchikan with out buying fuel in Canada I could achieve it by going slow on one engine. What applies to my boat does not necessarily apply to the next boat.
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Old 07-17-2018, 07:07 PM   #13
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My 37' sportfish with 3208Ts was exactly the same way. If I had kept the boat and cruised it, I would have run it single engine a lot.

As it was, I trolled for hundereds of hours at 5 to 6 knots fishing, on a single engine..
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Old 07-17-2018, 08:30 PM   #14
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I would accentuate the beat frequency of the two engines and raise some really awesome fish.


Man I miss trolling....
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Old 07-17-2018, 09:05 PM   #15
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I'm always amused by twin-motor boaters wanting to operate their boats like an unbalanced single, just trying to eke out the best of two worlds.
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Old 07-17-2018, 09:07 PM   #16
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Quote:
Originally Posted by tiltrider1 View Post
like everything in life there is a time to run on one engine and a time to run on two. Normally I run two engines at 10 knots and burn 1.1 gpm. If I want to max my gpm then i slow the boat down to 6.5 knots. Problem now is that I am out of safe operation parameters for my Cat 3208t engines. If I operate just one engine I can maintain proper rpms and temperatures while only going 6.5 knots there by halving my gpm. The cost in this case is time. It’s rare that fuel is more important to me then time. However if I was poor and wanted to get from Seattle to Ketchikan with out buying fuel in Canada I could achieve it by going slow on one engine. What applies to my boat does not necessarily apply to the next boat.
Trust you meant gph (gallons per hour) rather than by minute.
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Old 07-17-2018, 09:25 PM   #17
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Trust you meant gph (gallons per hour) rather than by minute.
You are correct. I got auto corrected and didnít catch it.
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Old 07-17-2018, 10:16 PM   #18
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If you’re engine’s too big why not downsize. Two smaller engines or for the out of the box crowd how bout one small engine on one side and a bigger engine w the prop closser inbd giving colse to asymmetrical thrust w both engines running.
Just say’in .. Just think’in.

Another thought is to only run one engine as a single. That way you probably won’t have to replace both engines at once. Again just think’in.
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Old 07-17-2018, 10:52 PM   #19
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Running two engines when only one is required makes no sense to me, unless the gearbox is not designed for this (most are). Many twin engined boats are overpowered and the extra horsepower does nothing other than creating a big bow wave.

Why not cut 40% off your engine hours and save a bit of fuel as well. Just because a boat was built with two engines, doesn't require you to use them. Many boats have two engines simply to promote sales to the buyer wanting redundancy.

Run two when you need the extra manouverability or power. Go single at other times.
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Old 07-18-2018, 12:46 AM   #20
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Trust you meant gph (gallons per hour) rather than by minute.
Actually I meant gallons per nautical mile. 1.1 gallons per hour at 10 knots in a 54í boat would be very impressive.
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