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Old 27-11-2016, 00:22   #1
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Motoring with 1 or both engines

I would like to hear from both camps here as to why you motor with one engine or why you motor with both engines.

Sitting on a fence
Thanks in advance
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Old 27-11-2016, 00:45   #2
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Re: Motoring with 1 or both engines

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Originally Posted by Code Blue View Post
I would like to hear from both camps here as to why you motor with one engine or why you motor with both engines.

Sitting on a fence
Thanks in advance
Simple really, one engine pushes the boat at say 6 knots 2 normally only 1-2 knots faster but you use double the amount of fuel and more engine hours for a gain of 15 - 25 % in speed (this depends on the boat , size of motors, hulls etc but as a non specific question this is the answer) Some cats have too smaller motors to only use one motor but generally most have enough grunt to use one at a time.
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Old 27-11-2016, 01:29   #3
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Re: Motoring with 1 or both engines

Like aclmck says.

For the vast majority of multihulls, you only need the full power of both engines in adverse conditions and when manoeuvering. The rest of the time one engine at higher revs is likely to be just as fuel efficient as two engines at lower revs and halves your engine hours.

Of course, that is up to a point - if you have to push one engine hard to make your desired speed, then two engines at lower revs may well be more fuel efficient. You need to know your individual boat to determine where the fuel efficiency cross-over point is and trade that off against increased engine hours.
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Old 27-11-2016, 01:50   #4
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Re: Motoring with 1 or both engines

In my case, in light conditions, my fuel consumption is about:

5.5 knots: 1 engine = 2.5 lph, 2 = 3 lph
6 knots: 3.5 lph either way
6.5 knots: 1 engine = 5 lph, 2 = 4.8 lph
7 knots: 1 engine = 7.7 lph, 2 = 6 lph

So one engine makes sense until I want to push over 6.5 knots or have adverse conditions requiring more power.

For my engines, that 6.5 knots on one is around the 80% RPM sweet spot which diesels like to run at and I base my rule of thumb on RPM, more than speed. I run with one engine until I need more than 2800 RPM to achieve my objective.
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Old 27-11-2016, 02:07   #5
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Re: Motoring with 1 or both engines

Stu, thanks for those numbers. I am a bit surprised because typically, engines are at their best specific fuel consumption at higher power settings. Could it be that the asymmetric thrust effect is somehow more pronounced at higher speed? Does your (single engine) helm correction increase at higher speeds?

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Old 27-11-2016, 03:18   #6
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Re: Motoring with 1 or both engines

I've never really analysed it but certainly: the faster I go, the larger the differential between the required HP on 1 vs 2 engines.

I suspect the major factor is that my fixed, two blade props are a lot less efficient at higher RPM and we need quite a lot more HP with one fast rotating prop compared to two slower rotating props to get the same effective thrust (any prop's efficiency decreases with both increased RPM and increased input power)
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Old 27-11-2016, 03:49   #7
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Re: Motoring with 1 or both engines

Lagoon 450, Yanmar 54hp, 2800RPM. 2 eng 8 kts, 1 eng 7 kts, rudder displaced about 3-4 degrees. Not just the fuel savings but halving engine hours so hourly maintenance savings.
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Old 27-11-2016, 04:15   #8
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Re: Motoring with 1 or both engines

Quote:
Originally Posted by StuM View Post
In my case, in light conditions, my fuel consumption is about:

5.5 knots: 1 engine = 2.5 lph, 2 = 3 lph
6 knots: 3.5 lph either way
6.5 knots: 1 engine = 5 lph, 2 = 4.8 lph
7 knots: 1 engine = 7.7 lph, 2 = 6 lph

So one engine makes sense until I want to push over 6.5 knots or have adverse conditions requiring more power.

For my engines, that 6.5 knots on one is around the 80% RPM sweet spot which diesels like to run at and I base my rule of thumb on RPM, more than speed. I run with one engine until I need more than 2800 RPM to achieve my objective.

+ 1 ,

exactly the same stats (Honda 20hp) for my Seawind 1160, the only point I would add is that 1 engine put more strain if you are hand steering to keep a straight line and definitely makes the autopilot work harder. Also, a cat is almost impossible to dock in a tight marina space with just one engine so alway use 2 engines departing and entering into the marina.
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Old 27-11-2016, 05:05   #9
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Re: Motoring with 1 or both engines

Usually I use one on passage and both in close quarters.
Important, alternate in which of the two you use from time to time.

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Old 27-11-2016, 06:11   #10
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Re: Motoring with 1 or both engines

I usually use one engine cruising on a passage, but I find that if I'm going into a stiff head wind that two engines at lower RPM work better though at slightly greater fuel consumption. My theory is that the surface area of two props is more effective under heavy loads.
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Old 27-11-2016, 09:04   #11
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Re: Motoring with 1 or both engines

I use single engine ( Yanmar 40hp) only if the sea is flat. If there is any sea running, the waves throw the boat out of course much easier than when using both engines. Than, under one engine, it takes more effort and time for the autopilot to bring the boat back on course. There is a similar problem when the wind is in front.
We are not very scientific about it. When the boat starts yawing a lot we deploy the second engine.
Under the described conditions we save about 25% of the fuel and of course 50% engine hours.
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Old 27-11-2016, 09:09   #12
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Re: Motoring with 1 or both engines

I had read somewhere that "it's better" for the engines to run diesels at higher revs, at least some of the time. Is there any truth to that?
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Old 27-11-2016, 09:15   #13
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Re: Motoring with 1 or both engines

Yanmar recommends in the engine manual to run it at full throttle for several minutes every month or so. I do not remember the exact details but basically speaking the answer to your question is positive.
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Old 27-11-2016, 09:40   #14
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Re: Motoring with 1 or both engines

Assuming you are talking a cat, I used one motor a lot. Why? Because I almost always motorsailed if motoring and just used the lee motor allowing the boat to point an easy 15 degrees higher.
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Old 27-11-2016, 09:48   #15
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Re: Motoring with 1 or both engines

Running 1 engine on a catamaran is the reason they are one of the most efficient boats ever made.

With folding props it even gets better speed and straight tracking as the off engine has very little drag.

Our 44 footer has 2 of the VP 55 hp engines with folding props.

We do 6.5 to 7 knots on 1 engine at 2000 rpm using 1 gallon (3.78 liter) per hour.
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