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Old 24-10-2013, 06:08   #1
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How well do cats motor on one engine?

How well do twin- engine multihulls motor on one engine?
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Old 24-10-2013, 06:12   #2
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Re: How well do cats motor on one engine?

Hi,

they do motor very well - in fact, we always use only one engine when motoring, because it gets fuel consumption way down. However, you do need both for close quarters, i.e. when docking or anchoring.

An exception to the above is when it's blowing like mad - I just prefer to have both engines running at times where I don't ever want to be stuck without at least one engine.

Oliver
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Old 24-10-2013, 06:14   #3
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Re: How well do cats motor on one engine?

Only ever use both when docking, when cruising, one motor sails up, using the motor to make apparent.I would have done the majority of my engine hours (which are not much as a proportion of the usage hours) on one motor.
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Old 24-10-2013, 06:57   #4
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Re: How well do cats motor on one engine?

With a clean bottom mine cruises at 7 knots on two and 6 knots on one. The bottom is dirty at the moment and I get 5.3-5.4 on one and 6.3 on two. So basically I use half the fuel for 1 knot difference in speed. The autopilot works a bit harder on one. If I look at the wake it is clearly not as straight as on two. In a strong head wind two engines definitely work better than one.
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Old 24-10-2013, 07:13   #5
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With a clean bottom mine cruises at 7 knots on two and 6 knots on one. The bottom is dirty at the moment and I get 5.3-5.4 on one and 6.3 on two. So basically I use half the fuel for 1 knot difference in speed. The autopilot works a bit harder on one. If I look at the wake it is clearly not as straight as on two. In a strong head wind two engines definitely work better than one.
Hold on a sec... 7 knots on two and 6 knots on one does NOT mean twice the fuel for 1 knot extra at all! It means that the engine have much less load when both are in use and thus each uses much less fuel than the one doing it alone.

You would need to measure fuel consumption , not speed, to find fuel savings. I think you may be surprised to find that fuel economie at 6 knots is better with two engines than it is with one.
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Old 24-10-2013, 08:13   #6
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Hold on a sec... 7 knots on two and 6 knots on one does NOT mean twice the fuel for 1 knot extra at all! It means that the engine have much less load when both are in use and thus each uses much less fuel than the one doing it alone. You would need to measure fuel consumption , not speed, to find fuel savings. I think you may be surprised to find that fuel economie at 6 knots is better with two engines than it is with one.
I concur
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Old 24-10-2013, 08:43   #7
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Re: How well do cats motor on one engine?

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Originally Posted by s/v Jedi View Post
Hold on a sec... 7 knots on two and 6 knots on one does NOT mean twice the fuel for 1 knot extra at all! It means that the engine have much less load when both are in use and thus each uses much less fuel than the one doing it alone.

You would need to measure fuel consumption , not speed, to find fuel savings. I think you may be surprised to find that fuel economie at 6 knots is better with two engines than it is with one.
Maybe not, but I base my estimates on engine hours per gallon. Distance of course varies a bit with conditions. One other complication is that my genset uses fuel from only one of my tanks and genset consumption is a bit hard to factor in. I like to refuel at about 80 engine hours and when I do so I put in 58-60 gallons of fuel. Since conditions are never exactly the same it varies a bit. I went a bit past 80 hours last tank and had a lot of genset time. I estimated it would take a bit over 70 gallons to fill it. It took 70.8 so I guess I have a pretty good handle on fuel consumption. You may be right on lower speeds on two engines consuming less fuel, but it would mean running outside of the recommended operating RPM range for the engines and it would mean more wear and tear on the engines. The engine hour meters are not calibrated to RPM, they are simply timers. It also puts less wear and tear on my SD40 hunk of junk sail drives. One of the other factors is that it takes a certain amount of fuel to overcome the internal friction of the engines and transmission. If you are turning two you are using more fuel for that. My boat cruises at 6 at 2500 rpm on two engines and 6 at 2950 on one engine. I seriously doubt that an extra 350 rpm on one engine doubles the amount of internal friction I have to overcome. I think it probably averages out with the required rudder drag to keep her going straight. The only real way to tell would be for me to install fuel flow instruments, but since I have a pretty good handle on my usage I'm not going to waste my money.
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Old 24-10-2013, 08:46   #8
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Re: How well do cats motor on one engine?

We motor on one engine all the time and keep meticulous track of our fuel usage. Motoring on one engine not only uses less fuel (not half, though, unless both engines are at full rpm) but it also doubles maintenance intervals and cuts engine hours in half. However, there is no free lunch - you must do twice the maintenance when it is time!

In other words, running two engines at 1400rpm is not equivalent to running a single engine at 2800rpm. We maintain 6kts with one engine at ~2600rpm, while that speed will require two engines at ~2300rpm. That 6kts on one engine is pretty much top speed even at 3000rpm, while we can go above 7kts with two engines at 3000rpm.

Sea state changes the equation some toward more equality for rough head seas, but we are mostly motoring in flat seas.

Mark
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Old 24-10-2013, 08:50   #9
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Re: How well do cats motor on one engine?

If you use your generator with a similar load on it most of the time, the generator fuel consumption is easy to factor because the rpm is constant. If the load is constant also, the fuel consumption is constant for any given time.

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Old 24-10-2013, 09:43   #10
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Re: How well do cats motor on one engine?

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If you use your generator with a similar load on it most of the time, the generator fuel consumption is easy to factor because the rpm is constant. If the load is constant also, the fuel consumption is constant for any given time.

Mark
While the RPM is constant, on my boat at least the load is all over the place. I could be bulking up the batteries, washing clothes, heating water, charging the rechargables, heating the cabin, running the microwave etc. Most of these things are on demand and often in different combinations.

My speed figures are for smooth water-no wind conditions. That is why I calculate gallons per engine hour, not miles per gallon. There is probably a bit of a penalty for running on one engine. I find that 80 engine hours on two engines requires me to put in closer to 58 gallons of fuel and on one engine closer to 60. Since I'm not always exactly on 2950 rpm and never exactly 80 hours it's a bit hard to tell, but it's close enough to half of the fuel consumption to make me happy.
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Old 24-10-2013, 09:57   #11
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Re: How well do cats motor on one engine?

We sometimes use one engine if we are on a long passage with no wind and every hour switch to the other engine. We like to keep both engine hours the same.

We usually cruise at between 4 or 5 knots and we use approx 1.5 litres of diesel an hour per engine

Peter
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Old 24-10-2013, 09:57   #12
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Re: How well do cats motor on one engine?

Quote:
Originally Posted by colemj View Post
We motor on one engine all the time and keep meticulous track of our fuel usage. Motoring on one engine not only uses less fuel (not half, though, unless both engines are at full rpm) but it also doubles maintenance intervals and cuts engine hours in half. However, there is no free lunch - you must do twice the maintenance when it is time!

In other words, running two engines at 1400rpm is not equivalent to running a single engine at 2800rpm. We maintain 6kts with one engine at ~2600rpm, while that speed will require two engines at ~2300rpm. That 6kts on one engine is pretty much top speed even at 3000rpm, while we can go above 7kts with two engines at 3000rpm.

Sea state changes the equation some toward more equality for rough head seas, but we are mostly motoring in flat seas.

Mark
We see similar ratios on our Catana 48 with two 35hp beta engines. One engine at 2600 rpm is using more fuel per engine than two at 2300 (or 2600 for that matter) but significantly less in total.

So if range/consumption is at all a concern we motor on a single engine at 2400 RPM at 5.5 knots. If it is not a concern or we are bucking headwinds rather than motoring through calm seas, we'll use both engines at somewhere between 2200 and 2800 RPM depending on circumstances.

Mark.
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Old 24-10-2013, 10:04   #13
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Re: How well do cats motor on one engine?

Mine motored well on one engine, spent a lot of time motor sailing with the lee engine running. They dont manuever at all with one engine in close quarters though!
Regarding fuel consumption, motors dont run for free even when unloaded.... there is a minimum fuel burn even with no load.
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Old 24-10-2013, 14:01   #14
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Re: How well do cats motor on one engine?

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Hold on a sec... 7 knots on two and 6 knots on one does NOT mean twice the fuel for 1 knot extra at all! It means that the engine have much less load when both are in use and thus each uses much less fuel than the one doing it alone.

You would need to measure fuel consumption , not speed, to find fuel savings. I think you may be surprised to find that fuel economie at 6 knots is better with two engines than it is with one.
Oh yeah, I'd be surprised all right!

We use considerably less fuel per mile on one motor that we do on two.
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Old 24-10-2013, 16:31   #15
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Oh yeah, I'd be surprised all right! We use considerably less fuel per mile on one motor that we do on two.
The example was that max speed on one engine is 6 knots. This means the engine is maxed out and this normally is far beyond it's good fuel economy numbers. With two engines maxed out, the max speed was 7 knots. What I said is that, for that boat with these measured parameters, you would probably get better fuel economy when running two engines to do 6 knots, then using one engine to do 6 knots.

In your case you would need to go full throttle with one engine and measure fuel economy, then add 2nd engine and throttle both back to same speed that you got with the one maxed out engine and then measure again.

We can make it a bet for a beer I'm pretty sure the one maxed out engine will use more fuel than the two engines in this situation.
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