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Old 24-10-2013, 16:53   #16
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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
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.
I read in the post that 7 knots was the cruise speed not max speed. Our max speed on one engine is 8 knots, 10 knots with both at full. We motor at 2200 to 2400 rpm's at 6 to 7 knots with one engine. Starting the second and putting it to the same rpm will only produce 1.5 to 2 knots of extra speed.

Regardless, I don't think you will find many cat owners who motor with both for long distances. We do run one for much longer periods, say 10 hours, then switch, when on long passages.

It takes about 10 degrees of rudder angle to correct the single engine push. The autopilot does have to work harder also. But it's 12 years old now and never had a problem so it must be able to handle it.
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Old 24-10-2013, 16:55   #17
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Re: How well do cats motor on one engine?

Okay so 44Cruising Cat has a few years of history and records and real life experience on his boat, and Jedi has theory on his side.

So after careful consideration I think I will go with 44C and say - JEDI, I will have some of that beer bet.

BTW, 44Cs experience is a mirror for my experience on my boat.
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Old 24-10-2013, 17:01   #18
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Re: How well do cats motor on one engine?

Maxing one engine on our boat is still much more fuel efficient than running two engines to reach the same speed. About 40% more. The boat gets up to a certain speed very easily, which just happens to be the max cruising speed of one engine. It requires a lot more power to go faster, and with two engines at full cruising speed only gains another 1-1.5 knots.

We only need 3 degree of rudder angle to correct a single engine - but I suppose the engines on a 56' cat are further apart than ours and would require more correction. Our autopilot never knows the difference and doesn't noticeably work any harder at all.

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Old 24-10-2013, 17:07   #19
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Re: How well do cats motor on one engine?

for our lagoon 380 2 30hp yanmars w/ folding props we motor on one engine at
2200 RPM /2litres/hr @5..5 knots
3000 RPM /3 litres/hr @7 knots

2 engines 3000 RPM 8 knots, not sure on fuel usage but I guess around 6l/h

also I wouldn't recommend running for an hour then alternating. Diesels like to be run long distance, not stop start, so run at least 8 hrs a side before switching
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Old 24-10-2013, 17:32   #20
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Re: How well do cats motor on one engine?

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We only need 3 degree of rudder angle to correct a single engine ...
Coincidentally, my single-engined, mono-hulled boat requires a 3 degree port rudder to counter the starboard propwalk in forward.
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Old 24-10-2013, 17:42   #21
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Re: How well do cats motor on one engine?

We only use one engine at 2400 rpm at 7.5 knots to cruise with a VP 30 hp engine with sail drive.

We have done 2400 rpm on both engines at 10 knots, but only once in 5 years.

We have VP folding props which make one engine motoring much better.

Only use two engines to anchor, dock or turn with no boat speed.
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Old 24-10-2013, 18:09   #22
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Re: How well do cats motor on one engine?

I've been reading a lot of boat test results on power catamarans. Every review that included single engine data showed increased economy at same speed on a single engine.

I think this is the 1st time I didn't agree with Jedi, sorry Nick.

One of the offshore capable powercats I was looking at addressed this issue by designing a new hull, single engine power proa where the engine hull is larger than the opposite hull. Did away with the added drag of the other's hull due to no appendages, i.e. rudder, prop, shaft, and supports. The hulls were shaped in a way as to allow no rudder input at any speed for steering correction, tracks straight. The powercat that I'm referencing is the Aspen C90, and even though 5 nmpg among sailboats under power isn't a good number, in a powerboat running at 15 kt is about as good as it gets. Larry Graf the designer and owner of Apen Catamarans in the PNW, is the same guy who started Glacier Bay Catamarans, who have sold more powercats than any other company.
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Old 24-10-2013, 18:26   #23
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Re: How well do cats motor on one engine?

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Originally Posted by markpierce View Post
Coincidentally, my single-engined, mono-hulled boat requires a 3 degree port rudder to counter the starboard propwalk in forward.
The rudder direction for us would depend on which engine is used. On the other hand, we have counter-rotating props, so I could probably switch them so that their propwalk would counteract their outboard placement and require no rudder correction at all.

Right now they are setup to counteract each other when both are in use, but we rarely use them that way.

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Old 24-10-2013, 19:31   #24
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Yes, I'm wrong. I had read the post as the boat not able to go faster than 6 knots with one engine, but it indeed says "cruise". The whole wording sounded like max. speed so I assumed that. I still think it is what was meant but I'll give in this time as I'm not so good when words and their meanings aren't clear

The whole theory is that fuel consumption is related to the amount of power required to propel the boat at a speed (i.e. Overcome the drag of the boat). When you switch from one to two engines, they share the load, each does half, except you get the losses (crankshaft bearings, prop etc.) twice. When both engines operate in their peak efficiency on their power curve, while one engine is at maximum load, then you can compensate for the double losses in bearings etc. by the gain in efficiency.

When the non-running prop is a folding type and especially when the one engine running is in a good efficiency range, then one engine always wins from two engines, like stated by the posters here.
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Old 24-10-2013, 20:50   #25
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Re: How well do cats motor on one engine?

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Originally Posted by colemj View Post
The rudder direction for us would depend on which engine is used. On the other hand, we have counter-rotating props, so I could probably switch them so that their propwalk would counteract their outboard placement and require no rudder correction at all.

Right now they are setup to counteract each other when both are in use, but we rarely use them that way.

Mark
Very interesting - I've never thought about that. Great idea. Do any manufacturers ever do this? I also wonder if my rudder correction is more pronounced depending on which is running.

I'm going to put new feathering props on in two years (once I leave the Med) and could do this. According to the mechanic who installed my Yanmars the rotation can easily be reversed.
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Old 24-10-2013, 20:56   #26
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Re: How well do cats motor on one engine?

Nick,

You might be surprised how rotating mass can add up. When I was on the drill ship I conducted a test on fuel consumption using (7) 4.6 mw gen sets. Had a long period of only 2 mw of loads which for safety would be running (2) gen sets lightly loaded. I asked the engineer to record fuel consumption and then switch to one gen set and check consumption. Then had him bring all 7 on line. Takes a lot of fuel to spin those rotating assemblies at 900 rpm.
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Old 24-10-2013, 21:35   #27
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Re: How well do cats motor on one engine?

I may not have made clear that my engines are cruisng at 2950 RPM and are rated at 3600 RPM Maximum continious power and 3800 RPM for 1 hr. 2950 is right at the maximum specific fuel consumption rating for these engines (Grams per hour per HP). Running them slower or faster burns more fuel per HP produced so that's where I run them. Of course there's not a whole lot of difference 100 rpm either side of that. My maximum speed on two engines is about 8.7. I have autostream feathering props which have flat blades and are not very efficient. In addition the faster you turn them the less efficient they get.
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Old 24-10-2013, 21:46   #28
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Re: How well do cats motor on one engine?

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I may not have made clear that my engines are cruisng at 2950 RPM and are rated at 3600 RPM Maximum continious power and 3800 RPM for 1 hr. 2950 is right at the maximum specific fuel consumption rating for these engines (Grams per hour per HP). Running them slower or faster burns more fuel per HP produced so that's where I run them. Of course there's not a whole lot of difference 100 rpm either side of that. My maximum speed on two engines is about 8.7. I have autostream feathering props which have flat blades and are not very efficient. In addition the faster you turn them the less efficient they get.
Yep, slip is a loser. Our prop's pitch is measured in feet, not inches, and max rpm is 118. Our slip is a single digit percentage.
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Old 24-10-2013, 21:53   #29
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Re: How well do cats motor on one engine?

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Originally Posted by colemj View Post
The rudder direction for us would depend on which engine is used. ...
Found having a rudder indicator a worthwhile investment.

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Old 24-10-2013, 23:10   #30
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Now that there seems to be consensus on the fuel consumption I have another question. When maneuvering in close quarters (ie parking) how much is done via the engines (ie throttles) and how much via the helm? I'm guessing 80% throttles when pulling alongside?
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