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Old 24-10-2013, 23:22   #31
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Re: How well do cats motor on one engine?

I have a small power cat (26'), and inside the harbor I just center the engines and adjust rpm to control the boat. But then I think of it as practice for the large power boat that has very minimal rudder control at slow speed.
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Old 24-10-2013, 23:34   #32
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Re: How well do cats motor on one engine?

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Originally Posted by rivonia View Post
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
I normally only use one engine unless I am docking or maybe backing down. I have been switching engines about every two hours but really have no idea I came up with that number. I have two 9.9 outboards in wells and raise one when not in use. Once I got some sea weed on one and switched to the other, that may be the origin of the two hour switching.

Any ideas about switching off on engines and what is a reasonable time for each. I kinda suspect the temperature may be a consideration but may be wrong. I mainly sail so don't have a lot of motoring experience.
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Old 24-10-2013, 23:51   #33
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Re: How well do cats motor on one engine?

So interestingly, with an electric twin drive, you can easily alternatively pulse each motor and therefore maintain a straight course without the drag of a rudder correction.
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Old 25-10-2013, 00:31   #34
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Re: How well do cats motor on one engine?

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Originally Posted by Olivergm View Post
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?
Under 25 knots of breeze 100% throttles and zero helm
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Old 25-10-2013, 02:14   #35
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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.
The post you quoted said:
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Originally Posted by Captain Bill View Post
With a clean bottom mine cruises at 7 knots on two and 6 knots on one.
CRUISES, not max speed. If it was max speed, you might have a point. Maybe. Depending on just how much fuel efficiency deteriorated at max throttle.
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Old 25-10-2013, 02:23   #36
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Re: How well do cats motor on one engine?

Quote:
Originally Posted by Olivergm View Post
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?
Typically leave the helm centered and just use the engines. Occasionally helm if for some reason need to rotate more aggressively e.g. strong cross winds. Certainly while teaching others to maneuver a twin-engined boat I focus on engines and leaving the helm centered. Only think about helm if engines alone will not suffice.

In very light to no winds, as a trick you can also make the boat move sideways by steering counter to the engines if your rudders are behind your props. For example port eng slow fwd, stbd eng slow reverse, steer to port will tend to crab the boat sideways to starboard.

Mark.
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Old 25-10-2013, 03:44   #37
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Re: How well do cats motor on one engine?

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Originally Posted by tomfl View Post

Any ideas about switching off on engines and what is a reasonable time for each. I kinda suspect the temperature may be a consideration but may be wrong. I mainly sail so don't have a lot of motoring experience.
Tom, 5 minutes is reasonable. Shutting off engines imediately after working hard will also shut off the cooling system (blower, impeller, turbo) and cause possible engine damage.
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Old 25-10-2013, 05:38   #38
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Smile Re: How well do cats motor on one engine?

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Originally Posted by tomfl View Post
I normally only use one engine unless I am docking or maybe backing down. I have been switching engines about every two hours but really have no idea I came up with that number. I have two 9.9 outboards in wells and raise one when not in use. Once I got some sea weed on one and switched to the other, that may be the origin of the two hour switching.

Any ideas about switching off on engines and what is a reasonable time for each. I kinda suspect the temperature may be a consideration but may be wrong. I mainly sail so don't have a lot of motoring experience.
Very good question. Diesel engines love to be worked hard. However the general rule is as follows; Always allow the engine to warm up before engaging the drive, this could be around 2 mins. When finished with the engine ALWAYS allow it to tick over in idle for approx 5+ mins. The main reason is o that you are not turning it off whilst it is very hot and it will also not leave residual acid in the rings.

Peter
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Old 25-10-2013, 06:45   #39
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Quote:
Originally Posted by 44'cruisingcat View Post
The post you quoted said: CRUISES, not max speed. If it was max speed, you might have a point. Maybe. Depending on just how much fuel efficiency deteriorated at max throttle.
Yep, clear. Running the angine at max efficiency is the way to go. All the excess power (the other engine) is only needed during close quarters manouvering or bad weather.
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Old 25-10-2013, 06:51   #40
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Re: How well do cats motor on one engine?

On long motoring passages I typically change engines every 12 hours. On shorter than 24 hr passages I just divide the trip in two. I've actually only had a few times I had to motor for more than two days. I check fluids in the cold engine about 20 minutes before the switch over and warm it up for 5-10 minutes before advancing the throttle. I then idle the original engine for 5 to 10 minutes before shutting it down.

I mostly use the engines for close maneuvering, but I will use rudder in the direction of turn to speed up my rate if needed and against the engines if I need to walk her sideways a bit.

I have a rudder position indicator.
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Old 25-10-2013, 07:04   #41
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Re: How well do cats motor on one engine?

Quote:
Originally Posted by Olivergm View Post
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?
Have to agree with Factor. 100% throttles
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Old 25-10-2013, 08:45   #42
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I have nothing to say. All you other cat people have already said it all. And we all agree.

Actually, It is fantastic to see such a beautiful consensus without any bitching - ok, ok, apart from from one lone mono bloke who interceded briefly.
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Old 25-10-2013, 09:24   #43
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Re: How well do cats motor on one engine?

Quote:
Originally Posted by Olivergm View Post
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?
I was always taught you should never approach a dock any faster than you are willing to hit it. I suspect most of us really slow down a lot when we are approaching a dock which results in a decrease helm control. Bottom line coming to a dock is probably almost all throttle because of the reduced effect of of the helm. So even if you turn the wheel or push a tiller it may not change the boats course much.

On the other hand leaving a dock, or a mooring ball, I will often have a course in mind, or a boat or piling I need to avoid so I will pre adjust the rudder so when I put both engines in forward (even if one of the engines has more throttle than the other) both the rudders and throttle make the boat more agile.
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Old 25-10-2013, 09:35   #44
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Re: How well do cats motor on one engine?

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Old 25-10-2013, 09:38   #45
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Re: How well do cats motor on one engine?

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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.
This was standard on Manta's - I don't know about others, but for Volvo saildrives it is only a matter of switching the shift cable attachment to the other side of the shift lever and using a different-handed prop.

If you have fixed props now, that is contributing a lot to your helm offset because that prop is responsible for 0.5kts of drag on that side that must be compensated.

Mark
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