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Old 05-02-2014, 07:20 AM   #1
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running on one engine

I have a 34T with twin 240 yanmar engines.

When I fish for groupers I drift over my area and then turn on only one engine to get back to original spot and drift again.

When I do long distance cruising I go ~1600 rpm at ~ 8 mph ( my trawler speed) with both engines only.

A little crazy thinking here, but has anyone considered or tried, for fuel economy reasons, running on only engine at trawler speed?
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Old 05-02-2014, 07:29 AM   #2
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Greetings,
Mr. j. Although they may be tough to find and possibly buried in a remotely related thread there are a few "in depth" discussions regarding this option. Try the "search" feature. ALMOST as controversial as, dare I mention (gasp) ancho......Nope, can't do it.
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Old 05-02-2014, 08:45 AM   #3
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Yes it can be more economical, do a search.
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Old 05-02-2014, 10:43 AM   #4
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I've checked quite a few consumption tables on this issue and my takeway was that there was really no benefit at normal cruise speeds however there was a benefit when you slowed to very slow speeds.

I attach a great table from another post. Grand Banks "Dreamer" which shows comparisons against port/starboard and dogging the props. A lot of data there.

To make my point, the GB does 1.6nmpg at 7.2 knots on two engines (1500RPM) and around 1.3nmpg on stbd engine only (2000RPM!). Not much in it.

At 5.7 knots she gets 2.38nmpg on 2 engines (1200RPM) and 2.4nmpg on stbd engine only (1500RPM). No benefit.

At slower speeds, say 1200RPM on stbd engine only, NMPG was 3.4nmpg at 4.1 knots but there is no data for two engines at the same speed (perhaps below idle). So one engine seems a good idea, but only at less than say 5 knots and probably not accounting for wind/sea etc. At least based on this data.

The 'penalty' for spinning a second engine is easy to measure. For those with a fuel flow system, simply rev up to cruise RPM in neutral and measure the engine unloaded fuel flow. It will most likely be less than 1gph. That's the energy cost of spinning the engine. The extra fuel burn when you put in gear is all going to the prop so once the energy cost of spinning the second engine is paid, there is not much benefit beyond that.

The only other factor is to look at BSFC (engine specific fuel consumption) which shows that engines are more efficient at producing power at around mid RPM engine range and with a well matched prop/hull that does not load up the engine too soon. On my engine (attached) BSFC is best at 1600-1800RPM even though full throttle is 2200. However on one prop only I cant make 2200 as the engine becomes overloaded before it gets there. And so running on one engine at higher than 1800 RPM puts that engine at a disadvantage as it is pulling harder, is lower on the efficiency curve and suffers a lower BSFC. It follows that running both engines at a better point on the BSFC curve, gives them an advantage which partially compensates for spinning the extra machinery.

To confirm, I note that the new Nordhavn's now have twin engines in the bigger models and that the designers are quoted as saying there is 'only a 10% difference' in fuel burn over the equivalent single engine models. It makes the case for two engines pretty well and is another way of saying that dragging a wing engine through the water without ever really using it, indeed does have a cost.

So if you are going to maintain 'trawler speed' I'd leave both engines on.

PS> Once I get my fuel flow sensors calibrated, I will publish data for my own (50 ton) case and make a comparison.
Attached Files
File Type: pdf Spec Sheets - Cat 3306B DITA Propulsion.pdf (549.8 KB, 27 views)
File Type: pdf GB Dreamer Fuel Study.pdf (74.3 KB, 32 views)
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Old 05-02-2014, 01:09 PM   #5
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What does the manufacturer say about free wheeling the reverse gear? It may be OK for short periods only. Also the stuffing box is a concern if it is water injected, you will need a cross over hose from the running engine to the free wheeling one.
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Old 05-02-2014, 01:58 PM   #6
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What does the manufacturer say about free wheeling the reverse gear? It may be OK for short periods only. Also the stuffing box is a concern if it is water injected, you will need a cross over hose from the running engine to the free wheeling one.

Twin Disc are OK with backdriving or windmilling as long as:

"
Start the engine and operate the marine transmission in
neutral at normal fluid pressures for five minutes, doing this
once every eight hours. Maintain the backdriven marine
transmissions oil level at the full mark on the dipstick."

See attached extract about windmilling.

I have a watercooled stern gland and I imagine you would want to check it occasionally with an infrared gun if you were running with no water flow. I think as RPM will be lower when windmilling, temps should be no problem.


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Old 05-02-2014, 02:24 PM   #7
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Two can play at that game, sir. ;-)


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Old 05-02-2014, 03:24 PM   #8
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Too much worry about fuel savings... Run both engines and enjoy them. Add up your fuel cost for an average of 150 hours per year. Then total your "other" costs; slip rental, maintenance and repairs, insurance, etc. You'll find that the cost of fuel is a small percentage of your total cost of being owned by a boat!
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Old 05-02-2014, 09:54 PM   #9
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This is a subject often discussed and the answer heavily depends on the engines and the drives. Many manufacturers have evaluated this on their own boats and I'd strongly recommend asking the boat manufacturer their recommendation on the subject. I've seen them say "no problem", I've seen them recommend steps to do it without problems, and I've seen them say "Don't do it."
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Old 05-02-2014, 11:56 PM   #10
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Let me chime in as I have first hand experience with this on my boat with twin Cummins 450C engines.

First of all, as mentioned by others, there are two areas of concern. Shaft cooling and transmission freewheeling.

Many newer vessels are equipped with cross-over cooling, so when one engine runs both shafts are water cooled. However, based on my research the freewheeling does not look so positive. I couldn't get a solid answer from ZF, so I assumed that my trainies cannot freewheel for long. Thus, my solution was to lock the shaft.

Regarding the benefits, my boat's hull speed is 7.4kts and I usually get the speed when two engines are running at 1000RPMs burning about 4GPH total. When I run on one engine (either side) I usually need to bump the working engine to 1200RPMs to maintain a speed of 6.5kts, which at that rate burns about 3GPH. So, as you can see the savings in fuel is not so great. But, IMO the biggest incentive is not fuel, but the engine hours. In the end, on a long runs you save some fuel and 50% of engine hours.

Another point worth mentioning, my boat would run in the range of 10-12 degrees angle on one engine. So, all would be fine when seas are relatively calm. But, when seas get too rough the autopilot works too hard and you would need to change something (either start tacking or engage 2nd side).

Hope this helps a little.
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Old 05-03-2014, 12:33 AM   #11
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So, as you can see the savings in fuel is not so great. But, IMO the biggest incentive is not fuel, but the engine hours. In the end, on a long runs you save some fuel and 50% of engine hours.
.
While it saves engine hours, I'm not convinced it saves much on engine wear, no more than it does on the fuel. There are those who argue, including manufacturers, that a better gauge of engine life is fuel used rather than hours. However, hours is easier to know so they've chosen hours as a convenient means. While saving hours impacts your service intervals, likely not extending your engine life much. Running the engine at 1000 rpm is very little wear on it.
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Old 05-03-2014, 05:20 AM   #12
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Too much worry about fuel savings... Run both engines and enjoy them. Add up your fuel cost for an average of 150 hours per year. Then total your "other" costs; slip rental, maintenance and repairs, insurance, etc. You'll find that the cost of fuel is a small percentage of your total cost of being owned by a boat!
I was a proponent of single engine cruising for several years. However the above statement is strikingly true. I don't do it anymore.
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Old 05-03-2014, 02:11 PM   #13
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Add up your fuel cost for an average of 150 hours per year. Then total your "other" costs; slip rental, maintenance and repairs, insurance, etc. You'll find that the cost of fuel is a small percentage of your total cost of being owned by a boat!

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I was a proponent of single engine cruising for several years. However the above statement is strikingly true. I don't do it anymore.
We use a lot more than 150 hours per year (Actually more than 1000) and not at fuel efficient speeds much of that time, but surprisingly to most, fuel is still a small percentage of our total costs. Yes, we could cut our speed in half and save a large amount, but take the cost of the boat and spread it over the years plus all other related expenses and fuel is far less of the total than people often believe. On the other hand it's the one people think they have the most influence over. That is also only partially true though as there are ways to address other costs.

In fact, looking at our current budget, fuel is 14% of are annual costs but if you take the price of the boats and spread it over 20 years then fuel is less than 8% of our costs. Now ours are somewhat distorted since we don't do maintenance ourselves so pay for it all.
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Old 05-03-2014, 03:39 PM   #14
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You've made my point. You're on the water more than 1,000 hours per year. I'm jealous! However, the "average" boater is having a good year with 200 hours. So your 14% or 8%, whatever, for fuel usage is even less for the average boater with fewer hours.
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Old 05-03-2014, 04:10 PM   #15
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You've made my point. You're on the water more than 1,000 hours per year. I'm jealous! However, the "average" boater is having a good year with 200 hours. So your 14% or 8%, whatever, for fuel usage is even less for the average boater with fewer hours.
Maybe theirs is less but we run up other expenses on the water though.

The 1,000 hours seems like a lot even with 270 days on the water. But what runs it up is the overnights. For instance we project 338 hours on our two month Alaska trip but that includes 40 hours from Bellingham to Ketchikan, 25 hours from Juneau to Glacier Park, at Glacier Park and then to Skagway, 46 hours from Haines to Valdez, 52 hours from Kodiak to Sitka, and 64 hours from Sitka to Port Angeles. So it's not as if we do 4 hours every day or 8 hours every other day.

But then our fuel is higher as a percentage than others might be because we typically cruise 15-22 knots.
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Old 05-03-2014, 04:29 PM   #16
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But then our fuel is higher as a percentage than others might be because we typically cruise 15-22 knots.
I may have missed it somewhere, but what boat do you guys have?
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Old 05-03-2014, 04:30 PM   #17
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So what is your fuel budget for that two month trip? Looks to be 5 figures if my imperfect quik calc brain is even close. And then what percentage of cost is fuel for that trip? Doesn't seem insignificant?
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Old 05-03-2014, 04:57 PM   #18
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So what is your fuel budget for that two month trip? Looks to be 5 figures if my imperfect quik calc brain is even close. And then what percentage of cost is fuel for that trip? Doesn't seem insignificant?
We're not using a trawler for this trip and have a large group of people including crew so our numbers won't be useful. That said our fuel costs for that boat including that trip are budgeted to be less than 15% of operating costs.

However, I'd estimate for most in trawlers that trip would be about four months and the fuel costs would be between $10,000 and $15,000. The other costs would depend on whether they anchored or used marinas. But if marinas on a 50' trawler you'd be talking $9,000 or so. Assume six people, then food and beverages at $10-15/day per person. That's $7,000-$10,000. What about communications. If they choose to have satellite television and phone and internet that could run as much as $10,000 for that period. Now for many it's no more than $2000 - $3000. Then there is just maintenance. Not just what occurs during that period, but the accumulation of wear and tear that leads to rebuilds, leads to replacing equipment, leads to replacing batteries, watermakers, leads to painting the boat. 4500 hours is a lot of wear and tear.

An interesting exercise one might do is go to luxyachts.com and under the Management tab at the top use the operating cost calculator and enter all the information for your boat. See how your costs compare, but it will also remind you of some costs. For instance it will put part of an engine rebuild into your annual costs, part of painting.

I'm not saying our costs are indicative of others. I am saying that for most boaters, fuel is a relatively small part of their total cost of ownership. I also recognize there are exceptions and you may well be an exception.
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Old 05-03-2014, 05:03 PM   #19
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I may have missed it somewhere, but what boat do you guys have?
Yes, we generally don't talk much about our boats and this trip isn't on a trawler but it's a very trawlerish type trip. We're actually doing it though in an 85' Westport/Pacific Mariner. Today though we're watching all the people going for shrimp. Apparently today is the first day of recreational shrimp season here in Washington. We decided not to move with all the traps today and just watch. Well actually right now we had to go pick up someone at the Seattle airport so still driving back to the boat. We intended to be closer today but changed our plans so a two hour drive.
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Old 05-03-2014, 05:05 PM   #20
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A lot of talk of average boaters. There's no such thing. And average numbers may not apply to an individual at all. We all have to look at their own situation. I do recommend a good budget looking at all costs if you can do it without scaring yourself too badly.
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