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Old 08-14-2013, 01:14 PM   #1
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One Engine

Is it possible to use only one engine to save fuel. We are buying a trawler in the next year and while I prefer a single engine, a lot of the boats I like have two. Are there drawbacks to shutting down one engine while on a voyage to save fuel? Is there additional strain on the rudder posts because of the yaw induced by using an off center engine?
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Old 08-16-2013, 06:21 AM   #2
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Most boats with 2 engines will have smaller high speed rudders. So the vessel would have to get underway on 2 then shut down one.

This requires a tranny that can be freewheeled (not common) or a prop locking mechanism.
The stuffing box may require an extra pump for the shut down engine.

The only folks that seem to have a twin and shut down one seem to be fish boats transiting to a new area , and they simply remove one prop for the voyage.

Far better to look at single engine boats , for the best low speed efficiency.
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Old 08-16-2013, 07:28 AM   #3
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Really depends on the boat, which engines, how propped, whether your boat's tranny's can freewheel, and what kind of speed you want....

Many twins run both the engines and get darn near the same economy as a single at really slow speeds say 6-7 knots. Try and go above...then it's really hit or miss depending your setup.
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Old 08-16-2013, 07:37 AM   #4
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You started looking over a year and a half ago, and you haven't found any deals yet?

Need help finding full displacement boat

Are you still on the west coast or the east coast?
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Old 08-16-2013, 10:15 AM   #5
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I wouldn't suggest using a single engine on a twin engine boat unless it was due to disablement. You will have the wrong prop and gear reduction ratio, which can overload and damage the engine you are using.

There are many single engine trawlers available. Nordic Tug and Nordhavn have some great models available. Ranger Tug has smaller single engine trawlers.
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Old 08-16-2013, 10:28 AM   #6
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Sure...any twin boat you run at the pin going single will probably overload that engine...but...

I'm pretty sure you would have to be pretty far up in the RPM range on most 30-50 foot trawlers to be overloading the engine...unless you were lucky enough to find one with "right" sized engines. Most will do 6-7 knots on less than 50hp from one or two engines.

A 30-50 trawler with twin 120 or so engines can run at 6-7 knots just fine on one engine ...but as I see with some other members here who report their findings on long cruises get pretty much the same mileage as single guys as long as they stay slow.

So yes you can do it without tearing things up if you do your homework...but then again do your homework and you may find a boat that is almost the same economically with twins....so look for a twin boat with really smallish engines and stay away from singles that are way overpowered.
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Old 08-16-2013, 11:02 AM   #7
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I do it all the time. I can run at 6-7 kts on one at hardly any more load than with both online.

I also have large rudders and can bring mine back to the slip with little drama or extra effort on one engine.
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Old 08-16-2013, 11:13 AM   #8
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Brian, We swallowed the hook 7 years ago to take care of ailing parents. We were living happily on our sailboat in the Sea of Cortez till they needed some help. Three years into this I began looking into our "next" boat and came to the conclusion we were going to get a trawler. Since then I've been poking around the forums and yachtworld looking for the right boat. We still have responsibilities here but think we can get a boat in the next year and keep it in the California Delta till we're able to cruise again. So, yeah. Been looking a lot longer than a year and still have questions.
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Old 08-16-2013, 01:57 PM   #9
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Have twin 6.354 Perkins, no turbo's, 130 hp, Borg Warner Velvet Drive trannys. If I'm not in a hurry, we will routinely run on one engine, with no adverse affect on engines or tranny's.

On one engine at 2200, we're running 7.5 knots. Boat handles well on a single engine and we usually swap engines after about 2 hours, just to keep the hours close and make sure the tranny bearings are lubed up. (Borg Warner’s recommendation??)

The big advantages are the security that twins offer. I have never been towed or adrift with a dead engine in over 35 years with this boat. But I've towed a dozen single engine boats in that predicament. The boat is much quieter running on one engine. Yet you can fire up the second engine and run up to 11 knots if the weather gets bad or want to get there a little quicker. Maneuvering while docking or in tight places is a big advantage with twins. Fuel economy while cruising may be a little less running on one engine, but to me it's not a consideration. We spend a lot of time fishing, idling one engine, which is definitely more economical than idling two engines.

There is nothing cheap about boating, it's an expensive hobby If your prime consideration is ecconomy then maybe a power boat is not a good choice.
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Old 08-16-2013, 03:36 PM   #10
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Originally Posted by Apophyge View Post
Is it possible to use only one engine to save fuel. We are buying a trawler in the next year and while I prefer a single engine, a lot of the boats I like have two. Are there drawbacks to shutting down one engine while on a voyage to save fuel? Is there additional strain on the rudder posts because of the yaw induced by using an off center engine?
The answer is ...it depends. On many twin engine boats, the engines turn more than a set of propellers. Big alternators and hydraulic pumps consume horsepower too. There are many reasons a "properly" (shaft brakes and large skeg hung rudders) set up twin engine boat may choose to run at times in single engine mode. A few of the reasons we choose to do it are more miles between maintenance cycles, lower er temps, less noise, engine spends more of it's life "properly" loaded and last the fuel economy is improved in the right conditions. As an aside when maneuvering and docking we routinely run with only one engine for propulsion, as our thrusters are hydraulic and can require more horsepower than can be generated by one engine at idle.

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Old 08-16-2013, 06:21 PM   #11
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Old 08-16-2013, 08:01 PM   #12
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Brian, We swallowed the hook 7 years ago to take care of ailing parents. We were living happily on our sailboat in the Sea of Cortez till they needed some help. Three years into this I began looking into our "next" boat and came to the conclusion we were going to get a trawler. Since then I've been poking around the forums and yachtworld looking for the right boat. We still have responsibilities here but think we can get a boat in the next year and keep it in the California Delta till we're able to cruise again. So, yeah. Been looking a lot longer than a year and still have questions.
No problem, take care of those elders, they took good care of you. I do find the Asian people tend to have even more respect and hands-on care for their elders.

I imagine there is more and more of this coming in the years ahead. And regrettably I'm sure a number of parents have had to sacrifice some of their retirement fun money to help their kids thru these recessionary times.
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Old 08-16-2013, 08:15 PM   #13
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BTW my suggestion, a good solid 6-cyl single engine. The in-line 6 is the most inherently balanced configuration for a 4cycle engine,....less counter-balancing weights required which gives it a smooth operation that is easier on its crankshaft bearings,...more longevity.

You may also need shaft brakes on the unused engine to keep its prop (and thus its gearbox) from rotating. Many marine gear boxes do not get proper lubrication in the freewheeling mode.
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Old 08-16-2013, 09:14 PM   #14
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There are a number of old strings on this subject. Timjet and I actually ran some performance tests on our boats that measured fuel burn at the same speed both single a twin engined. For a semi displacement hull running in the 8-9 MPH speed range with one prop freewheeling (lowest drag configuration), the fuel savings is in the 20% range.
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Old 08-16-2013, 09:15 PM   #15
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Personal observations, as I have had an engine down for most of the last month, all of which time has been spent cruising.

We have traveled from Saltspring Island to Desolation sound and back, mostly on one engine. There have been no difficulties with docking or anchoring, as the "down" engine has a problem only if allowed to run for more than 20 minutes, so I have been able to use it for manouvering. I have a good, new autopilot, so once up to speed the steering is not affected. Friends were with us for the first two weeks, who reported that there was black smoke when up to speed, but which went away as soon as I slowed a little. That taught me that the single engine was being overworked at anything over 6.2 knots. Our normal cruise is 8.2, with no smoke (on 2 engines). I expect the slower speed will result in lower fuel consumption, but I would prefer to cruise faster and when the second engine is back in service, I won't be going any slower than I used to. Fuel consumption is related only to the amount of work being done in the engine room. That, in turn is highly related to the height of the stern wave being created by your passage through the water. With one engine dictating a slower speed, a shorter stern wave will result in fuel economy. I am sure that if I wanted to save fuel, and I slowed to 6.2 knots all of the time, I wouldn't be able to tell how many cylinders were going up and down.

I tried putting this boat on the dock with one engine a few times and found that it can be done, but with only one direction of prop walk it isn't always easy. Any kind of a crosswind or current and I was regretting the loss of control.
When I checked the shaft on the idle engine, sometimes it wasn't turning, sometimes it was, but never fast enough to create any warmth in the transmission, so FF's warnings simply don't apply to my setup.

Like Idleweiss, I won't be going back to singles. I like the security of the second engine. I too have towed many broken down boats over the years, and almost all of them have been singles. I haven't yet had both engines down at once, and I doubt it will happen. I have had an engine down for various different reasons over the years, and every time, I was able to schedule the repair so that it never impacted on scarce vacation time. With a single, there is always a priority on getting the engine running again, as without it, you just don't go. If you are dependent upon a pro to do your repairs, you will find them busiest during that scarce vacation time, so your vacation may be over before you get slotted into their busy schedule. With twins, you can still go, and get the repairs done when you get home.
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Old 08-16-2013, 09:29 PM   #16
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Lots of good advice here, but let me add a couple of other considerations:

- Don't confuse the economy gained by going slower with economy gained by shutting down one engine. I've seen lots of claims of greater fuel economy by shutting down one engine, when 90% of the gain simply comes from operating at a lower speed. Run at that same slower speed with two engines and you will realize 90% of the gained economy.

- If your gear box and shaft seal cannot freewheel then you are likely to lose all gained efficiency by dragging around a locked prop. In fact, your efficiency may actually be worse than running with two engines.

- Check to be sure your gear box can be freewheeled, but also be sure to check your shaft seal. Many dripless seals require raw water flow which comes from a RUNNING engine. No running engine, no water flow, overheated shaft seal and possibly cuttlass bearings too. This is one of the biggest areas the check when people say "it depends".

My personal assessment is that any gains from running one engine is splitting hairs at best, and a false economy at worst. Throw in the risks and complications of damage cased by excessive freewheeling, and it's just not worth it.
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Old 08-16-2013, 09:31 PM   #17
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There are a number of old strings on this subject. Timjet and I actually ran some performance tests on our boats that measured fuel burn at the same speed both single a twin engined. For a semi displacement hull running in the 8-9 MPH speed range with one prop freewheeling (lowest drag configuration), the fuel savings is in the 20% range.
Thanks! It's great to get a good objective, apples to apples number on this.
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Old 08-16-2013, 11:28 PM   #18
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Lots of good advice here, but let me add a couple of other considerations:

...but also be sure to check your shaft seal. Many dripless seals require raw water flow which comes from a RUNNING engine. No running engine, no water flow, overheated shaft seal and possibly cuttlass bearings too. This is one of the biggest areas the check when people say "it depends".
I agree with all twistedtree says here, but the easiest one to deal with is the dripless log water flow. My boat has PSS seals and I simply placed a "T" into each water line and connected a crossover line linking the two. Now either engine can deliver water to the other dripless seal. Normally this is only an issue if traveling more than 7-8 kts, but I added it anyway just to be sure. It was a simple 15 minute job and cost less than $10.
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Old 08-17-2013, 02:26 AM   #19
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So, just for S & G - Please... do tell Marin...

Seeing as you are often sending PM's (i.e. "Private" Messages), and, you know that the people to whom you send will receive your PM and that they will know it is from you... then, why do you bother to tell us all on TF that you are so graciously sending a "Private" Message??

Inquiring minds want to know... at least mine does (bet a couple others do too!)
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Old 08-17-2013, 02:38 AM   #20
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We also stopped our loop trip in FL in mid 2010 to take care of parents in CA and luckily we owned a cute 34 CA while there for 2 plus years.

Today we left CA to drive back to the boat full time via AZ for a rest, wifes parents doing well and really appreciated our help while moving to retirement village and getting their house ready for sale.

I am sure we will be back there in a few years to help again but glad we could help.

Kinda goes with our boats name.
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