Alternating Engines while Underway

[SailorBruce]

I’m new to a twin-engine trawler but have a friend who owns a catamaran sailboat giving me USA (un solicited advice). He only powers up both diesels when departing or arriving or when he anticipates needing extra maneuverability. When in open water, he swears by using only one of his screws. He claims it halves the number of engine hours, lets the one engine operate at a proper load, and significantly reduces his fuel consumption.

Are there counterpoints that aren’t apparent to me? Is it a common practice in the trawler community?

Bruce

 

[kenk]

The only thing I would agree with is it halves the number of engine hours. I do not see how it would let the one engine operate at proper load when now that engine is doing the work of two engines. As far as fuel consumption, you might save a little, but your not going to save half of it. Another thing you could also be damaging the transmissions. Some transmissions require you to lock the shaft if that engine is out.
I don't think this is common practice with any twin engine boat.

 

[psneeld]

It does depend on how you would run the engines overall anyway.

Definitely lots of variables on speed, fuel consumption, where in the power curve(s) you are....

Usually one piece of advice with many variables not exactly like your situation is just that....one piece of advice.

 

[stubones99]

His cat may have folding props, and folding props don't free-wheel the shaft while underway. That might provide an advantage for one engine underway.

 

[wkearney99]

The cat has a sail. It's likely also got saildrive type folding propellers. This is key, otherwise the prop still spins, causing drag. A boat like that is designed to operate without an engine running at all. So, yeah, if the winds aren't sufficient it would make sense to only add as much engine as is desired for the conditions.

Be wary of sailors and their tales of savings. Cheap f'ers, will claim all kinds of nonsense, and jump through all sorts of stupid hoops in the name of 'savings'. I joke, a ragbagger can squeeze a nickel so hard you can hear Jefferson scream.

But I suppose, poking along at such slow speeds leaves them with too much free time to dwell on it.

I'm with Ken, powerboats with twins are generally designed to operate with both of them running. Steps may well need to be taken to avoid damaging the transmission and/or shaft bearings by leaving a prop spinning without the engine powering it.

 

[Jetstream]

We used to do it a bit and for us it did save a lot of fuel. With both engines cruising at 1200 RPM we got 7 knots and a fuel burn of 12 LPH. With one engine at 1200 RPM we got 6 knots and fuel burn of 7 LPH. She didn't handle as well with only one engine operating and in any sort of sea the autopilot struggled a bit because of the asymmetric thrust.
The first thing to do though is make sure your gearboxes are safe to operate freewheeling with one shut down. Some are not rated to freewheel or will have a time limit on them. Our ZF80's where fine but if you have to lock a shaft due to a lubrication or heat issue the drag would be huge.

 

[caltexflanc]

At the same speed through water there is typically little to no advantage.

 

[DDW]

Be wary of sailors and their tales of savings. Cheap f'ers, will claim all kinds of nonsense, and jump through all sorts of stupid hoops in the name of 'savings'. I joke, a ragbagger can squeeze a nickel so hard you can hear Jefferson scream.

But I suppose, poking along at such slow speeds leaves them with too much free time to dwell on it.

Tongue in cheek I suppose, but a very broad brush. I have a sailboat and a trawler. The sailboat cost way more than the powerboat, the sails cost more/mile than diesel for the powerboat, and the sailboat is faster under typical passage conditions.

A catamaran sailboat under power isn't really the same animal as a twin motorboat, unless that motorboat is also a catamaran.

 

[wkearney99]

Tongue in cheek I suppose, but a very broad brush.

Yes, all in jest.

 

[Ken E.]

Most but not all Twin Disc gears will tolerate free wheeling and their website has a list. Some gears, like Allison M20, can't be free wheeled at all and the shaft must be locked down.

 

[Tom.B]

I don’t want this to sound mean, but just use the boat the way is was intended. There is little advantage using a twin screw powerboat to save a few percent on fuel and stand to do more damage AND not have full thrust in the direction you need in case of an emergency.

 

[tiltrider1]

like everything in life there is a time to run on one engine and a time to run on two. Normally I run two engines at 10 knots and burn 1.1 gpm. If I want to max my gpm then i slow the boat down to 6.5 knots. Problem now is that I am out of safe operation parameters for my Cat 3208t engines. If I operate just one engine I can maintain proper rpms and temperatures while only going 6.5 knots there by halving my gpm. The cost in this case is time. It’s rare that fuel is more important to me then time. However if I was poor and wanted to get from Seattle to Ketchikan with out buying fuel in Canada I could achieve it by going slow on one engine. What applies to my boat does not necessarily apply to the next boat.

 

[psneeld]

My 37' sportfish with 3208Ts was exactly the same way. If I had kept the boat and cruised it, I would have run it single engine a lot.

As it was, I trolled for hundereds of hours at 5 to 6 knots fishing, on a single engine..

 

[what_barnacles]

I would accentuate the beat frequency of the two engines and raise some really awesome fish.


Man I miss trolling....

 

[markpierce]

I'm always amused by twin-motor boaters wanting to operate their boats like an unbalanced single, just trying to eke out the best of two worlds.

 

[markpierce]

like everything in life there is a time to run on one engine and a time to run on two. Normally I run two engines at 10 knots and burn 1.1 gpm. If I want to max my gpm then i slow the boat down to 6.5 knots. Problem now is that I am out of safe operation parameters for my Cat 3208t engines. If I operate just one engine I can maintain proper rpms and temperatures while only going 6.5 knots there by halving my gpm. The cost in this case is time. It’s rare that fuel is more important to me then time. However if I was poor and wanted to get from Seattle to Ketchikan with out buying fuel in Canada I could achieve it by going slow on one engine. What applies to my boat does not necessarily apply to the next boat.

Trust you meant gph (gallons per hour) rather than by minute.

 

[tiltrider1]

Trust you meant gph (gallons per hour) rather than by minute.

You are correct. I got auto corrected and didn’t catch it.

 

[Nomad Willy]

If you’re engine’s too big why not downsize. Two smaller engines or for the out of the box crowd how bout one small engine on one side and a bigger engine w the prop closser inbd giving colse to asymmetrical thrust w both engines running.
Just say’in .. Just think’in.

Another thought is to only run one engine as a single. That way you probably won’t have to replace both engines at once. Again just think’in.

 

[AusCan]

Running two engines when only one is required makes no sense to me, unless the gearbox is not designed for this (most are). Many twin engined boats are overpowered and the extra horsepower does nothing other than creating a big bow wave.

Why not cut 40% off your engine hours and save a bit of fuel as well. Just because a boat was built with two engines, doesn't require you to use them. Many boats have two engines simply to promote sales to the buyer wanting redundancy.

Run two when you need the extra manouverability or power. Go single at other times.

 

[tiltrider1]

Trust you meant gph (gallons per hour) rather than by minute.

Actually I meant gallons per nautical mile. 1.1 gallons per hour at 10 knots in a 54’ boat would be very impressive.

 

[psneeld]

Amused or amazed...running a rwin on one can have several practical purposes.

All boats are compromises and the nice thing about some, they have more options than others.

 

[Roger Long]

The only thing I would agree with is it halves the number of engine hours. I do not see how it would let the one engine operate at proper load when now that engine is doing the work of two engines.

That would be true if engines were properly matched to the hulls (speaking as a retired commercial power vessel designer). However, most yachts are way over powered, especially if you cruise at modest speeds for comfort and to save fuel. Our boat is an example on steroids. Two 130 hp diesels. At 6-7 knots, we are only taking 20 - 30 hp out of each engines. Diesels generally hate running this lightly but our Perkins seem happy. Doubling the hp by single engine running would get them closer, but still not into, optimum operating range.

I still generally operate with both engines running but I would want prop locks if I was going to operate single screw routinely. We also are generally in ICW waters where I might want the maneuverability at a moment's notice.

 

[catalinajack]

I don’t want this to sound mean, but just use the boat the way is was intended. There is little advantage using a twin screw powerboat to save a few percent on fuel and stand to do more damage AND not have full thrust in the direction you need in case of an emergency.

While I do not intend to ever run my boat equipped twin Lehman's on one engine unless I must, here is one man's recent experience doing so. In the Ottawa locks, we suffered a parted prop shaft due to a failed Drivesaver. I was forced to motor 18 miles on one engine on the Rideau to Hurst Marina for repairs. I ran at 1,250 RPM which yielded a minimum of 5 MPH up to 5.3 MPH. Two engines at 1,600 RPM gets me about 8.3 MPH.

Take from that what you wish, just one man's experience on one boat, a full displacement Defever 44 weighing 56,000 lbs.

 

[NightCrawler]

I ran across this article a few months ago. Found it very informative on fuel burn twin vs. single. It answered the question bugging me, how our fuel burn would be as low as it is when we have way more engine than needed.

 

[bayview]

I was curious with my 38,000 # twin 459 HP ACMY. I wanted to see what the best range would be for long passage. AT 5-6 KTS auto steering worked well trying to go much faster over drove the steering. Mileage increased to about 3 nmpg from 2+ at 7-8 KTS. I only went 60 Miles so it was a modest test. I usually slow cruised in water ways at 9 KTS at about 2 NMPG. Offshore 18-20 Kts was the rule.

IMO it was a good test of single operation ability but other than getting maximum range for a, never taken, long passage it was not worth the effort.

 

[Brooksie]

I've never owned a twin screw vessel but I have something to add:
My father in law when for years telling me how much safer he was with his twin screws instead of my single. That is until he lost an engine and found that his boat would only go in circles under one engine. And another time bad fuel killed both engines at once. Complete silence since then....

 

[NightCrawler]

I've never owned a twin screw vessel but I have something to add:
My father in law when for years telling me how much safer he was with his twin screws instead of my single. That is until he lost an engine and found that his boat would only go in circles under one engine. And another time bad fuel killed both engines at once. Complete silence since then....

I used to think twin was the bomb. Shangri-La came with two, so it's what we have. After spending months in Beaufort Yacht Basin watching all the shrimpers and fishermen come to the fish house to unload, incredibly tight dock space - I'm so impressed with single engine boat handling. Hats off to those who've mastered it. I saw things done in that space I don't think I could do with twins. They typically take good care of their equipment and for the thousands of hours of use, most get home just fine.

 

[bayview]

If he could only go in circles he was trying to go too fast. Tell him to try again, it works.

Twins should have independent electric and fuel. Of course if you fill both tanks with bad fuel then you are in a similar situation to a single.

Twins can be truly redundant if properly setup.

 

[Molly]

I tell my sailing friends. As soon as you get accustomed to the fuel costs related to a trawler, it's way better then a sailboat. (IMO) I owned 3 different sailboats and loved them all. This is my 3rd power boat. I really enjoy the Mainship 34. The twin models use (2) 220 HP Yammars. The single screw is a 370 HP Yanmar, The fuel burn is very close.

The type of cruising we do is, we go from point A to point B. Then we drop a hook or get a slip. Stay anywhere from 3 days to 7 days. The one thing I do miss with a sailboat is that "day sail". just go out and sail around for a few hours and come back.

95% of the time we are going somewhere. Instead of a 3 hour trip, it's 1 1/2 hrs. A grueling 10 hr. trip becomes a very manageable 5 hr. trip. You just need to open the wallet for the fuel. I think a good trade off.

I will run 2 engines and just back off the RPM's to save fuel. Just another opinion. Of course my opinion is the only one that matters! (to me)

 

[psneeld]

Way too many apples to oranges comparisons....and not all boats are the same.....

Discuss a specific setup with some real life experience and the discussion becomes manageable.