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Old 07-18-2018, 05:16 AM   #21
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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.
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Old 07-18-2018, 06:58 AM   #22
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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.
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Old 07-18-2018, 07:50 AM   #23
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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.
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Old 07-18-2018, 07:51 AM   #24
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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.
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Old 07-18-2018, 08:55 AM   #25
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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.
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Old 07-18-2018, 09:09 AM   #26
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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....
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Old 07-18-2018, 09:19 AM   #27
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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.
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Old 07-18-2018, 09:59 AM   #28
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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.
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Old 07-18-2018, 10:54 AM   #29
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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)
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Old 07-18-2018, 11:02 AM   #30
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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.
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Old 07-18-2018, 01:29 PM   #31
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“Not all boats are the same”
Indeed I may not even be a trawler guy if that was the case.

What would make this “run one of two” practice better or even possibly good would be a boat designed for that. A slight change may be a vast improvement and that’s positioning the propellers inbd quite close to C/L. May require angling the shafts and engines a bit to have good servicing space. With so many boats now having thrusters twins w the props well appart is’t necessary.
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Old 07-18-2018, 02:00 PM   #32
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I've got a bunch of dives on this one. Its got 4 prime mover diesels and just 2 shafts. Think of the efficiency experiments one could run... before it sank.

Was a much better dive in the late '80s, before the weather deck collapsed into the ER.
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Old 07-18-2018, 03:14 PM   #33
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Commercial fishermen having reliable diesels is based on reputations made when diesels were much simpler. Usually 100% mechanical, no circuit boards, electronic sensors and so on. Most current and new fishing boats are built with twins, especially those operating in remote dangerous waters. Almost all engines used commercially are heavy duty rated and can operate at light or heavy load without much, if any affect on engine life. Unlike small diesels.
My Detroit Diesels are 70 years old, overhauled once at 65 years. A previous owner cruised every year Canada to Mexico at 300 rpm over rated hp.

I get better economy with both at low rpm than one unless at trolling speed.
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Old 07-18-2018, 06:34 PM   #34
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We use two engines when needed, otherwise I travel using only one engine running between 1750-1850RPM. On our return trip, I just reverse the engines used. This RPM moves our boat at about 7K, now that is impressive when I consider we didn’t move much faster than 5K with our sailboat.

The up side is enormous. First the engine wear is greatly reduced because of reduced hours couple with reduced load when operating. Of course this is also reflected in our fuel consumption. At 2800RPM with both engines pushing the boat to about 12K our fuel burn is around 0.4 MPG. At 1800 with one engine fuel economy jumps to 1.5MPG. OK, under this condition our speed is slower than what most others travel at. That matters not, we are not racing anybody and find half the fun is just getting there.
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Old 07-19-2018, 06:12 AM   #35
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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.

The cost of change-over can be prohibitive.

Many (most?) folks -- especially those who've bought from the secondary (i.e., used) market -- are faced with running whatever engine(s) came in the boat. Removing fully-functioning engine(s) X to replace with engine(s) Y or Z or whatever can be fairly expensive.

For either little gain, or only significant gain over a really major length of time.

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Old 07-19-2018, 10:16 AM   #36
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I've got a bunch of dives on this one. Its got 4 prime mover diesels and just 2 shafts. Think of the efficiency experiments one could run... before it sank.



Was a much better dive in the late '80s, before the weather deck collapsed into the ER.


Okay that’s just gratuitous. Also, I loved it.

Proper frog kick. Proper stages. Hey, I see two vastly different cultures represented, given the gear configuration and you guys obviously get along. Impressive. Nice to see the dark period of the mid 90’s bearing fruit after all those that we lost.

Videos like this are superb.
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Old 07-19-2018, 11:48 AM   #37
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Easy when there is no current. I’ve since broke down my doubles and just do side mount now. Tougher in wrecks but makes more caves doable and much easier ground handling.
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Old 07-19-2018, 01:21 PM   #38
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The cost of change-over can be prohibitive.

Many (most?) folks -- especially those who've bought from the secondary (i.e., used) market -- are faced with running whatever engine(s) came in the boat. Removing fully-functioning engine(s) X to replace with engine(s) Y or Z or whatever can be fairly expensive.

For either little gain, or only significant gain over a really major length of time.

-Chris
Hey Chris I was talking about building new boats designed to run single or twin. Relatively small changes in design could vastly improve running on one engine when you have two.

Of course if you found the right boat say a twin w bad engines but was a very nice boat presumably you’d get it really cheap.
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Old 07-19-2018, 01:34 PM   #39
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Twins are alive and well whether planing or displacement M/V. In fact this is where the market is if one looks around. As we cruise Alaska the best guess I have is 19 out of 20 recreational M/V are twins.

BTW, the best example of running a twin engine boat as a single is a Nordhavn. Yup that well engineered smaller diesel provides a great example of shutting down an engine. Many Nordhavns though have and are being built with same size twins and no get home. Ditto the larger Krogens.

None of this means you can’t shut an engine down and ghost along to save a few bucks per day. But most twins are set up with alternators and electric that require both engines to be running.

I had an interesting talk with a commercial fisherman yesterday. On his Delta he had a genset that matched his main for HP draw. The genset runs hydraulics, freezer compressor as well as the high KW lights. Point being, there are many ways to provide power for a vessel. For the serious minded two or more forms of power generation are the rule. Yes, sails provide power too.

Last but not least, gosh there are lots of triple outboards around.
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Old 07-19-2018, 06:18 PM   #40
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I guess. It depends. If you are running your boat at slower displacement speeds than what it was designed for. Likely your Motors are not getting up to preferred operating operating temperature. Possibly leading to glazed cylinder walls. Running one motor at the same speed you would run your twins would put more load on that motor. Likely making the motor healthier. Probably not much difference in fuel consumption
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