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Old 09-27-2019, 12:58 PM   #1
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Twin vs single

Ok I know this is an age old debate- Twin engines vs Single. . I know most of the stated advantages and disadvantages like more maneuverability and back up power for twins. I get that . But given two identical boats w a full displacement hull., one with a single Lehman 120 and the other with twin 120s , how much extra fuel do the twins use at say cruising speed of around 6 nts? Twice? My gues is not that much more but does anyone have some real experience to answer that for me ? Also just how often really end up using the twin in a back up situation ? Iíve been told if one goes down the other is likely because itís usually fuel issues that cause a diesel to stop . Thanks for your help . Just trying to educate myself. I like the idea of a single primarily because itís easy on fuel. Done really have desire to go fast.
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Old 09-27-2019, 01:05 PM   #2
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Twins of the same size won't burn a ton more, but they'll burn somewhat more. However, smaller twins would probably be close to the bigger single.

As far as redundancy, I'm gas powered, but any failure that's left me running on 1 has not been fuel supply related but always something specific to one drivetrain. It's always been an un-forseen mechanical failure like a fuel pump dying, or in my most recent case, a transmission gear stripping its teeth with no prior warning or symptoms. So not issues that would take out both engines.

A good twin setup should have totally independent fuel supply to both engines (separate tanks, crossovers closed, etc.). So even a fuel issue is unlikely to take out both engines at the same time unless you fill all the tanks with a load of really bad fuel.
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Old 09-27-2019, 01:06 PM   #3
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But given two identical boats w a full displacement hull., one with a single Lehman 120 and the other with twin 120s , how much extra fuel do the twins use at say cruising speed of around 6 nts? Twice?

TTT, I don't have an exact answer to this question. I've run my boat on one engine when the other engine died or the transmission died. I don't really track my fuel consumption closely but I didn't notice a huge savings. Running on one engine in a boat designed for two will make that engine work harder than it would if it were a pair of engines.

Fuel is the cheapest part of boating. There's no way I would opt for a single engine boat if similar boats were available with twins.

My gues is not that much more but does anyone have some real experience to answer that for me ? Also just how often really end up using the twin in a back up situation? Iíve been told if one goes down the other is likely because itís usually fuel issues that cause a diesel to stop .
I've had to run three times on one engine because something died and caused the other engine to die. Once it was a starter issue and once a transmission issue. The other time was a hot engine. That's in 25 years of owning twin engine boats.

Thanks for your help . Just trying to educate myself. I like the idea of a single primarily because itís easy on fuel. Done really have desire to go fast.
Some times going fast can get you home when a storm is coming, and I believe the increased maneuverability of twins makes up for any extra fuel burn.
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Old 09-27-2019, 01:07 PM   #4
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If you go really slow the advantages of a single can make a difference...but many mid sized Trawlers, let's say over 40 feet.....and creeping up on hull speed tend to be just as efficient with two engines running slowly than a single running higher.
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Old 09-27-2019, 01:12 PM   #5
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The fuel savings would be the least of the advantages of having a single.
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Old 09-27-2019, 01:19 PM   #6
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If you can only go displacement speed anyway, I think I would definitely go for the single. I took a single engine sailboat all over the place, and while we could always have sailed if we had a problem, we never had a single malfunction with our Yanmar 4JH in 7400 hours.

With a semi-plaining hull, you're kind of stuck with twins, most of the time.
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Old 09-27-2019, 01:24 PM   #7
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Are you saying there are other advantages to a single? Maintenance costs?
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Old 09-27-2019, 01:25 PM   #8
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so much angst with 1 or 2. Think of the poor masses that worry with 3 vs 4!

https://www.thehulltruth.com/boating...0-vs-v300.html
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Old 09-27-2019, 01:28 PM   #9
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The fuel savings would be the least of the advantages of having a single.
Agree... my feeling is fuel diff is minor and likely less than the extra maint cost as that's 2X w twins.
I'm a fan of single and thruster(s) for maneuverability. Has many of the advantages of both. Only exception is redundancy for get home. If you do s lot of off shore cruising that may be a priority.
If somewhere where towing is avail I'd buy a no limit policy and sleep well while saving $ on several counts.
Just my opinion
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Old 09-27-2019, 01:30 PM   #10
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Are you saying there are other advantages to a single? Maintenance costs?
A single will generally be cheaper to maintain and depending on the boat, it may give more room to work on it. Personally, to me, the maneuverability and redundancy of twins is worth it. For fuel economy, engines should be appropriately sized to the hull and the desired speed. So twins will be smaller engines than a single and the engine-out cruise will generally be slower than normal cruise.

For example, with an engine out, I lose the ability to run up on plane, so no more 17 - 18 kt cruise. However, cruising along at 6.5 - 7 kts is still easy and if I really need to, I can push it to 9 kts or so before I'm starting to put a lot of load on the remaining engine. With a semi displacement boat you'd generally still be able to run hull speed on 1. With a displacement hull with not a lot of power left in reserve at hull speed, you might be limited to a knot or 2 below hull speed for a comfortable engine-out cruise.

Of course, any twin with an engine out sucks for maneuverability, but it's still better than a single with an engine out...
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Old 09-27-2019, 01:32 PM   #11
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At same boat speed below hull speed, the twin fuel penalty is only about 10-15 percent. Additional running gear drag, transmission losses, extra coolant pumps, and extra engine internal losses use a little more fuel than a single engine and drive train. Definitely not double.
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Old 09-27-2019, 01:50 PM   #12
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Twin vs single

I get just under 2 gph with twin lehman 90ís @ 7.5 mph. This is real close to a single Lehman 120 in a KK42. Iíve had two failures where the other engine saved the day. One was entering a lock on the Erie Canal when a coupling separated, the other was a filter clog going under the varrazano bridge. My speed drops from 7.5 to 5.5 on a single.
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Old 09-27-2019, 02:10 PM   #13
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We have twin FL 120s in our full displacement 40' trawler and see 3.5 GPH at 7.5 knots. Yes, great maneuverability and the "concept" of reliability but since most engine shutdowns are fuel related I'm not so convinced. Maintenance costs are X2 and actually doing the maintenance is a PITA with little access until you get up above 60' with proper engine rooms.

Our next vessel will be a single with a bow thruster. Protected prop, rudder shoe, etc.

Recently sea trialed a larger, heavier, full displacement vessel with a single screw that saw 2.5 GPH at 7.5 knots logged over a long passage. Not a huge difference in GPH but over a couple of thousand miles it adds up.
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Old 09-27-2019, 02:12 PM   #14
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Are you saying there are other advantages to a single? Maintenance costs?
I prefer a single due to the fact that the shaft and prop, are protected by the keel and the rudder can be hung off the trailing edge of the keel offering support on the bottom. I enjoy poking about so the added protection of a single vs the exposed running gear of twins is my main concern.
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Old 09-27-2019, 02:37 PM   #15
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Extra maint cost for twins: 1 more oil filter, 8l more oil, 1 more racor filter element. $ negligible.

Extra fuel for twins: some say 10%, no data to support this, some say none, again no supporting data. Jury should be out on this issue until there is some data. If 10%, $ is again negligible.

Extra maneuverability: indisputable

Redundancy: Indisputable.
I once had to run on one for a whole summer vacation, as the failure occurred at the beginning and I wasn't prepared to have my vacation affected. With a single, I would have been in the shop instead of enjoying my summer.

Failures mostly fuel related, so affect both: Not in my experience. Whenever I have had a dirty fuel filter, it has affected one side only.

Exposure to damaging logs, groundings, etc.
You quickly learn to be vigilant and avoid those things. In 25 yrs with twins, I have hit a log that damaged a prop 1 time. My vacation was not affected.
Colregs require you to maintain an adequate lookout, so these thing can be avoided in most circumstances.
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Old 09-27-2019, 02:38 PM   #16
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If somewhere where towing is avail I'd buy a no limit policy and sleep well while saving $ on several counts.
Agree. I have twins and that's my preference. But a strong case can be made for a single if reliable, timely towing is available where you use your boat.
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Old 09-27-2019, 02:48 PM   #17
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I'm surprised I haven't heard anyone mention that twins appear to have a shallower draft. I have no idea what I'm talking about - ask my wife - but when I look at KKs it appears twins give you about 11 less inches of draft. If you want to spend a lot of time in the Bahamas wouldn't that be a big benefit?
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Old 09-27-2019, 02:58 PM   #18
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Extra maint cost for twins: 1 more oil filter, 8l more oil, 1 more racor filter element. $ negligible.

An extra 17 quarts oil in our case. Plus extra secondary fuel filter. Plus extra coolant filter. Plus extra coolant flush and new coolant afterwards. Et cetera.

Not huge increase, though. But also plus longer-term maintenance items: valve adjustments, belt replacements, aftercooler and heat exchanger service, and so forth...

Were it me, and doing my as much of own maintenance as possible (as now), I'd lean toward single with thruster. Although some of that would be about access more so than cost.

Unlimited budget, twins, two thrusters, joy stick, etc.... and let somebody else deal with access issues. Probably not gonna happen, though...

In the meantime, twins are working for us.

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Old 09-27-2019, 03:18 PM   #19
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The old question of twin v/s single really isn’t aquestion of how many engines but how much power do you want.
The engines are all the same.
You wanna go slow you buy with one engine,
Wanna go faster you buy two engines.
It’s all about the speed you want. That’s the only choice you get.
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Old 09-27-2019, 03:24 PM   #20
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An extra 17 quarts oil in our case. Plus extra secondary fuel filter. Plus extra coolant filter. Plus extra coolant flush and new coolant afterwards. Et cetera.

Not huge increase, though. But also plus longer-term maintenance items: valve adjustments, belt replacements, aftercooler and heat exchanger service, and so forth...-Chris
OK, but you are near the top end of engine size among 35 to 45 ft trawlers.
Mine doesn't even know about coolant filters, has never heard of coolant flushing either. I did a valve clearance check on one last year, no adjustment needed, and this is the first time I have even checked in 25 years. Aftercooler and heat exchanger service are curiosity items as well. Having done both and finding nothing looking worse than brand new, no cost involved.
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