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09-27-2019, 12:58 PM
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#1
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Member
City: Anacortes
Join Date: Aug 2019
Posts: 13
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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 ? I’ve been told if one goes down the other is likely because it’s 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 it’s easy on fuel. Done really have desire to go fast.
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09-27-2019, 01:05 PM
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#2
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Guru
City: Rochester, NY
Vessel Name: Hour Glass
Vessel Model: Chris Craft 381 Catalina
Join Date: Aug 2019
Posts: 7,538
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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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09-27-2019, 01:06 PM
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#3
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Guru
City: Tri Cities, WA
Join Date: Nov 2012
Posts: 4,406
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Quote:
Originally Posted by TwoToTango
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? I’ve been told if one goes down the other is likely because it’s 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 it’s easy on fuel. Done really have desire to go fast.
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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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Mike and Tina
1981 Boston Whaler 13'
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09-27-2019, 01:07 PM
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#4
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Guru
City: Ft Pierce
Vessel Name: Sold
Vessel Model: Was an Albin/PSN 40
Join Date: Oct 2011
Posts: 28,119
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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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09-27-2019, 01:12 PM
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#5
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Guru
City: Adelaide
Vessel Name: Kokanee
Vessel Model: Cuddles 30 Pilot House Motor Sailer
Join Date: Jul 2012
Posts: 3,218
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The fuel savings would be the least of the advantages of having a single.
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09-27-2019, 01:19 PM
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#6
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Guru
City: Alexandria, VA
Vessel Model: 2000 Wellcraft
Join Date: Sep 2013
Posts: 1,467
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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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09-27-2019, 01:24 PM
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#7
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Member
City: Anacortes
Join Date: Aug 2019
Posts: 13
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Are you saying there are other advantages to a single? Maintenance costs?
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09-27-2019, 01:28 PM
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#9
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TF Site Team
City: Seneca Lake NY
Vessel Name: Bacchus
Vessel Model: MS 34 HT Trawler
Join Date: Nov 2014
Posts: 7,808
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Quote:
Originally Posted by AusCan
The fuel savings would be the least of the advantages of having a single.
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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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Don
2008 MS 34 HT Trawler
"Bacchus"
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09-27-2019, 01:30 PM
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#10
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Guru
City: Rochester, NY
Vessel Name: Hour Glass
Vessel Model: Chris Craft 381 Catalina
Join Date: Aug 2019
Posts: 7,538
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Quote:
Originally Posted by TwoToTango
Are you saying there are other advantages to a single? Maintenance costs?
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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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09-27-2019, 01:32 PM
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#11
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Guru
City: Cape May, NJ
Vessel Name: Irish Lady
Vessel Model: Monk 36
Join Date: Dec 2007
Posts: 4,947
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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.
__________________
Archie
Irish Lady
1984 Monk 36 Hull #46
Currently in Cape May, NJ
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09-27-2019, 01:50 PM
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#12
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Guru
City: Full Time Cruising East Coast
Vessel Name: Meridian
Vessel Model: Krogen-42
Join Date: Aug 2011
Posts: 1,014
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Twin vs single
I get just under 2 gph with twin lehman 90’s @ 7.5 mph. This is real close to a single Lehman 120 in a KK42. I’ve 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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Terry
Meridian
KK-42097
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09-27-2019, 02:10 PM
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#13
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Guru
City: Friday Harbor, WA USA
Vessel Name: FORTITUDE
Vessel Model: Kadey Krogen 54-8
Join Date: Jul 2017
Posts: 1,015
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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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09-27-2019, 02:12 PM
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#14
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Senior Member
City: Green Turtle Cay/Western NC
Join Date: May 2013
Posts: 182
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Quote:
Originally Posted by TwoToTango
Are you saying there are other advantages to a single? Maintenance costs?
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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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09-27-2019, 02:37 PM
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#15
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TF Site Team
City: Saltspring Island
Vessel Name: Retreat
Vessel Model: C&L 44
Join Date: Nov 2007
Posts: 5,656
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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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Keith
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09-27-2019, 02:38 PM
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#16
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Guru
City: Bellingham WA
Vessel Name: Hatt Trick
Vessel Model: 45' Hatteras Convertible
Join Date: Nov 2016
Posts: 1,971
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Quote:
Originally Posted by Bacchus
If somewhere where towing is avail I'd buy a no limit policy and sleep well while saving $ on several counts.
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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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Ken on Hatt Trick
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09-27-2019, 02:48 PM
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#17
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Veteran Member
City: Squam Lake
Vessel Model: Rossiter Coastal Cruiser
Join Date: Apr 2019
Posts: 43
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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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09-27-2019, 02:58 PM
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#18
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Guru
City: Annapolis
Vessel Name: Ranger
Vessel Model: 58' Sedan Bridge
Join Date: Aug 2013
Posts: 7,068
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Quote:
Originally Posted by koliver
Extra maint cost for twins: 1 more oil filter, 8l more oil, 1 more racor filter element. $ negligible.
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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...
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.
-Chris
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Chesapeake Bay, USA
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09-27-2019, 03:18 PM
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#19
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Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,738
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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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Eric
North Western Washington State USA
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09-27-2019, 03:24 PM
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#20
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TF Site Team
City: Saltspring Island
Vessel Name: Retreat
Vessel Model: C&L 44
Join Date: Nov 2007
Posts: 5,656
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Quote:
Originally Posted by ranger42c
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
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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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Keith
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