Operating Cost of Single versus Twins

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Whatever. You folks all know everything. It is and always has been your world, and we just live in it. But honestly, your responses are exactly what is wrong with this forum. Everyone on here knows everything. [MOD EDIT] Have a good one. And maybe, just maybe, someone here will learn something new from someone else. (Don’t actually believe it)
 
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High Wire said:
You're going to spend more on maintenance for twins vs single for obvious reasons. Fuel 10-20% more for twins at the same speed. Docking is pure preference.
Click to expand...


Yes more engine maintenance, but small, overall.


Fuel is virtually identical, if you compare the same boat single vs twin. We prooved that with two Mainship 400s, over a 500 mile run. The twin burned less fuel.
Even my 43 ft Mainship with twins, get about the same MPG as the single 400 did, both at hull speed.
 
Seevee said:
Yes more engine maintenance, but small, overall.


Fuel is virtually identical, if you compare the same boat single vs twin. We prooved that with two Mainship 400s, over a 500 mile run. The twin burned less fuel.
Even my 43 ft Mainship with twins, get about the same MPG as the single 400 did, both at hull speed.
Click to expand...

If just looking at DIY oil changes. differences are negligible. But over an extended period where other components need servicing, gap widens. Valve adjustments. Heat exchangers. Cutless bearings. Stuffing boxes. Through hulls. Starting batteries. Alternators. Starters. Water pumps. Lift pumps. Racors. Engine controls. Engine alignment. Rudder bearings. With exception of drive time and some set-up time to get tools out, all of which take exactly twice the amount of time and money to service.

For the most part, a boat is efficient within a relatively narrow range of use. The pervasive idea that running a boat designed to go fast at slow speeds is as efficient as running a slow boat slow is quaint but wrong. Notion that running a twin engine boat on one engine is "almost" as efficient as a single is quaint but wrong. The difference may be acceptable, but that doesn't minimize the difference.

There are mechanical and frictional losses that are simply a fact of life. A single large slow moving prop with fewer blades is more efficient than two smaller props. Then you get into optimized torque curve and what speed each was designed for.

If Mainship owners are not seeing a difference in efficiency between singles and twins, well, there are other explanations that don't involve bending the rules of physics and naval architecture. Running a single at the same speed as a twin could mean the twin is running at a slower S/L speed, but could also mean the single is running too fast for efficiency.

In short, saying they are equally efficient is likely not true. More accurately they may be equally inefficient - enough so that it clouds the comparison. Marketing decisions frequently override sound design choices, especially in horsepower selection installed in trawlers.

Peter
 
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JYoung said:
Whatever. You folks all know everything. It is and always has been your world, and we just live in it. But honestly, your responses are exactly what is wrong with this forum. Everyone on here knows everything. [MOD EDIT] Have a good one. And maybe, just maybe, someone here will learn something new from someone else. (Don’t actually believe it)
Click to expand...

Recommended to take two baby aspirin and hit the sack an hour early. If ill "filling" persists in the morning... call a doctor!

Hope you feel better soon! :dance::speed boat:
 
Looking for the ideal boat is never a bad thing, but reality dictates that you will buy whatever suits your needs, budget, market conditions, specific boat condition etc. I was looking at many boats before I bought the one I have now. I was focusing on a single for many reasons, but then I found a GB36 that was in my price range, in the general condition I was looking for, close to my home, and it was a GB, as I was looking "other" manufacturers for mostly price reasons. The drawback? twin engines that weren't what I was looking for, but this boat was the closest I was going get to my ideal without commissioning a new boat which I couldn't afford. So good luck with the search. You will just know when you find the one that best suits your needs and desires.
 
mvweebles said:
If just looking at DIY oil changes. differences are negligible. But over an extended period where other components need servicing, gap widens. Valve adjustments. Heat exchangers. Cutless bearings. Stuffing boxes. Through hulls. Starting batteries. Alternators. Starters. Water pumps. Lift pumps. Racors. Engine controls. Engine alignment. Rudder bearings. With exception of drive time and some set-up time to get tools out, all of which take exactly twice the amount of time and money to service.

For the most part, a boat is efficient within a relatively narrow range of use. The pervasive idea that running a boat designed to go fast at slow speeds is as efficient as running a slow boat slow is quaint but wrong. Notion that running a twin engine boat on one engine is "almost" as efficient as a single is quaint but wrong. The difference may be acceptable, but that doesn't minimize the difference.

There are mechanical and frictional losses that are simply a fact of life. A single large slow moving prop with fewer blades is more efficient than two smaller props. Then you get into optimized torque curve and what speed each was designed for.

If Mainship owners are not seeing a difference in efficiency between singles and twins, well, there are other explanations that don't involve bending the rules of physics and naval architecture. Running a single at the same speed as a twin could mean the twin is running at a slower S/L speed, but could also mean the single is running too fast for efficiency.

In short, saying they are equally efficient is likely not true. More accurately they may be equally inefficient - enough so that it clouds the comparison. Marketing decisions frequently override sound design choices, especially in horsepower selection installed in trawlers.

Peter
Click to expand...


Considering the number of Mainship singles I've seen with high blade area 4 and 5 blade props either to reduce vibration or improve performance, I'd bet the extra efficiency on the twin comes from the single having insufficient prop blade area or worse prop efficiency with the prop behind the keel, making the twin setup better optimized in that hull. Definitely not equally applicable to every boat out there though.



As far as costs, yes, many of those things listed above are doubled on a twin. And labor costs (or DIY time) will be more. But if DIY-ing, the parts cost isn't nearly double, as many of those parts are smaller and cheaper on a twin (assuming the single and twin are both well powered).



In some cases, design considerations of a boat make the single vs twin choice for you and don't really give many options. My boat couldn't reasonably be a single with the same performance it has now, as I can't think of a single 650 - 700hp engine that would fit within the height envelope of the current engine room (and salon headroom is only 6'2", so you can't just raise the floor to make more space). You'd also add potentially as much as a foot of draft with a bigger single rudder, much bigger prop that's mounted lower on the boat, etc.


For something like Weebles, the hull shape is well suited to a single, power requirement is low, etc. Putting twins in a boat like that would likely be a pretty terrible compromise.
 
ronobrien said:
Looking for the ideal boat is never a bad thing, but reality dictates that you will buy whatever suits your needs, budget, market conditions, specific boat condition etc. I was looking at many boats before I bought the one I have now. I was focusing on a single for many reasons, but then I found a GB36 that was in my price range, in the general condition I was looking for, close to my home, and it was a GB, as I was looking "other" manufacturers for mostly price reasons. The drawback? twin engines that weren't what I was looking for, but this boat was the closest I was going get to my ideal without commissioning a new boat which I couldn't afford. So good luck with the search. You will just know when you find the one that best suits your needs and desires.
Click to expand...
Why were you leaning towards a single? Personally, I simply love the way the GB36 single handles - there is a certain "zen" to GBs that is (as they say) often immitated, never duplicated.

Even an avowed single-guy, I would have made the same choice on a twin.

Anyone have any idea what percentage of GB36s we're singles? Guessing pretty low.

Peter
 
Been boating for 35+ years. Full time cruiser for a decade. Learn something new every time I sign in here.


Been on single screw only except when on center console outboards or crewing for others. Singles will back and fill better either CW or CCW. Singles will have prop walk in one direction only. So with slips it’s either easy going in or easy going out.
Most multi engine boats I’ve been on have counter rotating props. Few have no shaft angle (except outboards). There’s more tools available with twins when docking.
 
Hippocampus said:
Been boating for 35+ years. Full time cruiser for a decade. Learn something new every time I sign in here.


Been on single screw only except when on center console outboards or crewing for others. Singles will back and fill better either CW or CCW. Singles will have prop walk in one direction only. So with slips it’s either easy going in or easy going out.
Most multi engine boats I’ve been on have counter rotating props. Few have no shaft angle (except outboards). There’s more tools available with twins when docking.
Click to expand...


I think the best explanation I ever heard for docking a twin is "it's just 2 singles with really bad prop walk tied together". Thinking of it that way tends to smooth out maneuvering a lot, vs how many learn with a twin (forward, reverse, spin in place as the primary maneuvers).
 
The Nordhavn 41 is a good example of this subject. It won the Power and Motoryacht magazine as boat of the year. It is a fresh design, well laid out and super popular.

The twin Betas usurp Nordhavn’s get home designs (twins in disguise) that have proven quite capable for decades.

As we currently sit in remote AK I note a nearby Beebe Passagemaker with a reliable Gardner and get home engine. Then there is the penultimate twin powered world cruiser proven by thousands of globe cruisers, a well founded sailboat.
 
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sunchaser said:
The Nordhavn 41 is a good example of this subject. It won the Power Boating magazine as boat of the year. It is a fresh design, well laid out and super popular.
Click to expand...

I haven't paid too much attention to the N41, but, as a guy with only fair mechanical skills, I was really impressed by the selection of a pair of small mechanical Beta's. I have no bones with twins in displacement boats - the Defever 44 is a good example of a pair of small twins.

I agree that the Achilles Heel of a single for distance passagemaking is the single-point-of-failure. I know all the retorts about the world's fishing fleets; and I can think of how far my autos have gone without a hiccup. But still, the notion of being offshore with a DOA engine is chilling. Back when PAE did the Atlantic Rally, they required entrants to have a wing or twins. I don't blame them one bit.

Peter
 
Hippocampus said:
Most multi engine boats I’ve been on have counter rotating props. Few have no shaft angle (except outboards). There’s more tools available with twins when docking.
Click to expand...

With rare exceptions, ALL twins have counter-rotating props. With rare exceptions, ALL twins will have the port engine with a LH prop, starboard with a RH prop. The beauty is it makes the boat's prop-walk ambidextrous - when coming along a starboard side-tie, drop the port engine into reverse to slow the boat and simultaneously pull the stern into the dock. Mirror image for port-side tie. Can't do that with a single. It's a strong advantage to twins (the other advantage as pointed out by rslifkin is ability to pack more horsepower into an engine room).

The rare exception I've seen was an older Taiwan Trawler where apparently the builder had a pair of the same engine/gears so just installed them as same-rotation. Was a PITA.

Peter
 
Peter
Some AK friends who are successful single engine commercial fisherman have 3 ways of addressing single engine problems:
- hands on ability to dip into their, or nearby boats, extensive onboard spare parts
- towed by their on board large net tender
- float plane parts or mechanic delivery

The bigger concern these guys face are the hydraulic, electrical and mechanical issues with refrigeration, net pulling and pumping.
 
Being a pleasure craft, working lobsterman, boat-yard crew and new boat building - i.e., a heavily marine influenced person - since birth [70 yrs]. And, being in the masonry, concrete, tile and stucco trades since late teens with contractor license since mid 70's... I say the following:

I liken the maneuverability ratio between boats in water that have twins and those with only a single - in similarity to maneuverability ratio between vehicles in the snow or slimy mud that have dual axel 4 wheel drive and those with single axel drive.

Twin engines [as compared to a single engine] give a far greater number of water thrust alternatives for turning and forward-reverse movements. 4 wheel twin axel drive [as compared to rear or front wheel single axel drive] gives far greater number of traction alternatives for turning and forward-reverse movements.

Pretty simple stuff!
 
On the horsepower thing, many of the twin engine Grand Banks, Taiwan Trawlers, etc. with "SD" hulls are oddly both underpowered and overpowered at the same time. They're overpowered for use at displacement speeds (as is every fully planing boat like mine). But at the same time, the hulls are often capable of at least somewhat planing, yet there's typically not enough power to do so, meaning they're underpowered for being able to use the full capability of the hull under every possible set of conditions.
 
mvweebles said:
Why were you leaning towards a single? Personally, I simply love the way the GB36 single handles - there is a certain "zen" to GBs that is (as they say) often immitated, never duplicated.

Even an avowed single-guy, I would have made the same choice on a twin.

Anyone have any idea what percentage of GB36s we're singles? Guessing pretty low.

Peter
Click to expand...
I was leaning towards a single partly for fuel economy, although that may not turn out to be much of an issue. I do all my own engine work and working around the twins is WAY less than ideal, so that was the main reason. But as I stated when I found this one that fit all my other criteria except a single, I decided to move forward on it. I don't regret that decision but the ease of working on a single would be nicer.
 
sunchaser said:
The Nordhavn 41 is a good example of this subject. It won the Power and Motoryacht magazine as boat of the year. It is a fresh design, well laid out and super popular.

The twin Betas usurp Nordhavn’s get home designs (twins in disguise) that have proven quite capable for decades.

As we currently sit in remote AK I note a nearby Beebe Passagemaker with a reliable Gardner and get home engine. Then there is the penultimate twin powered world cruiser proven by thousands of globe cruisers, a well founded sailboat.
Click to expand...

Tom
Nice to see that you and Scot have found one another, so far from home.
 
JYoung said:
If you know what you are doing, single is the advantage every step of the way. Today, so many seem to need thrusters. Do not understand why. If you know how to drive a boat and you know what you are doing, single is a snap. As far as cost, efficiency and overall value, one engine is all you need.
Click to expand...
.

JYoung said:
Whatever. You folks all know everything. It is and always has been your world, and we just live in it. But honestly, your responses are exactly what is wrong with this forum. Everyone on here knows everything. [MOD EDIT] Have a good one. And maybe, just maybe, someone here will learn something new from someone else. (Don’t actually believe it)
Click to expand...

EVERYONE is entitled to voice their opinion, provided it is within the scope of the forum rules. You voiced you opinion, others politely explained why they like a bow thruster.

If you insist on voicing your opinion in the form of criticizing others, and are then vehemently opposed to other people voicing their opinions.......then maybe online forums aren't for you.
 
Hydrospud said:
We are new to this type of boating. Stepping up from a 19.5' open bow to hopefully a 36'-44' trawler. I would prefer a single engine but there are several nice vessels for sale with twins.
What would the operating cost difference be between the single verses twins.
Standard Preventative Maintenance Costs?
Fuel Consumption?
What motors to stay away from?

Thanks for your input and this forum is great!
Peter and Deb
Click to expand...

Peter and Deb ,

You didn't say what your budget is.
If you are planning on buying new or used.
Doing your own maintaince or have the shop do it.
Fuel consumption is how hard your on the throttles.
Can you cruse comfortably at 7-8 knots.

Several of these questions are ones only you can answer. :facepalm:
 
Regarding engine make, model etc., though there are not a lot of V-8 diesels out there They should as a rule be avoided. (Cat 3208 comes to mind). The majority of diesels are I-6 (in line 6). There are issues with most engine manufacturers of different eras and models. If you are considering a specific boat, research the engine make/model & generation. Problems found in some earlier generations may have been resolved in later years. (Cummins B5.9 early casting issues). Some engines were great but the injection systems were a problem (Roosa rotary injection pumps have had their issues). That said a blanket rule is not always going to be true. Look at maintenance history, the more records the better. Have the engines surveyed for sure. Best of luck.
 
for me is the main thing that i haven redundency with 2 motors. in the past 14 years i have needed it.
1 time a propeller bend,
2 times a gearbox problem
on our new (old) boat a engine heat alarm afther 20min.
it was the temp sensor, but you cant take any risk. so we carry on on one engine
i also are a fond of 2 complete seperated engine system, each his ouwn start battery, electronics and fuel tank

my boats where second hand and you never now the exact state of the engines.
i had detroid diesel and carterpillars

Best regard
Patrick
 
ronobrien said:
Regarding engine make, model etc., though there are not a lot of V-8 diesels out there They should as a rule be avoided. (Cat 3208 comes to mind). The majority of diesels are I-6 (in line 6). There are issues with most engine manufacturers of different eras and models. If you are considering a specific boat, research the engine make/model & generation. Problems found in some earlier generations may have been resolved in later years. (Cummins B5.9 early casting issues). Some engines were great but the injection systems were a problem (Roosa rotary injection pumps have had their issues). That said a blanket rule is not always going to be true. Look at maintenance history, the more records the better. Have the engines surveyed for sure. Best of luck.
Click to expand...

Not sure Cat 3208s as a rule should be avoided.

Many have provided long, hard service in everything from trawlers to sportfish to fishing vessels to commercial work boats......mine and many I know were relatively low maintenance also.
 
Hippocampus said:
Had several long conversations with the gentleman who consults at present for Lugger/NL. He said singles fail less often and are rarely towed. He thought with a single any new noise, vibration or hiccup is identified and addressed ASAP. He thought this was both due to a higher level of concern if failure occurred as well as it being easier to be aware of new deviations from normal.
.
Click to expand...

Certainly my way of thinking
And with the big single, ease of access has regular checks and maintenance performed.

Never like doing miles with Genset running and very rarely do due to second engine noise in the mix.
 
I’d avoid twins if I a single engine can get me everywhere I want to go. With practice you can learn how to maneuver anything. It might take longer or conditions may need to change at worst. Huge savings in time and money.
 
JRO said:
I’d avoid twins if I a single engine can get me everywhere I want to go. With practice you can learn how to maneuver anything. It might take longer or conditions may need to change at worst. Huge savings in time and money.
Click to expand...
Savings are modest, not huge. And you can get everywhere you want to go with a single until it craps out and leaves you stranded.
 
Simi 60 said:
You make it sound like a regular occurrence?
Click to expand...
It has happened to me twice, both at very inopportune times so regularity is not the issue. Do you start your engines from your house bank or do you have separate starting batteries, for redundancy don't you kmow?
 
catalinajack said:
It has happened to me twice, both at very inopportune times so regularity is not the issue.
Do you start your engines from your house bank or do you have separate starting batteries, for redundancy
Click to expand...
We have seperate starts with both alternator and smart charger attached to keep them in peak condition.

At the flick of a switch we can get a boost using the house bank if required.


don't you kmow?
Click to expand...
Yes I do know.
 
1v2

Hydrospud said:
We are new to this type of boating. Stepping up from a 19.5' open bow to hopefully a 36'-44' trawler. I would prefer a single engine but there are several nice vessels for sale with twins.
What would the operating cost difference be between the single verses twins.
Standard Preventative Maintenance Costs?
Fuel Consumption?
What motors to stay away from?
Deb and I have been happily married for 28 years. I don't think the docking learning curve with either (single or twin) is going to have a negative effect on our marital bliss.
Are there any other +/- of single engines verses twins?
Thanks for your input and this forum is great!
Peter and Deb
Click to expand...

All great questions. One of my major criteria when buying my boat was 1 engine.
Backing a single screw boat takes practice but you can learn it easily.
One engine has 1 motor, 1 transmission, 1 shaft, 1stuffing box, 1 cutless bearing and the screw is protected by the the housing that holds the shaft and screw.
Two engines don’t have that.
I have my motor, transmission, shaft,cutless bearing and gen professionally maintained. A professional will find issues you will never see if you do it yourself. Just a standard annual maintenance with no issues will be $1000. Double that for 2 engines. When it’s time for serious maintenance, replacement or repair you are looking at thousands for two engines.
With one engine you can literally move around the motor and transmission with ease. With 2 motors you can barely get into the engine room.
The myth that two engines are more reliable than one is tempting but doesn’t hold water. A well maintained Diesel engine will probably only fail for 1 reason; fuel. If you have bad fuel you can have 2 engines or 10.
They will all fail. I have one engine with 7700 hours and it purrs like a kitten. This is my advice. I’m a one engine guy. Listen to all advice and make your decision.
Stay away from Volvos. Expensive to maintain and very difficult to get parts. I have 50 years boating experience, for what it’s worth.
A Ford Lehman 120 burns 2 gals/hr at 8 kts in calm seas.
 
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