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markpierce
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Al, it is a bottomless pit.
Single vs Twin: It's Baaaaack!
- Thread starter markpierce
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The friendliest place on the web for anyone who enjoys boating.
If you have answers, please help by responding to the unanswered posts.
If you have answers, please help by responding to the unanswered posts.
FF
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Al, it is a bottomless pit.
NO it is a religion , just like anchors, fear of gasoline or propane , or for some leaving the dock
NO it is a religion , just like anchors, fear of gasoline or propane , or for some leaving the dock
Anode
Guru
I actually figured it out 3 or 4 boats ago....
bfloyd4445
Guru
as Eric pointed out in order to answer this question we will have to establish the criteria upon which we will base our answer then graph the results. We would also need a list of the advantages and disadvantages of twins and singles. If we do this it appears that there are only two tentative advantages of twins over singles and those are: 1) manuveurability < trumped by singles with thrusters>
2) less chance of being stranded because of a redundant complete engine system
The advantages of a single according to this thread are:
1) greatly improved fuel efficiency
2) prop and rudder protected by keel
3) less weight allowing for greater cargo
4) half the maintenence costs of a similiar vessel with twins
5) Much quieter operation than twins
6) More room in the engine compartment
7) More storage area for gear
8) initial cost much less than the same boat with twins
have i missed any?
SomeSailor
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Can't get a thruster on a twin?
Define "greatly"
Bigger boat... no problem stowing everything the wife has shown up with yet.
Much better resale value, greater perceived market value among those looking for twins over a single for a 40,000# trawler.
RT Firefly
Enigma
Greetings,
Mr. FlyWright. Nope THAT question will NEVER be settled to the satisfaction of some and there's no RIGHT answer. It's like at the strip. Ya run wut ya brung. The vessel I "babysit" was never offered, as far as I know, with a single option. Would i have preferred a single if I had to make the choice? Yes and I concur with some of Mr. 45's points but I got what I got and that's just fine with me. One can ponder and rationalize until they're blue in the face but, in this the 306th post, I just don't care. By the way the answer to the ultimate question is 42.
Just saw Mr. SS's post to which I might add; singles or twins are as quiet as the sound insulation allows. I have to listen quite intently to synchronize the engines at cruise.
Mr. FlyWright. Nope THAT question will NEVER be settled to the satisfaction of some and there's no RIGHT answer. It's like at the strip. Ya run wut ya brung. The vessel I "babysit" was never offered, as far as I know, with a single option. Would i have preferred a single if I had to make the choice? Yes and I concur with some of Mr. 45's points but I got what I got and that's just fine with me. One can ponder and rationalize until they're blue in the face but, in this the 306th post, I just don't care. By the way the answer to the ultimate question is 42.
Just saw Mr. SS's post to which I might add; singles or twins are as quiet as the sound insulation allows. I have to listen quite intently to synchronize the engines at cruise.
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bfloyd4445
Guru
Bigger boat... no problem stowing everything the wife has shown up with yet.
Much better resale value, greater perceived market value among those looking for twins over a single for a 40,000# trawler.[/QUOTE]
Wife good point and so is the resale value. I guess since the inititial cost is more for a twin that the cost savings mentioned in my list as an advantage for the single is canceled out.
new list
advantages of twins over singles and those are:
1) manuveurability
2) less chance of being stranded because of a redundant complete engine system
3) better resale value
The advantages of a single according to this thread are:
1) greatly improved fuel efficiency
2) prop and rudder protected by keel
3) less weight allowing for greater cargo
4) half the maintenence costs of a similiar vessel with twins
5) Much quieter operation than twins
6) More room in the engine compartment
7) More storage area for gear
8) initial cost much less than the same boat with twins
Much better resale value, greater perceived market value among those looking for twins over a single for a 40,000# trawler.
[/QUOTE]Wife good point and so is the resale value. I guess since the inititial cost is more for a twin that the cost savings mentioned in my list as an advantage for the single is canceled out.
new list
advantages of twins over singles and those are:
1) manuveurability
2) less chance of being stranded because of a redundant complete engine system
3) better resale value
The advantages of a single according to this thread are:
1) greatly improved fuel efficiency
2) prop and rudder protected by keel
3) less weight allowing for greater cargo
4) half the maintenence costs of a similiar vessel with twins
5) Much quieter operation than twins
6) More room in the engine compartment
7) More storage area for gear
8) initial cost much less than the same boat with twins
Tad Roberts
Guru
I was thinking about this sometime in the night with regards to modern tugs, which almost without exception have multiple engines and drives. While they certainly do this for the aforementioned manoeuvrability and system redundancy, I think there are a couple of other good reasons as well.
Draft......Overall draft (with equal power and thrust) will be less with twin drives than with a single prop. This adds versatility to the job envelope......
Power Density.....In general it's possible to cram more horsepower into a smaller hull using twins. This pays off in smaller build costs and lower operating due to manning requirements based on tonnage. It doesn’t really apply to recreational boats except that (at least in theory) there could be more accommodation space and a smaller engine room.[FONT="][/FONT]
Draft......Overall draft (with equal power and thrust) will be less with twin drives than with a single prop. This adds versatility to the job envelope......
Power Density.....In general it's possible to cram more horsepower into a smaller hull using twins. This pays off in smaller build costs and lower operating due to manning requirements based on tonnage. It doesn’t really apply to recreational boats except that (at least in theory) there could be more accommodation space and a smaller engine room.[FONT="][/FONT]
psneeld
Guru
Bigger boat... no problem stowing everything the wife has shown up with yet.
Much better resale value, greater perceived market value among those looking for twins over a single for a 40,000# trawler.
Wife good point and so is the resale value. I guess since the inititial cost is more for a twin that the cost savings mentioned in my list as an advantage for the single is canceled out.
new list
advantages of twins over singles and those are:
1) manuveurability
2) less chance of being stranded because of a redundant complete engine system
3) better resale value
The advantages of a single according to this thread are:
1) greatly improved fuel efficiency
2) prop and rudder protected by keel
3) less weight allowing for greater cargo
4) half the maintenence costs of a similiar vessel with twins
5) Much quieter operation than twins
6) More room in the engine compartment
7) More storage area for gear
8) initial cost much less than the same boat with twins[/QUOTE]
by PSNEELD- Your lists are inaccurate because there are too many examples of one being just the opposite....go back to square one....nevermind, don't start over...it's a waste of bandwidth to come up with absolutes that aren't...compare two boats side by side..but two general categories and all you can produce is generalities.
bfloyd4445
Guru
Are you saying that the addition of a second drive system's added weight results in less draft?
Power density? Hadn't thought of that. I tend to think of fuel economy and hull speed but then i am thinking of recreational boats
bfloyd4445
Guru
you got a point there, so how to we answer the original question?
Sailor of Fortune
Guru
Tads right in respect to less draft with twins. Think deeper keel with the single engine. To build to a required Bollard pull is easier with twin engine setup. Because of the configuration of the engine room setup, it is usually more tonnage (Think documentation tonnage). Remember, tonnage is a volumetric calculation rather than a weight calculation.
psneeld
Guru
Nomad Willy
Guru
Like TAD said some time ago. Singles are about CHEAP. They cost considerably less. So the greatest advantage of singles is that ther'e cheaper.
And Ilm not convinced singles are more efficient. Twins are more efficient in one significant way. They don't need to deploy their rudders 3 to 6 degrees to keep the boat going straight. Prop walk in forward gear is at least as effectual as it is in reverse but it's effects are felt all day long. And other than the drag of shafts and struts ther'e is no difference except w our typical double power twins except that they are over a ton more in weight to the singles. For slow vessels the amount of power required is most often calculated by the amount of power per ton of displacement so the extra weight is basically directly proportional to the extra fuel burn.
So if you realistically compare twins w the same disp and power (no other way is valid) I think personally that twins will come out ahead. I know most disagree w me but that what I think and why I think it. It all hinges on the question of "is the drag of the prop walk greater than the drag of the struts.
So to compare them honestly you'd need to create a pair of boats both halving their shafts going through a keel and their keel area must be the same and their total power must be the same. Or none of their shafts located inside their keels. I know of no such pair of boats
So if one compares the typical trawler w it's single engine shaft going through the keel w much less weight ... To ... The twin w both shafts dragging through the water and w all that extra weight
SURE the single will be more efficient!
But if tou want to compare singles and twins on equal terms I suspect the twin would come out more efficient.
And Ilm not convinced singles are more efficient. Twins are more efficient in one significant way. They don't need to deploy their rudders 3 to 6 degrees to keep the boat going straight. Prop walk in forward gear is at least as effectual as it is in reverse but it's effects are felt all day long. And other than the drag of shafts and struts ther'e is no difference except w our typical double power twins except that they are over a ton more in weight to the singles. For slow vessels the amount of power required is most often calculated by the amount of power per ton of displacement so the extra weight is basically directly proportional to the extra fuel burn.
So if you realistically compare twins w the same disp and power (no other way is valid) I think personally that twins will come out ahead. I know most disagree w me but that what I think and why I think it. It all hinges on the question of "is the drag of the prop walk greater than the drag of the struts.
So to compare them honestly you'd need to create a pair of boats both halving their shafts going through a keel and their keel area must be the same and their total power must be the same. Or none of their shafts located inside their keels. I know of no such pair of boats
So if one compares the typical trawler w it's single engine shaft going through the keel w much less weight ... To ... The twin w both shafts dragging through the water and w all that extra weight
SURE the single will be more efficient!
But if tou want to compare singles and twins on equal terms I suspect the twin would come out more efficient.
bfloyd4445
Guru
I wasn't saying he was wrong i just didnt understand. Thinking about it the light bulb has lit
thank youbfloyd4445
Guru
are there any single engined duo proped trawlers? There are boat power system manufacturers that use two counter rotating props on the same shaft with a single engine. Volvo and Merc both make I/O systems like this
Prop walk loss is still there with a twin just that the off tracking is canceled by the other engine. So wouldn't you still have a net loss in efficiency with the twin?
bfloyd4445
Guru
1) manuveurability < trumped by singles with thrusters>
2) less chance of being stranded because of a redundant complete engine system
The advantages of a single according to this thread are:
1) greatly improved fuel efficiency Wrong Based upon what others have said in this forum there is a fuel savings
2) prop and rudder protected by keel Many twins have deep keels True but all single engine displacement trawlers have protection
3) less weight allowing for greater cargo Silly concept on a trawler No so, i can think of many trips in which every available bit of space was filled with food drink toys for the raft up party on the 4th[FONT=Arial, Helvetica, sans-serif]. One can always use extra space[/FONT]
4) half the maintenence costs of a similiar vessel with twins Baloney, price out oil changes on a pair of JD 4045s vs a JD 6081. Thats like comparing an apple to a pear. I was thinking like one lehman as opposed to two
5) Much quieter operation than twins Two engines at 80 dba or one at 80 dba is same noise I find it hard to believe that a 36gb with twins has the same level of cabin noise as one with a single
6) More room in the engine compartment If you are comparing a 34' sincle to a 48 foot twin, baloney I'm comparing like boat to like boat. Not accurate to compare a porsch to a vw
7) More storage area for gear Really, gear or junk .....your right it will likely end up filled with her junk
8) initial cost much less than the same boat with twins I have recent new boat price lists with a large single vs a set of smaller twins, not much difference overall interesting idea, i will have to check that out[FONT=Arial, Helvetica, sans-serif]. Thanks[/FONT]
have i missed any Yes, there are few single engine choices above 45' unless you go with a very expensive builder such as Nordhavn or Selene. Many twin alternatives are available at much lower costs for a similar sized boat Thats another varible we havent nailed down, vessel size. I'm thinking under 49ft
Eric--- With regards to your above post, it would be interesting to know how the drag of a twin engine boat like our GB36 compares to the drag of the single engine version of the same boat.
The reason I wonder about this is that while our twin engine boat has a pair of shafts, struts, and spade rudders, the deep keel of our boat is cut short and reverse-curves up to blend into the bottom of the boat. As opposed to the deep keel of the single-engine boat which is carried a fair amount farther back to culminate in a vertical trailing ege and a shoe supporting a big rudder.
Don't know how it works with hydrodynamics but the single engine GB36 has, I think, more wetted surface than the twin. Would this result in more drag?
The reason I wonder about this is that while our twin engine boat has a pair of shafts, struts, and spade rudders, the deep keel of our boat is cut short and reverse-curves up to blend into the bottom of the boat. As opposed to the deep keel of the single-engine boat which is carried a fair amount farther back to culminate in a vertical trailing ege and a shoe supporting a big rudder.
Don't know how it works with hydrodynamics but the single engine GB36 has, I think, more wetted surface than the twin. Would this result in more drag?
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markpierce
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Definitely. More wetted surface creates more drag.
A keel-less boat will have less wetted surface.
A keel can provide protection to shaft, propeller, and rudder.
A two engined/propellered boat will need at least two keels to provide protection. Expect two keels to have more wetted surface then one keel.
If two smaller engines were more efficient than one larger engine of the same total horsepower, wouldn't there be more dual-engined automobiles?
A keel-less boat will have less wetted surface.
A keel can provide protection to shaft, propeller, and rudder.
A two engined/propellered boat will need at least two keels to provide protection. Expect two keels to have more wetted surface then one keel.
If two smaller engines were more efficient than one larger engine of the same total horsepower, wouldn't there be more dual-engined automobiles?
RT Firefly
Enigma
Greetings,
Hmmmm...two engines in a car eh?.......Prius, Lexus CT200h, Ford Fusion, Honda Civic, Hyundai Sonata, Ford C-max, Chevy Malibu....Nope, it'll NEVER work!!!!!
Hmmmm...two engines in a car eh?.......Prius, Lexus CT200h, Ford Fusion, Honda Civic, Hyundai Sonata, Ford C-max, Chevy Malibu....Nope, it'll NEVER work!!!!!
Chuck Gould
Senior Member
'Twas said: "So if you realistically compare twins w the same disp and power (no other way is valid) I think personally that twins will come out ahead. I know most disagree w me but that what I think and why I think it. It all hinges on the question of "is the drag of the prop walk greater than the drag of the struts."
Doesn't your theory presuppose that a pair of 125-HP diesels will produce the same effective thrust per gallon as a 250-HP single? As every engine suffers from some inefficiencies in converting potential BTU's injected as fuel into thrust, why would each engine in a pair of "twins" be only half as inefficient as an engine used in a single application?
Having seen a few twin screws sent to the bottom when a strut is ripped off to leave a 3 or 4 sq ft hole in the hull, I'm personally a fan of single engines. Getting back to port after an engine shut down is academic if hitting a deadhead tears away your running gear and opens your bottom like a can opener.
Doesn't your theory presuppose that a pair of 125-HP diesels will produce the same effective thrust per gallon as a 250-HP single? As every engine suffers from some inefficiencies in converting potential BTU's injected as fuel into thrust, why would each engine in a pair of "twins" be only half as inefficient as an engine used in a single application?
Having seen a few twin screws sent to the bottom when a strut is ripped off to leave a 3 or 4 sq ft hole in the hull, I'm personally a fan of single engines. Getting back to port after an engine shut down is academic if hitting a deadhead tears away your running gear and opens your bottom like a can opener.
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markpierce
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Greetings,
Hmmmm...two engines in a car eh?.......Prius, Lexus CT200h, Ford Fusion, Honda Civic, Hyundai Sonata, Ford C-max, Chevy Malibu....Nope, it'll NEVER work!!!!!
Was counting engines, not motors.
The Coot has one engine (JD 4045 diesel) and at least 16 motors (windlass, bow thruster, air compressor, two bilge pumps, two fuel pumps, toilet pump, two water pumps, and six fans).
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CPseudonym
Moderator Emeritus
markpierce said:
I think he has you Mark. Engine, motor, or sail are all propulsion units.
When ya get right down to the bottom line the Coot has twin propulsion.
Nomad Willy
Guru
Chuck Gould wrote
"why would each engine in a pair of "twins" be only half as inefficient as an engine used in a single application?"
Not sure I follow. Could you state that another way?
Floyd,
No duo-props.
"why would each engine in a pair of "twins" be only half as inefficient as an engine used in a single application?"
Not sure I follow. Could you state that another way?
Floyd,
No duo-props.
RT Firefly
Enigma
Greetings,
Mr. Mark. So what do you put into your JD 4045 for lubrication?
By the same token...
Mr. Mark. So what do you put into your JD 4045 for lubrication?
By the same token...
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markpierce
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Chuck Gould
Senior Member
Restating:
A certain percentage of the fuel burned in any engine is not going to produce thrust. Some of the energy extracted from the fuel heats up the engine block, head, etc. Some of it is used to turn an alternator. There is additional efficiency lost in the gear box.
While there are fairly good models for the amount of fuel consumed per HP produced, to theorize that the only difference between a pair of 125-HP engines and a single 250-HP engine is whether one large rudder creates more drag than two small rudders and a pair of struts (which, I believe is an example of your point) depends on the assumption that each of the 125-HP engines is losing only half as much potential energy in each gallon of fuel to mechanical inefficiencies.
In so many cases with modern diesels, the displacement of the 250-HP single engine can be pretty similar to the displacement of each of the 125-HP twins. Two engine blocks are going to radiate a lot more heat than a single engine block of similar size. Two alternators will require more fuel to turn than a single alternator, etc.
The additional weight of the second engine is yet another factor that cannot be ignored when considering efficiencies.
A certain percentage of the fuel burned in any engine is not going to produce thrust. Some of the energy extracted from the fuel heats up the engine block, head, etc. Some of it is used to turn an alternator. There is additional efficiency lost in the gear box.
While there are fairly good models for the amount of fuel consumed per HP produced, to theorize that the only difference between a pair of 125-HP engines and a single 250-HP engine is whether one large rudder creates more drag than two small rudders and a pair of struts (which, I believe is an example of your point) depends on the assumption that each of the 125-HP engines is losing only half as much potential energy in each gallon of fuel to mechanical inefficiencies.
In so many cases with modern diesels, the displacement of the 250-HP single engine can be pretty similar to the displacement of each of the 125-HP twins. Two engine blocks are going to radiate a lot more heat than a single engine block of similar size. Two alternators will require more fuel to turn than a single alternator, etc.
The additional weight of the second engine is yet another factor that cannot be ignored when considering efficiencies.
Ocean Breeze NL
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There is also a twin to single conversion in a mid-40' "trawler" that uses a silent chain drive (like an auto timing chain) to distrubute power from a single engine to the two original transmissions. Ramsey Silent Chain did a great deal of the design work (mostly for the pressure lubed chain enclosures). The chain drive is much simpler with fewer failure points than the shaft/gearbox arrangement shown bove. Compact and robust with low losses. Changes to overall drive ratios as simple as changing sprockets. Same concept was used in reverse for a Navy test program where four 600 HP diesels were harnessed to two props in a fast assault boat.
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