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Old 01-08-2013, 09:24 PM   #321
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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!!!!!
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Old 01-08-2013, 09:30 PM   #322
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'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.
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Old 01-08-2013, 09:33 PM   #323
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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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Old 01-08-2013, 09:44 PM   #324
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Originally Posted by markpierce

Was counting engines, not motors.
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.
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Old 01-08-2013, 09:47 PM   #325
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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.
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Old 01-08-2013, 09:55 PM   #326
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Greetings,
Mr. Mark. So what do you put into your JD 4045 for lubrication?



By the same token...

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Old 01-08-2013, 10:01 PM   #327
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Mr. Mark. So what do you put into your JD 4045 for lubrication?

Ask my mechanic.

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Old 01-08-2013, 10:05 PM   #328
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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.
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Old 01-08-2013, 10:06 PM   #329
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Old 01-08-2013, 10:36 PM   #330
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are there any single engined duo proped trawlers?

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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Old 01-08-2013, 10:47 PM   #331
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The one engine- two shafts/props makes absolutely no sense to me. In terms of engine redundancy you have none so no advantage there.

And in terms of maneuverability, you don't have one of the most effective maneuvering tools of a twin which is not only to be able to use differential thrust but to use differential power, too. I am very often adding power to one engine and not the other to speed up a maneuver, counter a wind or current, or improve the accuracy of the maneuver.

While I guess you could split the thrust with a pair of transmissions driven by a single engine, you could not add power to one side while remaining at idle on the other.

So other than having redundancy in the shafts and props I see no advantage to it whatsoever.

A lot of good single-engine boat handlers know how to take advantage of the propwalk from their one propeller. Rather than bemoan it they have learned to take advantage of it and are able to do things that single-engine drivers who don't understand how to make propwalk work for you have no idea how to do. If the two props in the single engine-dual prop setup are counter-rotating, the setup even takes away a good maneuvering tool from the experienced single engine driver.

The cons greatly outweigh the pros in this case, so far as I can see.
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Old 01-08-2013, 11:00 PM   #332
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Greetings,
How about two engines/motors ONE variable pitch prop just to complicate matters further? I looked into x Swedish rescue boats about 8-10 years ago and a LOT of them had this set up. I suspect for towing or light ice breaking.
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Old 01-08-2013, 11:06 PM   #333
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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?
Re the drag of GB36 twin.
There's a lot more wave making resistance from the 3000# (approx) of the additional weight required from the extra engine, sea cock, strainer, control cables or hoses, larger tanks and more fuel, hoses, shaft, prop rudder ect ect ect. Perhaps 4000# would be closer.
There is the drag of the propeller shafts and struts.
Decreased wetted surface of the keel.
Engine inefficiency reduced due to engine underloading (25 to 35hp per engine)
Twice as much surface area for rudder drag.
Rudder drag to compensate for prop walk eliminated.
Less weight = lower in the water and more wetted surface.


The wetted surface of the keel and extra rudder could possibly eliminate each other but the extra rudder is in the propwash so probably more drag.

One could find out how much the extra disp from the weight of the extra propulsion system by computing said weight and putting it aboard a single and testing it.


Re the drag of the GB 36 single.
Much reduced drag due to much less wave making resistance due to less weight.
With less weight boat rides higher and has less wetted surface.
half as much rudder drag.
More drag from larger keel.
Engine not over loaded so thermally considerably more efficient.
Increased rudder drag compensating for prop walk.


Marin I almost told you to find an apples to apples pair of boats to compare and that your question would only shed light on the efficiency differences between GB boats. No mater what the above shows it will NOT tell much of anything about wether or not a single engined boat is more efficient than a twin. Twin engined boats in general may be more efficient or singles may be but comparing boats with twice as much and half as much total power is rather pointless. Compare a single w a 120hp engine w a same boat w two 60hp engines and you'd be close to a meaningful comparison.

Anyway I humored you and gave you what you asked ... I think.

Marin re the geared up one engine 2 screws ... ya basically need to read the article. Trust me Marin the maneuvering capabilities of the geared up system is almost as good as the regular twin. And they concluded it is more efficient that either the single or traditional twin. It's worth reading for those that are interested. It's in a PMM.

Chuck Gould,
I'm trying to keep this about twins v/s singles not the efficiency of one engine over another. I did indeed include in my post/statement that the thermo loss of the twin would be far greater ... in the GB example. But if your'e going to compare the drive system efficiency of singles and twins I would think one would want engines w the same disp per hp, weight per hp, power per pound so only the difference in efficiency of the drive system emerges. But first of all I think one needs to compare boats w the same total power. And most all the trawlers on this forum have twice as much hp in twins and half as much hp in singles. So even a good comparison won't be applicable to the boats on this forum.
But re the above basically most all the boats considered here have singles being more efficient that twins, mostly due the engine loading and the drag from increased weight that causes greater wave making losses.
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Old 01-08-2013, 11:48 PM   #334
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Eric--- While I don't dispute your drag comments there are a couple of points to consider. The spade rudders on the twin GB are each considerably smaller than the one rudder on the single.

The hp of each of our FL120s at our cruise rpm of 1650 with the props we have is about 60 hp per engine. So I'm not sure where you got that 25-35 hp figure unless you are saying that the loading of EACH engine in the twin is 25-35 hp LESS than the loading of the ONE FL120 at the same cruise speed in the single-engine version of the boat.

I don't have PMM magazines to reference, but the only question I have with regards to the single engine-dual shaft/prop setup is: can one of the props be spun up to a high rpm while the other prop remains at a low rpm regardless of which direction the props are turning or if they are opposing each other with one generating forward thrust while the other is generating rearward thrust?
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Old 01-09-2013, 12:01 AM   #335
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Most everything said here I have heard in other single vs. twin arguments, but Chuck Gould offers something new in that twins are more likely to sink and qualifies it by saying that he has seen a few sent to the bottom. Should't that end this debate? Since twins are likely to sink, what use will having a redundant engine be?

So to those still shopping for a boat, do not buy one with twins unless you are a strong swimmer.
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Old 01-09-2013, 12:17 AM   #336
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Well, now wait a minute, Mahal. I am aware of some single-engine boats that had their hulls punched open by running into deadheads and reefs and stuff and they sank, too.

So I'm thinking that this is a FAR bigger issue than any of us first thought. Perhaps what we are discovering here is that BOTH single AND twin engine boats can be sunk by slamming into something. In which case the only sure-fire way to avoid this potential catastrophe is to STAY THE HELL AWAY FROM BOATS.

What do you think? Has that finally put the single vs twin issue to rest?
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Old 01-09-2013, 12:49 AM   #337
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C. Marin is absolutely correct that single engine boats can also be holed by a deadhead and sink.

The critical difference isn't a case where some deadhead comes rocketing vertically up from the depths and makes an unwelcome intrusion into the bilge. There would be no clear advantage, single or twin, in such situation.

Consider the far more common encounter with a deadhead. There's a submerged log, floating more or less horizontally, just below the surface. Strike that with a single, and it will make a lot of noise as it thumps its way along the keel and pops up (giving you the equivalent of a deadhead finger) immediately astern. No holes in boat, nothing damaged, usually, except your pride. (Don't ask me how I know).

Strike the same deadhead with a twin screw trawler. It thumps its way along the keel, but can't pass harmlessly astern because it runs into the strut and the exposed shaft. Hey, maybe 19 out of 20 times even *this* isn't going to tear out a strut. Then comes the guy unlucky enough to statistically be #20. One 12 x 18" hole in the bottom of your boat can wreck your entire day.
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Old 01-09-2013, 01:14 AM   #338
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The one engine- two shafts/props makes absolutely no sense to me. In terms of engine redundancy you have none so no advantage there.
Agree, and two engines and one well-protected shaft/propeller/rudder makes more sense even at the expense of paranoic redundancy.
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Old 01-09-2013, 01:29 AM   #339
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...., maybe 19 out of 20 times even *this* isn't going to tear out a strut. Then comes the guy unlucky enough to statistically be #20. One 12 x 18" hole in the bottom of your boat can wreck your entire day.
Every time you pass an oncoming car on a two lane road there is a chance that the driver will be drunk or strung out or falling asleep and he/she will swerve over and slam into you head on. Given the number of times this actually happens on just one stretch of I-5 here, I think it is actually a much greater statistical risk than hitting a deadhead with your boat and having it take out the running gear in such a way as to sink it. So are you going to stop driving your car?

This "it could sink your boat" reasoning for preferring a single to a twin is just dumb as far as I'm concerned. If it was really a credible threat, you would think this would have long been figured out by everyone including the boat manufacturers, yet Grand Banks, Fleming, and just about every boat manufacturer on the planet keep on turning out twin-engine death traps and people keep buying them up and--- gasp-- NOT SINKING.

For another thing, deadheads are called that because they float vertically, not horizontally. That's why they're so hard to see--- just the top end of it is visible lifting in and out of the water. Hence the "head" in the name. And sometimes they float vertically just below the surface of the water.

As such, they can and have done considerable damage to the props and rudders of single engine boats, too. We know one couple who used to be on our dock who had the prop and rudder ripped off their sailboat, of all things, by a deadhead they failed to see. If there's anything out there with a protected prop and rudder it's a sailboat but....... you just never know.

Deadheads almost invariably are hemlock logs. So if you really want to protect your boat from deadheads, the best thing is to figure out a bug or a poison that will kill off the hemlock population of the PNW.

So while I certainly agree that twin drivelines can be more vulnerable to some kinds of waterborne debris, this notion that they are putting every twin engine operator in dire peril is silly and is certainly no reason to pick a single over a twin. There are plenty of valid and sensible reasons for making the decision one way or the other, but i sure don't see this as being one of them.
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Old 01-09-2013, 01:41 AM   #340
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Tad is right BUT Tugs are not cruising boats.
Our tug fleet consists of Z Tech tugs and Rotor Tugs.
The Zedies have 2 main engines with azmithuing props ( ie 360 deg drive)
Highly manoueverable and with 60 Tonne bollard pull draught of approx 4.5 mts
Rotor tugs have 3 main engines even more highly manoueverable and with a bollard pull in excess of 80 tonnes.
BUT none of you would like to go to sea in one. When we have to deliver one of these harbour tugs down the coast it is difficult to get even seasoned seamen to man them.
Look up Ztechtug.com
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