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Old 08-11-2011, 09:57 AM   #21
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Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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DavidM wrote:
The "hard numbers" you want are in the link I posted early on in this thread. No opinions, just numbers. They were obtained by the guy who ran a boat yard in Oak Harbor, Wa. His*boat is an Alaskan 45, a wooden hull Grand Banks with twin*Lehmans*equipped with*Flowscans.
Makes one*wonder what the fuel efficiency increase and maximum-speed reduction*would be with the same boat/engine with a single engine on the centerline.


-- Edited by markpierce on Thursday 11th of August 2011 10:04:21 AM
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Old 08-11-2011, 12:30 PM   #22
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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Here is a link to the data that Marin cited- http://www.mv-dreamer.com/SpeedTrials.htm

_______________

In case there are new people on the string, this is the link to the data DavidM is referring to above. *Certainly not a scientific study, but a heck of a lot better than I could have done!!

David a couple of questions for you; In your opinion, what explains the difference in the results between port and starboard engine? *Drive train performance, hull design, data collection errors due to enviromental changes?

Also the last column for port and starboard engine "Range % Increase Freewheeling vrs running twins". *45.96% and 55.36 % increase respectively, seems substancial. *Am I reading that correctly?

Larry B

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Old 08-11-2011, 01:27 PM   #23
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Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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*Sorry Marin, but I can't quite reconcile the above with the fact we all know you just love narrow-boating in this very bollocking UK you speak of...

Ahh, but narrowboating is not "boating" in the sense that we think of it on this forum.* Narrowboating is running a recreational version of what was originally a cargo-hauling watercraft being pulled along by a horse in a ditch not much wider than the boat.* The canal system itself came out of a need to move stuff faster and cheaper than it could be moved by handcart, then pack horse, and then wagon.* Narrowboating is much more akin to a walk through the country than it is to boating.* The only real difference is you don't have to walk, and you take your bed, kitchen, and bathroom along with you.

You couldn't go fast in a canal if you wanted to.* For one thing, the canal itself would not hold up, which is the primary reason for the 4mph speed limit.* The narrowboat we use will actually go much faster than that--- it has lots of power--- but the wakes from this kind of operation would erode the canal banks in short order, to say nothing of the damage caused to bridge abutments, moored boats, docks, etc.

And in the UK, the moment someone invented the railroad, the canal system--- while it continued to be used to haul low-value bulk commodities like coal until the 1950s--- immediately began to decline.* Even back then, people knew that fast was way better.

This is a far cry from going out in a boat on a body of water like Puget Sound or the Pacific.* There you can blast along as fast as you can afford, and the faster I can get across the big open stretches to the places we want to explore, the better.


-- Edited by Marin on Thursday 11th of August 2011 01:30:56 PM
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Old 08-11-2011, 02:34 PM   #24
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

For the record, Bob Lowe's Dreamer does not have Ford Lehman 120s. The boat is fitted with John Deere Ammarine engines.
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Old 08-11-2011, 04:48 PM   #25
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Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Regarding my answer to Peter about what I believe to be the difference between*boating and narrowboating, here is an illustration of what I said.

Photo 1.* Apples

Photo 2* Oranges


-- Edited by Marin on Thursday 11th of August 2011 04:49:56 PM
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Old 08-11-2011, 05:24 PM   #26
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Darn go fast Mule. Overpowered.
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Old 08-11-2011, 06:23 PM   #27
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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Darn go fast Mule. Overpowered.
*If it was a mule, you'd be correct.* Way overpowered.* But they used horses on the canals which by Eric Henning's power to speed to weight to drag ratios are much better suited for the job.
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Old 08-11-2011, 10:36 PM   #28
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Stan,

Numbers are not very meaningful as there are too many variables. A narrow boat w props close in toward the keel v/s a wide boat w engines far apart. Big props small props. Small rudders big rudders. There was a good thread in BoatDesign.net. Go there and join* ..free. Find the thread under "Power boats". There was a comment about a boat that routinely removed one prop and went to Juneau from Seattle saving considerable fuel. Seems like a sorta crosseyed thing to do but w most boats it can save some fuel. But I think John P has it right saying "just buy a single engined boat" or better yet just get one more fuel efficient. There are some that think a twin is more fuel efficient so the theory that a single is more efficient isn't exactly cast in stone either.
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Old 08-11-2011, 11:58 PM   #29
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Problem is that virtually all the boats we're looking at are twins.* aonly single screw boats in the 50-60' range are Selenes and Nordhavens.* Don't like the latter and can't afford the former.*
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Old 08-12-2011, 12:14 AM   #30
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

If you can lower your size requirement a little, Krogens are single-engine boats and very good ones indeed. There is also the 49' Victory Tug, which I believe, like the 37' VT, is a single. Nordic Tug makes at least one model over 40' and I'm pretty sure it's a single. There are 42' Grand Banks that are singles although the singles tend to be older boats.
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Old 08-12-2011, 12:26 AM   #31
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

How about a 2011 52' Nordic Tug, new, It's a single, first 52' single I've seen. *It's on G dock here at Cape Santa marina in Anacortes. *Go see Greg at the Nordic Tug office, I'm sure he would give you a tour.

*

Oh, asking price is 1.3 big ones
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Old 08-12-2011, 12:26 AM   #32
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

We have very specific requirements for liveability aboard the boat.* One of those requirements is that the boat have a good size salon and a good size outside area (e.g., sundeck, flybridge, cockpit).* The boats you listed have a tendency to chop up their interiors with large foredecks and a good size pilot house but leaving one with a small galley, a small salon, small flybridge and either a reasonable cockpit or one only useful* as a fishing platform.
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Old 08-12-2011, 12:28 AM   #33
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Ouch!* If that was doable I wouldn't be worrying about fuel costs.
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Old 08-12-2011, 05:57 AM   #34
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Here is what I dont understand....
A 40 footer runs just fine on a single 120HP engine. If You have twins and only run on one, the fuel consumption is almost the same? Does that mean if you run both, you are running each one individually on only half the consumption rate as a normal single would be running to give approximately the same overall consumption rate converted to MPG? Does this mean that the 2nd engine is only needed to provide enough power to turn it's own prop? Using the same logic, does this mean that if I had a single screw boat and were to drag something in the water behind me (Like a dinghy with maybe some weeds caught in the prop and just dragging) to create the same drag equivelant as a twin engine running only on one with a non-rotating prop and some rudder drag, that my fuel consumption rate would actually almost double?
I know there are always going to be some losses in power running on one but find it difficult to believe that it would be almost the same as running 2 considering the first engine is pushing up to 30,000 lbs of boat and the second engins only has to spin it own prop and overcome some drag on the rudder.
When in a sailboat, the rudder is typically around a 5 degree off of normal. When more than that, you have to trim your sails. There is a loss in power, no doubt, if you are more than 5 degrees off but not twice as much loss. You can feel it when you have a tiller on a heavy sailboat.
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Old 08-12-2011, 08:27 AM   #35
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Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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rostan wrote:
Ouch!* If that was doable I wouldn't be worrying about fuel costs.
*Yeah, I agree!! *For 1.3 Million I wouldn't be looking at a Nordic Tug either. *Maybe a 61" Tolly for half the price or something like that. *They seem to be a niche boat and attract a certain croud. *My friends who have bought them were all long time sailboaters who finally came over from the dark side.** That's a joke, don't get excited Marin. *A 39' has an asking price of 1/2 million!!

The part about them being sailboaters is true. *When asked why they chose a Nordic, they always point out the amenities, style, simularities to a sailboat (HUH), etc. *I don't get it, but "to each their own, said the man as he kissed the cow!"

LB


-- Edited by Edelweiss on Friday 12th of August 2011 08:28:05 AM
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Old 08-12-2011, 09:51 AM   #36
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Tony,

The practice of running one engine on a twin overcomes the losses of asymmetrical thrust, prop drag ect by reducing the heat loss of one engine. Diesel engines are heat engines and any heat lost without doing any work is a total loss. The combustion chamber (including the piston crown) and cylinder area (the length of the stroke) is a very high heat loss as all this area is exposed to combustion temperatures. The heat goes into the cooling system and into the sea. There are many other ways heat is lost while an engine is running but when you stop one engine on a twin a great deal of heat loss is eliminated. Two engines producing 30hp each produce much more heat than one engine producing 60hp.
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Old 08-12-2011, 12:45 PM   #37
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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Does this mean that the 2nd engine is only needed to provide enough power to turn it's own prop? ....I know there are always going to be some losses in power running on one but find it difficult to believe that it would be almost the same as running 2 considering the first engine is pushing up to 30,000 lbs of boat and the second engins only has to spin it own prop and overcome some drag on the rudder.

*I don't have anywhere near the technical knowledge about this sort of thing as Eric, but the notion that in a twin one engine is doing all the work and the other engine is coasting is not correct.* Both engines are working equally hard (assuming the right prop pitches, gear ratios, etc.) to move the boat.* But since you have two engines moving the boat instead of just one, each engine does not have to work as hard as the one engine in the single-engine version of the same boat.* This is why the fuel consumption of a twin is not double that of the same boat with a single.

If you run a twin engine boat on one engine this one engine now has to do ALL the work of moving the boat PLUS overcome the drag caused by the freewheeling or locked unpowered prop plus the drag from the rudders that will now be at some angle to keep the boat going straight plus the drag caused by the hull not being evenly driven.* So this one engine now has to work harder--- perhaps much harder--- than the same engine in the same boat but configured as a single engine boat.* Whether the amount of fuel burned by the one operating engine in a twin equals the amount of fuel both engines would burn under normal conditions will depend on how much drag is being produced by not running both engines.

This is why to really get a benefit from running on one engine, the unpowered prop should be removed (or at least feathered if the boat has that kind of capability).

But I believe the objective of putting two engines in a boat has more to do with things like redundancy, maneuverability, and---* if the hull configuration makes this possible--- speed.* Maximum fuel efficiency is not the objective with a twin-engine boat.
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Old 08-12-2011, 01:59 PM   #38
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Thank you Marin.

I think if one had lots of variables stacked up against running single like a wide spacing between screws (props) small or otherwise undesirable rudders ect there may be a point where running single on a twin will burn more fuel instead of less. Another element that I think has'nt been mentioned is the question of the fact that without counter rotating props one engine will run more efficient than the other. Think of a single screw. If you had a right hand prop the boat would want to turn to port and the rudder needs to be deployed to some degree to counteract that tendency. So if you were trying to run a twin with the port engine only and it's prop was right hand the "prop walk" would be turning the boat to port and the asymmetrical thrust would be turning the boat to stbd so it would require less rudder deflection to hold a course than if that port prop was left hand. If you had counter rotating props both would be advantageous or disadvantageous. Can't remember which way most twins turn but most all are one way. But if you had all the variables stacked up in your favor it could be almost as efficient as a single engined boat. So if you had a short wide boat w counter rotating props very far apart and the port prop was left hand and the rudders were small and the keel was small.........toooo many variables. It may work great and it may not work at all but I think (opinion) that on most boats it will work to a significant degree.
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Old 08-12-2011, 02:18 PM   #39
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Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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nomadwilly wrote:Can't remember which way most twins turn but most all are one way.
I'm not sure if this is correct, Eric, at least not with more modern producton twins.* Twin engine GBs, for example, have counter-rotating props as have most of the other twins I've seen in the Seaview Yard in Bellingham (when I've bothered to check).* The prop configuation is always the same.* The port prop is a left-hand prop (turns counterclockwise when viewed from the rear) and the starboard prop is a right-hand prop (turns clockwise when viewed from the rear).* There is a reason for this specific rotation setup that has to do with what the prop wash does when the transmission is put in reverse and the direction of the combined prop walk when one transmission is put in forward and the other in reverse.* In short, the boat is more maneuverable with this rotation setup.

Some twins have counter-rotating engines.* The gas-powered 28' Uniflite I fished on in Hawaii had a pair of counter-rotating Chryslers.* I believe most twin-engine diesel boats have engines that turn the same way.* FL120s, for example, all turn the same way (counterclockwise when viewed from the rear).* Reversing the direction of the starboard prop is accomplished by an extra gear in the starboard BW Velvet Drive which is why the final drive ratios of the port and starboard transmissions are different.

Twin engine boats from the 60s or so back may tend to have props that all turn the same way.* The three props on the PT boats of WWII all turned the same way, which when combined with the boats' tiny spade rudders made them all but unmaneuverable at idle speeds.

PS--- The prop shop where we had our props re-pitched and balanced a few years ago gave me a tip on how to tell if a prop is right or left handed.* If you face the prop from the rear, if the blade on the left is pitched so you can reach out with your left hand and put your palm on the blade with your hand slanted away from you, it's a left hand prop.* If the blade on the right is pitched so you can reach out with your right hand and put your palm on the blade with your hand slanted away from you, it's a right handed prop.


-- Edited by Marin on Friday 12th of August 2011 02:23:49 PM
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Old 08-12-2011, 07:32 PM   #40
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RE: Cruising a twin screw Semi-displacement boat for minimum fuel consumption

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Marin wrote:nomadwilly wrote:Can't remember which way most twins turn but most all are one way.
I'm not sure if this is correct, Eric, at least not with more modern producton twins.**The prop configuation is always the same.* The port prop is a left-hand prop (turns counterclockwise when viewed from the rear) and the starboard prop is a right-hand prop (turns clockwise when viewed from the rear).* There is a reason for this specific rotation setup that has to do with what the prop wash does when the transmission is put in reverse and the direction of the combined prop walk when one transmission is put in forward and the other in reverse.* In short, the boat is more maneuverable with this rotation setup.

*Any twin screw commercial vessel will have opposite turning propellers.* As Marin says, the port prop is left-hand and the strb prop is right-hand.* This is referred to as outboard turning.* Interestingly, this tends to be more of a West Coast occurence as most Gulf of Mexico built twin screws will be inboard turning (just the opposite).

While there is a design argument for outboard turning props to help the twisting motion, I handled an inboard turning tug years ago, and the twisting effect did not appear to be significantly lessened.* Any measureable difference may depend on a number of external factors.
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