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Old 01-19-2008, 09:26 PM   #41
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RE: Electric Boat Engines

Eric:

I've been interested in DE for longer than I care to admit, but as of yet, have done nothing about it. It makes total sense to me and is not that far off.

Do you remember that article in PMM a few years ago about a single engngine driving 2 screws on a Mainship 34? I almost opted for one but decided I'd let someone else to the R & D.

Walt
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Old 01-21-2008, 10:10 AM   #42
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Electric Boat Engines

Nordhavn is currently building a 73 footer with D/E power. Guess we'll see how the technology works after all.
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Old 01-21-2008, 11:47 AM   #43
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Electric Boat Engines

As has been noted, there were many World War II tugs that were configured with diesel electric propulsion. Only the government could afford the inefficiencies of the system in those days, all subsequent commerical towing companies built tugs with conventional propulsion systems. Cheaper to build, cheaper to operate and maintain.

However, technology has improved generally over the years, and many vessels have alternative propulsion systems such as voith schneider, azmuthing drives and azipod propulsion. Lately, there have been a class of tankers and cruise ships where diesel electric is a viable option due to the desire for redundancy, configuration of propulsion machinery that permits improved cargo/passenger handling, and improved maneuvering capabilities with azipod propulsion.

Whether such efficiencies exist in recreational vessels will be determined by a few builders and the marketplace. There will always be a place at the high end of the market for this type of experimentation.
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Old 01-21-2008, 04:47 PM   #44
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RE: Electric Boat Engines

Hey Walt,

I tried to post a pic but lost the whole post so heres a redo. I remember it well. Unbeliveably they did'nt talk about swinging the boat with one eng in fwd and one in reverse. One would need to go in and out of fwd gear to equal the thrust of the reversing engine more rapidly than one goes in and out of gear with an over proped boat to go dead slow in a marina. Since the clutches are in the gearboxes this should work at least and possibly even well. For those that arn'nt famillar with the system it's a twin screw system, including the gear boxes, but driven by one engine in the center of the boat and coupled to the gear boxes by several other right and left angle gears. Comments?

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Old 01-21-2008, 05:12 PM   #45
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The more shafts and gearboxes you add to a system the less efficient and the more maintenance-intensive it becomes.

In WWII the Elco PT boat had three engines. The center engine was direct drive, the two wing engines were reversed in the boat and drove through V-drives. At the very end of the war, in the ongoing drive to get more power, Elco eliminated the V-drives and managed to mount all three engines in direct drive configuration without changing the size of the engine room. They gained sevearl hundred horsepower simply by eliminating the V-drives.
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Old 01-21-2008, 07:26 PM   #46
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the buyers of Nordhavn 72/76 hull#4 specified an electric wing motor
Nordhavn 72/76 hull#6 is entirely hybrid diesel electric including the main engine
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Old 01-22-2008, 04:00 AM   #47
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RE: Electric Boat Engines

As Scotty would say on Star Trek, "The more you overtake the plumbing, the easier it is to stop up the drain..." Great wisdom for boats or starships!
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Old 01-23-2008, 10:08 PM   #48
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RE: Electric Boat Engines

Walt and Marin,
I looked up that article and yes I agree with you Marin, more gears shafts and bearings should be less efficent for sure. I thought it was just an apples and oranges thing but it seems they are fairly close to apples and apples. I wright off the twin screw twin engine sample as it was 100hp above the control unit. Of the SE SS and SE TS the SE TS ( gear drive ) was 12% more efficent. I do think a twin screw should be more efficent as it does'nt have to cope with the lateral thrust of single screw propeller walking. My outboard boats indicate that is a lot of force to correct. What do boat designers say about this?
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Old 01-23-2008, 11:30 PM   #49
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I don't have any scientific knowledge to back this up, only logic, but I would think a propeller is going to generate the same forces whether there are one, two, or ten of them. The props on a twin "walk" in exactly the same way as the prop on a single. The only difference is that they are made to turn opposite directions so the sideways thrust of one of them is opposed by the sideways thrust of the other one and the boat does not yaw. But the sideways thrust from each propeller is still being generated. It just doesn't move the boat because there is an equal force from the other propeller in the opposite direction. But if you pull one transmission into neutral the boat instantly behaves like a single and the stern will move in whatever direction the prop walk of the still turning prop wants it to move in.

So having two props doesn't eliminate prop walk or lateral thrust or whatever you want to call it. It's still being generated. It's just not yawing the boat because of the opposing force from the other propeller. So in terms of lateral thrust I don't think there is any reduction of the work an engine has to do to turn a prop on a twin compared to the work an engine does to turn the prop on a single. They're both generating an equal amount of lateral thrust (assuming the same props, same engines, same rpm, etc.). It's just that the lateral thrust coming off a prop on a twin doesn't accomplish anything.

-- Edited by Marin at 00:31, 2008-01-24
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Old 01-23-2008, 11:35 PM   #50
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Marin, I am not sure I am following you and I don't have a twin so I truly dunno. BUT, I was under the impression that most twins(I do know there were some trannies that were counter-rotating) turned their props in the same direction....not counter-rotating???

Seems to me that the BW counter-rotating trannies were not very highly regarded???* Maybe I am confused....

-- Edited by Baker at 00:37, 2008-01-24
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Old 01-24-2008, 02:42 AM   #51
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RE: Electric Boat Engines

I've never heard of a twin prop boat with screws turning in the same direction, although I'm sure there might be a few. They would always be "walking" to one side or another. The vast majority are counter-rotating.... one scew turning clockwise and the other counter-clockwise.
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Old 01-24-2008, 04:02 AM   #52
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Cool....I didn't know this.
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Old 01-24-2008, 09:45 AM   #53
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It's not the trannies that are counter-rotating, it's the engines themselves. One engine is right hand rotation and the other is left hand rotation. The right hand rotation engine is considered the "normal" one.* When you buy a LH rotating engine it usually costs more.* At least that's how it used to be....maybe these new electronically controlled things are different.

-- Edited by gns at 10:48, 2008-01-24
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Old 01-24-2008, 10:11 AM   #54
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RE: Electric Boat Engines

On my boat the transmissions do the counter rotating. Both Yanmars rotate the same direction. I think this is the way most newer builders have dealt with this.
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Old 01-24-2008, 01:29 PM   #55
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RE: Electric Boat Propulsion

I wish somone would change the topic to " Electric Boat Propulsion "...quick before any guests see it. Marin is right. Equal yawing forces are created with twin screws but since they oppose one another no correction is needed. I'ts the correction that hurts the single screw boat. Some rudder needs to be applied to counter act the prop walking force. Outboards and high pitch propellers create high levels of this yawing force. Signifficant ammounts of drag result from the applied rudder to counteract the propwalking ( yawing ) force. If a single screw is more efficent it would need to first overcome the prop walking losses just to be even with the twin screw. The infeed water to the propeller on a SS suffers from the turbulence created from the keel as well. I don't see how a SS could be more efficent but I'm almost certian thats what I've heard. Does anyone know the truth and the facts on this question?

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PS I changed the title
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Old 01-24-2008, 02:24 PM   #56
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First off Eric, prop walk is most pronounced when the prop is highly loaded....high power low speed situation. Like when you are moving forward and go into reverse and power up to slow/stop your progress. When the boat is underway and all things basically being in equilibrium, then prop walk is almost non-existant.

A single engine boat is more efficient simply because a twin basically doubles your fuel consumption without doubling your performance. Now if you were to compare the efficiencies of a single engine installation against one engine in a twin engine installation(no keel turbulence) you mught have an arguement.
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Old 01-24-2008, 02:56 PM   #57
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Gary---

Actually, most diesel engines in twin-engine boats rotate the same direction. One of the tranmissions will have an extra gear in it to reverse the direction of the output shaft so the props are counter-rotating. But counter-rotating diesels in boats are very rare. One exception is an older John Deere engine from the 60s that was used in some American Marine Alaskans.

I know some gas engines used in twin boats are counter-rotating. I used to fish in Hawaii in the 1970s in a 28 foot Uniflite that had two Chrysler V-8s driving through V-drives. One engine turned one way, the other turned the other way. I don't know if this is common today in gas engines or not. But I don't know of any diesels--- at least from the main suppliers, Cat, John Deere, Cummins, Lugger, Yanmar, etc.--- that supply engines with a choice of rotation. The ubiquitous Ford Lehman 120 does not have a counter-rotating model. They all rotate the same way, that way being counterclockwise when viewed from the rear of the engine.

Keith---

Counter-rotating props have not been around as long as one might think. For instance, the three-engine PT boats in WWII had all three props going the same direction. This was one reason for the boat's terrible low-speed handling. The British had a number of twin engine craft for rescue service and torpedo/gunboat service, and their props all turned the same way.

Counter-rotating props don't have a long history in airplanes either. I remember in the 1970s when Piper came out with the twin-engine Seneca, one of the big deals about this airplane was it had counter-rotating props. I think virtually all previous multi-engine planes, from Cessna 310s to B-17s and everything in between had props turning the same direction.
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Old 01-24-2008, 03:10 PM   #58
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Eric and John---

A twin engine boat does not double the fuel consumption compared with the same boat powered with one engine. It does use more fuel than the single, but the engines in a twin don't have to work as hard to achieve the same speed as in a single, so the fuel consumption of each engine is less than the fuel consumption of the only engine in a single. So you do use more fuel, but not twice as much fuel.

What IS twice as much are the maintenance costs--- twice as much oil, twice as many filters, twice as much time to change oil, etc..

I agree wth John that underway, prop walk is negligable or non-existant. On the single engine boats I have run, there was no need to hold rudder to compensate for it. Also, a properly designed keel does not mess up the waterflow into the prop, at least not to a degree worth worrying about. Or at least no more than the shafts and struts on a twin mess up the waterflow into their propellers.

What I have experienced with a large outboard is not prop walk but torque. The rotating masses of the engine are mounted vertically. The action-reaction thing (Newton) means there is a force opposing the rotation of the engine. Because the engine is bolted to the back of the boat, this force (torque) tries to rotate the boat as well as the engine, so the stern yaws to the side. So a correction is needed, either by holding pressure on the wheel or,, more normally, adjusting the trim tab on the bottom of the engine to counter the force of torque. Kind of the same deal as reving a car engine up and down and watching the car lean to the side away form the rotation of the engine, only in this case the rotational forces are vertical.
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Old 01-25-2008, 03:11 PM   #59
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RE: Electric Boat Engines

Marin, Naturally if a twin dos'nt have CR props it will prop walk just like a single. I'm only talking about proper twin engine boats with CR screws.
John, Seems a no brainer to me John that the prop is more highly loaded the more the throttle is advanced but prop walk is less noticeable at speed as boats have more directional stability there.Any time we discuss twin screw v/s single it's stupid not to assume each boat has the same power and all engines are of the same type ( TC NA ect ). Marin is silly comparing a 240hp twin against a 120hp single. If you had a 240hp single compared to a twin w 2 120hp engines maint costs would be similar. Marin .. on an outboard as on all other propeller driven craft torque produces roll, not yaw. My intention on this thread was to get something from a Marine Architect or equivolent. I may have Skeenes " Ellements of Yacht Design "....I'll try that. The more I think about this the more I need to find out about Efficency twin v/s single.
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Old 01-25-2008, 04:05 PM   #60
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Eric, when a boat is stabilized in cruise it has a tendency to be closer to equilibrium....ie thrust and drag are equal. When this is the case, prop walk is not as pronounced.

The idea is that the water close to the hull is going slower than farther away. When the water goes into the upper and lower propeller blades at different speeds, the forces generated by the vector sum of the speed of the water in and the speed of the blade around are then different and act in different directions on the top an bottom blades. Since the top blade is going the opposite direction than the bottom, the net force is somewhat off the centerline.

Okay, I aint that smart and I pirated that off of a boat design website....but you get the idea. When you are going faster, at cruise, for the reasons above, prop walk is not as pronounced.
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