Discussion in 'Technical Discussion' started by Crewagency, Nov 26, 2004.
None of which are expected to have a long life ...
The strength of the US / RIckover approach was the search and achievement of "excellence" of developing proven reliable conservative technology and concentrating just like the French on one...
The weakness of the Soviet approach was the same as the losers in WW2 the search and implementation of cutting edge ideas without adequate development... in a hellter skelter manner.
Actually your idea of the torpedos mounted outside the pressure hull was only not considered need to servicing the torpedos. The complication of launching them from inside is very very heavy, complicated and prone to disaster.
Glad you posted your picture of the silent chain driven pod... thumbs up on that!
I am still in awe of the SWASH pods in the Expedition Yacht thread.
Diesel electric versus diesel mechanic
There was a field study in Germany about diesel electric propulsion for inland cargo vessels.
Taken was a standard inland bulk carrier type cargo vessel of the Europe size, 280 ft LOA, 32 ft beam, 8,5 ft draft, airdraft 13 ft. 1500 metric ton displacement. 2 x 370 KW main engines with gear on fixed propellers, 50 KVA Genset.
This vessel was on regular service from Rotterdam, NL to a factory near Braunschweig, Germany. This route via the Rhine River and different canals took during the test about 68 hours and 4460 Liters of Diesel on average.
This vessel was then modified with diesel electric propulsion concisting of 4 x 230 KWe diesel gensets and 2 x 2 x 230 KWe electric motors driving the same two fixed props. That means 2 electric motors gearless on each shaft, one motor per shaft could be disconnected if not needed. Not at its optimum, but modular in design.
The (except for the propulsion) identical vessel was brought back to service and did the same route with the same load again. The result was a fuel consumption of 3300 Liter per trip and a reduction in fuel consumtion of 26 % and a reduction in carbon dioxide of 3082 kg.
How come? The reason one could think is, DE propulsion is more effective. Wrong, it is not. The lower fuel consumption was only caused by the excisting diesel power not used (switched off) when not needed. So, it is only more efficient under the bottom line.
That means, a normal cargo vessel with mechanical diesel power train is by far overpowered.
Testing of the vessel prior modification gave the result that: Only 60 % of the existing mechanical diesel power was needed, to reach the fully loaded hull speed. But the engine power had to be 25 % bigger, to compensate for the necessary torque at low RPM. The last 15 % was needed as reserve power for fast floating rivers and acceleration. On the average, only 45 % of the excisting mechanical diesel power was used. But the big lion had to be fed.
Only the DE setup of using 4 identical diesel gensets and inline electrical motors, which were only partially used, gave the reduction in fuel consumption. The max number of gensets used at one time was only 3. During maneuvering in the harbour, only 1 genset was necessary. Downhill the Rhine River with empty ship, max. 2 gensets at one time were used. The electrical power produced, included the hotel power for the ship.
Result: A diesel electric ship has a lower fuel consumption only because you switch on just the gensets needed (and run them at optimum load) for the specific situation. Fuel is only saved by the sleeping gensets.
The above would support the reasoning that multiple smaller Gensets are better for load regulation than a few large ones on a DE Vessel.
Do you have any drawings or a link showing how this was all arranged?
Or more accurately, on a DE vessel used in inland waters or in the type of service where nearly constant maneuvering is required. That is kind of a no-brainer and doesn't really turn the marine propulsion paradigm on its head.
Cruise ships and product tankers also benefit from similar installations but a boxboat in the liner trades would be an economic disaster with the same system installed.
The majority of yachts simply don't operate enough to make it worthwhile. The power required to keep heaters on to avoid low IR on the generators and motors alone would make it a silly proposition.
It would be a boom time for shoreside electrical contractors though.
Maybe we should promote the idea and start specialist DE shops at the big yards.
This field test was (for us) the first example of a true comparison between a diesel-mechanical setup and the same ship converted to a diesel-electric setup without any improvement of hydronamics by different hull shapes and modern props and pod drives. Even the profile of usage was identical.
The results with modernisation of the hull design and props or even with pod drives would have given even better results. But one could not have said, what was the cause of the DE setup and what was caused by the external improvements.
Given the fact, that inland cargo vessels and their design are a total different world, I still believe, that part of this results could be taken for yacht or ship design.
You would hardly believe, what kind of a big subject shore power supply for cargo vessels and cruise ships is at the moment in Europe. The number of very large cruise ships docking in Hamburg or Kiel over the year is enormous. Taking the cargo vessels in the Hamburg harbour in account too, air polution is a factor, people are concerned about. Politicians are under big pressure to find a solution ASAP. The problem is not providing the shore side of the power supply. The problem is the standarization. Voltage, Frequency, DC, AC, quantity of power needed and the type of plug and socket are the question. This problem can not be solved on the national level. IMO / Marpol has here to set standards as soon as possible. No ship owner voluntiers to impliment a shore power system in his ship without legal pressure.
If you watch for example the mighty Cunard Line cruise liner Q2 at the Hamburg cruise terminal every year and her smoking funnel showing her attemps to produce the necessary hotel power during her stay, one would not like to know the quantity and composition of her exhaust gases from "burning" heavy fuel oil within the boundaries of a large city.
Marmot, you are right, the effectiveness of DE propulsion depends on the type of use of a ship. A cargo vessel on longe range trading routes with constant low RPM 2-strokes does not need DE. On a ship with this type of use, a DE setup will only be choosen for constructional reasons (f.e. engineroom to far away from the prop or whatever).
But for the "typical" Megayacht, when getting in the appropriate size range, a DE setup will be both effective and efficient and more enviromental (and user) friendly.
I am absolutely confident, that (for a large yacht) a setup of multiple smaller and identical diesel gensets has more advantages, than having two large gensets and a few smaller ones. Example would be Topaz with her 10 MTU gensets of the 4000 series. They rarely travel, if they travel, the are mostly trolling or they are only in DP mode. That means, all 10 Gens are very seldom used at a time. But they still have full power available, when needed. And the sleeping onces save fuel.
I would agree DE makes a lot of sense on a yacht actual perfect application.
But the choice of number of generator sets and size all depends...
Diesel Electric... reasonable number of identical gen sets, for the size of yacht, for all power on the yacht makes the most sense... this coincides with a cruise ship technology.
Diesel Electric Hybrid... with battery storage... then fewer and larger gen sets make sense because the battery storage if sufficient will allow periods of long off like overnight and quick charge scenarios... this more coincides with a submarine technology.
Fuel cells in a hybrid... not feasible for yachts right now.
Comment on shore power nuclear power both solves the pollution problem but complicates the shore power problem... typical naval conventionally powered ship 3-4 shore power cables... same size ship nuclear... 16-22. But no port pollution if you run the plant... in port... if they will let you... N. Europe is touchy on that one.
One more comment... to make the 'bomb grade' nulcear material for a naval plant takes as much fossil fuel as running a conventionally powered ship for the duration of the core life of the nuclear.... hummm interesting isn't it!
I am curious HTM, what is the benefit to having two stators per shaft rather than just one?
I believe to reduce the mass of stator, when only half of the power was needed? It was probably part of the test setup to find the smallest propulsion power necessary for that ship.
One of the phenomenas of an electic motor driving a shaft in comparison to a diesel engine is, that you need only app. 40 % installed power with an electric motor, because of its ideal linear torque curve. The trick behind this test setup probably was the modular design of the whole propulsion system.
Have a look at this website: www.torquemarine.de
They explain it more deeply. I am a mechanical engineer. Electricity for me is walking on thin ice.
The bit that is attached to a shaft is a rotor. A stator just sits there and occasionally hums.
I am aware of that. I just wasn't sure of the nomenclature when you have two motors on one shaft, are they considered to have one shared rotor or two? The way I said it was the least inelegant way I could come up with to ask.
Diesel-mechanic versus diesel-electric
If you look at this chematic flow chart of a typical DE setup, even a stupid mechanical engineer like me can understand, that the losses within a DE setup are higher than within a simple mechanical diesel propulsion where the only losses after the engine output are in the reverse gear and the shaft bearings (friction).
The fuel savings of a DE drive are located only in the diesel genset power not being used (switched off). And the working gensets hopefully run at optimum load. All other tricks are only optimization.
The real advantages for a (large) DE yacht are located in more flexibility of the operation and with the internal design and layout, comfort (quiet and low vibration) and the very low fuel consumption, when trolling and under dynamic positioning. The higher cost are of much lower priority with a larger yacht.
But this numbers will change in favour of the DE propulsion with the full DC ship.
A couple of comments. In a much simplified explanation.
Electric motors and Generators... are most efficient at or near full load.
The why of that is the "flux density" or that the magnetic forces are strongest at that point of operation and the gaps necessary in the moving parts of the machine are fixed. This means the more magnetic force... that is created by current flow... in that given machine the more efficient it will be as it is better magnetically coupled together. Decreasing the gaps also works.
So why, two motors... to tailor the sweet spot of efficiency to the anticipated load.
two motors on a single shaft
I was able to confirm the reasons for these specific setup. One of the special effects of the operation of large inland cargo vessels, which operate both on rivers with high current and canals, is the total different power requirements. Going full speed uphill on the large Rivers, those vessels need quite amount of propulsion power. But going downhill or on the canals with the speed limit of 6 Kts, only a fraction of power is needed. By having two electric motors on one shaft, the mass of the moving part of one motor can be disconnected and the remaining electric motor drives the propeller more efficiently.
The modular design was the whole trick behind this setup. This system is aimed, not exclusively but mainly, for the retrofit market. These inland cargo vessels last a very long time, in comparison with ocean going cargo vessels. Their normal life is calculated for about 50 years. Our ocean vessels go to the srapyard after 15 to 20 years maximum. So, retrofitting the propulsion train of one of those cargo vessels at mid life, is worthwhile and a large market for these companies. We will at least will do so, because it makes sence.
But those very high skilled engineers in the DE business agreed on the fact, that the full DC ship will be the future of diesel-electric propulsion. The propulsion setup will be much simpler (less components) and less space consuming, has lower losses and is less expensive. And the DC powertrain can be used in combination with variable speed gensets, which are smaller and very efficient.
They confirmed, that one of these downsclaled systems, would also work on smaller displacement yachts in the size range of 30 to 50 feet. Two DC variable speed diesel gensets, a long lasting set of traction batteries and one PM brushless DC motor are the main components (plus some black boxes, I do not understand). When on the hook, one of the gensets serves as the hotel generator. There are some examples of this type are on the market in Europe already. The Dutch company Mastervolt and the German company Fischer Panda are producing the technical components for that systems.
After all those years, the famous Mr. Thomas Alva Edison will have a final victory over Mr. Nikola Tesla, his great opponent, with the return of the DC electrical power.
HTM09... ah ha... you're the one that inherited from Huguette Clark! But what did you do with the doll collection?
Seriously... all basic generators and motors are AC at heart... as batteries are DC at heart... and fuel cells are DC at heart. So you have to live with some conversion either electronic or mechanical.
AC is more complicated to understand but so is real life.
Your comments as to Edison and Tesla... some interesting observations:
Edison the great inventor... paid off Maxim (yes of the Maxim Gun Devils Paintbrush fame) when he borrowed Maxim's filament to make his light bulb work. Really, the big invention that Edison came up with was the screw thread for the light bulb... or was it really one of the guys working for him. Edison was an executive inventor... meaning he comes up with a concept and someone else figured out the details of how to do it. Edison was a hugely successful self promoter that occasionally let that overcome him.
Tesla was a genius... came up with the polyphase electrical system the world works on today but hugely erratic as a person. What is not commonly known Tesla theorized the concepts, defined the quantitative aspects (all the math), invented and designed all the basic equipment and control theory... motors, generators, transformers, voltage control the whole shebang in about 6 months.
As to black boxes... I have been against the plug and play concept as it dumbs down the technicians and drives up costs. I don't know how many times someone who works for me has thrown away a very expensive black box... which I subsequently, if I have time to investigate, I nearly always find a component costing less than a coke-a-cola burned out... usually it is in the power supply of the thing... meaning it likely saw some kind of input overload... .
If we had the internet and forums around 50 years ago I bet the forums would be a-wash with discussions of diesel electric power. That is in a diesel/electric locomotives forum.
Try 60 yrs ago...
The idea of two electric motors suppling the shaft when the load varies from about 1/2 load continuous to full load continuous based on, for example, running up river loaded and down river unloaded... is a electrical efficiency problem not an inertia of the motors rotor problem...
Here is a nice discussion this motor efficiency at low loading problem and a solution... other than two motors.
I know this applies to single phase motors but it illustrative of the issue... and points to solutions.
Nola power factor corrrector
Frank Nola of NASA proposed a power factor corrector for improving the efficiency of AC induction motors in the mid 1970's. It is based on the premise that induction motors are inefficient at less than full load. This inefficiency correlates with a low power factor. The less than unity power factor is due to magnetizing current required by the stator. This fixed current is a larger proportion of total motor current as motor load is decreased. At light load, the full magnetizing current is not required. It could be reduced by decreasing the applied voltage, improving the power factor and efficiency. The power factor corrector senses power factor, and decreases motor voltage, thus restoring a higher power factor and decreasing losses.
Since single-phase motors are about 2 to 4 times as inefficient as three-phase motors, there is potential energy savings for 1-φ motors. There is no savings for a fully loaded motor since all the stator magnetizing current is required. The voltage cannot be reduced. But there is potential savings from a less than fully loaded motor. A nominal 117 VAC motor is designed to work at as high as 127 VAC, as low as 104 VAC. That means that it is not fully loaded when operated at greater than 104 VAC, for example, a 117 VAC refrigerator. It is safe for the power factor controller to lower the line voltage to 104-110 VAC. The higher the initial line voltage, the greater the potential savings. Of course, if the power company delivers closer to 110 VAC, the motor will operate more efficiently without any add-on device.
Any substantially idle, 25% FLC or less, single phase induction motor is a candidate for a PFC. Though, it needs to operate a large number of hours per year. And the more time it idles, as in a lumber saw, punch press, or conveyor, the greater the possibility of paying for the controller in a few years operation. It should be easier to pay for it by a factor of three as compared to the more efficient 3-φ-motor. The cost of a PFC cannot be recovered for a motor operating only a few hours per day. 
I also invented a partial solution but it is tied to motor construction so you cannot use standard motors and have to go to a custom... and that is not real efficient to do money wise. Besides... I did not want to add another type of electric motor for the poor overloaded electrical engineering students to have to learn about... too many already.
OK Karo1776, you are making use of the fact, that I am only a stupid mechanical engineer. I will have your arguments checked. If you fool me, shame on you.