MHD Propulsion : 
Yamato-1 Study

      IN 1985, the Japanese shipbuilding industry was operating at a loss due to a recession. However, in order to inspire new engineers and students the ongoing researchers led the establishment of a new driving force. This force is named magnetohydrodynamic (noted : MHD). Throughout this post, the MHD concept will be explained alongside the world's first superconducting MHD propulsion ship named : Yamato-1. 

Additionally, the sources are all attached at the end of this post. The overall project can be recognized as outstanding work provided by research, engineering, testing and so forth. 
 

Part 1 : The introduction

         It was during the 1960s that Lt. Doragh (a researcher from MIT) concluded that the only way to induce a magnetic field powerful enough to propel a ship was by the use of a superconducting coil which had been just made viable, luckily. However, it will be a decade later that the professeur Saji decided to set out on the research and development of an MHD propulsion ship using the superconducting coils. This was tested with the model ship SEMD-1 (length : 1,25m), and was thereafter recognized as a successful test run. Thus, Prof. Saji and his research team led a second test shortly after on a bigger scaled model, the ST-500 (length : 3,6m), which operated very successfully as well.

(Proof of concepts are posted below, in respective order) 

The tests were ground breaking as the superconducting MHD thruster could propel a ship without a screw propeller. A very different approach from the habitual propulsion structures. This defined a new driving force which provides a lot less noise & vibration, more speed with less cavitation (bubbles which reduce efficiency). These are only a few benefits of the MHD propulsion system. 

With the initial success of both models, it was time to create a viable ship that was seaworthy. In other words, it was time to take the theory out of the laboratories and put into practical usage. It was with great enthusiasm that the project began in 1989. A year later, the hull constructed in aluminium alloy (a non-magnetic material) was finalized. The Yamato-1 had come to life. This vessel has a length of 30m LWL, a beam  of 10m, and displacement weight of 185 tons. The displacement weight is fairly significant, and the reason why will be discussed later on. Finally, the ship was built at the MHI (Mitsubishi Heavy Industries) shipyard. The vessel had operated successfully on the 16th of July 1992.
As a fun fact, the team has named the vessel Yamato-1, as it signifies Japan in ancient words and symbolizes the infamous Yamato battleship.
 

Part 2 : Experiments

    As we noted previously, the final displacement weight had come down to 185 tons. Prior to this result, the ship would have weighed a lot more. To illustrate, only the thrusters had a significant impact. The projections done by researchers set the vessel to cruise 8 knots with 2 thrusters on each side (port and starboard). This was an optimistic idea but unrealistic, since they found that a single thruster weighed 50 tons. In addition, this didn't include the installments of a power generator or even the electric power supply. Thus, the case in question was to rightfully reduce the overall weight of the thrusters. This was effectively done and passed from 50 tons to 20 tons. Still, to this day the on going challenges for a superconducting MHD propulsion system is the large size equipment and heavy weights. 
Additionally the Yamato-1 was a test vessel, so the thrusters were manufactured by two different companies to enhance competition (in a good way) and technology advancements.

The superconducting MHD propulsion system can be viewed as a more sophisticated water jet system. Firstly, the MHD system sucks water underwater and thrusts it aft, then it discharges water in an electromagnetic kinematical manner. In other words, the concept is that seawater is sucked in at the intake, and is then accelerated by the MHD thruster. And finally, the water is then discharged out at a high speed.  

The idea behind the propulsion system is not new as it uses the basic theory known as : Fleming's Left Hand Rule.

The rule can be explained as follows : if we arrange our thumb, forefinger and middle finger of the left-hand perpendicular to each other, then the thumb points towards the direction of the force experienced by the conductor, the forefinger points towards the direction of the magnetic field and the middle finger points towards the direction of the electric current.

Part 3 : The overall MHD concept

    In order to fully understand the concept of the superconductive MHD system, we must explain the aspect of superconductivity. So, It is the point of zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a critical temperature. Now, this is a big topic in itself and I will not go in depth about it, but I recommend viewing Boaz Almog's explanation (link in references). In parallel to Fleming's Rule we can add Bernoulli's equation which explains that pressurized seawater passing through the thruster is converted from pressure energy to kinetic energy. This allows to move forward. 

In the case of the Yamato-1, they had to add liquid helium (noted : LHe) to maintain the superconductivity. Therefore, it was needed to place a cooling system, or a 'helium vessel', which kept the 4.2 Kelvin temperatures (or -269 degrees celsius). To understand the concept of the MHD, it is important to comprehend the behaviour of the electrons. For the electrons to react, they must be placed in ionized water (i.e seawater). The ionized water will play the strongest influence on the electric and magnetic fields. Once, a system is structured around a magnetic field, electric current and ionized water, then you can have a MHD concept. As for the case study, they brought it a step further with the use of a superconductor making the system more complex and a much more rigorous experiment. 

Is magnetohydrodynamics a propulsion with potential ?

On the general sense we can note that the practical usage of the theory is fairly new, therefore we can conclude that there is a lot of room for progress. The MHD reaction is rewarding and is a different form of propulsion. This force can lead to new ideas and inspiration for engineers. Additionally, as we have seen it can allow more efficiency due to less cavitation. However, the obvious states that the technologies involved are weight consuming which, in definitive, reduces the general usability of such a vessel. I believe, it can become a propulsion with important potential and the Yamato-1 vessel proves this point. After all, ship building is a lot about trade offs but as long as the issues of heavy and large size equipment are not advanced, then the true potential of MHD is limited. 

 

References :

  1. Yamato-1 : https://www.spf.org/en/_opri_media/publication/docs/yamato-1.pdf
  2. https://www.popularmechanics.com/military/navy-ships/a34878023/jet-ships/
  3. Fleming's rule : https://byjus.com/physics/flemings-left-hand-rule-and-right-hand-rule/
  4. MHD explained : https://www.youtube.com/watch?v=LS3GQk9ETRU
    https://www.sciencedirect.com/topics/materials-science/magnetohydrodynamics
  5. Superconductivity : https://www.youtube.com/watch?v=PXHczjOg06w
  6. Lorentz Force  https://www.britannica.com/science/Lorentz-force
     

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