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What Makes A Ship Move?

DrD

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Mechanics Corner

A Journal of Applied Mechanics and Mathematics by DrD, #47

                                                              What Makes a Ship Move?

One of the problems that often confronts engineers is the description of large, interconnected systems. Engineers tend to specialize, so that one is very knowledgeable on gears, another knows bearings, a third knows pumps, but none of them are comfortable with the whole system. In the automotive context, this is often expressed as, "How does the engine cause the car to move forward?" On the one hand, most engineers can describe the process in words, but they are far less quick to talk about equations for the whole system. Here, I will address a similar modeling question in the marine context, "What makes a ship move?"

WhatMakesAShipMove.pdf



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I agree that the description of large interconnected systems is difficult for most of us. In my opinion, this lack of skill responds to two main factors:


First, the mechanical engineering curriculum usually devotes more time to the teaching and evaluation of very specific topics and less time to the analysis of large interconnected systems.


Second, the industry, for the most part, demands specialized personnel either to solve problems or to make small improvements in subsystems in order to obtain a small but continuous improvement that allows them to remain competitive at the lowest possible cost.


Regarding the issue of education, I do not have any complaints, after all, whoever wants to learn has all the resources and tools at their disposal to do so. Similarly, regarding the behavior of the industry, I do not complain, it is a behavior that responds to more factors (economic, social and political) which I do not intend to mention here.


It is known that one can succeed in the exercise of the profession in different ways and one of them is to become an expert in a specific area. So if the educational system, neither the industry nor the economic incentive are of interest for the analysis of large interconnected systems, I suppose that only people who have a desire to understand and describe these systems will do so.


I believe that we all have the ability to describe those systems and if so, we would surely notice an incredible advance in the technological development since great advances are a consequence not of the improvement of very specific things but of the proposition of new approaches for the solution of a problem.


The main idea that I take away after reading your article is to try to describe through equations a complete system and in doing so identify which areas I lack knowledge or skill.


One point that caught my attention is the fact that all the thrust generated by the turbine reaches a single point (the seat of the thrust bearing) that although it was obvious I had not stopped to think about it and its importance in the structural design of the ship's hull.


Finally, I have a question, if one wanted to develop the ability to describe systems at that level, is it enough to take systems at random and try to describe them with equations or are there books that teach methods of analyzing those systems that would be worth studying?


If so, I would appreciate a recommendation.


Best regards,

HKS.

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What has happened? Is the sky falling? This is unprecedented!! A well thought out response to one of my posts!  I am simply stunned!!!

Thank you, HKS! This is the first time I have ever seen such a good response to a post.

You asked about learning whole system level modeling. I suggest that you might simply want to pick some everyday systems and try to model them. A car or motorcycle is an obvious choice, but there are others such as a railroad train (starting/stopping/running down the track), a rolling mill (if you are familiar with how such systems are built), a diesel engine driven generator starting a pump motor load, or perhaps a soda bottle filling machine.

At the time I first wrote this article, some 15 years ago, I was working for the US Navy in a propulsion engineering group. Most of the engineers around me were EEs, because we were looking at electric drive for ships. I found that most of them did not know much about motors, much less the rest of the system. Their knowledge was largely limited to power electronics for speed control of electric motors. They has no idea how that related to ship motion.

 

DrD

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DrD, first, I want to apologize for the lack of response my colleagues and I have shown to your great content, is just a sad behavior of recent generations, to show indifference even to things that interest us. One can argue that in a greater extent is due to the massive distractions we are exposed (primarily social media) in current times and I don't see a practical solution to that problem in the near future. In any case, I'm not better than anyone in that regard, just happened that I don't want to continue wasting time in trivial things and that's the reason why I decided to take the time to respond to your post this Sunday.

Second, I thank you, all of your posted content is enriching, especially from a professional growth point of view.

I will do as you say, starting with the modeling of simple everyday systems while I develop my analytical skills. If it isn't a burden I will be pleased if you can review the models I'll do. I don't want you to waste time so I will take the time to produce something worth reviewing.

Lastly, I can relate to your experience with the team of EEs and I could not agree more. Two years ago I was looking for design a centrifugal separator but I didn't want to use a motor and multiplier (neither gear or pulley) but directly design (and build) an electric motor capable of high rpm (I know that they already exist but I was looking for built it specifically for the work at hand), so I seek for an EE to help me and to my surprise he just relates to a lecture where they "learn" the principles of electric motor design and that was the limit of his help. Certainly, I could have done that by myself in the library.

It appears that specialization along with a lack of knowledge in the basics and the general perspective of things is not exclusive to mechanical engineering.

I only remain to say that I just want to be a better engineer and your content help me (and others that are silent) in that direction.

Best Regards,
HKS.

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I think the education we get in school is good and it should not be specialized in any area.  There are people who specialized in some area but this happens in large companies where they can afford one person devoted a specific area.

 

in most companies as, I found out, as an engineer you got to be able to respond everything.  Of course we don't know everything so we need to change our method in such places from specialization in narrow areas to knowing a little about a lot of areas.

 

The method I use is know at least a little about many areas.  If something comes up in most cases you can talk to all kind of vendor who are specialized in the area and get the correct device or system.  You don't have to be an expert in motor all you need is to describe what you want.  You don't need to be an expert in temperature controller just ask companies who makes them.  And so it goes.

 

As for the specific problem of what pushes the boat then we can write all kind of differential equation or other equation but we can't solve any of this because we don't know the material properties, the friction, all kind of other resistive elements, and many other thing that we don't know.

 

In most cases you got examples that can fit you situation so you can design and get an idea about the various elements.  Then you buy the parts or make some, assembly them and test.

 

I have done many calculation in the past be it in finite elements, finite difference, specializes method and others.  One thing I know for sure is that there are many properties that I don't know.  Also most of the calculations do not take into consideration many small variation in the design because it takes very long to code them.  It would be easier to make the part then to calculate it.

 

So to conclude I suggest learn about many thing and you do not have to an expert on each one of them.  Build the assembly and test it.  You can do some calculation but don't look for an accurate mathematical model of assembly.  However if you need an accurate mathematical model then do it and based on the test results of the assembly tweak the model to behave like the tested assembly.

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What a delight and a marvel, to have two people comment!! Thank you both.

Yuli makes the point that we still cannot solve this system because we lack many of the necessary parameters. That is certainly true, but the point is simply this: Now we know what we are lacking and go to seek further information on these matters. As I mentioned in my previous comment, this was originally written for some EEs who had no concept of the whole system. In particular, they did not comprehend the critical role of the thrust bearing in pushing the boat forward. In their minds, motors only produce torque, not thrust.

HKS mentions developing some models himself, which is a good idea. Rather than me reviewing them, why not post them here so that the entire community can review them? If I see them (which I probably will), I will comment also. Go to it, and see what you can create.

DrD

 

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Thank you.  Similar arguments apply to the case of a shaft driven by steam (or gas) turbines and a gearbox.  One could argue that the vessel is driven by the reaction from the mass of water driven astern by the screw.

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The nature of the prime mover really does not matter from the system point of view. Steam turbine, gas turbine, diesel engine, whatever, all simply provide shaft torque. It is the critical interaction between the thrust bearing and the hull structure that drives the ship.

 

DrD

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As I am not good in studies but what does ship move is that question which I was asked to my marine master teacher and he replied the propeller designed in such a way that they cut water like nut move on bolt, water be consider as bolt and cutting of water by propeller is nut so propeller cuts water and move forward. Similarly when ship move astern its moves in backward direction,  

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If you want experience analyzing a large, interconnected system, how about this??

https://newatlas.com/apollo-11-moon-landing/59108/?utm_medium=email&utm_campaign=2019-05-03%20111624%20USA%20Weekly%20Basic%202019-05-03%20111915%20How%20to%20land%20on%20the%20Moon&utm_content=2019-05-03%20111624%20USA%20Weekly%20Basic%202019-05-03%20111915%20How%20to%20land%20on%20the%20Moon+CID_1bba05cc647cbc6b9e9ef805ca92a427&utm_source=Campaign%20Monitor&utm_term=Read%20more

(From NEW Atlas)There were MANY application-specific problems that had to be solved - orbital mechanics, propulsion, life support, communications....  It took MANY engineers and scientists, over a period of twenty years or more, to make it happen, starting with basic flight/aerodynamics, guidance, communication, Atmospheric re-entry and landing - heating, cooling, life support .  The development of the on-board computer for guidance and docking procedures led to the development of the microcomputer that is  at the heart of every device we make and use today.  These people did not have all of the specifications for the material properties; they had to do laboratory work to get an answer -  or at least a best estimate.  Yes, there was much activity to build  analytical  ( and physical ) to do simulations and models of systems or parts of systems to look at alternative ways of doing things.  I remember seeing some of these problem statements in some of my engineering tests.  Or we did lab exercises that looked at some element of the system, say, a power supply (using vacuum tubes in that day). 

If you want to be a good/useful engineer, you have to know how the system you are looking at as it relates to the other systems around it.  If you are the expert on connecting rods (stress analysis/FEM, materials....), you still need to understand bearings, lubrication, crankshafts, pistons, combustion, fuel injection.....all the way to the wheels that touch the road surface (assuming you are in the internal combustion engine business. I would liken the design and development of a machine/system to an orchestra - The French horn player does not necessarily know what the cellist is doing, but he hears it and relates to it.  The Conductor is the Chief Engineer who brings all of the experts together to ensure that the "parts" fit together the way the composer intended.  Whether you make beautiful music together depends on the collective expertise and execution of all of the members of the orchestra.   Likewise for the design/development and manufacturing teams.  

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