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Author Topic: Standard Water Drag Coefficients  (Read 10256 times)

AriesDW

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Standard Water Drag Coefficients
« on: March 30, 2008, 05:20:42 »

I know that the drag a ship experiences is a lot to do with the design of the craft - however, I am curious, is there an average coefficient available to use when considering acceleration tendancies of vessels. For the most part I can figure acceleration, however, I would like to better figure it by having a little more information regarding water resistances and what not in a very generic sense (because swell, wind, current speed/direction, etc. all have a lot to do with it as well, I know.)

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tugruloktas

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Re: Standard Water Drag Coefficients
« Reply #1 on: March 30, 2008, 13:33:40 »

Dear Dave,

This is Suhulet:)

You should be able to get such data from Fluid Mechanics.

I will advise some of my feedback in the forthcoming.

Best regards,
Capt. Tugrul Oktas
Oceangoing Master
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LucAtC

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Re: Standard Water Drag Coefficients
« Reply #2 on: March 30, 2008, 13:35:44 »

Hello Dave,
There is no such average (global?) coefficient, alas. Ships obey to the rules of Newton, and do accelerate if the thrust is greater than the resistance of the medium (?), as the ratio of (forces balance) vs (mass of the ship). 
If the thrust of propellers is relatively easy to find (thanks to screw and engine manufacturers), the water resistance, which is of the same order of magnitude, is more complex. It is made of water friction and of what is called residual resistance, having different names because they follow very different rules.
Also, there are some excellent sources of information to be found on the internet, particularly from TU Delft. Also, if you are more interested in the relation between design and drag aspects, the freewares "Freeship", and its freeware brother Delftship will give you some answers.

Regards,
Luc
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AriesDW

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Re: Standard Water Drag Coefficients
« Reply #3 on: March 30, 2008, 21:18:18 »

Thanks, Luc . . . As always you make me realize I often make too complicated of requests for simple answers. LOL!  ;D
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-Dave

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LucAtC

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Re: Standard Water Drag Coefficients
« Reply #4 on: March 30, 2008, 23:53:58 »

Surely, Dave, I understand what you mean, my answer was indeed  ;) too vague. This link to how MAN B&W sees the question is much more  ;D precise and circumstanciated. And more colored too  :D.

If you wish to have an idea of how much water resistance a ship encounters, you have the choice of estimating the power used, for instance through fuel consumption for a given speed and an estimation of the propulsive efficiency, that can give you a good guess of the resistance at this speed (and under the sea conditions you mentioned, including water depth etc..) as thrust times speed is power.

 :D A fully wild -mad  >:(- guess is that each 100 bhp of a well adapted screw propeller can deliver 1 ton thrust (or even more for tugs) on its shaft, of course only to validate your refined computations and check that you stayed in possible real world values.

 ;D From that, of course, initial (maximum, that is) acceleration is derived, as it is the ratio of the thrust to the ship mass.

Regards,  ;)
Luc
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AriesDW

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Re: Standard Water Drag Coefficients
« Reply #5 on: March 31, 2008, 00:29:00 »

LOL! You did not need to go into more depth, but certainly thank you, I appreciate it. You are very, very kind.
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-Dave

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santoshkumbhar

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Re: Standard Water Drag Coefficients
« Reply #6 on: December 21, 2012, 06:30:51 »

I know that the drag a ship experiences is a lot to do with the design of the craft - however, I am curious, is there an average coefficient available to use when considering acceleration tendancies of vessels. For the most part I can figure acceleration, however, I would like to better figure it by having a little more information regarding water resistances and what not in a very generic sense (because swell, wind, current speed/direction, etc. all have a lot to do with it as well, I know.)


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