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Author Topic: Block coefficient  (Read 4954 times)

Bryan Young

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Block coefficient
« on: February 11, 2008, 10:52:31 PM »

I prefer a more integrated approach but I do have an iron rule that what goes in must be able to come out - because at some point it will need to! My models tend to be a lot smaller than yours Bryan so weights can be more of an issue in terms of stability etc. No point in building the thing and then finding that the working bits won't fit or are too heavy. With a bigger model the motors etc. are a lesser proportion of the whole displacement.
Missed this one earlier, but although "Havelock" is at 1:48 she is (was) a small ship so the model is only 51" long..and much narrower in the beam than I would have liked. But that's the legacy of the sail days..
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Colin Bishop

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« Reply #1 on: February 11, 2008, 10:56:39 PM »

Nevertheless she does have a fairly full hull form My fishery cruiser "Brenda" is 45 inches long but I'd say that the hull lines give her only half the displacement of Havelock - if that.
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Bryan Young

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« Reply #2 on: February 11, 2008, 11:08:23 PM »

Nevertheless she does have a fairly full hull form My fishery cruiser "Brenda" is 45 inches long but I'd say that the hull lines give her only half the displacement of Havelock - if that.
Yeah, OK..I guess I am looking at a draught of around 3.5" or so, giving a total weight of about 40lb...but don't quote me.
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bigfella

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Block coefficient
« Reply #3 on: February 12, 2008, 12:04:50 AM »

Hi Bryan

Just found this build (where has it been hiding) and all I can say is wow.

Regards David
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Peter Fitness

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« Reply #4 on: February 12, 2008, 02:11:23 AM »

My pleasure Bryan, glad I was able to help. May I say what a beautiful example of craftmanship your model is - and your reference to an Oscar speech is quite appropriate, you deserve the modelling equivalent for you work  O0
Peter.
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Bryan Young

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« Reply #5 on: February 12, 2008, 07:07:57 PM »

When I woke up this morning I just KNEW it was going to be "one of those days".
First, it turned out to be my (our) 42nd wedding anniversary (which I had forgotten), then the little milling cutter I was using to make the windlass gear wheels snapped.....and then the lathe motor blew up big style. (Only 75 for a new one though, which is peanuts compared to the smaller. weaker Unimat item).
A great day. But not is all lost. The temporary loss of the lathe forces me to think about fitting the running gear, stern frame and rudder. Should have done it sooner, but the didn't. And chickens come home to roost. To make and fit the stern frame the model has to be upended...but the foremast and its rigging are fitted. What a rotten day.
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Colin Bishop

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« Reply #6 on: February 12, 2008, 07:32:09 PM »

Even worse for the wife.... :(
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Bryan Young

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« Reply #7 on: February 12, 2008, 09:19:00 PM »

Even worse for the wife.... :(
I guess by now she is used to me....perhaps that is why I am imprisoned in the "little" bedroom with the bolt on the outside.
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Colin Bishop

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« Reply #8 on: February 12, 2008, 09:41:49 PM »

Sounds like you got off lightly then...
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Bunkerbarge

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« Reply #9 on: February 12, 2008, 09:59:39 PM »

You could always make it up to her by spending a fortune on a couple of wilting red roses on valentines day!! O0

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Bryan Young

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« Reply #10 on: February 12, 2008, 10:11:40 PM »

You could always make it up to her by spending a fortune on a couple of wilting red roses on valentines day!! O0


Tried that and it wasn't just the roses that were wilting.
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Bryan Young

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« Reply #11 on: February 13, 2008, 08:48:37 PM »

Nevertheless she does have a fairly full hull form My fishery cruiser "Brenda" is 45 inches long but I'd say that the hull lines give her only half the displacement of Havelock - if that.
Trying (mentally) to work that one out. She must have had very fine lines. Or maybe both of us are wrong.
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Colin Bishop

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« Reply #12 on: February 13, 2008, 09:16:51 PM »

Very fine lines indeed!

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Bryan Young

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« Reply #13 on: February 13, 2008, 10:48:19 PM »

Very fine lines indeed!


I see what you mean now. I guess the cf must be around 0.5 as opposed to Havelocks 0.75 or 8 (ish). No doubt there will be guys "out there" wondering what on earth we are talking about. (I am going from 00.0 to 1.00)
So fo those who don't know...
cf is the coefficient of fineness. Still bamboozled? OK.
Look at a ships hull as a solid chunk of whatever. Square on all sides. A brick. That will have a cf of 10. Now equate that to a racing skiff and you can possibly bring that figure down to 2. A Destroyer will possibly be between 4 and 6. A large tanker will be between 8 and 9. The general cargo ship would be in the region of 7.5 until "they" decided to put the engines at the back end. The modern Cruise Liners (etc) may look huge and bulky but that belies the very sophisticated underwater hull form. I would guess that the better ones will be around 6.5.
But that is only a rough idea as to what cf means. The weight is in the girth...if you get my drift.
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Bunkerbarge

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« Reply #14 on: February 14, 2008, 09:58:17 AM »

How strange, I always thought it was coefficient of form.  Maybe things have greyed over a bit since my time at college, which was, I admit, a few years ago now!!  You're probably right Bryan as my memory is noted as deteriorating nowadays.

I reckon my coaster must be knocking on for about 0.85-9.0 which is up there with the supertankers!!
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Colin Bishop

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« Reply #15 on: February 14, 2008, 11:47:19 AM »

I'd always heard it referred to as Block Coefficient but, hell, it's all the same thing isn't it?
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banjo

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Block coefficient
« Reply #16 on: February 14, 2008, 02:06:36 PM »

I lifted this from Wickipedia:-

Coefficients help compare hull forms:-
 
1) Block Coefficient (Cb) is the volume (V) divided by the LWL x BWL x T. If you draw a box around the submerged part of the ship, it is the ratio of the box volume occupied by the ship. It gives a sense of how much of the block defined by the Lwl, Bwl & draft (T) is filled by the hull. Full forms such as oil tankers will have a high Cb where fine shapes such as sailboats will have a low Cb.
 
2) Midship Coefficient (Cm or Cx) is the Bwl x draft divided by the cross-sectional area (Ax) of the slice at Midships (or at the largest section for Cx). It displays the ratio of the largest underwater section of the hull to a rectangle of the same overall width and depth as the underwater section of the hull. This defines the fullness of the underbody. A low Cm indicates a cut-away mid-section and a high Cm indicates a boxy section shape. Sailboats have a cut-away mid-section with low Cx whereas cargo vessels have a boxy section with high Cx to help increase the Cb.
 
3) Prismatic Coefficient (Cp) is the volume (V) divided by Lwl x Ax. It displays the ratio of the underwater volume of the hull to a rectangular block of the same overall length as the underbody and with cross-sectional area equal to the largest underwater section of the hull. This is used to evaluate the distribution of the volume of the underbody. A low Cp indicates a full mid-section and fine ends, a high Cp indicates a boat with fuller ends. Planing hulls and other highspeed hulls tend towards a higher Cp. Efficient displacement hulls travelling at a low Froude number will tend to have a low Cp.
 



4) Waterplane Coefficient (Cw) is the waterplane area divided by Lwl x Bwl. The waterplane coefficient expresses the fullness of the waterplane, or the ratio of the waterplane area to a rectangle of the same length and width. A low Cw figure indicates fine ends and a high Cw figure indicates fuller ends. High Cw improves stability as well as handling behavior in rough conditions.

 O0
 
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Colin Bishop

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« Reply #17 on: February 14, 2008, 02:52:26 PM »

So, to sum up, I suppose you could say that all these coefficients will give you some idea of how "thick" the model is - but not the builder?   ;D ;D
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banjo

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Block coefficient
« Reply #18 on: February 14, 2008, 02:53:58 PM »

Very droll
 ;D
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Bunkerbarge

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« Reply #19 on: February 14, 2008, 03:25:28 PM »

They are all a ratio of how closely the shape or volume conforms to a theoretical regular shape or volume. 

It brings back fond memories of my Naval Architecture classes at college.
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Bryan Young

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« Reply #20 on: February 14, 2008, 06:40:47 PM »

I'd always heard it referred to as Block Coefficient but, hell, it's all the same thing isn't it?
Yep. But I seem to recall that their is a nuance about the definition....not that that concerns us today. Call it what yo will, but a brick is a brick and a yacht is a yacht. Cheers.
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Bryan Young

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« Reply #21 on: February 14, 2008, 06:50:13 PM »

I have (now and again) pondered the question as to how much some of the huge bulbous bow extensions fitted nowadays will affect the cf (or whatever), not quite part of the hull but attached to it ...so to speak. I guess there must be a mathematical relationship between hydro-dynamics and simple (huh?) hull form.
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banjo

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Re: Block coefficient or what ever....
« Reply #22 on: February 20, 2008, 01:11:47 PM »

The dilemma occurs at the transition from fluid to air.  The naval architect has to concern himself (sexist) with the performance of the hull at the water line.  How simpler if cargo ships, all carriers, were designed to transit completely submerged.   Then the teardrop shape would suffice.
Comments?
 8)
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bigH

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Re: Block coefficient
« Reply #23 on: February 20, 2008, 04:00:28 PM »

  Now my head hurts even more, I'm going to have to lay down with a box of aspros and a shot of malt, just when I thought I was learning all there is to know about this boating thingy.........   :'(
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Bob

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Re: Block coefficient
« Reply #24 on: February 21, 2008, 03:46:40 AM »

Which M"Malt" BigH? They're good just some better than others.

 "Water is the drink if it taken in the right spirit."

Slang, BF
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