Coefficient of Scantling

Discussion in 'Boat Design' started by Annode, Apr 3, 2020.

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TANSLSenior Member

This is my opinion and maybe someone wants to comment on it. Currently, with all the means available to the designer, calculation facilities, software, the correct thing is to do a quick calculation of the boat's scantlings and, created the 3D model of the boat, obtain a weight of the structure and a CoG pretty good for the early stages of design. It's not like in the past that the designer had no choice but to use those generic numbers. In any case, given the great variety of types of existing boats, it is difficult to find numbers that are valid for any boat and, above all, the enormous weight savings that are being achieved with the new materials and working methods, make them of little use. those old numbers, if they exist.
I repeat, it is only my opinion.

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DCockeySenior Member

How do you "do a quick calculation of the boat's scantlings" without an initial estimate of the boat's mass? And how do obtain an initial estimate of the boat's mass without an estimate of the mass of the hull and structure? How do you start?

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TANSLSenior Member

With the hydrostatics calculated and knowing, more or less, the draft to be obtained, I take a margin of 10% in the draft and with this I obtain the main dimensions to enter them in the calculations of the scantlings. With current programs it is very fast to obtain thicknesses of various shell panels and decks, and dimensions of a few reinforcements. With this, a CAD program and the 3D model, I get good enough weights of the structure, to which I add a 10% margin for errors and welding. This is what I do and the practice with many scantlings calculations tells me that it is a much better procedure than others based on magic numbers. All this does not take me more than one job afternoon.
It may seem wrong or what you want, but it is what I do and it works very well for me.

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gonzoSenior Member

Most people start by looking at similar designs that have proven successful.

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TANSLSenior Member

I think that, now, is less true than before. Similar projects are not your project, a similar weight or CoG are not those of your project and, with the number of drawing and calculation tools at our disposal, it is preferable to make your boat and not one similar to yours.

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MikeJohnsSenior Member

Yes entire structure including all framing bulkheads and deck.

For one vessel with a moderate sensible practical layout, from the W&M data I have:

All plating and framing not including rudder. 17 Tons
Aft double cabin full fitout and furnishings, double bed, seating, lockers and drawers, doors and electrical fittings 230kg
Midships single cabin fitout, one bunk one seat and lockers incl furnishings 135kg
Fwd Lounge seating for 8, tables and lockers incl furnishings 270 kg
WC head and shower linings shower tray and porcelain bowl 120kg
Moderately well appointed Galley incl cooker, fridge and freezer with integral compressors 421kg

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DCockeySenior Member

TANSL, am I correct in understanding that you start with the size and shape of the hull and draft, and then calculate the displacement? Is that displacement then used as input for the initial calculation of the scantlings?

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MikeJohnsSenior Member

David

For a vessel under 24m design pressure dictates initial scantlings. That pressure is dependent on the location, it ranges from the maximum hull slamming loads fwd to the lowest on a cabin top.
Hull pressure depends on the shape speed and displacement of the vessel.

Last edited: Apr 7, 2020
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DCockeySenior Member

My question for TANSL is where does he get the displacement from using his design methodology. It sounds like he designs the shape of the vessel, then determines the displacement. If so I am curious how he designs the shape without an initial assumption about the displacement.

Added: Design is frequently considered an iterative process alternating between synthesis and analysis. That leaves open the question of what information is used to start the process, and the source of that information.

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MikeJohnsSenior Member

And as Gonzo said before, usually it's a case of basing the initial parameters on a similar design. That usually simplifies the design spiral a lot.

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rxcompositeSenior Member

There is an existing method used. It is the WBS or Weight Breakdown System/Structure and is defined in Taylor's or DGM Watson Practical Ship Design.

There are two types, the Weight based design- where the weight it will carry is placed is first and the Volume based system. Volume based is when it is going to carry passengers or light bulk cargo.

The ship is broken down into detailed structures and weight/volume is calculated according to formula or coefficients outlined according to the materials used.

For example- Shell displacement= 1/380 (displ x Length)^1/2.
Then:
" 1 ton of high tensile steel will replace about 1.3 tonnes of mild steel"
"1 ton of aluminum will replace about 2.9 tonnes of mild steel"
" 1 tonne of FRP to abput 2.9 tonne of mild steel"

Attached is an example system that posted here in the forum a long time ago. I have the Taylor's system but will search for it.

Attached Files:

• ESWBS.xls
File size:
195 KB
Views:
22
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12. AnnodePrevious Member

TANSL, you appologize for wasting my time (evidently somewhat insincere), yet here you are again redirecting this thread into a discussion about "Why is this the question?" or ""best practices for the design spiral" If you want to discuss the merits of your OPINION re estimating weights, great, go start your own thread and do it there. PLEASE.
I am not interested in answers to your questions, I am seeking answers to MY questions. OK?
So please, stop with the thread redirection attempts. That is the definition of trolling.

Also, you also presume that everyone has your training and access to the software (that you do not name). Marine software is expensive! Even if I had that software, with my lack of experience, I would waste a month getting up to speed and working through my first scantling design. I dont want to do that! I am working on hull design right now. Not scantling design.

THIS IS A WEIGHT ESTIMATION THREAD

Last edited: Apr 7, 2020
13. AnnodePrevious Member

Fantastic response. Thank you. Thank you.

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TANSLSenior Member

That "initial displacement" is the result of various previous calculations and drawings, and, of course, observation of similar boats, when they exist or we have their data at our fingertips. I think, like everyone else, I start with an SOR and a General Arrangement plan. In that GA there is usually a depth and a draft (and an LWL, and a waterplane breadth and a death rise, and a speed, etc ...), initials, which will later be confirmed or changed. And yes, indeed, I use that draft and those "initial" shapes as input to calculate some "initial" scantlings.
It should be recognized that in all this there are many variables that depend on the "intuition", based on experience, of the designer, but I repeat that the procedure I follow gives me good results. Of course, I would like to know other procedures and adopt what is good in them.

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TANSLSenior Member

@Annode, I did not want to trolling anything, so if you allow me, I apologize for expressing my opinion and my working methods. I didn't want to brag about anything because I think that nothing I've said can teach anyone to design boats. It is simply a work procedure, not a master lesson, which, if it is not worth it, you can immediately forget it. Since I think you are very misguided and you are going to need a lot of help, I sincerely offer you mine if you want to request it. Good luck with your project.

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