# Coefficient of Scantling

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

1. ### AnnodePrevious Member

What is a good number for the coeficient of scantling for a steel sail boat 20-40m?

Yeah. So there is the question.
CofS = weight of hull structure - OKA scantlings / weight of hull
Its a thumbnail metric just like Prismatic coeficient.

THIS IS A WEIGHT ESTIMATION THREAD

If your answer is: why is that the question or there is no such thing, or you need to design the entire ship including plumbing and HVAC to answer this question, or consult a NA, then please go here:
http://tiny.cc

Last edited: Apr 3, 2020
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### TANSLSenior Member

A steel sail boat, 20-40m long, is a very serious thing and you shouldn't be looking for a number that will allow you to get scantlings. That number doesn't exist, but given the size of your boat, you should consider making the design as professional as possible.

3. ### AnnodePrevious Member

A non answer. I didnt ask for opinions. Weight guestimating is a valid process, so post a number or begone. thx

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### TANSLSenior Member

Since you seem to care very little about technical rigor (this is also an opinion), I'll give you a number : 7/156

5. ### AnnodePrevious Member

great. numbers, not invective... and speaking of technical rigor, might I trouble you for some units to go with those numbers..

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### TANSLSenior Member

The figure that I have given you is a dimensionless coefficient. It is just as useful if applied to the submerged volume as it is to the drift area. Like the number you are looking for, it is useless. But hey, that's what you wanted, a number. My number is as valid as any other, to do a very little rigorous calculation of scantlings.
Sorry, in my post # 2 I was trying to be useful to you, but....

7. ### AnnodePrevious Member

A coefeicient is obviously dimensionless, but this coef pertains to STEEL SAIL YACHT of the order of magnitude 20m to 40m. Not cargo ships, not navy vessles, not dingys, not 20ft trawlers, and not canoes. Just as the prismatic coeff is different for different classes of vessles, so is this. But then I suspect you are fully aware of all this.
So, your demominator is in ft for a steel sailig yacht? ie in the relevant class?
Now that info would really be helpful

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9. ### AnnodePrevious Member

ya, dont need opinions, thx, just numbers )
so feet? or nonsense?

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### TANSLSenior Member

I can not do anything else for you and sorry for the time I have made you lose. I guess you will know how to forgive me. My intention, honestly, was to help you but I think that my intervention has been counterproductive. If you find something, not an opinion, but a number, I hope you are so kind to share it with me

11. ### AnnodePrevious Member

Sure:
From an NA that has built many boats in steel, and has a ton of helpful information and numbers from which to estimate all manner of steel project metrics:

Very roughly, around 12% to 15% of a small yacht's as-built structural weight will be in the form of extrusions such as longitudinal stringers and pipes which will not be NC cut. It is assumed that all other structure will be NC cut. In order to estimate the weight of the gross NC cutting order, a 25% to 30% waste allowance is assumed on the material that is to be NC cut (after deducting the appropriate percentage for the vessel's extruded members).

Designing for Offshore Construction http://www.kastenmarine.com/offshore.htm

Estimating CofG requires extimating structural weight which all over the place. Since this is not know when doing preliminaray calcs for a given hull form, and the location of he hull plate IS known
http://www.kastenmarine.com/_screenshots/fantom.jpg
a reasonable aproximation would be to add weight to the hull plating. But how much?

Another number is given here in Table 3 for different materials

But why am I pointing this out to you? You guys are the NAs with decades of experience desiging...

"Scantling design is essentially a spiral process," explains DLBA's Doug Blount. "You input the structural forms and materials, knowing the overall weight limits you have to meet and the loads you're designing to, and oftentimes the materials and structural framework change slightly to make all the pieces fit together."

Hull design is a spiral process and you dont want to wast hours completing scantling deisign just to see where the ACTUAL waterline is so you can iterate the hull design

Makes sense to me. But then what do i know.

Last edited: Apr 3, 2020
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"Hull design is a spiral process and you dont want to wast hours completing scantling deisign just to see where the ACTUAL waterline is so you can iterate the hull design".................................

A competent NA would not hesitate to spend (not waste) hours to be certain that his/her scantlings are appropriate for the design and function of the boat. A boat intended for leisure cruising in a river would survive with lesser scantlings than the one that is expected to round the Horn, explore icy waters or be exposed to vicious sea states.

There are no cook book scantling rules for 20 to 40 meter boats. For a pond dinghy there are fairly well established conventions for skin and frame dimensions that are suitably reliable. Not so much so for sea going vessels of power or sail...................or Ice breaking.

hoytedow and bajansailor like this.
13. ### AnnodePrevious Member

A competent NA wouldnt misunderstand the process of design (iteration) and assert the last stages of design (final calca) for the first stages of design (estimating weight and CG) for MANY changes in hull design

If you have any actual numbers
• Hull weight
• Scantling weight
for a steel hull of roughly this length, lets see them.

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### MikeJohnsSenior Member

For a start at 20 meters displacement around 30 to 45 tons is common for a steel sailboat, 40 tons laden is a safe estimate, 50 tons is on the heavy side.

The major contributors: ballpark figures for the steel hull is 15 to 20 tons of steel. Ballast 8 to 10 tons, machinery 2-3 tons, Mast rigging and sails around 1 to 1.5 tons. Tankage 1 to 5 tons depending on intended use. Interior Fitout 1 to 2 tons woodwork for a fully fitted boat depending whether it's lightly or heavily timbered fitout, deck gear incl anchoring equipment around 1 ton.

Is that any use to you ?

15. ### AnnodePrevious Member

Hell yes ! Any numbers are better than no numbers. Thank you.
In your hull number you are including the scantlings? (all stiffening structure)
And at the risk of pushing my luck, assuming the majority of the hardware is in the kitchen bathrooms and engine room, how would you compare the interior spaces SqM to SqM. Would you expect a galley to be 1.5x or 2x an ordnary living space?

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