# Boat Strenght's Scantling Number vs Displacement

Discussion in 'Boat Design' started by TealTiger, Apr 20, 2015.

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### TealTigerJunior Member

Are all Scantling Numbers calculated the same as they are in Dave Gerr's Boat Strength?

I'm guessing that since a designs' displacement can't initially be directly calculated, the calculations must initially proceed based on volume.

In any case, volume seems, at least initially, to be a stand in for displacement.

Does anyone know what the relationship between Sn and D is?

I'm guessing ever 1 of Sn may be equal to approx. 50,000 lbs D.
(10 ft x 10 ft x 10 ft = 1,000 cu ft / .5 (approx. Cp) = 500 cu ft x 100 lbs/cu ft (approx. density of water) = 50,000 lbs)

I'd like to be able to compare a known production boat's scantlings to those suggested in Boat Strength. It doesn't seem logical to calculate a Sn that estimates displacement, when the actual displacement of the boat is known.

I would appreciate any considered and courteous contributions.

Last edited: Apr 20, 2015
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### DCockeySenior Member

Do other scantling rules use the "Scantling Number" concept? I read the chapter in Principles of Yacht Design 4th edition which describes the ISO rules and calculation method for scantlings and I don't recall a "scantling number" being used.

Why can't displacement be directly calculated as the sum of the weights of the individual components of the boat?

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### messaboutSenior Member

TT, displacement can, and must, be calculated. In fact that is one of the first considerations of the contemplated design.

Displacement does have a habit of growing when the boat owner continues, over time, to put heavy things on board that were never intended during the design stage.

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### TealTigerJunior Member

I appreciate your input DCockey, thanks you.

Ok, so a "Scantling Number" may be Mr. Gerr's own creation? Curious if anybody else uses the same or a similar concept.

After I posted, I realized that an initial Displacement may be estimated about as easily and accurately as an initial volume. Which leads me to believe that maybe Mr. Gerr's number is intentionally based on volume. Thinking.

Approx. displacement can be directly estimated, of course. I meant the final displacement can't be until all the scantlings are completed.

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### TealTigerJunior Member

I appreciate your input messabout, thank you.

Again, I was referring to final displacement, not the initial estimated displacement.

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### DCockeySenior Member

"Displacement" is frequently expressed as a weight but also a volume. The term "displacement" when applied to a boat comes from the fact that a boat or any other floating object when at rest displaces a volume of water with a weight equal to the weight of the boat. So the volume of the displaced water equals the weight of the boat divided by the density of the water.

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### nzboySenior Member

My take on Gerrs scantling numbers is it is based on volume of the structure
displacement as such is not a good way to base scantlings because boats have different requirements .Gerrs scantlings are like a ready reckoner and generally are quite conservative based on experience .Eg some years ago an Australian Americas cup contender broke in half. Another well known designer has plans for 50ft trawler 25mm cold moulded ply with no frames only longitudinal stringers and a few tabbed in bulkheads I dont want to be the first one to drop off a 6 metre wave

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### TealTigerJunior Member

I appreciate your input DCockey, thanks you.

I am aware of that; but in regard to my original question, the displacement seems far more 'accurate' than the LOA x Beam x Depth of Hull รท 1000. Is there a relationship between Gerr's Sn, and the displacement that would allow me to calculate more accurate scantlings from his formulae?

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### TealTigerJunior Member

I appreciate your input nzboy, thank you.

Yes, his Sn is definitely based on volume. And, as we note, it may have been his intention. But it could also just have been to approximate displacement. Does anybody know?

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### nzboySenior Member

It is a good question .But for example I want a heavy displacement trawler say 50 ft or I want a 50ft planning launch .Of course I could build the trawler heavy as I want .The planning launch I would need to think about the forces at high speed .because I maybe installing large motors I need to take that into account .Of course you could build a trawler with steel but not a planning launch. So that is where Gerrs offers coeficients to that end So my guess the answer is yes sn has a certain relationship to displacement but its not linear .The trend I see is manufacturers building light as possible in some cases But equally there are makers building very heavily for a select market but asking a price in keeping

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### RurudyneSenior Member

TT, I think the emphasis on volume of the hull (at the extremes amidships) may have been intended to allow for how this usually reflects the use a boat. By this I mean that most smaller boats aren't like houseboats or paddlewheelers that stack a large percentage of their volume on top of the hull. And those that do aren't usually called on for shifting heavy freight as commercial paddlewheelers used to commonly do. They make scant difference (plus to or minus 10% IIRC based on the intended use) between a very light displacement hull, like a Bolger Tennessee, that's barely in the water relative to one that, as a commercial craft, sits low in the water.

Here I wonder: try to imagine what Gerr would have to do to attempt to work in a nuanced modifier for, essentially, the block coefficient/displacement into a book that will probably be used for quick comparison purposes by many pros and by home builders just wanting good enough? Most pros designing heavy displacement commercial craft or super light LDL boats, or relatively towering houseboats I suppose, are going to have either/and experience or resources so they won't be depending on Gerr for the final word even though they find his book useful.

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### Ad HocNaval Architect

In a word, no!

I personally have no idea what his method is, nor read that book, for calculating boat strength. But is it in fact very easy to design scantlings, given the right tools. Many Class societies, for example, now provide their rules for free. Also there is the ISO standards etc.

Any of these shall be far superior to what you are currently reading.

What is your objective? What type/size/speed of boat are you wishing to calculate the scantlings? Is if for personal curiosity or for a uni project or an actual build?

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### TealTigerJunior Member

I appreciate your input nzboy, thank you.

Interesting possibility about a non-linear relationship between Gerr's Sn and D.

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### TealTigerJunior Member

I appreciate your input Rurudyne, thank you.

I can't help wondering if his Sn is really 'about' D, length, bean, or 'draft' aren't differentiated in any way. He just assigns a number to a volume.

Actually nzboy, I just realized the relationship between the Sn and the scantlings is, of course, non-linear. But the relationship between Sn and D is proportional and linear.

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### TealTigerJunior Member

I appreciate your input Ad Hoc, thank you.

It's peripheral research for a patent. It's unfortunately impractical for me to divert into to those types of research now. But I have read and reread Mr. Gerr's book many times between hearing about it here, and my earnest pursuit of this patent.

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