ABS Shell Longitudinal Spacing

In the attachment which is the correct definition of the longitudinal spacing "s" in the ABS Guide for Building Yachts

 

I believe "A" is the correct answer. Some might argue that it should be "B". It's certainly not "C".
It looks as if you're trying to determine the scantlings of a multihull using ABS' rules. You cannot do that - the ABS rules are only for monohulls! Multihulls are subjected to loads that are not covered by these rules.

 

"C" would be correct, subject to soren's caveat that multihulls are not covered. Of course, you offset that humungous distance by being able to take a nuge curvature allowance Or, at least, you would be if that was a monhull....
If it IS a monhull, all I can say is "Can we see more?"
Steve

 

Oops - quick clarification - It would be "A" if there was a centreline structural member.

 

Correct me if I'm wrong, but I think it is common practice among naval architects to use "A", even without a structural member at the centerline.
You could use "B" and get the huge curvature allowance, but I don't think ABS would allow it (were they still in the yachting business).
According to Larsson & Eliassons "Principles of Yacht Design", the new ISO-standards state that "A" should be used.
Anyway - "s" is always the shortest (i.e. straight-line) distance between two supporting members.
And by the way: The ABS Guide doesn't specifically state that it doesn't cover multihulls. According to Section 1, Part 1.5.1, the Guide "...is applicable to offshore racing yachts..." and multihulls are yachts too, aren't they? ;-)
Like Steve said: If it IS a monohull, let's see it! If it's a multihull, use the ISO-standard (it DOES cover multihulls!)

 

Soren,
I'd correct you if were wrong, but...
I wouldn't use "A" as drawn, simply because I would have at least thinckened the laminate in that area to provide some sort of longitudinal structure at the most efficient point. This would at least make "A" valid. "C" is, as you say, plain wrong (I plead sickness from it being a Friday afternoon), and "B" is just simply the correct (by the book) one to use, except that ABS wouldn't let you get away with it. They don't even let you get away with following the rule properly - it's always "Add a layer of 12oz mat"

STeve

 

I'm glad we agree! All our designs have some sort of stiffening along the centerline or close to it. If not for any other reason, then because it is common sense!

When ABS published the Guide and it became mandatory for offshore racing yachts, I was very enthusiastic about it, but after having used it for about ten designs I'm not so sure... The above is just one example of the Guide being unclear.

I think we all should use the ISO Standard in the future, or maybe Lloyd's SSC-rules (at least for cruisers)

 

Here follows what the ISO Rules call up which is also ambiguous, but as I am studying the ABS solution that is the one I need some clarity on. I have recently seen some sandwich cored racing monohulls that do not have any stiffening in the hull or deck for the first 25% - 30% part of the vessel, so I was wondering how where they got there "s" dimension from, especially considering that there is no hardspot at keel cl area (it is actually flat there in most cases) similar to most powerboat hulls.

In the “ISO 12215-5” rule “Section 7.1.2 Sandwich Construction” it states:
In general the sandwich area of the hull forward of 0.6*Lwl shall be supported by one centerline stringer and two additional stringers port and starboard side fitted at 0.25 and 0.5 times the half girth from centerline. Arrangements that provide the same strength and stiffness are acceptable.

In the “ISO 12215-6” rule “Section 5.4 Sandwich Construction” it states:
Sandwich construction can be regarded as an effective means to achieve overall rigidity and avoid deflection beyond permissible limits. This construction method may be suitable with or without additional stiffening.

 

What you've seen are so-called monocoque designs, that is designs that rely on the shell plating to take all (or at least most) of the imposed loads.
I have no experience with monocoque designs, but I believe that they fall under the category "Arrangements that provide the same strength and stiffness..." mentioned in the ISO Standard. If that is true, then "s" doesn't come into play at all, since you won't be using the formulas of the Standard - you'll probably have to use direct calculation instead.
By the way: Have you seen the latest issue of Seahorse? In it, there is the first part of a series analysing composite raceboat structures. Might be interesting to follow...

 

The "monocoque" method also looks similar to boats with "encased" stringers - i.e. the stringer is sandwiched with, and the same thickness as,the core, and covered by the inner skin, giving the look of having no structure.
Steve

 

...and then there's "semi-monocoque" where parts of the interior is used as stiffening members.