Engineering Frameless Topsides

[Willallison]

All of the rules that I've come across - ABS, ISO etc - use the panel length (L) as one of the basic parameters for determining panel thickness. This is generally stated as the distance between frames &/or bulkheads.
However, the topsides of most cored FRP boats have little or nothing in the way of transverse framing above the chine.
When calculating the scantlings for such a vessel, do you simply use the (MUCH) longer panel length in your calculations and thus have much thicker topsides? Or is there another way of going about this?

[Raggi_Thor]

Can you use the distance from chine to deck?

[Willallison]

That would effectively be the case when determining the allowable deflection - you choose the shortest span and apply a deflection limit - in this instance probably L/50 or L/100.
But in the case of determining the bending moment, you would use the maximum unsupported span. In the case of a vessel with no transverse frames between the bulkheads, this would be measured in metres. I haven't worked it out but I suspect that the required section modulus - and subsequent required thickness would be enormous.
Granted, you would normally include appropriately fixed interior joinery etc as stiffening members, but there would be at least some areas of the topsides - the engine spaces for instance - where no such fixtures are installed.
Perhaps it is a matter of adding frames in these locations.......?

[Raggi_Thor]

I don't know but it's interesting.
DNV seem to expect close frames.

[fcfc]

It is not what I have understood from ISO 12215-5.

My own personnal understanding is :

A plating panel must supported on the 4 sides by stiffeners.

There are real stiffeners, and "natural" ones. Chine, keel line, deck edge junction, or even curvature in the panel (under conditions) are natural ones.

A panel has a short dimension (b) and a long one (l).

The thickness of the panel is ONLY determined by b (the short dimension). In cored plating only, SM and I have minimum values, again against b only.

l is hidden in the required pressure. A ratio between panel area and waterplane area is used for slamming pressure correction.

It is up to the two stringers along the long dimension of the panel to have the correct bending characteristics.


For the initial case, of no transverse framing on topside, the influence on the plating thickness is not so big. It is up to the chine log and sheer clamp to be strong enougth.
Now, if you have no real stiffeners, but use just the hull chine and deck edge as "natural" stiffeners, it is possible by the ISO rule. But computing the required strength is an exercice left to the reader

[Willallison]

Ah yes, but the problem is that the allowable deflection is calculated based on the longer span (L) so if you have a span of say 3.5m (the distance between bulkheads) then with an allowable deflection of L/100, then your allowable deflection is 3.5cm - clearly too much.
Plugging in preliminary numbers to a spreadsheet, suggests to me that in order to reduce this deflection, then thickness must be increased to an alomst ridiculous extent.
Why then do I see so few boats with transverse framing in their topsides...?

[bhnautika]

Will I think you are confusing frames and the hull shell and about which direction you are working with .Go back to your Westlawn course on fibreglass construction, the “One Inch Strip Analogy”. Longitudinal frames are supported by transverse supports; the hull shell spans the longitudinal frames or chine and gunnels.
House analogy, span of the bearers, joists, floor sheets

[fcfc]

The deflection of 3.5 cm is not supported by the skin only. But the skin and the two longitudinal (real or natural) stringer at the chine level and sheer level.

From what I have understood in iso 12215, deflection in the short dimension is supported by the skin only.

Deflection in the long dimension is supported by the stringers mainly. (in fact, a part of the plating ("effective plating", the stringer + plating forming kind of I beam) is included in the computation. But not the whole panel.