Structural stiffness related to material and scantling rules

@baeckmo I think you need to explain a bit more what you are looking at. The answer depends a lot on topography. If you have a measurement ruleset where a hull must not have any hollow in excess of .001m with a 1 meter edge and .002 meter with a 2m edge, and you need to preserve that standard for ten years of hard ocean racing on the international circuit, Then you need to look at the problem of flat panel rigidity rather closely. It you are looking at a panel with positive Gaussian curvature as molded, then it's a different problem related to how the strain from global load imbalances gets distributed or concentrated. Localized loads from rigging points and dynamic shocks are yet a third problem. Negative Gaussian panels are very common in slamming locations (Carolina Flare), but there is very little analytic data or model verification on these shapes.

Each of these needs to be looked at with respect to benefits, risks, costs, and inconveniences. Flexing dissipates energy.

That's bad if there is the potential to recover that energy. But if there isn't the potential to recover that energy, not so much. At the waterline, you really can't expect to recover pressure. The speed of sound is effectively zero. When you slam, you don't get that back. You could make the boot stripe out of marshmallows. There's nothing to be gained from high rigidity in the first few inches of depth.

That's bad if the flexing causes stresses to concentrate at hard spots or happens in buckling modes where stresses are repeatedly concentrated in the same pattern even when there is variation in the load onset. Buckling is often progressive in composites and is notoriously difficult to model for anisotropic materials. When pressure is applied to a convex surface, deflection doesn't normally act to distribute the stress and strain, but instead tends to concentrate the stresses and strains in narrow bands. Conversely, when applied to concave surfaces, defections do tend to distribute stresses and strains in a nice manner.​

So your question about acceptable stresses begs the question of whether you can predict the stress map from the panel topography, laminate schedule, and dynamic loads. To put this in perspective, try to find the stress concentration factor for a anisotropic flat plate with a circular hole or square side notch. This is engineering 101 for isotropic plates. Assuming the boat owner may wish to add a depth sounder, through hull, or hull lights, it'd be nice to know what concentration factor to allow for and whether the appliance mounts can bridge the loads.

 

For what it's worth, which may not be much, a prototype high speed outboard powered, composite hull open "patrol" boat with an intentionaly flexible bottom was exhibited at IBEX around ten years ago. The hull was designed to deflect much more than the standard practice, with the claimed benefit being reduced peak accelerations. from impacts with waves. There may have also been a claim of a lighter structure. That's the limit of my recollections.

 

Lindsay Lord published scantlings for intentionally flexible high speed planing hulls.

 

maybe look at British Standards just google B.S for your needs
widely adopted for alu construction