After personally repairing more than 7 boxes, installing 5 from scratch, and overseeing another 50+, I put together these free-body diagrams to make sense of what’s actually happening under your feet.
Key takeaway:
Take your full rig weight — rider, board, wetsuit, everything — and multiply it by ~5x.
That’s the compressive load acting on the front edge of the mast plate and, by extension, your foil box. For a 100 kg system, you’re looking at ~500 kg of compression during a hard front-foot pump — countered by roughly 400 kg of tension on the back bolts. This is the cyclic load you’re applying with every pump. And it’s why boxes break.
Static Load Diagram (Steady State Glide)
In steady-state foiling, the compressive load is about 2.6x the rider’s weight.
This can already cause stress fractures in poorly supported boxes.
There may not be a lot of builders on this site, but ideally feedback gets to the manufacturers. Historically ~ 1/2 of the builders had no concept of these forces and boards would fail within 10 sessions. If there wasn’t more than low density foam between your feet and the foil base you were doomed.
You should be able to put your board on the grass, climb on top and stand there. That is exactly what is happening in a steady state when you are gliding. Pumping, turbulence or a steep drop that ejects you multiplies the forces.
See this review of Diving Board Physics on moment arms, tensile and compressive mechanics.
These diagrams don’t just illustrate the physics — they explain why even high-end boards built around “Beer-Cooler” foam can fail under moderate use.
I think the big issue is that “failure” is rarely dramatic or obvious. I’ve encountered bad flex (loss of responsiveness, control, and esp pump) much more frequently than outright failure. The only real box failure I’ve ever seen was one where the reinforcing PVC foam plate was bonded in with too hot epoxy that melted a 1/2 in void in the EPS righ on top of the boxes.
For feeling the flex you have to be really in tune with you gear to identify it on the beach. I get a lot of shit for “man handling” my setup on the beach but as you said the foiling loads are huge and hard flexing is the only way to reveal issues before they happen.
I think for prone and dockstart, where pumping is most important, flex is the biggest factor. I think for DW and wing the moment of inertia of the longer and heavier board dampens the input somewhat so it’s less of a factor.
For prone, my solid Divinycell boards are great. Hopefully building a hollow-tech wing board next. I’m thinking leverage a hard chined flat bottom design that lets me make a melamine mold and 1/2” d-cell for sandwich and a shoebox style connection for the deck - thinking it’s a lot easier than replicating a rounded rail (surfboard affectation that doesn’t add a lot of value to foiling) and has the added benefit of spray deflection!(getting flashbacks to racing my sunfish 25 years ago)
Last thing I’d add is I wish I had a code mast for that bigger plate. It’s almost double the surface area of a NL mast and 50% bigger than many others. Huge advantage!
This moment arm is extreme. And it’s only from one pump. Imagine that happening 500 times a session.
Design Implications
Ask yourself: Where is the board strongest? If it’s just carbon skin over foam — it won’t be enough.
