Compound curves

As a boatbuilder I know a little about compound curves. In my experience flat panels (FRP or ply for example) will not bend into such curves without kinking. I see there are number of great shapers / builders here who seem to be able to make a flat skin (often bagged, wood or FRP) and then bond it to a shaped blank. I am sure I can see curvature across the deck (and concaves on bottom) as well as the lengthwise rocker.

My question, how the hell is that done? I have bagged all sorts of shapes over the years and have yet to achieve this trick. If it is secret or trade only, so be it, I’ll keep experimenting. If not, please would somebody give me a lead on this perplexing issue?

A razor cut, criss cross pattern of scoreing on the contact side, will allow the panel to conform to both concave AND convex compound curves.


The skins are 1/40 in, so being flexible is not an issue. Also when fiberglass and epoxy is added to the underside of the skin, the moisture from the epoxy adds to the flexibility…and the glass reinforces the veneer from cracking byadds thickness and strength…but not much weight.

The issue at hand is the force from the skin in the bag will sometimes pull the rocker out of the board if you try to run the skin too far out and down the rail. There are two ways around this that i use. 1) is a rocker table made out of eps cut offs from the blank. or 2) I do the bottom first because it is flat and has no real compound…let it cure, then do the deck.



1/40 inch? Damn, I was hoping to use 3mm Paulownia or HD corcell.The rocker table I will use, have already an adjustable one made from MDF. I like the idea of doing the bottom first. I was also goint to glue a strip of Paulownia along the rail before skinning it to reduce the likelihood of over compressing the edges when bagging hard.

Bill, I guess that’s like the cut core foams in boatbuilding but the ones I have used have always been 15mm + thick.

     Yes, exactly the same principle, just on a smaller scale.       Both the depth and width of cuts, as well as the pattern, have an impact on the ability of the panel to conform to the desired form.

Bill, I suppose the same problems can arise. If the cuts are not filled with resin / bonding paste the gaps act as stress raisers and shear failure can start.

PVC foam can be thermoformed easily, corecel maybe too. I use 1 or 1.5mm thick wood i put some paper scotch on top side where compund curve are max (nose) no wood crack anymore

However thick the panel material, I don’t think you’d want to have the cuts be more than 50% of the thickness.     I’d butter or squeege the scored surface, before adding it to the core material, and vac bag it down.     Likelyhood of a stress point would be quite low, seems to me.     I’d also be inclined to have some glass under the panel material too.     I’d rather overbuild, than underbuild.

Hi Rikds , basically , standard plywood is not designed to take compound curves ( certainly not tight ones), because of multiple grain directions opposing each other to keep it structurally flat …even bending ply becomes difficult when the compound radii get smaller, because it’s designed to bend easily in one direction only…its the reason ply boats with a flaired bow use the double diagonal system to form the smaller radius compound curves. Vacuum forming to a solid substrate is a bit different…if you use relief cuts on the inside of the panel , as Bill suggests ,the cuts relax the surface tension enough to take the curves (depending on how close together you put the cuts and where you put them … Under vacuum , the relief cuts actually fill with the epoxy , forming an internal “skeleton” that gives the shape added strength.

What ResinHead said.  My bamboo skins are 1/40 inch and will conform to compound curvature after saturation with expoxy and some vacuum bagging.  Actually, I was surprized when I first pulled it off.

all the best

OK, I’m gonna be totally off topic, but since you’re likely to see this Mr. Rikds (by the way, I’ve always wondered how to read your screen-name), anytime you want to talk about infusion, I’m all ears. 

Just sayin.  It’s been a few years since you did your longboard infusion thread.  At the time, I think it was over most of our heads.  Now, not quite as much.  If you prefer not to go there publicly, I respect that.  But if you want to, . . . oh boy oh boy would that be something.

Digressive hijack over. 


Hi Newschoolblue. That takes me back, longboard infusion? People remember that thread? Funnily enough I’ve just come in from the first surf in 6 months and took that board for old times sake. Still goes well but has been borrowed a few times by some less than careful surfers and it now has war wounds aplenty!

Now you have really put the cat amongst the pigeons, I was going to try a compsand but - infusion, hmmmm. After what Bill said above it has crossed my mind that all those little knife cuts would make great resin paths. So in theory EPS core (sealed), bond rail, glass, Paulownia (knife cut), glass, bag, infuse! Will have to try some experiments, I love experiments.

Where you want to go with infusion. It seems everybody is doing it these days.

Rikds? It’s a nom-de-nyme or pseudoplume or maybe an acronym, break it down into manageable bits, just don’t try to pronounce it!!