I'm one of the people wresting with hatch warp now.  It happened in spite of the doubler -- although I think it's mostly on the part that isn't doubled.  I think if I were starting over, I'd apply the glass to the undoubled hatch and either clamp it to something flat or put weights on it while it was drying, then add the doubler after it had dried.  Also, make sure your doubler is truly centered -- I was just enough off with one of them that I needed to trim it back with a dremel tool so it wouldn't get caught on the sill rim.

Thanks for the reply, ELB.  I am curious if your hatch is plywood or cedar strip.  I am wondering if warp is more common on one than the other.  I ended up putting two layers of fiberglass on the hatches for additional strength.  I epoxied the sills in place today, so tomorrow I will learn how well the hatches fit.

The deck, including hatches, of a hybrid shearwater are cedar strip.  the stitch and glue deck is plywood (sappelle in the case of a shearwater).

here are some observations from my experience having built a lot of kayaks with these two types of hatches....

the plywood hatch is much more susceptable to 'warping' than a strip hatch.   

my approach to minimizing warping of a shearwater hatch has been to store and build them on a completely flat surface and always keep them flat with weights until both sides have been glassed and the edges have been sealed. at this point, once completely sealed, their shape is pretty much the shape that they will maintain.   warping, to a large extent is caused by different moisture contents across the plywood allowing a little bit of expansion/contraction across and through the wood, which causes it to bend out of the plane non-uniformly. 

the 1/4 inch thick cedar strip hatch of a shearwater hybrid, while not likely to warp like plywood, will often 'spring back' (or flatten) a bit from the deck curvature, as when it is cut out, there is not that little extra pressure pulling it down towards the shearlines. you can minimize the spring back by securing them in place as much as possible...(e.g., don't store them unattached).

the toggle system for holding hatches on the shearwater (strip or hybrid) is a strong, agressive system that can hold the hatches in the correct position even with a bit of spring back or warping of the hatch.  so if you use this system, even though you may have to exert a bit of muscle to get the hatch in position, once you flip that toggle over it, it will be just fine in almost all cases

h 

My hatch is plywood, not strips.  As mentioned elsewhere, I had used a heat gun to try to soften the epoxy and clamped it down to try to straighten it. I think it made some progress -- not completely flat, but close enough that I think the toggles will hold it in place fine.

Speaking of which, the shearwater sectional manual does not include instructions for installing the toggles.  I get the general idea, but am curious if there are instructions about drilling the holes, what hardware to use, etc.   Could someone please post a picture of that part of the instructions?  Or email them to me at elizabethlb at gmail dot com.

thanks