I would think that the stresses of plywood trying to return to it's original shape would cause the cutouts to flatten out in time. 

if you cut out your hatches nice and clean and like the shape of the hatch and the camber is nicely matched....i would go ahead and convert to 'flush' hatches.

once you complete the flush hatch project, keeping the flutch hatches in place for the better part of the time you store the boat will keep the hatch in the correct shape.

to convert to a flush hatch system is pretty simple.  it is described nicely in the clc boat-builder tips section:

http://www.clcboats.com/shoptips/fitting_out/flush_hatches.html

the only place i don't really follow/agree is the step 2 (laminating a second piece of 4mm okoume) but other's may have their own experience.

i do have a strong preference for wooden cams as giving the best, most water-proof result.  if you have camber (but not too much camber)  i would ensure i had a cam along the centerline to ensure that the hatch does not 'bubble' up in the middle and you have consistent pressure onto the gasket all the way around the hatch.

this is a very slick and nice approach.....so go for it....if you want.  totally doable and acceptable.

howard

   Thanks for the advice. I read the CLC article on building flush mounted deck hatches, but was unclear as to whether step 2 only pertained to building a hatch cover from scratch or whether it also pertained to using the deck cut-out as the hatch cover. My biggest concern is keeping water out, and I am afraid that the slightly warped (non-flush) deck covers I built won't press the foam evenly enough to create a waterproof seal. The cut-outs, on the other hand, match the deck camber perfectly (at least for now). So, Howard, have you had success using unreinforced deck cut-outs as your hatch covers? I'll probably go this route. I can always add the hatch rims and standard hatches later if this doesn't work out, but removing the hatch rims if I don't like them would be a lot of work.

John

   yes, i have had success on using 'unreinforced' cutouts.  that said, a couple key points.

- hatch is 4 mm okoume

- hatch is glassed on both sides.

- hatch is not excessively large (larger than the 'as-designed' and you should be fine)

- position of wooden cams is important and you needs them close enough together to get equal pressure.   most hatches are elongated rectangles and would recommend no less than 6 cams.   its important that a pair of the cams are along the center-line.

it is ok if the hatch is a bit flexible.  what gives it water-proofness and ultimate shape is cam's holding down again a very rigid (three layers of 4 mm okoume - deck, spacer, sill) opening/sill with weather-stripping.  as long as there is not too much distance between the cams you will be fine.

howard

I'm also in the process of doing flush hatches. On my Ches 17 and my pax20, the foam gasket was fixed to the hatch lid. I see in the link above that the foam is fixed to the lip rather than the lid.

Any opinions on which is best? Gasket on lid or lip?

Cheers

for the regular hatch (not flush), the foam is typically on the hatch.   since the lie-on-top hatch is larger than the hull opening, as a practical matter, it is easier to attach it to the hatch and know you followed the outline of the hatch.  also, if you have a hatch lip, like most of the current clc designs, the lip is specifically designed to help get a waterproof seal by pushing up into the foam in a way that cannot be duplicated if you put the foam on the hull unless you put a lip on bottom of the hatch

in most flush arrangements, the foam is on the sill probably becuase it is protected there and its simply easier to place correctly in this arrangement.

that said, i have seen people switch it around and they seem to make it work. but if you do flip it around, you need to pay attention to things like lips and other little details which may have been engineered into the recommended approach that help it maintain water-tightness that you may not be aware of and may be compromized by the switch.

howard