The general sentiment seems to be to go lighter and I concur.
Quarter inch ply seems heavy to me.
I'd aim for 4mm where you need the strength and 3mm where it's not as critical.
Fiberglass inside and out seems a bit of overkill.
I'd think about glassing the outside only and just the seams inside.
That's what I've done on my two boats.

: First of all the plywood adds stifness to the boat it does not make it stronger - i.e. resistant to wear.

: 1 layer of 1/8" (3mm) material is plenty for a fishing boat if you do not abuse it.

: 1/8" material will bend much better than 1/4" material.

: I would suggest that you use 6oz cloth on both the inside and outside (for wear).
: I would suggest that you try the boat and epoxy in 1/8" x 2" ribs (tapered across the width) covered with 3"x6oz tape if the boat has too much flex.

: : I have been absorbing all the great information available on kayak construction. I noticed that most stitch and glue applications use 1/4" plywood. I am in the process of designing a kayak for fishing etc. Approx. 14' long with a 34" width. I am curious to know your thoughts on using two layers of 1/8" plywood -vs- one layer of 1/4 plywood. My thoughts are I may be able to aviod seams (except for the length, then the seams will be overlaped). I may be able to bend the 1/8" plywood to shape, and keep the whole width as one piece. I would like to read anyone's thoughts on this process. I think that my bottom design may allow for this type of construction. If the plywood will not easily conform to this shape it could be possible to cut lengthwise notches on the inside of the hull with a Skilsaw. These notches will give the plywood more flexability to conform to the hulls shape. I plan to use temporary "ribs" on the inside of the hull until the epoxy and fiberglass cure properly. Am I crazy or could this work???? Thanks in advance!