Peter,
The lashings should hold them which is why they are at at pretty good angle from the beam to the gunwale. They also help hold the gunwale angle. The best way to hold the deck beam tenons in the mortices are pegs. Drill for a 1/8" peg down through the upper edge of the gunwale through the tenon and into the gunwale wood below the tenon.

: I finally at the point where the keel is lashed to the ribs and am getting
: ready to install the stringers (Using Morris' book). I have found however
: that when I put it on the floor to test fit the boat, the pressure on the
: ribs causes the gunwales to pull apart a bit from the deck beams. The
: don't pull apart so far that the beam pegs are completely out but it is
: noticeable. I am thinking that once the stringers and skin are on this
: won't be a problem. Is this normal or should I look at re-lashing the deck
: beams?