Good idea Bob, unfortunately the only access to those compartments now is through the 5 mm vent slots. In any case I am going to have seal those compartment joints.
The three stage plan is:
1) Use Captain Tolley's penetrating sealant.
2) Coat the inside of the tanks with liquid rubber.
3) If all else fails, f/glass in the vent slots for a fixed waterline hulled 'surface runner'.
Step 1, then 2, lastly if I have to go to 3 I will still have a working vessel, albeit without variable depth trim.
If I have to lose the vent slots the hull reverts to 'standard' with only prop shafts and rudders penetrating.
The hatch covers are gasketed screw-down, same as the big Engel's.
Fingers crossed
Polyphemus will be sailing, hopefully in variable depth mode.