On the inside of my boat under the deck / gunwale there is a removable GRP section that has no apparent structural purpose, I’m not sure why or what for but it seems logical to me that this would contain buoyancy bags or foam. I wondered if anyone knew any more?
The removable sections in your gunwales are indeed intended to hold some polystyrene blocks in place. It is important that you place those blocks again, because they are the last resort when one of the buoyancy compartments fails. GRP does not float! The foam will keep your boat afloat though you cant bail it anymore because the CB case remains submerged. And don’t think that it won’t happen to you, I have seen hatches fail on boats of some very experienced and well known wayfarer sailors within our community.
The exact specifications of the foam blocks are found in the class rules:
34.4 Positive buoyancy units of closed cell plastics foam. Shall be securely fixed within the hull of G.R.P. and composite boats, as follows (lift refers to buoyancy when submerged in fresh water):
(b) [Mk II (except SD versions)]. One unit providing not less than 40.82 (90) lift in the forward compartment. Units providing a total lift of not less than 40.82 (90) under each side deck, aft of the main shrouds
(c) Alternative for MK IA and MK II (except SD versions). One unit providing not less than 40.82 (90) lift in the forward compartment. Units providing a total of not less than 13.6 (30) beneath each side deck and two units providing a total of not less than 54.42 (120) lift in the aft compartment. The units shall be positioned according to the official drawing
Note that you have two options to choose from. Also note that the lift is defined in kg (with lbs in brackets).
By the way, lift in kilogram is the same as foam volume in litre because the specific mass of sweet water is 1 kg/l. Seawater is a bit heavier than sweet water, hence you will get a bit more lift at sea when using the sweet water volume.