The hull is obviously the single most important part of any floating structure, it is the hull which fulfils the requirements of the Archimedes Principle and allows the structure to sit on the water instead of under it. To achieve this the hull needs to be watertight.

In a sailing yacht however there are more requirements, it needs to be hydrodynamically formed to reduce resistance and more importantly it needs to be built so it can take the loads of the keel, mast and also forces from waves without getting damaged. A yacht up to about 38ft can have loads of up to 14t acting on any part the rigging, and so also parts of the hull when the sails are up, while slamming in waves and grounding create more forces in other places of the hull. It is obvious then that the hull gets a lot of abuse. And so it needs to be looked after.
 

There is an illness in GRP hulls that is called Osmosis. This is when water manages to creep underneath the Gelcoat, the out layer of the hull and sometimes also creep into the inner layers of the laminate. Over time this water pulls in more water (especially when there is salt in the water) and forms bubbles within the fabric of the hull. This can vary in severity, but in its worst case it can weaken the hull and make the boat dangerous to sail.
In most cases though there are ways to get rid of Osmosis, some are more time consuming and require the top layers of laminate to be scraped off, others just require a coating of epoxy coating to stop the water being able to get in. 

On inspecitng Fritha's hull I found a few small bubbles, which were located in the outermost layers of Gelcoat, indcating that she does not need extensive work done to protect her against Osmosis. This is a tribute to the manufacturers; a 40 year old boat which has spent 95% of the time in water has only got slight Osmosis. Many newer boats are worse off!
Therefore it will be necessary to sand the underwater hull right back to bare Gelcoat, remove any bubbles there are and after enough time to allow the bubbles to dry, filler the hull before coating it with an epoxy protection paint. This will get done once the keel has been sorted out and is reattached.