Learn more about your boat:

Dry Rot is a fungus that arises from wet wood. It only needs a small amount of water to gain a foothold and send out rhizomorphic runners in search of more moisture. It thrives in poorly ventilated areas, grows in temperatures above 40º Fahrenheit, and literally eats up the cellulose of its host, destroying the wood in the process. Unless stopped, the fungus can spread very quickly, causing the condition commonly referred to as "dry rot".   Dry rot can be your boats worst enemy.  Moisture causes wood to break down in composition and It generally will require professional attention.  All damaged wood must be identified and replaced to maintain the structural integrity of your boat.

Some boats come with wood framing and others come with teak trim.  A lot of the framing wood isn't teak because of the higher cost of the wood and is prone to dry rot.  Tap questionable areas with a hard instrument and listen for hollow sounding dead spots. Probe suspect areas with an ice pick or sharp knife. Check areas where fresh water is likely to accumulate. Particularly check the following: hatches, deck seams, transom, companion slides, planking, toe rails, stem, and ribs.  You can fix small "hollows" with liquid epoxy which will saturates and restores rotted wood by penetrating the rot.

What does all this have to do with a fiberglass boat? 

Many boats have transoms (sterns) made of plywood encapsulated in fiberglass. The reason for this practice is the need for strength where an outboard engine or stern drive unit are supported. Decks may also be made of plywood (or even balsa wood) covered with fiberglass. Also engine stringers, the structural members that run longitudinally fore and aft the length of the boat on the inside of the bottom can develop dry rot.  They are often made of solid wood such as Douglas fir or pine, and then covered with fiberglass, they stiffen the hull and support the engine.

As long as the fiberglass covering on a transom, deck, cabin top or engine stringer doesn't crack or have a hole drilled through it, all is well. If water is allowed to enter into the wood and get trapped inside, however, dry rot will occur.

Drilling holes and then not sealing them off properly will allow moisture into the wood.   Boat owners, unaware of this problem, install equipment on their transoms or decks and soon develop dry rot.