- 1). Look for discoloration in the fiberglass or fiberglass-reinforced polyester around any deck fitting or hardware, and at the edges of the deck or upper surfaces of the boat. If water has penetrated the glass or FRP and the stringer beneath has begun to oxidize -- rot -- the discoloration is a byproduct of that process.
- 2). Inspect the fiberglass surface for peeling or spot delamination, where the layers of fiberglass seem to come apart. This may be evidence of a rotted stringer or the beginning of a blister in the gel coat. Gel coat is semi-permeable; it takes years to cure fully, as evidenced by the waxy secretion in spots, on the surface of a new boat. If water penetrates the gel coat but not the glass or FRP, it may eventually form blisters, but a blister is not indicative of a rotted stringer.
- 3). Walk your deck and the upper surfaces of the boat, putting your weight on the parts of the deck where you know there is a stringer. If a place doesn't "feel right," you might try sounding it with a rubber or plastic hammer: a clear "thunk" indicates the wood beneath is probably sound. Soft spots over a stringer's location clearly indicate stringer rot.
- 4). Tow your trailer and your boat -- empty of gasoline, supplies, leftover clothing or anything not bolted down -- to a truck scale and weigh the boat and trailer. Record the weight in a safe place. The next year, repeat the process. If the boat weighs the same, you're probably in good shape; if not, your boat is accumulating water, possibly in the stringers, possibly in the foam. If your boat is surprisingly lighter, you may have to consider, in conjunction with other evidence, that parts of the stringers have rotted away.
- 5). Drag a moisture meter along the path of known stringers on the completely dry hull of the boat. The meter will not read in areas where the fiberglass or FRP is peeling, but the meter will give you an indication of the moisture content of the stringer. Where stringers are fully enclosed in fiberglass, this is the only non-invasive method available.
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