In an old article entitled “Why we tap your hull” we discuss reasons that we tap on or percussion test boats. Percussion testing is used on wooden and fiberglass vessels, both single skin (solid) and composite (cored) laminates.
Many times we find small innocuous problems such as voids in the deck, but occasionally percussion testing reveals a more significant condition.
During a recent pre-purchase inspection of a 50’ luxury sportfisherman an anomaly was found during percussion testing. Initial tapping was done with a plastic/phenolic hammer (less cosmetic damage) and after an anomaly (different sound) was detected, a metal hammer (more definitive sound) was used to confirm the finding.
The area was on the starboard hull bottom and was approximately 3’ in diameter. The area sounded different than the rest of the hull bottom and subsequently we noted that it would flex with manual pressure applied.
The broker attended but neither the buyer nor seller were present during the survey. We requested that the fiberglass expert from the boat yard have a look at this problem while the vessel was hauled.
An agreement was reached between buyer and seller and a week later the potential buyer asked that we monitor the repair. The boat yard began by removing a coupon of the laminate with a hole saw. In this area the external fiberglass skin was not attached to the foam core, but the foam core was attached to the internal fiberglass skin. A few cuts later and the “never bonded” external skin was removed, leaving foam core exposed over a 3’ x 5’ rectangular area. No water was found in this area.
Three layers of a modern laminate were applied using a vacuum bag, the area was faired and several days later the hull bottom percussion tested normally.
Not all surveyors perform percussion tests. Most of the time the entire boat is not percussion tested, i.e. the upper portion of the hull sides that are inaccessible without moving a ladder slowly around the boat. We usually tap out the deck and bottom and the hull sides we can reach, but we certainly don’t tap every square inch. In this instance we performed our normal inspection and fortunately found this problem area.
I wouldn’t go so far as to use the cliché about the blind squirrel (who sometimes finds a nut), but luck is a useful tool in this surveyor’s bag.