To Tap, or Not to Tap?


Get Your Boat Surveyed — By Mike Smith

If you’ve ever watched a surveyor at work, you’ve probably seen him going over every square inch of the hull with a small plastic hammer, tap-tap-tapping away in search of something amiss. But is the hammer necessary in the 21st century? Hasn’t it been superseded by the moisture meter? Or is there a better way altogether?

When a surveyor applies hammer to hull (technically called “percussion testing”), he’s listening for a change in tone, just like you do when you tap your wall to find a stud underneath. A dull “thud” can mean water intrusion into the core, from osmosis or via poorly bedded fittings, delamination due to imperfect manufacturing or repair, or maybe just a bulkhead or stringer on the other side. It takes an educated ear to make sense of percussion testing.


Most surveyors today also carry a moisture meter, an instrument invented to measure the moisture content of wood, but one that also works on fiberglass laminates, with one big limitation: The surveyor has to know how to interpret his meter’s readings. “I have two moisture meters,” says Joe Lobley, adding that each requires a good deal of experience and practice to use accurately. Lobley also says he hasn’t put away his hammer yet and still percussion-tests every hull along with doing meter readings.

There are some very valid reasons for this. At a recent SAMS continuing education class, held at a salvage yard, some 20 surveyors were able to check a variety of meters by destructive means. After taking numerous readings on derelict vessels, the surveyors cut plugs out to visually inspect the laminates. Often, areas that read ‘dry’ on a given meter were actually soaking wet, so wet that water literally ran out around the hole saw. 

Of course, some of the meters were consistently more accurate, maybe because their signals probed deeper into the laminates, reaching past the outer skins. But still, the most accurate “meter” of the entire lot seemed to be the hammer, when used by a skilled surveyor with excellent hearing. 

“Younger guys use moisture meters,” says Ted Crosby. “Older surveyors use hammers.” Maybe the old guys have the right idea.