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Application Notes
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Thickness Gaging of Fiberglass Boat Hulls

boat hull testing 38DL PLUS

Background: Fiberglass reinforced plastic (FRP) is used extensively in the boating industry for both work boats and recreational craft. Boat manufacturers and marine surveyors need to obtain accurate hull thickness measurements. In the past, unless there was a place to put a micrometer, there was no way to determine thickness except to drill a hole in the hull. Modern ultrasonic thickness gages, however, make it easy to check the thickness of boat hulls.

Ultrasonic thickness measurements are particularly useful with fiberglass mat/roving lay-ups. Variations in layer thickness make it necessary to periodically check thickness. Sometimes boat hulls are made of fiberglass skins sandwiched around an inner core of balsa wood, cork, or foam. In those cases, the core material makes it impossible to make total hull thickness measurements, since it does not transmit ultrasound, but the outer fiberglass layers can be measured very effectively with ultrasonic equipment.

Equipment: The Olympus 38DL PLUS® with the High Penetration software option and 45MG with High Penetration software are portable gages capable of making digital thickness measurements on fiberglass. They are commonly used with an M2008 transducer (500 kHz) to maximize the penetration of ultrasound in thick or attenuating fiberglass, providing a thickness measurement range of approximately from 0.150 in. to 3 in. (4 mm to 75 mm), depending on the type of fiberglass. In especially challenging cases, the advanced 38DL PLUS thickness gage with the high penetration transducer option and large waveform display is recommended. This configuration enables operators to adjust setup parameters on site. Transducers such as the M1036 (2.25 MHz) or M109 (5 MHz) can measure down to approximately 0.050 in. (1.25 mm) in fiberglass.

Procedure: The 38DL PLUS gage with the HP option and the 45MG gage with the HP option both have pre-programmed default setups for the two transducers most commonly used for fiberglass measurement, the M2008 and M1036. In many cases, these default setups can be used with no further adjustment, just the two-point velocity and zero calibration that is recommended for all ultrasonic thickness measurements.

The fiberglass used in boat construction is generally of high quality, and unlike some other fiberglass laminates it usually does not produce high amplitude internal reflections that can cause false readings. However, if false readings are observed in a particular case, the situation can usually be corrected by small adjustments of the initial gain and TDG slope as outlined in the gage operating manuals. The 38DL PLUS thickness gage waveforms below show a typical hang up on internal reflections, and a correct reading of total hull thickness following a simple gain adjustment.

False reading due to a hang up on internal reflections

Correct reading following gain adjustment

Any delaminations that may occur in the hull will appear as strong, well-defined reflectors accompanied by a complete loss of the back wall echo. While ultrasonic flaw detection should always be performed with instruments specifically designed for that purpose, such as the EPOCH® series, a trained operator can use the waveform display of the 38DL PLUS and 45MG gages to make initial judgments about the presence of delaminations in a hull.

Gelcoat measurement: A closely related application is measuring the smooth gelcoat applied over fiberglass. For detailed information on ultrasonic measurement of gelcoat thickness, please refer to the separate application note Measurement of Fiberglass Gelcoat.

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Products used for this application


The 38DL PLUS is an advanced ultrasonic thickness gage. Uses dual element transducer for internal corrosion applications, and has features that include THRU-COAT technology and echo-to-echo. Uses single element transducers for very precise thickness measurements of thin, very thick, or multilayer materials.


The handheld 45MG ultrasonic thickness gage is packed with measurement features and software options. This unique instrument is compatible with the complete range of Olympus dual element and single element transducers, making this gage an all-in-one solution for virtually every thickness gage application.

Magna-Mike 8600

The Magna-Mike is a Hall effect thickness gage that uses a magnetic probe to perform accurate measurements on nonferrous and thin materials such as plastic bottles.
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