Methods for Determining Moisture in Fiberglass Hulls

I had recently responded to a blog post on Linkedin about the effectiveness and accuracy of moisture meters on fiberglass boat hulls. My post had to be approved by the moderator which was ran by a yacht broker. Which it never was approved or posted, however other posts condemning the use of moisture meters and insinuating in some other posts that marine surveyors make hasty assumptions from moisture meter readings were quickly posted. I suspect because of my knowledge and detail on this subject was so informative it was not posted as I did not find one post that separated the good marine surveyors from the not-so-good marine surveyors. I thought the subject was so important that it needed to be discussed here so I am writing what I wrote a few days ago. Here it is: As a marine surveyor I wanted to weigh in on this. I am an independent marine surveyor. I am very honest with my Clients.

Moisture meter readings on the hull of a sailboat.

Moisture meter readings on the hull of a sailboat

I have been a full time marine surveyor now for five years with an extensive past in boats and yes, I have even worked in boatyards and have experience in laminating fiberglass. That being said, remember that moisture meters only pick up conduction. That means any water, metal, or even certain elements will make the moisture meter read something. Below the waterline there is anti-fouling paint which most contain copper or some other metal composite anti-fouling agent. The moisture meter will pick this up and read high so I do not use the moisture meter for anything below the water line. Also some topside paints will contain metal composites which will also read high on the moisture meter. If you are getting high moisture readings everywhere on topsides, lets not be naive. Start asking the owner if the topside was painted, and if so what was used. Refer to the paint label or MSDS sheet to search for the components of that paint. I have seen some surveyors, much older and have more years at it than me, just slap the moisture meter on the boat and when it reads high in a few places and say, “the boat’s hull has water in it” and without any other testing walk away. Now I can tell you meeting and working with other surveyors there are two types of marine surveyors I see. One group that are true professionals and are intuitive surveyors and the other group of surveyors that treat the profession as a hustle. It is the second group in my opinion that give the rest in this profession a bad name. I use four methods to determine moisture in the hull; 1. Visual inspection, 2. Phenolic hammering of the hull (IE percussion testing), 3. Moisture meter testing, 4. Infrared thermal imaging. With the combination of these four methods you can make a pretty good determination of the hull and if there is or is not moisture trapped in the hull.

"Picture in Picture" thermal imaging on trapped water near the chine on a fiberglass power boat.

“Picture in picture” thermal imaging on trapped water near the chine on a fiberglass power boat

I however strongly suggest that any use of the thermal camera be done by a surveyor that has at least a level one thermography certification from a reputable training center. There is a science to it and the surveyor needs to distinguish between actual anomalies consistent with water versus reflective and temperature related anomalies. You can not just point and shoot the thermal imaging camera. Each image needs to be tuned properly for analysis. I have only recommended core sampling twice in all of the vessels I have surveyed. Both were to confirm the findings of core moisture / damage from two very stubborn insurance companies that argued there was no damage to the core after testing and thermal imaging. Both times they were wrong and the core showed moisture and damage.

"Picture in Picture" Thermal Image of a Fiberglass Patch Under the Gel Coat with Residual Trapped Water Around the Patch.

“Picture in picture” thermal image of a fiberglass patch under the gel coat with residual trapped water around the patch

I do not think core sampling needs to be done on most boats with moisture / core issues. We are in the 21st century ladies and gentleman. We have all kinds of non-intrusive technology that out perform moisture meters alone and take all the suggestive work out of marine surveying (such as thermal imaging). I do not rely solely on thermal imaging, but it is just one of the tools I use out of the four methods to prove or disprove the presence of water or core damage in the hull. The main reason I believe that more surveyors are not using thermal imaging is for two reasons: 1. The older surveyors in my experience cling to their own methods and are unwilling to consider other testing methods, 2. Thermal imaging is expensive and the certification classes are not easy.

Trapped water under the fiberglass near the hull bottom (no blistering was seen yet).

Trapped water under the fiberglass near the hull bottom (no blistering was seen yet)

Currently to date, I hold a level two thermography certification through ITC and own two thermal imaging cameras (the Flir i5 and the E50 cameras). My total investment in thermal imaging since I got into it about four years ago is approximately $15,000.00 between certification courses and the purchase of the cameras. I would not survey without them, but I would not solely rely on them either. However when I use all four methods I am about 99% – 100% confident of my findings on the condition of the hull.


Until I write again, be well and fair winds!


Very Sincerely,


Capt. John Banister, SA

Suenos Azules Marine Surveying and Consulting

4521 PGA Boulevard, Suite 461 Palm Beach Gardens, Florida 33418

Member SAMS®, ABYC®, IAMI®, and NFPA®

ABYC® Standards Accredited

USPAP® Certified Appraiser

ITC® Certified Level II Thermographer

USCG Licensed Master Captain

(561) 255-4139

Level II Thermographer Certification

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