Rotten to the Core

Posted by Darrell Nicholson at 11:58AM - Comments: (7)

This leaky lifeline stanchion that has caused the coring beneath its base to rot.

Cored decks are a soggy subject for many owners of older boats, but owners of new boats have to be wary too. I turn to one of our regular contributors, Capt. Frank Lanier, for this week's blog on this topic. Captain Frank Lanier is a 27-year Coast Guard veteran and Accredited Marine Surveyor with over 30 years of experience in the marine and diving industry.

Mounting hardware directly through a cored decking without taking the proper precautions to prevent moisture entry is asking for trouble. Photo 1 shows a leaky lifeline stanchion that has caused the coring beneath its base to rot. The mounting nuts are drawn up so tight they're crushing the panel, no doubt caused when the owner tried to stop the stanchion from wiggling, and to try and stop the leak as well. Note the inadequately sized washers and lack of a backing plate.

Failure to seal the coring during installation of this hawse pipe resulted in a rot and delamination of the entire foredeck of this vessel.

Anytime you screw or drill through a cored panel, the first rule is to properly seal the core against moisture entry. There are a number of ways to do this, but the best one is to avoid breaching the core material in the first place. In a perfect world, your boat's manufacturer has anticipated where all deck penetrations are necessary (stanchion bases, cleats, etc) and has "de-cored" these areas by reverting to solid fiberglass, allowing you to mount hardware without drilling into the core.

In the case of new installations however, chances are slim-to-none that any of these areas will coincide with whatever aftermarket doo-dad you want to mount, meaning you're going to have to seal and protect the core the yourself. There are a number of ways to handle this problem, and how you deal with them will depend on a variety of issues, such as what sort of hardware you are mounting, what kind of access you have below decks, what sort of loads are expected, etc. In all the recommended approaches, the object is to make it impossible for any leaks to get to the core.

For the nitty gritty details on how to seal core for hardware mounting (and a range of other repair and maintenance issues), check Chapter 7 of the free book "Fiberglass Boat Repair And Maintenance" published by Gougeon Brothers. You can order a hard copy or download a free PDF. It's a good how to booklet and well worth space on any vessel's bookshelf or hard-drive.

And for a more detailed discussion on the pros and cons of cored construction, check out our October 2007 feature "In cored hull construction, does high-tech mean high-quality?" Don Casey's book "This Old Boat," also offers some tips on how to address cored deck installations.

Comments (7)

I have a question about a method that I've used for through-deck installations that appears to work. My decks are plywood cored, and when I drill a whole, I tape the bottom with masking tape or duct tape; and then fill the whole completely with Git Rot, a high viscosity two-part epoxy. The Git Rot is absorbed into the plywood, such that when it's dry, there is almost none in the hole. I then bed with a good bedding compound. Is there a reason why this wouldn't work as well as creating a solid core of epoxy?

Posted by: Rigoler | February 7, 2016 3:39 PM    Report this comment

Compass marine offers an excellent how-to using a special butyl compound available from them:
I used this technique on a 30 year old neglected boat with many through-deck leaks. EVERY leak was easily and flawlessly sealed. It is a pretty foolproof technique.

DO NOT USE SILICONE as a bedding compound (except for where it is intended like plexiglass windows) as my previous owner had. 1) It doesn't work. 2) It will make subsequently doing the bedding correctly 20 times more difficult because all of the silicone must be removed because the proper butyl (or life calk if you prefer) will NOT stick to silicone.

Posted by: S2Pilot | November 4, 2015 9:23 AM    Report this comment


Core thickness varies depending on the boat -- common thicknesses are usually 1/8" to 1"
Most boats we look at are somewhere in the middle range.

Balsa is still common in many decks. Our October 2007 feature (link provided above) offers a pretty good look at the technical challenges of constructing with core, and some bits on repairing. Before you begin any repair projects, you should also take a look at some of the publications I mention in the post

Posted by: sailordn | October 19, 2011 8:28 AM    Report this comment

I am new to doing alot of the fiber glass work on a boat. I was told that the core material is very thin, and is made of Balsa Wood in a lot cases, is this true?

Posted by: bill t | October 19, 2011 7:46 AM    Report this comment

How thick is the core material usually?

Posted by: bill t | October 19, 2011 7:42 AM    Report this comment

The two sources mentioned at the bottom of the blog post offer good guidance; Gougeon also has other free booklets to help. Bruce Niederer has an article in the Spring 2002 issue of Gougeon Bros. Epoxyworks magazine (Google replacing damaged core epoxyworks and a PDF will come up). The photos are helpful. Casey describes two options coming in from the bottom (saving you the trouble of dealing with matching non-skid etc.) or through the top (easier since you are working with gravity). If you've got a soggy deck, you've probably have other issues, making Casey's book a good investment. Our October 2007 article illustrates the importance ensuring a good bond between the core and skin laminates.

Posted by: sailordn | May 5, 2011 7:27 AM    Report this comment

Do you have any suggestions on a book or manual that would explain how to replace a cored deck where most of is is soaking wet and no good? I have replaces a 1.5 x 1.5 foot area and was suprised to see that it was so wet and rotten that i could grab the wood core and squeeze it like a sponge.

Posted by: STEVEN B | May 4, 2011 11:17 AM    Report this comment

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