A question that I've had for a while: the Taiwan Trawlers (CHB's and kin) seem to have lots of trouble with their deck coring getting squishy because of the fasteners for the teak decking allowing water to get in.
But other boats (I'm thinking of the Grand Banks, in particular) don't seem to have the same issues.
Anyone know why? Solid decks? Properly prepared holes in the deck? Bettter sealing of the deck boards???
The issue with water getting down through the teak planks is more to do with the seams separating from the sides of the grooves than with water getting down from the
surface of the deck past the screws. Some water can get down this way if the plugs are missing, but the much greater culprit are the seams. It can sometimes be hard to see if they've separated unless you get down and examine them closely.
Separated seams will allow a lot more water down past the planks than missing plugs. Once the water does get under the planks, however, depending on how well the planks were bedded, the water can then migrate out under the boards and it's at that point the screws become a problem. The moisture will migrate down along the screws into the wood deck core and once there it will stay there a long, long time and promote rot.
Grand Banks use the same subdeck construction as most other boats of this type--- a fiberglass-plywood-fiberglass sandwich. There are several reasons why GBs may be less susceptible to subdeck rot than some Taiwan Trawlers (BTW all fiberglass GBs were made in Singapore, and today both Singapore and across the strait in Malaysia).
1. GBs tend to be owned by people who keep them up. So they tend to stay on top of things like separating deck seams and missing plugs.
2. GB did a very thorough job of bedding the teak deck planks when they were first installed. So much so that when a teak deck is removed from a GB, even a very old one, so well adhered and bedded to the deck are the planks that they are usually destroyed in the removal process even after all the screws have been removed.
3. GB uses high quality materials. So the plywood in the subdeck, besides being quite thick, is of a high quality to start with. So it may be more resistant to rot than lesser or thinner grades of wood.
None of this is a guarantee against rot in the core and if the deck, seams, and plugs are ignored for a long time on a GB the subdeck can have the same problems as any other boat built the same way.
This water-under-the-planks thing is why a teak deck should always be washed with salt water. Since there's a good chance on an older boat that some water will work it's way down under the planks, using salt water at least delays the onset of rot in the core because salt water is much less conducive to the formation of rot than fresh water.
So the key to maintaining the integrity of a teak-planked deck is to make sure all the deck seams are good and adhered to both sides of the grooves (but not the bottom which is why bond-breaking tape is a critical component of a seam replacement or repair), that there are no missing plugs over the deck screws, and that the deck is washed only with salt water and a detergent like Lemon Joy which suds up nicely in cold water.
If you have a teak-decked boat on a lake or river, make your own salt water. The wood won't care where the salt water came from.
Obviously you can't do anything about rain unless you keep the boat in a boathouse. But the frequent washing of the deck (wash it, don't scrub it, and always wash across the grain unless deck hardware prevents this) with salt water will help stave off rot if there are leaks down through the planks.
This whole issue of water migration down along the screws is why manfacturers like Grand Banks, Fleming, and the custom yacht people glue their teak decks down now rather than screw them down. Also, the deck seam material used in the 60s, 70s, 80s, and 90s was not as long-lived as the material used today. Today there is only one off-the-shelf deck sealant worth using and that is TDS. It's the deck sealant used by Grand Banks, Fleming, etc. in their new-builds. Even TDS has a finite life but it is a lot longer than the older materials like Thiokol and such.
If you have to do some seam repair or replacement and you can't get TDS (although you can order it directly from Teak Decking Systems), whatever you do don't use Lifecaulk. I tried it for awhile before we had our deck re-grooved and re-seamed and as far as I'm concerned, where TDS is the best seam sealant on the planet, Lifecaulk is the worst. Very, very short life in the weather.
The one use I have found for it is when I reset a deck screw I use a "trick" the shipwright who re-grooved and re-seamed our main deck taught me, and that is to dip the end of the screw in a sealant before re-installing it. The sealant will help prevent moisture that gets under the deck from migrating down past the screw threads into the core. Lifecaulk works fine for this and a tube of Lifecaulk has a much longer shelf life than a tube of TDS, which the manufacturer says is only good for a year as long as it hasn't been opened and exposed to air.
