Finding Mystery Rot and What to Do

Wood in a marine environment will eventually rot. Choosing rot resistant species will prolong the process, but eventually, it will succumb to the effects of decay. 

Rotten wood is soft, moist, and squishy. When you prod it with a metal probe, the probe will sink into it without resistance. When you tap on it with a hammer, it will not make a bang sound like sound wood would, instead it will make a dull thud. Rotten wood in cosmetic structures like cabinets and interior joinery is unsightly, but rot in structural members is devastating to the structural integrity of your yacht.  


When you find rotten wood, what should you do? Should you cover it up and ignore it? Should you sell the boat?  Should you fix it yourself?

Well all of these question’s answers depend on how you comfortable you are at working on your own boat. If you are willing to take on the restoration of this rotted section, then have at it! If you are tired of boats and all the time they consume, then now would be a good time to sell (but you might get a lot more for the boat if the rot is fixed first).  

Rot is not that big of a problem when it comes to repairing. All the wood has been removed for you! The big picture here is you are going to remove the bad and replace it with new. The decay process of wood is very much like the decay process in teeth, and being a dentist, treating rot is a part of my job. 

When you look at wood, just like with teeth, you will have two types of wood: damaged and ok. Ok wood looks new. It is dry, feels normal, and sounds solid when tapped on. Damaged wood will be wet, look dark, and can be further subjugated into two more categories: Infected and Affected.

Infected wood is rotten and disgusting. It will resemble mulch in the way it just falls apart. It will be wet and make a thud sound when you tap on it, as it is no longer solid. Rotten wood can be removed by scraping with a metal instrument, as the weak remains will just fall apart and come out in pieces. 

Affected wood is simply close to the infected wood, but it doesn’t need to be removed. Sometimes, keeping a bit of wood structure will make your life so much easier. Think about it, if you have some rot in a bulkhead, you don’t need to replace the entire bulkhead, just the rotten portion and scarf in a replacement piece. Affected wood is the transition between the infected and the normal wood. It will be stained, wet, possibly slimy, but it will sound solid when percussed and will not hole when struck with a screwdriver tip. This wood just needs to dry, be wiped down with bleach, and given some time to restore itself before the new wood can be scarfed to it. 


To assess the damage, it is best to sand away any paint that way the full extent of the area can be assessed. They say the first three rules of surgery are:
1. Access
2. Access
3. Access

You need to see what you are working on to figure out how you want to fix the problem. The rotten section is the void of a hole. From there, the affected wood is located somewhere in the transition between the good wood and the infected wood. The problem is, this bulkhead is made up of two layers of 3/4” plywood, so access to the underlaying layer is only attainable by removing the top layer first. 

You can see the large section of wood that has been cut to reveal the underlaying bulkhead plywood. When you have access, you want to treat the wood the wood as you would a cancer in a biopsy: cut everything you suspect and leave only clean margins. With the wood, if your cut section has signs of rot still left behind, cut it back a bit further.  

When you don’t have such liberal access, you will have to be a bit more judicious in your selection of what comes out and what gets to stay. The wood in front of the mast was too close to the mast to get the saw in there, so I was forced to use a drill (dentist joke) to remove the rot. A 1/2” drill bit will do wonders at ripping all the rotten wood out of the plywood, and won’t be able to easily drill into the good wood you want to leave behind.  

When you are removing the rotted wood, it pays to keep in mind that you will need to rebuild next. Severely rotted wood is actually easier to replace because the tabbing on the hull will be left intact. All you need to do is measure the size of the wood that once occupied that space and have it milled. Then you can glue it back in place with gobs of thickened epoxy that has been smooshed into the tabbing. When the wood is partially decayed, it can pose a challenge as you can’t get it to come out as easily. 

If you have a single sheet of rotted wood, consider scarfing in a replacement piece that is held in place with thickened epoxy, followed by a few layers of fiberglass to help tab it all into place. If you have laminated layers of plywood, like in this case, you can simply cut the access hole a bit larger than the deep layer. Now, the deep layer will be glued to the remnants as well as to the access hole’s board, and the access hole’s board will be glued to the deep layers remnants. This stepped approach gives you plenty of surface area to glue everything together while still giving you the ability to put everything back together in a structurally sound method. 

When rebuilding, it is always important to look at the cause of the rot and figure out how to prevent it from happening again. Frame heads are notorious for holding water if they are not beveled towards the midline of the hull. This bulkhead abutted the shower and had no protection from the water that seeped into the wood for decades. 

Figuring out the cause will then let you plan the solution so that the future you or the next generation won’t have to carry out this same repair.