Rot Repair in Fiberglass Boats
   Stringers and Supports
Stringer Next to transoms, stringers are the glass boat owners' biggest concern. Transom rot and stringer rot are often associated. Again, this is primarily a power boat problem because the impact forces generated on the bottom of the hull require substantial stringer reinforcement. Nowadays these are usually wood chopper-glassed over, although a few use a glass molded unit and on older boats they laminated a mat, cloth, or roving mold over the wood to create a strong structure (see photo at right and Diagram A below). These are better, of course, than the cored stringers where the wood is the main structural support, but result in a more costly boat. Where cloth or roving is used over wood, most of the strength is in the glass. If you have about 1/4" or more of glass around the wood in a "hat" section and the glass is securely bonded to the hull then the wood becomes almost redundant (see photo below left). Even so, you often get rotted Diagrams wood because of poor lamination. If it is not laid down carefully and there are waterways left, long gaps that allow water to work it's way along the wood as in Diagram B. Sometimes the boat manufacturers drill or notch them (Diagram C) and the water gets in. When you inspect your boat, these are the areas which need a thorough CPES treatment, even though there is no evident sign of rot. Stringer

Click on image for closeup view.
Stringer Cross Sections
You can see from the photo (top far left) how a typical small hull is stringered. The bottom left photo is closer and shows the floor over the stringer. The top right photo is a close-up and gives you a picture of a bare wood stringer on top of a glassed stringer. You can see the gaps between stringer and hull, a perfect water channel. The glass laminate over the stringer is not thick enough to provide structural support if the wood rots. The bottom right photo shows where they glassed the stringer and then actually cut away a section so the floor could be mounted. Duh! An open invitation to wood rot.

Glass Boat Cross Sections

The rot process is often from the top down. Water on the floors leaks through and gets into the stringers. You go to fix the floor and find that the glass is delaminating around the stringers and the wood inside is rotted. The illustration above shows the layout and you can see how the pattern progresses. Too often the manufacturers fasten the floor on with screws, right through the glass on top of the stringer. A perfect channel for water. On a sailboat hull this is less a of a problem because the floors are generally
Stringer W/Top Cut Away
Stringer Replacement
covered and the water doesn't get there. The cored decks and cabins of sailboats are another problem which we'll get to later.

Getting to the stringer is often the problem. You may have to rip out sections of the interior. You then have to cut away the top of the stringer. Pick your tool. There are some specialty instruments that are capable of making these cuts, from Dremmel tools to small battery circular saws to hole saws. The photo (top right) shows a stringer with the top cut away and the wood showing. Any wood that is left should be reasonably dry and treated with CPES™ to greatly reduce the rot fungi problem. The CPES™ should be allowed 3-5 days for the carrier solvents to evaporate away and the epoxy to start to cure. You can then glass everything back in. You must use epoxy resin because polyester resin will not bond well with epoxy, which you now have on and around the wood.

How do you repair a rotted stringer? Ideally, you replace the wood. This is what we recommend. It's worth extra rip and tear work to do it this way. You pull out the rotted portion, cut a long angle on the remaining good wood and the new wood, and then splice it in, using Layup & Laminating Resin™ to join the two pieces. Gaps can be filled with the Fill-It™ epoxy filler. The new wood going in should be thoroughly saturated with CPES™, and then all vacant areas filled with the L&L Epoxy Resin™.

There is an alternative to the above process, but it's not nearly so thorough. Still, in some cases it's enough to keep an old boat going for a few more years. Basically what you do is drill access holes in the top of the stringer 5 to 7 inches apart, allow the wood inside to dry (or blow dry it), treat with CPES™ injections, and then fill all voids with Layup & Laminating Resin™. Make your holes large enough to accept the tip of a turkey baster--it's the simplest way to get the resin in. It's not a complete solution because you can't see what you're doing, but it will supply some substance and stiffness to the stringer. You will have to allow at least 2 weeks after the CPES injection for solvent evaporation and epoxy cure before putting in the Layup & Laminating Resin™.

The wood that was above the stringers (floors, spacers, etc.) should probably be replaced as well. Treat the new wood thoroughly with CPES™, especially the edges. If you want complete protection, you will pre-drill fastener holes and treat them with CPES™ before installing the screw or nail.

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Stringer Repair
Support planks and sections should be torn away and replaced if at all possible. These are normally plywood which has been chopper-glassed, as you can see in the photo (left). If the moisture gets in then the ply delaminates and rots. This is repairable with epoxy if absolutely necessary. The wood must be dry and the top must be open. It's then just a matter of soaking the inside ply with CPES™, allow about a week for the carrier solvents to evaporate away, and then follow up with the Layup & Laminating Resin™ to fill interior voids. You can then re-glass the top edge. It is possible to leave the top of the plywood un-glassed, as the CPES™ and Layup & Laminating Resin™ will now pretty much waterproof and substantially protected from any future rot problems, but only it the top glass is not required for structural stiffness.

All of the interior wood repair on a glass boat is best done in the spring, after the boat has been out of the water, under cover, and dried out. No epoxy effectively adheres to wet wood, and CPES™ certainly will not displace liquid water. A bit of moisture is okay, for there are carrier solvents to help displace them.

Engine Beds and Mounts
Engine beds and mounts can be a problem on older glass boats. Running hours alone tend to shake things loose, and then add a little water to soften things up and the engine starts hopping around. Here is a typical inboard engine installation for a shaft drive. It is the usual 4-point mount onto a steel frame over wood beds. Other mounting configurations are shown below. The photograph shows a typical I/O mount with steel pedestals through-bolted on a chopper-glassed wood beam.

I/O Engine Mounts
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Eng Mounts Diagram

The wood is usually integral to the mounting system; if it goes bad then things shake loose. Repair will depend on the extent of the damage. If the wood is totally shot then it should be replaced. This can be a hassle because you've got to pull the engine. Just as often the damage is a matter of stripped-out mounting hole wood that has gone soft. This can sometimes be repaired with the engine in place, or the engine can be jacked up and skidded over to give access to the hole. Repair is then usually a matter of cleaning out the hole (drilling or chiseling), saturating the newly exposed good wood with CPES™, inserting new wood which has also been CPES™ treated, and then embedding the new wood in Layup & Laminating Epoxy Resin™. After this you re-drill the mounting hole, tap for threads if you need to, and then fasten the engine back down. You may have to do a bit of re-glassing as well, being sure to use epoxy resin, not polyester.

And you might as well repair all the mounting holes as long as you are involved in the process. If one is bad then the others are likely not in great shape either.

For the above process to work the wood must be reasonably dry and oil-free. (Wood with oil on it should be drilled or chiseled away.) Blow the holes with a hair dryer for a few hours before proceeding with the CPES™. Allow at least 3 days to pass after the CPES™ treatment for the carrier solvents to evaporate away before proceeding with the wood and L&L Resin™ rebuild. After that 48 hours between steps is sufficient.

One way or another the repair process outlined above will work on just about any wood/steel engine mount configuration.