I mentioned earlier that ski boat manufacturers have gone away from using wood in their stringers, floors, and seat backs. But for a long time, that was the industry standard. When your stringers suffer from rot, and/or delaminate from the hull, they need to be replaced. It seems like the decision of what to replace the stringers with should be easy, replace them with what they were made of in the first place. For example, in my boat I replaced the main stringers using 2x10 fir. The secondaries I used 1x6 pine.
It isn't that simple. I contemplated using other materials to build the stringers. What comes to mind first was to enlarge the stringers themselves and encase a foam core in fiberglass, similar to how some boats are built today. Other folks have come up with a variety of materials that would occupy the same space as the original stringers but are more lightweight, stronger, or whatever. Another option is the pourable stringer using a compound that would just be poured into the cavity left where the wood stringers were removed leaving the fiberglass shell in tact.
In the end I decided that my boat was twenty years old. It made it that long with the manufacturer using wood stringers. The guys who had it before me smashed it into a rock at high speed and stored it outside without so much as a tarp for cover. If I rebuilt the boat using the same stringer system it was guaranteed at least another twenty years of service.
You have to decide how you plan to rebuild your stringer system before you make the first cut. I started with the main stringers, removing one at a time. Then building a new stringer, laminating, installing, and fine tuning the dimensions. That is one technique. It it also possible to remove all of the stringers first. Then build new stringers and dry fit the entire new assembly before laminating and installing them into the boat. Again, you need to decide what road you plan to follow.
The front of my stringers were separated from the hull leaving a big gap. I started cutting at the exposed end of the stringer (bilge side of the stringer). When I cut beyond the point where the stringers were now touching the hull I could feel when the saw blade was in that pocket between the stringer, the hull, and the bilge. I chased that pocket all the way to the back of the boat. The main stringers were held together with a cross brace in the stern. I cut it away. I followed the same technique on the other side of the stringer. There were a few places where I had to cut away small perpendicular ribs that connected to the stringer. Like I said earlier, I had to switch tools a few times.
When the stringer is cut free it comes out with some twisting force. Remaining in the hull is a channel with a fillet on each side where the stringer used to be. Against the hull are the remnants of where fiberglass was starting to fail from moisture. I used an angle grinder to grind down the channel to where it was even with the hull. At the stern, the stringer was bonded to the boat. That had to be cut away and ground down. This was very hard because of some odd curves and blocks that were precisely in the way. Next, I ran an orbital sander over the length of the stringer using 80 grit sand paper. I finished by vacuuming and wiping the repair area clean with acetone.
It is difficult to see in this pic, but on the right side the main stringer is removed. You can just make out the outline of where it belongs. On the left side of the pic is the lateral rib that had to be cut away from the stringer. I repeated this process on the left side once I replaced this stringer.
If you are fortunate, then the stringer you pulled out is in good enough condition to use as a pattern to build the new one. Take into consideration that there is about 1/8 inch to 1/4 inch of glass over top of the actual wood stringer. I suggest that you first cut away the fiberglass from the top of the stringer in order to have a more accurate template. Pay attention to the bevel at the bottom of the stringer, it changes in severity from stern to bow.
I spent about an hour at Lowes unstacking a pallet of ltwo inch by 10 inch by 14 foot fir beams. I selected the two lightest, straightest beams with the least amount of knots. Then I restacked the pallet. These were the foundation for my new main stringers.
I used the old stringer as an outline. The easy part was cutting along the outline. The top was not an issue. The bottom is beveled at an ever changing angle. That was not easy. I tried to summon some ancient math skills I learned in grade school to come up with a system to calculate the angles. I failed. I settled on this technique: I placed a carpenters square with the head flush against the bilge side of the stringer and the ruler running under it. Then I measured the gap from the ruler to the edge of the bevel. I marked that position on the new stringer. It is important to note that this would have been very easy if I had carefully cut out the stringer. I did not do that all the time. This means in some places I cut through the stringer during removal. I had to piece that back together to figure out what it was supposed to look like. Don't do that. Once I marked the stringer, I used an old fashioned hand plane to remove the material. I shaped it as close to the original as possible.
The next step was to test fit the new stringer. You would think the dumb thing would fit properly. Nope, that beveled edge was a monster. I finally settled with small gaps 1/4 inch or less in a few places. I also confirmed that the stringer was properly lined up with the existing stringer so I was not going to effect the location of the engine cradle or anything.
Before installing the new stringer, I laminated it using epoxy resin and fiberglass cloth. Cut out a pretty close outline in the glass cloth. I brushed epoxy onto the stringer in a thick application. I then placed the cloth onto the stringer. Then I brushed epoxy over the top of the cloth. You will be able to tell when the cloth is absorbing the resin. Finally, I rolled out all of the air bubbles. The cloth has loose ends that tend to get caught in the roller. I cut them as they come along.
Once the resin was dry to the touch I came through with a razor knife and trimmed the edges a bit. I let it cure all night. Then I flipped it over and did the other side. I left the top and bottom alone.
You see in the picture that I placed the stringer on a ladder. I wanted to support the stringer as much as possible to prevent bending. I spent a lot of time finding two straight planks. I wanted to keep them that way.
I made sure to trim the excess glass from the bottom of the stringer. I ran an orbital sander across it to knock down any run off drips. This is important. Experts recommend washing the stringer with soap and water to remove an effect called amine blush that develops with epoxy resin. Then use a heat gun to dry the exposed part of the stringer where there is no resin and glass. I also cleaned the bilge with acetone again.
Some people say not to bond the bottom of stringers to the boat. Everything I read told me otherwise.
I used fiberglass mat. I cut the mat into strips just about as wide as the stringer. I saturated the mat strips in epoxy resin. I brushed on epoxy resin the length of the repair. I moved the stringer inside the boat. Then Ibrushed on a layer of epoxy resin on the bottom of the stringer. Then I placed two layers of fiberglass mat strips the length of the stringer. (There were two places where I left a gap because the stringer actually has a channel cut into it for water to drain into the bilge). I rolled out all of the air bubbles in the mat. Then I set the stringer on the mat. I adjusted the position of the stringer so that it was properly aligned front to back and was perpendicular to the floor.
I placed large weights on the stringer in several places to force it downard and hold it in place while the epoxy resin set over night.
The next day I filleted the edge of the stringer where it meets the hull. I used epoxy resin mixed with aerosil cabosil (as a thickener to prevent running) and I used West System 403 (Should have used 404).
To do this, Just mix the resin, aerosil cabosil, and WS 404 in a bucket until it is the consistency of mushy oatmeal. Then press it into all of those little gaps I mentioned earlier and build a fillet along the stringer. The fillet should be 1/2 inch thick and at least as tall and wide. Press it in tightly to form a good bond. This needs to set over night.
Again, after any application of epoxy resin it is recommended that you wash the area with soap and water first. Then wipe clean with acetone and let dry. Now you can begin to laminate the stringer onto the hull. Start by cutting 2 inch and 4 inch strips of fiberglass mat (you can buy it in rolls precut). Cut a strip of 8 inch fiberglass cloth. Use aerosil cabosil to thicken the epoxy to a consistency that is very slow to run down your stir stick, like cool honey.
Apply resin along the entire side of the stringer and 4 to 6 inches down the hull (on each side). Saturate the strips of mat. Apply the 2 inch strip first. Center it along the joint of the stringer and hull. Follow with the 4 inch strip. Then the 8 inch fiberglass cloth. Roll out all of the air bubbles. Let this cure over night.
This is a solid enough installation of the stringer onto your boat. Many times more solid than the factory.
Despite my best efforts, I was not able to make the new stringer exactly the same as the old. I determined this by placing a block across the unaltered secondary stringers and measuring down to the unaltered driver side stringer and the new passenger side stringer.
What I did next was transfer the measurements over to the new stringer (taking into account 1/8 inch thickness for fiberglass on top of the existing stringer). Then I used my hand plane, a drill with a sanding drum, several hand files, and whatever else I could find to adjust the new stringer.
In some cases I actually had to add about 1/4 inch of material. I bonded fiberglass mat onto the stringer until it reached the proper height. Then I sanded it down level with the existing stringer. In the pic below the red outline is the new material built up on the stringer.
Eventually I was satisfied with the dimensions of the newly installed stringer. This would become the baseline for tuning the new driver side stringer.
I will not waste your time repeating the steps to remove and install the new driver side stringer. The steps are the same as the observer side stringer.This is the point when I test fit the engine cradle to make sure everything was right. It is better to change it now than later. I recommend being about 1/2 inch wide to allow for additional fiberglass over the stringers later in the build.