Replacing Rotten Stringers In A Small Boat

Many small boats are built with plywood or wood-reinforced stringers. After a couple of decades, these often rot away. But they're critically important – they're what gives the hull a structural backbone, preventing twisting and flexing. Try running a boat with rotten stringers and you'll likely see the hullsides ripple and shift as the boat hits waves. Complete structural failure becomes a very real possibility. This isn't just bad for the boat, it's utterly unsafe.

While replacing stringers in a boat may seem like a big deal, it actually isn't that much of a complex process for small 14- to 18-footers that commonly have just one to three stringers under the deck. Anyone with a basic knowledge of working fiberglass and wood and the ability to read and follow instructions can get the job done. That said, it is a lot of messy, hard work.

As a general rule of thumb, always try to mimic the boat's original construction. Replacing stringers with significantly larger or stronger ones can cause hard spots in the hull, which could lead to future hull damage. And replacing them with significantly lighter stringers could lead to structural failure. So, it's always best to stick with the original designer's intent unless that design has been proven to fail already.

1. Cut off the deck. This is one of the most stressful parts of the job because if you cut too deep (particularly near the sides and bow) you can pierce the hull. And while many small boats have portable fuel tanks kept abovedecks, if yours has a fuel tank built in, there's a danger of cutting that tank or its lines, too. Also be concerned about wiring or cable runs, if any. To be safe, first delicately drill a hole in the deck and measure its thickness with a probe. Then you can set the saw blade to cut through that depth and no more. Note: In most cases, if plywood was used for the stringers, it was used for the deck as well. You may be able to salvage the deck if it's in good shape, but that's unlikely. In the vast majority of cases, removing the deck will mean destroying it and replacing it along with the stringers. Generally speaking, the best move is to break out the crowbars and hammers, rip it out in pieces and chunks, and plan to start fresh. This isn't a time to be gentle.

2. Measure the old stringers and cut the core for new ones with marine plywood. You want to duplicate them as closely as possible, so it's a good idea to make in initial rough-cut, put the new stringer core next to the old stringer, then modify it as necessary. Don't forget that the fiberglass skin will increase the size of the stringer when you wrap the new core. Cut away a sample of the old fiberglass stringer skin to ascertain the thickness of the glass used and the number of layers you'll be adding. As closely as possible, you should also duplicate any limber holes (holes at the bottom of the stringers that allow water to move to the low point in the bilge). Coat the inside of the limber holes with resin to avoid water soaking into your new stringers.

3. Remove the old stringers. The wood that used to be inside them has turned into a gelatinous mess of rot, so this usually means dealing mostly with the fiberglass shell that used to surround them. Depending on how well the boat was laminated and how it has withstood the test of time, it may be possible to simply rip the remainder free or bang them out. Otherwise, you'll have to shave them down along the hull and then grind away the remnants. In most cases it's a combination of both. Plan to get very, very messy during this stage.

4. Grind and sand the inside of the hull. To get a good secondary bond between the new stringers and the hull, you'll need to create a clean surface. That means grinding away all the glass from the old stringers and sanding, sanding, then sanding some more until you're working with a "new" surface free of contamination and obstructions. But be careful with that grinder, because if you apply too much pressure in one spot for too long it may be possible to grind right through the hull.

Also, be sure to mark and, as necessary, re-mark the stringer location(s). The easiest way to do so is often to make your marks well outside the area being prepped (for example, make marks four inches apart to center a stringer at two inches) so you don't sand away your markings.

5. Prepare the surface for bonding. Choose solvents and prep methods following the fiberglass resin manufacturer's instructions. When the surfaces are ready, pre-prep a jig to support the stringers as the bedding dries, and set the pieces aside.

6. Install the new stringers. Start by bedding the ply core in thickened epoxy (which is expensive but generally does the best job) or fiberglass putty (inexpensive and generally does a great job), putting the stringers in place, then adding the pieces of the jig to support them. While the epoxy or putty is still pliable, form a "fillet" (a radius) along both bottom edges of the plywood. Fiberglass cloth doesn't make sharp bends very well, and rounding out the bottom edges so the fiberglass has a curved surface to adhere to is very important. Add more epoxy or putty as necessary so the fillet is at least a half-inch.

The red arrows show the fiberglass tabs going from stringer to hull. (You can see the white edge where they end.) The white behind the fiberglass that has been laid down (blue arrows) is the fillet.

7. Apply the fiberglass stringer skin. Cut fiberglass cloth sized to wrap the stringer and extend to either side of it ("tabbing") the same amount as the original stringers had. When applying multiple layers, make each consecutive layer about a half-inch smaller than the previous one so the tabbing is stepped.

Again, mimic what the manufacturer originally did; don't add additional layers of glass or use fewer layers. When the fiberglass cloth is all prepped, wet out the stringer cores thoroughly with fiberglass resin, then center the first cloth layer over the core and wet it out with fiberglass resin as per the manufacturer's instructions, including but not limited to those concerning skin contact, fumes, and dust. Apply additional layers while the resin on the previous layer is still tacky. (Note: If a layer cures before the next can be applied, the surface will have to be sanded and prepped before adding the next layer.)

8. After the fiberglass has cured, apply some finishing touches. You'll almost always end up with a jagged edge here or a fiberglass hair sticking up there. In accessible areas and bilges where you may one day be reaching around, you don't want to find any of those jagged edges the hard way, so sand or grind them until smooth.

With your new stringers in place you're ready to replace the deck, do some finish work to make it look nice, and then enjoy some well-deserved time on the water.