Installing a skid plate to protect a rudder’s bottom

To the editor: Go through any marina and look at the bottom edges of the rudders, and you will see that many are fractured. The same rudders will have horizontal stress cracks on the sides indicating further trouble. These rudders are weakened from water that has seeped its way up from the cracks on the bottom. Further delamination is common, especially if the boat is exposed to a freeze/thaw cycle. Water may find its way in by passing around the rudder post seal.

To protect the bottom of the rudder on our boat Vespera, a 51-foot fiberglass cutter that will be used for a sailing school, we fabricated a thick aluminum shoe and through-bolted it to the rudder. The shoe was purposely made with a 1-inch gap between the bottom of the shoe and the bottom of the rudder so that polyurethane caulk could be injected into the gap to act as a rubber shock absorber.

When Vespera was pulled from the water for the first time after we purchased it, water drained out of the rudder for about a week. Water that had accumulated inside the rudder slowly found its way to the cracks in the bottom. Like many rudders, much of Vespera’s rudder was hollow; the boat had a hollow, stainless steel inner framework welded to a 2.5-inch solid stainless rudder post. This inner structure was covered in cold-molded wooden layers. But no matter how your rudder is constructed, if you have a hollow space inside that fills with water because of a cracked bottom, the cure is the same. You need to dry out the cavity and fill it.

Drying the cavity

To dry the rudder cavity, map out where hollow spots are by tapping or drilling small holes. A moisture meter may assist as well. Wet cavities will show very wet readings (a moisture meter will also show a wet reading when it detects metal such as a rudder post). Next, drill a pattern of 1/2-inch holes into the cavity about 6 inches apart and rig a shop vac so that it sucks air through a few of the lower holes, one at a time. Let the shop vac run over night on each hole. Do this on two or three of the lower holes on each side of the rudder. The polyurethane sealant that will be injected into the rudder will set up in high humidity so there is no need to get the rudder completely dry. However, you do not want standing water or saturated fiber inside the rudder. You can use a moisture meter to check your progress. When finished drying, fill the 1/2-inch holes with epoxy/micro balloons or an appropriate under-water filler.

After the rudder has dried somewhat, you are ready to fill it with polyurethane sealant. Initially, you will be filling the rudder with Vulkem 45 under low pressure, straight out of a 1-quart caulk cartridge. After that you will be using very high pressure from a grease gun. Low pressure will be used to fill the large cavities. High presure will be used to force the sealant between the rudder post and the rudder structure.

Filling the large cavities is fairly simple. Cut the caulk tube so that the outlet has about a 1/4-inch diameter. Drill 1/4-inch holes about 6 inches apart. Start pumping from the bottom holes until caulk drips out of the upper holes. Cap the hole you just filled temporarily with the right size machine screw (bolt). You will need to practice a little to get the right size hole drilled for the bolts you are using.

Sealing the rudder

Sealing the rudder post using highpressure injection is trickier. Here you need to make sure that a good seal is made 360° around the post. Vespera’s rudder has a skeg so the rudder post has three entry points. All three needed a 360° seal. Drill rows of 1/8-inch holes about two inches apart. Drill the first row of holes about an inch from the rudder post entry. Start drilling holes on one side of the rudder and work your way around the front of the rudder to the opposite side, a 270° arc. Make sure the holes that you are drilling are smaller than the diameter of the needlepoint grease fitting that you will be using to inject the sealant. Drill rows of holes all the way down to where the rudder post stops inside the rudder. It will be obvious when you hit the rudder post with the drill. If the post goes two feet into the rudder and you drill a row of holes every two inches, you’ll have 12 rows of holes all in a 270° arc.

You’ll need to purchase a pointed grease fitting for your grease gun. Drill the tip out of the grease fitting using a 1/8-inch drill. The cone of the grease fitting will be forced against the holes in the rudder to get a good high-pressure seal. Also buy a small, handheld grease gun. The grease gun will be difficult to clean, so buy an economy model. Load the Vulkem 45 sealant into the grease gun body directly from the sealant cartridge.

Inject each hole starting from the bottom until sealant comes out of adjacent holes. Plug each hole with a machine screw and work your way up. Some holes will not take any sealant from the grease gun because the bond is too good between the rudder post and the rudder. That’s OK. Some holes will make cracking sounds as you pump grease – a little of that is OK, too. You are just forcing sealant into a weak joint. When you get to the top of the rudder, ideally you will see sealant ooze out of the rudder post all the way around. The end result should be a completely solid rudder. There will no longer be cavities for water to seek.

You may wonder why we did not fill the rudder with a solid material such as epoxy mixed with micro-balloons. According to boatbuilder Alan Vaitses’ theories, dissimilar materials bonded together will eventually shear, or the stronger material will break the weaker. We did not want a solid core material bonded to the inside of the rudder. We wanted a material that was flexible so that it could expand and contract, yet displaced air and water.

A metal shoe

Fabricating a welded metal shoe requires a visit from a professional welder. The shoe should fit sloppy on the bottom with about an 1/8-inch gap on the sides. The bottom of the shoe should be about 1 inch below the bottom of the rudder. This gives a rubber shock absorber after you fill the shoe with polyurethane. Temporarily attach the shoe to the rudder using four wood screws.

Polyurethane is pumped into the shoe by tapping a hole in the shoe for the grease gun outlet. The grease gun is pre-filled with Vulkem 45. As the 45 is pumped in it will ooze out of the top edge of the shoe. To ensure that the Vulkem is distributed everywhere equally, pound a rag or soft line into the top edge of the gap between the shoe and rudder when a leak occurs. This will force the Vulkem to ooze out somewhere else. Stop that leak and continue until you’ve worked your way around the rudder. After the assembly has had time to cure, bolt it to the rudder.

The shoe is through-bolted to the rudder using oversized holes also filled with polyurethane. Drill holes through the side of the rudder about 1/4 inch larger than the bolt. The holes will pass through the rudder and both sides of the shoe. Pre-fill the holes with a thicker polyurethane such as 3M 5200 and force the bolt through while holding your finger on the opposite hole so that all of the sealant is not pushed out the other side. The sloppy shoe, once sealed in polyurethane, will take a good bump without damaging the rudder.

By Ocean Navigator