A Rudder Skid Plate—Off Road Vehicles Have Them, Why not Off Shore Boats?

Go through any marina and look at the bottom edge of the rudders and you will see that a lot of them 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 and hydrolysis is common especially if the boat is exposed to a freeze/thaw cycle. Probably nothing is more vulnerable on a boat than the bottom edge of the rudder. Sometimes water may also find its way into the rudder by passing around the rudder post seal. We will deal with this as well.

To protect the bottom of the rudder, we have 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.

Filling a hollow rudder with Polyurethane

When Vespera was pulled from the water for the first time after I purchased her, 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. Vespera 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 on a cracked bottom, the cure is the same. You need to dry out the cavity and fill it.

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. Note, a moisture meter will also show a wet reading when it sees metal such as a rudder post. Next, drill a pattern of ½ inch holes into the cavity about 6 inches apart and rig a shop vac up 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. A reading of moist is OK but a reading of wet is not OK.. Letting the rudder, with holes, sit through a Minnesota winter also works well. When finished drying, you will have to fill the ½ inch holes with epoxy/micro balloons or an appropriate under water filler.

After the rudder has been 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 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 pressure will be used to force the sealant between the rudder post and the rudder structure.

Filling the large cavities will be fairly simple. Cut the caulk tube so that the outlet has about a ¼ inch diameter. Drill ¼ 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 post using high pressure injection is trickier. Here you need to make sure that a good seal is made 360º around the post. Vespera’s rudder has a skeg so the rudder post has three entry points. All three needed a 360º seal. Drill rows of 1/8 inch holes about 2 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 270º degree arc. Make sure the holes that you are drilling are smaller than the diameter of the needle point 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 rudder post goes two feet into the rudder and you drill a row of holes every two inches, so that you will have 12 rows of holes all in a 270º 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, hand held, 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. See the write-up for Polyurethane injection for more details.

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 grease gun because the bond is too good between the rudder post and the rudder. That’s OK. Some holes exhibit 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, 360º. 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? Remember Alan Vaitses’ basic theory, dissimilar materials bonded together will eventually shear or the stronger material will break the weaker. We did not what 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.

Adding 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 1/8 inch gap on the sides. The bottom of the shoe should be about 1 inch below the bottom of the rudder. This will give you a rubber shock absorber after you fill the shoe with polyurethane. Temporarily attach the shoe to the rudder using 4 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. So that the Vulkem is distributed everywhere equally, as a leak occur pound a rag or soft line into the top edge of the gap between the shoe and rudder. 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 for a couple weeks, bolt it to the rudder.

The shoe is through bolted to the rudder using oversize holes also filled with polyurethane. Drill holes through the side of the rudder about ¼ inch larger than the bolt. The holes will pass through 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 of the other side. The sloppy shoe, once sealed in polyurethane, will take a good bump without damaging the rudder.


Stan Sroga



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