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Old 02-13-2002
Mark Matthews Mark Matthews is offline
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Rudder Problems

A small section of the metal rudder post on my boat has been deteriorating. There is a half-inch-wide section that looks bad. Can this be repaired?

Mark Matthews responds:
Thanks for the question. While it’s hard to see how bad the corrosion is from where I sit, over time all thru-hulls, shafts, props, and other submerged metal parts can lose their structural integrity. If you can’t remember the last time you changed the zincs on your boat, or if you suspect your vessel has a checkered past regarding maintenance, chances are you'll need a new rudder shaft or need to repair the one you've got. At the very least you need to take a closer look at what’s going on. 

If the boat is out of the water, tie off the tiller or wheel and then climb back down and have a look. I mention this because you'll want to determine if the rudder stock has come loose from the rudder blade. So try to move the rudder on its stock. If there’s any play at all, the rudder is likely separating from the stock and it will need repairing.

If, however, you don't discover any play when you moved the rudder with the helm tied off, then you probably just have some serious pitting in the rudder stock. I'd remove the rudder and cart the whole thing to a machinist or a welder so that they can look at it and see if they think that the structural integrity of the shaft is at all compromised.

Another thing to check if you suspect further deterioration is to look for tell-tale bubbles at the top of the rudder where the stock exits. If you've got those, it's almost unavoidable that the rudder will need to come off and be taken apart. Depending on the arrangement, you may be able to take the rudder off while the boat is in the water. (I’ll leave that up to you to decide.) If you plan to take it off while the boat is in the water, it's a good idea to keep a line tied on the rudder just in case, even though many rudders are foam-filled and thus have a near neutral buoyancy.

Most rudders are generally constructed of two halves that are glassed together. I’ve had to repair my rudder by cutting it along the central seam using either a sharp chisel, circular saw, or grinder. If your rudder is built like this, once you've got it in two pieces, you’ll be able to get a better idea of the problem and how to fix it. You may need to take the rudderstock to a machine shop for welding, which might seem like a big job, but remember, every boat crisis is an opportunity to improve things in disguise, and now is a great chance to improve the rudder design.

The rudders on many boats have a skeletal framework that is made up of short welds sticking out like unsupported fingers. This means that stress can be concentrated on several short welds rather than the entire length of the stock. Consider adding more supports to the rudder stock to strengthen it. Once you have this fixed, glass the skeletal structure into one half of the rudder. Place the other half on top of it and fill the rudder with expanding foam, and try to fill it so there are no voids. Glass the two halves back together, and after some fairing and painting you should be ready to reinstall your rudder.

You might also want to consulting either Nigel Calder’s Boatowner’s Mechanical and Electrical Manual or Don Casey’s This Old Boat before you embark on your repair. Both these books are invaluable resources for boat owners, and both are available in SailNet's on line Store. Have a go at it, and let us know how everything works out.