• Sailing Koinonia

How to select, remove, replace, and eliminate thru-hulls and seacocks - Part 2

Updated: May 22, 2020


Welcome to Sailing Koinonia! In last week’s post, we shared some of the do’s and don’ts of selecting and installing thru-hulls and seacocks and some common signs it’s time to replace them. This week, we’re going to share a step-by-step guide for how to remove a thru-hull. This can be a frustrating project, so we’re going to break it down and hopefully make it easier for you!

Step by step guide to remove a thru-hull and seacock


1. Our lawyers said we had to include this step: Removing a thru-hull, especially below the waterline, should only be attempted with your boat OUT of the water (sheesh)!


2. Before removing a thru-hull, make sure replacement is actually necessary. Under normal circumstances, with routine maintenance, bronze thru-hulls have a very long life, are expensive to replace, and a pain in the butt to remove and install. Dezincification is a type of corrosion that dangerously compromises the strength of a bronze thru-hull and is arguably the most common issue that warrants replacement. You can learn about other common signs of thru-hull and seacock issues in Part 1 of this series. As pictured above, you can check for signs of dezincification by using a wire brush attached to a drill to clean the mushroom head down to bare metal. If you see pink spots or the color looks like a copper penny instead of gold/bronze, this is a good indication of dezincification. Be sure to check the seacock as well; it too may be compromised.


3. If your thru-hulls are brass or an inferior plastic, neither are intended for use below the waterline, so don’t ask questions — replace them STAT!! Marelon seacocks or specialty polymers that are ABYC approved are the only exception.


4. If you confirm removal is necessary, inspect both sides of assembly: the seacock and thru-hull. Make sure the seacock is either securely attached to a backing block to prevent it from turning or, if possible, recruit another set of hands to hold it with a pipe wrench. If the seacock can be kept from turning and is in good condition, you can try removing and replacing only the thru-hull.

5. While you are checking the seacock, be sure to inspect the hose and clamps. If you find signs of dry rot, corrosion, cracks, leaks, or rusty clamps, it’s a good idea to replace the hose and the clamps. Be sure the hose is meant for use below the waterline. We strongly recommend Trident 250/100 wire reinforced hose.


The seacock pictured below is secured to a plywood backing block but it’s in very bad condition. The only way to keep it from turning would be to use a pipe wrench.


6. Inspect the mushroom head on the thru-hull. If there is a strainer cover, remove it. Take a look inside the thru-hull. Clean any debris or hard marine growth. It‘s a good idea to soak the inside threads with PB Blaster or another penetrating oil thoroughly before attempting removal. Usually, newer thru-hulls have two wings inside for fitting a step wrench (a special tool designed for removing and installing thru-hulls). Older versions may look more like the one pictured below with slots. Either way, the idea is to fit a step wrench or another tool to back the thru-hull out of the seacock.


Warning: depending on the type of sealant used to install the thru-hull, the type/ amount of corrosion, and type of thru-hull, breaking the seal and removing by unscrewing may prove difficult, or even downright impossible, however this is the least invasive approach and the best starting point.


7. If you cannot break the thru-hull free and unscrew it from the seacock, the next step is to get out the trusty angle grinder with a metal cutting blade attached. The idea is to cut slots through the mushroom head only, taking care to minimizing damage to the hull (see image below). Don’t worry if you happen to nick the hull a bit, you will have to sand this entire area anyway. Cut the slots in the mushroom head to keep the size of each piece wide enough to get a large flathead screw driver or crowbar underneath, but small enough to make prying them away from the hull easy. Bronze is a relatively soft metal that is cast and it doesn’t bend well; it breaks. For this situation, that fact makes the job of removal surprisingly easy.


8. Once you finish prying away all pieces of the mushroom head, there is nothing to prevent removing it from inboard. Simply back the seacock out by removing the screws attaching it to the backing block and break the seal. The whole assembly should come off with minimal fuss.


9. Finally, if you plan to salvage the seacock, you need to remove the thru-hull body. This may be challenging but we have a few tips to make the job easier:

  1. Drill a hole through the side of the thru-hull body all the way through to the other side. This will allow you to insert a large Philips head screwdriver.

  2. Spray the threads of the thru-hull body inside and out with penetrating oil like PB Blaster, WD-40, or the like.

  3. Put the seacock into a bench vise to hold it steady. Insert the screwdriver and begin backing out the thru-hull.

  4. If necessary, carefully apply heat using a propane torch to the thru-hull body only for 1-2 minutes. Allow to cool for 10-20 seconds and apply more penetrating oil. Repeat if needed, until you are able to turn the thru-hull.

10. Polish the seacock using a wire brush kit and solvent and be sure to grease the ball valve using U-Lube waterproof grease. This seacock is still in great shape and is ready to reinstall for decades of dependable service!


Congratulations. You’ve just successfully removed a thru-hull and seacock. Stay tuned for Part 3 of this four-part series on how to select, remove, replace, and eliminate thru-hulls.

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