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Let’s talk about seacocks and thru-hulls. No, contrary to what my coffee-deprived early-morning photoshop image below might imply, seacocks are not some strange water-based fowl. They are instead valves which control the flow of liquids or gases through the thru-hulls (through-hulls) in a boat.

When it comes to thru-hulls, they can be located either above the waterline or below it. The ABYC marine code dictates that all thru-hulls that could end up under water need to have a seacock attached to them. In a cat, that’s pretty easy to determine but in a monohull’s case, thru-hulls that are located above the waterline when the boat is at rest may in fact be underwater when the boat is heeled over. Something to consider.

In our boat’s case, there are only 4 below-the-water thru-hulls: one for the watermaker/head input, one for waste discharge, and one each for the speed and depth transducers. In the post where I described our latest head troubleshooting job, I mentioned that we had T-ed off the head intake for our watermaker input. In my mind, the fewer holes in the boat we have the better, especially ones below the waterline. As a comparison, the boats belonging to some of our friends have 14 or more!

This images shows three thru-hulls above the waterline and two below it. The other two below-the-water thru-hulls are located in the same area but are not visible from this angle.

Fortunately for us, all four of our below-the-water thru-hulls are confined to the same forward locker making it easy to inspect and control them. Thru-hulls are like any other moving part on the boat, they need to be exercised. At some regular interval, it is a good idea to open and close the seacocks that are typically left in one position, just to make sure that they do not seize up (something which is reportedly quite common). It’s also prudent to have nearby some method for plugging the holes in the event that the seacock breaks off, the traditional method being to have a wooden plug fixed nearby each of them that you could hammer into the hole to stem the flow of water. Because our thru-hulls are all confined in that one location, we go one step further and keep nearby a container of Stay-Afloat in addition to the wooden plugs.

Another prudent thing to do to prevent water intrusion from these sources is to use only high-quality stainless steel clamps on each of the hoses attached to them, and also to double up on them, using two clamps on each hose. Make sure that you inspect these too as just because they are labeled as stainless, it does not mean that they won’t rust. Remember, it’s stain-LESS, not stain-PROOF. During my last trip into that locker I found that the screw portion of many of the hose clamps had indeed rusted considerably, warranting replacement.

Hungry for more reading? A lot of info on thru-hulls and seacocks, including some excellent photographs, can be found on this page.


  1. When I saw holes, I thought the post was about me…Phew……All good points…Holes (and ass-holes) dont go hand and hand on boats..

  2. Do you keep a check list of things you have to do on an on-going basis? If so
    how is it kept (computer?), how is it updated (automatically or by hand), What is on it and the time schedules.

    • Yes and no. Rebecca has just recently put together a spreadsheet list on her computer. Up to this point though we have simply gone by memory, possible only becuause we have a small and simple boat.

  3. Thanks again for the read .I went around our boat 1988 41 morgan classic sloop rig. 16 thru hulls . And yes two needed attention (frozen) .I closed them and opened many times this has made them easy to move. I also looked at our selection of wood plugs .All is good however they needed to be moved from under a lot of stuff and placed with safety items in an easy to get at location. I am now putting together a once per month list of things to look at. I also found a clamp at the base of our bildge pump one of three. The clamp looked new untill you turned it over and it had rusted through. Good call . Thanks for making me think to look.

  4. oh wow, that link is so helpful. I’m going to be installing some soon…it’s so fun to drill holes in the thing I spent 5 years building to keep water out.

  5. Hummm, no seawater supply for the engines? Aircooled?

  6. Any hole below the water line makes me nervous. The fewer the better. Anything mechnical can and will fail. I could live with your set up, but more than that would make me unhappy.

    • If we had 2 diesel engines we’d have at least 2 more I assume.

    • As Mike said, they are in a bulkheaded compartment. I had the good luck to have one fail during the delivery trip when we bought our PDQ 32. Actually, it was a cracked speed transducer plug. The result was that we flooded the compartment to the water line in January (Chesapeake Bay) when dealing with water is unpleasant. The damage? We were down 1/2-inch on the water line and fixed it a week later. No biggy.

      THAT is a really nice design feature.

  7. I’m still curious about my PWC propulsion system. No one commented. I know personal watercraft are the bane of “serious” boaters. However, how cool would it be to have 2 PWCs docked into the stern of each hull that become the boat’s propulsion system.

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