• Installing a Thru Hull

    Installing a thru-hull may seem like one of the most daunting boat repair task to handle, but it is not a very difficult job for someone with basic mechanical skills and the proper tools. However, a successful installation does require a bit of preparation and planning. Just drilling a hole in the boat, applying silicone and sticking in the thru hull will lead to disaster.
    To give you an idea of how important it is for the thru hull installation to be done correctly look at these numbers. A three inch diameter hole twelve inches below waterline can emit 177 gallons of water per hour (gph), double that distance to 24 inches below the waterline and the rate increases to approximately 250 gph. This may not sound like much water at first but when you consider all of the variables a clearer picture forms. That Superpumper 1000 you have installed below, how old is it? Five, maybe six years old and when was the last time you checked it?

    Some manufacturers are more than generous in the ratings they stamp on the side of their bilge pumps, now add a few years of age, throw in some dirt and corrosion and the pump?s true output may dramatically decrease. Also consider the vertical distance or lift the pump has to overcome to discharge the water overboard and in most cases through several feet of corrugated hose. Hopefully you can see how the actual flow rate can be considerably less than the pumps rated gph. It is not uncommon to have an actual overboard discharge rate of one half of the pumps rated output. That Superpumper 1000 may be more appropriately named the Supersquirter 300.

    Common Terms

    It is not uncommon to confuse the various components or use terms interchangeably, so to alleviate any confusion we will discuss the components before moving on.

    Attachment 759


    The American Boat and Yacht Council (ABYC) defines a thru hull as a device that allows the passage of gases or liquids through a hull. Thru hulls may be located above or below waterline depending upon purpose and may be made of bronze, stainless steel, plastics or fiberglass reinforced nylon (Marelon).

    Attachment 760

    Additionally some thru-hulls are designed with a scoop and strainer. These types of thru-hulls are generally used for items that do not have self priming pumps, for example marine air conditioners. Do not use a scoop thru-hull unless specifically suggested by the manufacturer. Scoop style thru-hulls are not recommended for most generators and if used will damage the unit.


    ABYC regulations require any thru hull below the waterline or below the heeled waterline for a sailboat to be fitted with a seacock. A seacock is a flanged ball valve fitted to the inboard side of a thru hull or fitted to the hull that is operated by a lever type handle usually operating through 90 degrees giving a clear indication of whether it is open or shut. They can be used to control the inflow and outflow of water within a boat. You may find seacocks made from a number of different materials; the most common are made of bronze, stainless steel or Marelon. For below waterline installations, bronze is the preferable choice due to its corrosion resistance and strength. Marelon seacocks may also be used for below waterline installations, however there have been some instances of their handles breaking. This not a good situation if you need to close the valve in an emergency.

    Attachment 761

    Ball Valves

    Ball Valves normally operate with a quarter turn of the handle and provide visual feedback whether open or closed like seacocks. However they are not designed to be attached directly to thru-hull fittings (notice the lack of a support flange). They are commonly used to control fluid or gas flow within a boat and made of bronze, brass, plastic and Marelon.

    Attachment 762

    Gate Valves

    Gate valves come in all shapes and sizes and are not recommended for the marine environment. They are more prone to corrosion, leakage and becoming jammed because of trapped debris. They also do not provide an obvious visual indication of the valves position.

    Attachment 918

    NPT vs. NPS Threads

    National Pipe Thread Taper (NPT) and National Pipe Thread Straight (NPS) threads are both identical in thread angle, shape and threads per inch, however NPS threads remain straight or parallel while NPT threads taper toward the end. Most seacocks are threaded NPS on the flange end for proper mating with the NPS threaded thru-hull while the tailpiece end will be NPT threaded to properly mate with NPT fittings. Marelon seacocks are NPS on both sides while most ball valves are NPT threaded. Why is this important?

    Attachment 764

    Notice in the figure above we have a NPS threaded pipe screwed into a NPT coupling. The NPS pipe is unable to fully seat due to the taper of the threads in the NPT coupling. That is what happens when a thru-hull (NPS) is screwed into a ball valve (NPT) and leads to a weaker connection. Notice the other side where a NPT fitting is screwed into the NPT coupling, it has full penetration and good thread contact. This is why a seacock is the best choice for thru-hulls below the waterline. The bottom threads on a seacock are NPS threads to match the NPS threads on the thru-hull while the top threads are NPT threads to match various fittings (see figure below).

    Attachment 919


    The first thing to consider is whether you actually need a new thru-hull. In some situations installing a ?tee? into an existing line may be a better option. For instance, some boats use raw water to feed the lavatory, this water is drawn in through a thru hull in the bottom of the boat. You could install a ?tee? in this line to provide a water source for other items, a deck wash down system for instance. Installing a ?tee? into the water intake lines for engines and generators is not recommended.


    Once you decide that a new thru-hull installation is required, the next step is to select the proper location. This step is more important than many people realize. Anything that interrupts the smooth flow of water along the hull of the boat could potentially cause problems. These interruptions cause the water flow downstream to become aerated and turbulent. This turbulence creates havoc for sonar transducers and reduces the efficiency of impeller pumps on engines and generators causing overheating problems. For these reasons not only do you have to pay attention to what is downstream of the proposed thru-hull installation, but you have to check upstream as well.

    Attachment 766
    In scenario 1 above the proposed thru-hull is placed in the disturbed water-flow pattern (gray) created by the existing thru-hull. This scenario should be avoided but if this is the only location available attempt to increase the distance between both thru-hulls. In scenario 2 the proposed thru-hull is upstream of the transducer. The disturbed water-flow pattern (red) produced by the proposed thru-hull will create havoc with the transducer. Scenario 3 shows a better location for the proposed thru-hull.

    From the interior of the boat, determine the general location where you want the thru-hull to be installed, keep in mind the thru-hull must be placed in a position that is easily reached so the valve can be closed in an emergency and ensure there are no obstructions in the path of the handle from fully closed to the fully open position. Once you determine a likely place mark it with a pencil. Now take measurements from your chosen position to objects that will be easily identified from the bottom of the boat such as existing thru-hulls, transducers or the transom of the boat. Now get underneath the boat and using those measurements mark the spot that you selected for the thru-hull. This may take several attempts to get it close. Once you feel confident that you are close to the correct spot there are several things to consider. If you are going to trailer the boat you want the thru-hull to remain clear of all bunks and rollers. Look around and note objects fore and aft of the selected position that could cause turbulence as described previous.

    Drilling The Hole

    Once you are satisfied that you have chosen a good location, from inside the boat drill a small pilot hole at the marked location. You may want to leave the drill bit sticking in the hole to make it easier to locate from beneath. Now go back under the boat and check the location one final time. With the appropriately sized hole saw, drill the hole from outside the hull. Using the proper sized hole saw is very important, you want the thru-hull to be a snug fit with no wiggle room.

    Sealing The Core

    If the hull is cored the next steps are crucial to protecting the integrity of the hull. You will need to dig out the coring material around the perimeter of the hole to hollow an area at least as large as the flange of the seacock. Once the coring is removed you should fill the area with an epoxy resin paste or a mix of epoxy resin and colloidal silica. This has two purposes, it will strengthen the area where the seacock is to be installed and it will help prevent moisture from entering the core of the boat. Consult the epoxy manufacturers? documentation for the proper cure time. The polyester resin typically found at local auto parts stores is not as strong as epoxy resin and less water resistant, for more information on resins visit the Boatinghowto.com forum fiberglass repair section.

    Attachment 767

    Backing Block

    While the epoxy resin is curing you can proceed with the next step, creating a backing block. The backing block will add additional support to the thru-hull installation and should be used for all installations. The thickness of the backing block depends on the installation, a backing block of the proper thickness should allow a minimum of 5 threads of the thru-hull to penetrate the seacock. (see example 3 below)

    Attachment 768

    You can use trial and error to determine the backing block thickness or use the method described below. Install the thru-hull fully into the seacock and measure from the head of the thru-hull to the base of the seacock, now subtract the thickness of the hull. Add at least 1/4 of an inch to this value and this is the minimum thickness that your backing block should be. The thickness should be adjusted to allow the thru-hull to penetrate the seacock as shown in example 3 figure 10. In most cases ? inch plywood will work.

    Once you have chosen a suitable piece of wood, cut a ring 2-3 inches larger than the flange on the seacock and then use a hole saw to cut out the center for the thru-hull forming a ring. Now using a sander or wood rasp, shape the bottom of the ring to conform to the hull of the boat. The objective is for the backing block to conform the curvature of the hull. Once you have the backing block shaped properly place it over the hole drilled for the thru hull, install the thru-hull and seacock and hand tighten to check for proper fit. Now place a mark on the backing block and an adjacent one on the inside hull. This will aid in proper alignment later. Coat the backing block with epoxy resin and allow it to cure.

    Attachment 770

    Alternate Method

    The procedure above is one of the simplest ways to fabricate a backing block, another more complicated method is to form a resin-fiberglass backing block lay-up on the inside hull which forms the strongest mounting surface for a seacock. To accomplish this, once the first pilot hole is drilled and verified in the correct location, the gelcoat is ground away around the pilot hole to match the desired size of the fiberglass backing block. This ensures good adhesion when the resin mix is applied. Use acetone to clean up the area.

    Next place a small piece of tape over the hole to prevent the resin from dripping through. To form the mold for the epoxy various methods have been used with modeling clay being one of the easiest. Using the clay form a perimeter the desired size around the hole. Mix epoxy-resin with chopped fiberglass to form a mix the consistency of syrup. Pour this into the hole and allow it to cure. Once cured using the hole saw drill from outside the hull using the pilot hole. The top of the resin-fiberglass block may need tapering once the hole is drilled so the face is perpendicular to the thru-hull to ensure the seacock has a flat mounting platform.

    Another optional method is create a fiberglass backing block. Do a fiberglass lay-up to a thickness of approximately 5/8 of an inch in thickness and slightly larger than the desired backing block size. When cured you can cut it to the desired shape and cut a hole for the thru-hull and you have a very strong and long lasting backing block.

    Dry Fit

    Once the backing block has cured, it is time for a dry fit. Install the thru-hull from outside the hull, if you don?t have anyone to assist with holding the thru-hull, cut a piece of 2x4 to a length that will allow you to use it as a wedge to hold the thru-hull in place. From inside the boat place the backing block around the thru-hull in the proper position and thread the seacock onto the thru-hull and hand tighten. Ensure the seacock handle will be unobstructed and if the seacock has a drain, attempt to rotate it so it is on the lower side.

    Drilling The Mounting Holes

    Once you are satisfied with the position, use the holes in the flange to mark the backing block for drilling. If the seacock does not have pre-drilled holes in the flange drill 3 holes equidistant around the perimeter. Remove the seacock, ensure the backing block is still properly aligned and drill holes through both the ring and hull. Make sure you are holding the drill perpendicular to the hull.

    Once the holes are drilled, countersink the holes on the outside of the hull to allow the heads of the mounting bolts to fit flush. Also countersink the holes on the backing block slightly, this will allow a small amount of sealant to form a gasket once the bolts are tightened. Re-install the seacock hand tight and install the mounting bolts to ensure correct fit and alignment. Caution: If you are using a bronze seacock, you should use bronze bolts for the installation to avoid galvanic corrosion.

    Sealing The Assembly

    Once you have verified everything will fit correctly it is time to apply a polyurethane sealant. 3M 4200 is commonly used and some opt for 3M 5200, the 5200 forms a more permanent bond and will make future removal of the thru-hull very difficult. Remove all of the parts in preparation for application of the sealant.

    Install the thru-hull and support with a 2x4 then install the mounting screws far enough so they will extend into the backing block to prevent it from spinning. From inside the boat apply sealant to the backing block and place it over the mounting screws. Now apply sealant to the bottom of the seacock and thread it onto the thru-hull. Tighten the seacock so it fits snug against the backing block and the screw holes align properly.

    Now back outside remove the screws and apply sealant to the threads close to the head. Reinsert the screws into their holes, apply the lock washers and nuts then tighten them firmly, don?t over tighten if using bronze screws. Now remove the thru-hull and apply a liberal amount of sealant around the shoulder and a light coating to the threads. Tighten firmly using a step wrench or similar tool. Sealant should squeeze out from all joints and can be cleaned up with a rag, apply some of the excess sealant over the heads of the screws to help protect them from corrosion.

    Attachment 771

    You are now ready to install the tailpiece and other components as necessary, remember to use two hose clamps on all hoses below waterline.

    Bung Plugs

    Bung plugs are a safety device. They are normally made of softwood and tapered to fit into most normal sized fittings. They are designed to be driven into pipes, fittings or small holes with a hammer, as they absorb water they will swell to seal the hole. Normally they are attached to the seacock with a piece of string and you should have one within easy reach of each seacock in your boat.

    Here is how it works. You have neglected to regularly service your seacock to ensure it functions properly and debris or corrosion has blocked the valve. A hose has just ruptured and the boat begins to fill with water, you grab the seacock handle and it will not budge. What do you do? Quickly remove the ruptured hose from the tailpiece, place the bung plug in the end of the tailpiece and drive it in with a hammer. Step wrenches are tools specifically designed to tighten or loosen thru-hulls. If you don?t have a step wrench others have had success with a block of wood lightly driven into the thru-hull, vice-grips or even crossed screw drivers for nylon thru-hulls.

    Attachment 772


    These same methods can be used to successfully install transducers. Transducers normally do not require mounting screws and have a lock nut to secure them to the hull. Avoid using 3M 5200 as a sealant for the transducer since it forms a more permanent bond and transducers may need to be replaced.

    Above Water Line Thru-Hulls

    Thru-hulls located above the waterline are for discharge of gases and fluids. In many cases these lines do not need a method to shutoff the flow and therefore do not have a valve installed. If a valve is desired ball valves are commonly used to accomplish this function. Installation procedures are very similar to below waterline procedures with the exception that mounting screws are commonly not used. If the sides of the boat are cored it is still necessary to protect the core with sealant or resin after drilling the hole, however you only need to remove approximately ? inch of the coring since structural strength is not as much of an issue.

    Hopefully this article will help ease the trepidation many folks have that are contemplating installing a thru-hull. The process is not very complicated but does require a basic understanding of what you are trying to accomplish. The key is to take your time and do it right, after all you are cutting a hole in the bottom of your boat.

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    Comments 2 Comments
    1. gasngo's Avatar
      gasngo -
      I thought I would revive this one and ask a ? I read Cap N' Rays build a thru-hull backer block with left over glass strands. Great idea by the way. Im curious anyone offended with using fiberglass insulation. You could pack it in the bottom of a bucket for sure. Integrity verses regular glass? I have some from the remodel project thought it would achieve the same results. The cost aspect is ideal.
    1. TimG's Avatar
      TimG -
      I would be hesitant to use it because I don't think the strength is there and I am not sure how the fiber would hold up if exposed to moisture.
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