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Drill Fiberglass and Install Thru-Hull Fittings

If the thought of drilling a hole in the bottom of your boat conjures terrifying images in your head, you're not alone. We sell a lot of automated ballast systems and ballast systems components, so it's no surprise that we hear from a lot of customers that are uncomfortable with the idea of drilling one (or more) rather large holes through the bottom of their boat.

We figured it would be beneficial to post a little how-to that outlines the process of drilling through the fiberglass hull of the boat, and then installing the thru-hull fitting. This article is specific to an intake for a ballast system, but the same procedure can be applied for any hardware that is installed in the boat's hull. Alright, let's get on with it!

Required Tools & Supplies

In addition to the actual hardware you'll be installing, the following items are necessary:

  • Measuring tape
  • Masking tape
  • Felt-tip pen
  • Power drill
  • Sharp drill bits
  • Appropriately sized hole-saw bit (refer to documentation included with your thru-hull for the correct size)
  • 100 grit sand paper

Step 1: Planning the Install
The first step is to determine where the fitting will be installed, and as the old adage goes, you definitely want to measure multiple time to make sure you won't be encountering any problems down the road. When locating your thru-hull you first need to ensure the location you choose will have a sufficient supply of fresh water at all times, and that it will not interfere with any other systems on the boat. This includes, but is not limited to, the engine's raw water intake, the paddle wheel speed sensor and the depth finder transducer. You want to make sure that the intake will not be located directly in front of any of those items, as the disruption to the flow of water can alter their performance.

Also, be sure you have enough clearance on both the inside and outside of the hull. It helps to pick a landmark, like an existing fitting or component, that is visible on the exterior of the boat as well as in the bilge compartment.  Usually the outside isn't an issue because you'll be doing the install with the boat on the trailer, and bunks are the biggest obstacle here. Inside you need to have sufficient room not only for the components that will attach to the thru-hull, but also for the installation. Remember, you'll need more room for all of your parts to be assembled than they will actually occupy once installed.

Measuring for the location of the bronze thru-hull intake

Once you've determined the ideal location for the intake, apply masking tape to the exterior of the hull extending out from the intended location by at least six inches.  Applying masking tape will serve two purpose; first, it will prevent damage to the surrounding are if the bit happens to slip, and second it will prevent splintering of the gel-coat (which is the exterior colored surface of the boat's hull that gives it the smooth glassy appearance). If your boat has a finished bilge liner (Malibu direct drive models for example), you'll also want to mask of the interior where the penetration will be for the same reasons outlined above.

Mask surface to prevent damage

Step 2: Drilling the Pilot Hole
Well, the hard work is done, now it's time to drill the hole.  While incredibly simple and straight forward, this step is obviously the most intimidating, so just make sure you've placed the fitting correctly above and take your time.  The first step in the drilling process is to drill a pilot hole for the hole saw bit you'll be using.  Starting from the exterior, use the same size drill bit as the pilot bit on the hole saw you'll be using to drill through the hull of the boat.

Step 3: Drilling the Final Hole
Now that you've got a hole that extends all the way through into the bilge of the boat you can score the fiberglass on the inside to prevent chipping when the hole saw bit finished the cut.  To do this start from inside the bilge compartment and insert the hole saw bit into the hole you drilled in the previous step.  Either using your hand, of if you have clearance, the drill running in reverse, lightly score the fiberglass inside the bilge by applying pressure rotating the hole saw bit.  This will result in a cleaner hull when the hole saw finishes the cut.

Predrill the hole from the inside to prevent damage

Now you're ready for the big moment, let's finish drilling the hole from the outside of the boat.  Gelcoat is actually a very brittle material, so we need to be very careful not to damage or chip the gelcoat around the perimeter of the hole we're going to drill.  With the drill in reverse, start from under the boat and apply pressure to the drill to use the friction of the bit to wear through the gelcoat completely.  This will control how much "bite" the bit has, which will prevent chipping of the gelcoat.  Once you've worn through the gelcoat, which is only a couple millimeters thick, and "feel" much different than fiberglass, switch the drill to forward and continue drilling completely through the hull.

Drilling the hole for the thru-hull intake fitting

Step 4: Chamfering the Hole
The final, and perhaps most important step before actually installing the thru-hull fitting it to chamfer the gelcoat around the perimeter to prevent superficial cracks from occurring in the future.  Because gelcoat is brittle, if it is subjected to any force parallel to the finished surface of the hull small spider cracks can occur and radiate out away from the center of the hole.  The preventative fix is simple, we just remove the gelcoat around the edge of the hole so the fitting doesn't make contact with it at all once installed.  The easiest way to create this chamfer is to use sandpaper around the circumference of the hole until the gelcoat makes a 45 degree angle with the fiberglass underneath it.

Chamfer the hole to prevent damage to the gelcoat

Cut Away of hull with correctly chamfered hole ready for fitting

Now all that's left to do is install the thru-hull fitting following the recommended installation method, and using the appropriate adhesive/sealant to ensure you'll have a leak free setup.  For more information refer to the instructions included with the fitting you are installing (where applicable).

Conclusion
Hopefully this guide has proven beneficial, if you have any questions feel free to contact us and we'll be happy to provide assistance.  If you need to purchase any of the items discussed in this guide you can view them by clicking on the image below.

Buy Bronze Mushroom Thru Hull Intakes Buy 3M Marine Adhesive
Our Price $13.95-$57.95
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Our Price $14.99
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Scared of doing this to your boat? Install thru-hull fittings the right way for a leak proof install.
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