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Install A Simple Ventilation Or Access Port


Need more breeze below decks or a way to access fittings and tanks? Installing a port is not a complicated project.

Photos: Cliff Steele

Boatbuilders do their best to provide access to critical components under decks or behind hull liners, but sometimes the unexpected happens and you find the need to get at something behind fiberglass. Or in other cases, you simply want to improve the ventilation belowdecks. If your boat could use a new port, follow these steps for a smooth install.

Location, Location, Location

Choosing the correct placement before installing your new port is very important. You must avoid cutting into your boat’s structure, wire runs, piping, or any other mechanical functions. If you don’t know what’s behind the spot you want to cut, ask your manufacturer. Find out before you begin whether you’ll be cutting solid fiberglass or a cored portion of your console, deck, or cabin sides, for example. Solid glass is easier to deal with because coring will require proper sealing before you can move ahead with the install. Similarly, if there is a void, say between the inner liner of your boat’s cabin and the outside, you’ll need to reinforce the area around the port with coring to prevent flexing.

Think ahead: Before picking a spot for your new hinged-window port, check for any obstructions that will be above it to make sure it can open. The same goes for access ports that tip out. If you can’t open it all the way, it won’t be much use. Spin-out ports typically don’t require the same clearances.

Measure, Trace, And Measure Again

Most ports will come with a template that you can trace onto the spot you’ve selected that will help you cut the proper-sized opening. To ensure you get the opening square and level — or at least even in relation to the rest of the boat — you’ll need to carefully measure both, from either the top or bottom of the template, on both ends to a fixed reference. The difficulty is that the reference itself, whether it’s the top of the console or the deck, may not be level. Traditional bubble levels aren’t much help, either; boats are rarely sitting in the water or on the hard in a level position. Choose carefully — if you’re cutting into a console or cabin, a position that looks good outside might not look good inside.

Use the template provided to trace a pattern onto your boat where you will make the opening.

Once you’re happy with the port’s position, trace the template onto the boat using a marker.

The Moment Of Truth

Now comes the part that makes most boat owners squirm: cutting the fiberglass. With a bit large enough to accommodate the blade of the cutting tool of your choice, drill a hole at one corner of the template you’ve traced onto the boat. Insert the blade of your jigsaw, reciprocating saw, or similar tool, and begin cutting along the traced lines, running the tool at full speed to reduce splintering and chipping of the gelcoat. Which tool is best for the job? The one that gives you the greatest control while fitting in the space where you are making the cut.

Run the saw at max speed to avoid splintering the fiberglass.

Next, drill the holes for the mounting hardware following the recommendations in the instructions for the port. Be aware that if you are using screws rather than thru-bolts or well-nuts to hold your port in place, you will need to drill pilot holes that are slightly larger than you would for screwing into wood. You want the threads of the screws to just engage the edges of the hole in the fiberglass.

Once you’ve made the holes, go back with either a countersink or an oversize bit and chamfer the holes just enough to remove the gelcoat. If you’re afraid of drilling too deep, run the bit backward. Chamfering will prevent the threads of the screw from pulling the gelcoat up away from the glass, which can make it impossible to properly seat and seal your port.

If you had to cut through coring, or if your holes for the mounting hardware pass through coring, now’s the time to seal the edges with epoxy or similar to prevent any leaks from saturating the core.

Dry Run

Now it’s time to dry-fit your port and, in the case of some ventilation ports, choose which way to install the trim ring. If you cut your opening properly, the port should fit in easily with about 1/8-inch all the way around to allow space for the sealant plus expansion to take place. If it’s tight, you’ll need to file the edges to make the hole large enough for the port to fit easily.

Dry-fit the port before using any sealant. The port should fit easily with about 1/8-inch clearance all around.
Use sealant liberally and then tighten the port into place until the sealant just starts to ooze from the edges. Don’t tighten it all the way! We used black sealant for this demonstration. White or clear are the usual choices.

Seat And Seal

After you’re satisfied with the fit and alignment of the predrilled holes (you may need a helper on the opposite side to assist you here), snug up the port temporarily. If everything fits and looks as it should, you’re ready for sealant.

Using a wet sponge, wipe liquid dish soap around the outside of the trim ring of the port before applying sealant. The soap will prevent excess silicone from sticking to the fiberglass around the port.

Most port manufacturers recommend Dow 795, or any other 100-percent silicone long-life exterior sealant, but butyl tape is a good alternative that some do-it-yourselfers find easier to work with. Don’t skimp on the sealant and be careful not to seal any drain holes. Apply sealant to both the sides and surface of the port, filling the gap. Do not tighten the port all the way down. You don’t want to squeeze out all of the sealant, just enough to ooze around the edges. Trim up the excess after it cures.

Michael Vatalaro

Contributor, BoatUS Magazine