Drilling and Filling Holes in Your Boat - SailNet Community
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Drilling and Filling Holes in Your Boat

When drilling large holes, measure twice and drill once.

Your heart races and your hands tremble. Beads of sweat form at your hairline then trickle their salty droplets onto, of all things, the electric drill that lies quietly on the deck by your feet.

“Let’s do it!” You tell yourself, trying to sound a little like your old high school football coach. “Go for it, meathead; it’s only a small hole!” Your coach would bellow.  “Yeah, but it’s not your boat, Coach,” you yell back.

“The heck with it!  I’ll just call the boatyard next week while I’m at work and have them send someone over.  That way I won’t even have to watch.”  

If the idea of drilling holes in your boat scares the living hell out of you, don’t worry—you’re not alone.  “Hole Phobia” is a normal affliction experienced by even seasoned sailors who think they’re about to ruin their boat by drilling in the wrong place or by using improper technique resulting in expensive, and possibly irreversible, damage.  

We used to feel this same anxiety ourselves.  But now, we’re 12 months into our refit of Serengeti, our 1978 Formosa Peterson 46, and Sue and I have drilled and filled enough holes to last a lifetime.  Today, we don’t hesitate when it’s necessary to drill large holes for mounting the foot switches of a new electric windlass. The filling and fairing of four-inch diameter holes through the deck where chimneys used to reside doesn’t phase us a bit.  With the removal of teak decks, and the replacement of most hardware, handrails, and stanchions almost complete, we estimate our combined total for both drilling and filling of holes is well over 3,000!

Assemble the fasteners, hardware, fairing blocks, and back-up plates before picking up the drill.
It’s not that we no longer make an occasional mistake—we do. In fact, just earlier this week, I drilled eight erroneous holes in a newly painted fiberglass hatch frame before I realized my wrongdoing. The hatch wasn’t aligned correctly,and in my rush to complete the job,I drilled in haste.  A couple of years ago this would have been devastating to me, but not anymore. We’ve learned that any hole drilled can also be filled without weakening the integrity of the boat.Within 30 minutes, my erroneous holes were repaired with epoxy, and I was able to drill again and correctly mount the hatch.  This time,I proceeded more carefully and first made sure that the hatch was aligned properly. Yes, the old carpenter’s adage of “measure twice, cut once” is certainly also applicable when it comes to drilling holes in your boat.

To help you win that battle of nerves with your drill, we’ll share some of the tips, tricks, and techniques that we’ve discovered.

Fiberglass    Drilling into fiberglass is pretty easy.  It’s a relatively soft material and cuts cleanly and evenly.  For holes up to a half-inch in diameter, you can effectively use a traditional drill bit. If your hole needs to be larger than a half inch, such as those needed for installing foot switches for an electric windlass, or for adding a water tank fill, you should consider using either a hole saw or a paddle bit. Because of the way they cut, traditional twist drill bits of very large sizes can damage surrounding fiberglass and should not be used.

Before you start drilling, determine what’s on the other side.  Is there any wiring, a bulkhead, or anything else you need to be careful not to drill into?  If your hardware will be under load when sailing, make sure that there’s space below for a backing plate and access for tightening the nuts on the thru-bolts.

When drilling into the deck, all holes should be perpendicular to the surface. Nothing’s more frustrating than going down below to slip on that backing plate and tighten the nuts only to find that you drilled the holes crooked, and now the backing plate holes don’t line up with the mounting bolts anymore. A straight hole will also ensure that the fastener head seats neatly into the hardware, resulting in a professional look.  To help you drill these holes straight, you might want to add a valuable tool to your kit—a portable alignment tool that works like a miniature drill press attaches to your existing drill and keeps your bit at the proper angle.  Without it we never could have drilled the 68 6.5-inch-long holes through our bulwark for the two new 10-foot genoa tracks without this tool.

Attaching an alignment tool to your drill provides true, straight holes with a professional look.

When mounting no-load deck hardware like a through deck fitting that you screw into the deck to accept wiring,it’s necessary to take an additional step in your preparations. After drilling the appropriately sized hole for your fastener, take a slightly larger bit, and lightly drill out the top of the hole at the gelcoat.  This will allow the screw to bed securely without the gelcoat immediately around the hole cracking and popping up when the screw is tightened.

If you want to move deck hardware from one location to another, or if you’ve drilled in error like I did, don’t worry.  Filling small screw or boltholes in fiberglass is quick and easy.  Your repair will be water tight and actually stronger than your boat originally was.  To begin, the tops of these holes should be beveled slightly to best receive and hold the filling material. (A Dremel tool works well here.)  Mix epoxy resin with a colloidal silica thickener, like Cabosil, to achieve a mayonnaise consistency.  Force the thickened resin into the hole with a plastic spreader. If the hole is really small, the epoxy can be injected with a plastic syringe made for this purpose. As the epoxy may shrink a little after it has cured, you’ll need to come back a second time with more epoxy putty.  This time thicken with an easy-to-sand filler such as micro-balloons.  The area can now be sanded smooth and touched-up when the time comes for prettying up your boat.

Filling larger holes in a fiberglass deck needs a little more work than just epoxy.  You’ll also need to replace the fiberglass and some of the coring material that may be missing.  Once again, you start by aggressively beveling the edges of the outer skin.  Cut circles of fiberglass matt of incrementally smaller sizes from just inside the perimeter of the bevel down to the exact size of the hole itself. The fiberglass matt will go on top of a replacement core, sized slightly smaller than the hole and already held in place by thickened epoxy, completely sealing the inner skin.  Next, add the epoxy and fiberglass.  Begin by “wetting out” (soaking with epoxy) the largest circle of matt in place over the hole, and repeat with progressively smaller circles of matt until it’s level with the surface.  Finish by sanding lightly and covering with the same easy-to-sand epoxy putty mixture  used above.  Again, you’re ready to paint.  Epoxy is not UV stable and must be coated.

An excellent reference for fiberglass repairs is Fiberglass Boat Repair & Maintenance by Gougeon Brothers Inc, makers of West System epoxy.  

Filling, fairing, and sanding large holes is messy but not hard.
Aluminum and Stainless Steel    When drilling into an aluminum mast or through stainless steel stanchions or rails, having the right tools can greatly speed up the process and result in a better overall job.  For drilling metals use only cobalt drill bits.  Being much harder than other bits, cobalt ones speed up the process of drilling aluminum, and are an absolute necessity for drilling through stainless steel.  Sue and I keep both a set of steel bits and a set of cobalt drill bits on board.

Hardware on an aluminum mast or boom is usually fastened by pop rivets or by tapping the holes to receive machine screws. In either case, it’s important to ensure that the hole is drilled straight and that any dissimilar metals,like stainless steel screws in aluminum, are isolated from each other to help prevent electrolysis. Drilling straight into a curved extrusion is sometimes difficult. Have a partner stand back and advise you whether you need to raise, lower, or move your drill from side to side to obtain a straight hole.  To isolate dissimilar metals, apply Tef-gel or a marine sealant to the fastener before insertion into the mast.

After painting, the results are spectacular.
If you drill in the wrong spot, or remove old hardware and are left with holes in the mast, your options are somewhat limited.  It’s best not to weld the holes closed, since welding weakens the spar. To fill the hole, tap it and insert a screw, or pop rivet it closed.

Drilling into stainless steel presents a real challenge.  Use a sharp cobalt drill bit, and drill at a slow speed while applying heavy pressure on the drill.  If you run the drill fast, the tip of the bit will heat up and render its cutting properties useless.  You can also apply oil or cutting fluids to the bit to reduce friction and heat buildup.

If you’re drilling railings or stanchions, indent your starting spot for the bit with a sharp blow to a nail set.  This dimple will keep the bit from wandering along and scarring the side of the round tubing. Drilling all the way through the tubing and exiting the other side at 180 degrees from your starting point can be difficult. It’s very easy to ruin the installation by letting your bit angle vary greatly as you drill through the backside of the tubing. Fortunately, the same alignment tool we mentioned earlier allows us to drill straight into metal tubing. It really makes the job easier!

Teak is very easy to work.
Exterior Teak Trim    Teak is very soft and easy to drill and repair.  If you’re moving hardware from one place to another on teak surfaces, there’s no need to leave ugly holes in the wood that will eventually turn black and begin the rotting process.  Teak bungs or plugs are available in sizes from 1/4-inch to one-inch and even larger.  These bungs are easy to install and with little effort can be finished to a beautiful surface.

To fill a hole, drill out the opening to the size of the bung you’ve selected.  The depth of the hole should be about 3/4 the depth of your bung.  Dip the bottom of the bung in epoxy, wood glue, or varnish before seating it by gently tapping with a rubber hammer.  Remember to align the grain of wood in the bung to that of your repair area.

A little sandpaper, a few coats of varnish, and a first-rate result.
Once the adhesive has dried, it’s time to remove the top of the bung. The safest way to achieve a smooth surface is to sand it with a sander.  Another method of leveling the bung is to whack off the top with a hammer and chisel. When using this technique, we found that sometimes part of the bung below the surface chips away too, leaving a small indentation.

Once the bung is level, you’re ready to varnish, oil, or leave natural as you wish.  Your repair will be watertight and you’ll be amazed at how good it looks.   

Bedding Deck Hardware    Any time you drill a hole for mounting new hardware, you are going to need to seal that hole with more than just the screw or bolt you attach it with.  Water has a wonderful knack of finding its way into the smallest imaginable openings on boats.  Doing the job right the first time may save you many frustrating years down the line seeking out the source of annoying little leaks.  It will also ensure that you preserve the integrity of your deck, especially if it’s cored.

With my past experience as a general contractor building custom homes and using a variety of sealants, I used to think that the special marine caulking and sealants were just overpriced hype.  I’ve discovered now that I was wrong.  Don’t be tempted to use the local hardware store varieties that say “for indoor/outdoor use.”  The marine environment is very different, and your boat is a moving, flexible platform that requires a sealant designed for this very task.

In the marine line of sealants, there are several different choices. Silicone and polysulfide sealants can often be used for many of the same applications, except for underwater situations, in which case the polysulfide is preferred.  Since polysulfide provides much greater adhesion, lasts four times longer, is sandable, paintable, and can be used above and below the waterline,it is our choice for most applications. Polysulfide products,such as 3M 101 are very good. They provide an excellent seal that remains flexible for 20 years or more.

Polyurethane adhesive/sealants, like 3M’s 5200, are very popular but often misused and even overused by many boaters.  These products provide incredible adhesive strength as well as watertightness, but are not necessary unless you require the extra bonding strength such as in hull-to-deck joints or if laying in a keel.  You’ll see firsthand this great adhesion if you ever need to remove a piece of deck hardware that was bedded with this type of sealant. If often rips the gelcoat right off!

Once you’ve overcome that very normal fear of drilling holes in your boat, a whole new range of projects will open up to you. Start small and your confidence level will grow with every hole drilled and each one filled.  Don’t let the worry of making a mistake stop you from trying.  Repair skills are not difficult to master and are a valuable asset for any sailor to acquire.  Soon, you’ll have the capability to do many of those custom jobs you thought only possible at an expensive boatyard and you’ll be pleading, “Put me back in, Coach. I’m ready to drill!”

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