How to drill a hole in a cored deck?

Drilling holes in your boat

Making holes in the boat and drilling through the deck is probably one of the most terrific experiences of any sailor. But it does not have to be. If you know how to do it correctly, you will do it with confidence and peace of mind. Correct and professional procedure will provide you with permanent deck mounting, without any leaks and problems in the future.

Drilling through the cored deck

Special attention is required when you plan to drill through a cored deck. Core, which is usually a foam or balsa, is a middle layer between two layers of fiberglass. It is sensitive to humidity and absorbs water easily. Water trapped between two layers of laminate can penetrate further and cause a lot of damage, if not sealed properly. 

Should I use sikaflex to seal the holes in a boat?

It is not a good solution to use sika or any marine sealants only in such vulnerable areas as cored decks are. These kinds of sealants get old and their sealing properties can deteriorate with time. 

Proper sealing will also do its job when drilling through teak decks. It will not only protect the core, but also prevent water getting under the teak planks. 

Drilling through the deck – step by step guide

In this guide we will show you how to drill a hole in a cored deck to prepare it for new equipment or re-seal the old holes before rebedding deck hardware. Ready?

1. Drill an oversized hole using a good quality drill. If you drill through teak, we recommend placing masking tape on it to protect wooden edges from being damaged. 
2. After the hole is drilled, you will clearly see laminate layers and the core. Next step is to remove the core within 1-1,5 cm beyond the hole edge. You can do it using a bent nail inserted into a power drill. Use vacuum cleaner and/or compressed air to remove core leftovers from the hole cavity. 
3. Now it’s time to start sealing. Place a duck tape to the bottom hole. Apply catalyzed, unthickened epoxy to the hole using a syringe until it’s full. Puncture the duck tape so the epoxy flows out of the hole. Unthickened epoxy better penetrates the core. Retape new duck tape over the old one.

REMEMBER! Do not remove duct tape, just puncture it. If you remove it the area is contaminated with epoxy and it will be impossible to retape it. New duck tape will not stick.

4. Next step is to thicken the epoxy with colloidal silica or microballoons so it has peanut butter consistency. Fill each hole with it using new syringes, up to the deck level.
5. Now you need to wait until the epoxy cures. We wait a minimum one week before we start drilling through the cured epoxy. Then you have a hole ready for new equipment, and teak and core are protected from the leaks. Before mounting new hardware, now it’s time for using high quality sealant or sika.

Drilling through the deck – summary

As you can see, the devil is not as black as it’s painted. Making holes can be time consuming and it is important to stick to the proper technique. Leaks into the core or under the teak are what you need to avoid at all cost. It can definitely be done as a DIY project without much experience! 

Using this technique we mounted our watertight compressor box on a teak deck. There will be a separate post about it later. Here, you can read more about our boat upgrades we did last winter!