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Finding Leaks and Choosing The Right Sealants

Here’s how to find leaks and choose the proper sealants for them on your boat.

Fresh water leaks come in, typically at windows but just about any hole in the cabin, from the companionway to mast partners will, at some point, leak. Keeping the water on the outside of your boat is the way to stop it from entering the boat. That may sound facile but once water has gained entry, it will find a way to spread throughout your boat, emerging in the most unlikely of places: Through a combination of wicking and unknown pools of water trapped between liner and hull, I have had a drip in my nav station from a leak by my mast! By water pooling in a tight vertical space, a leak can drip from higher than one would think possible and water flowing along electrical wires can travel in unexpected directions and cross even large gaps. The main point here is that once water has entered your boat, it will make a mess. It must be stopped from entering and that means correctly sealing entry points, from chainplate holes to windows, with the appropriate sealant, from the outside.

When you find a leak, trace it as far as possible and then look for the most probable source of the leak. It’s usually something with old caulk that has aged to brittleness, shrunk and cracked. If possible, run water over it alone, to see if the leak reappears. This often takes a while and sometimes only works when water enters the area from a particular direction or when the wind loads the rig from a particular direction and enough force. When you have a likely suspect, remove it, clean it carefully and completely, sand if necessary, wipe clean with acetone, seal and attach it. After it has cured, test to see if that fixed it. If not, repeat the process with the next culprit. It’s a good way of catching up with your caulking maintenance - Most everything needs to be rebedded every five to ten years.

Some things need to have an excellent seal but are incompatible with more aggressive or similarily-based products, such as polycarbonate (plastic) windows. The best sealant (silicon) does not bond at all for this application but it does seal perfectly and it won't crack, craze or fog the plastic and destroy the window, as just about any other sealant will.

Most sealants will crack these polycarbonate windows: Use silicone for them.

This crack in a hull (You can see the water welling up through it!) needed a super sticky and very strong bonding and completely waterproof sealer: SplashZone temporarily stopped the leak, until the boat was hauled and repaired properly.

More and more people are confusing sealing with bonding. A sealant (usually liquids) prevents something from entering or exiting a joint. A bond holds the joint together. Some sealants also bond, some more than others. It’s necessary to be able to take some things apart for regular maintenance and these should not be permanently bonded. An example of this is a thruhull, which needs to be taken out of the boat for inspection and replacement. It’s underwater and therefore Dolphinite is the right sealant to use, not 5200, which makes the maintenance a nightmare and does not make the seal any better. Bonding the fairing block for the seacock to the hull, however, if materials are compatible, such as wood or fiberglass, is best done with epoxy, with barrier coat additive and once painted wet on both surfaces, thickened to a paste with colloidal silica, to fill all the voids. With this structure, we then have a fairing block that is part of the hull and completely sealed to it. The thruhull and seacock are thoroughly sealed in place with very strong mounting and easily removed for regular maintenance.

These are fairing blocks, roughly cut and showing how they assemble with thruhulls (the hollow mushroom that penetrates the hull) and seacocks (the valve with the integrated base).

Dolphinite is used to waterproof the seacock and thruhull to the fairing block and hole in the boat and the fairing block is bonded with epoxy with structural grade thickener and barrier coat additive, to create a strong pad to distribute the force of working the valve directly to the hull, not the hole or the fragile thruhull. This allows for easy disassembly and replacement: The right sealant is crucial and that's not 5200!

Very roughly, here is a brief list of marine sealants and adhesives I use regularly, with their typical uses and sealing/bonding properties:

Bonding Effectiveness
Sealing Effectiveness
Underwater Thruhulls
Very Poor
Very Good
Windows & Polycarbonates
LifeCaulk MultiCaulk
Decks Fittings
Very Good
Temporary Flexible Joints, eg. Hose Barbs
Permanent Flexible Joints, eg. Mounts
Fair - Excellent
(depends on application)
Poor - Excellent
(depends on application)
Thin Underwater Cracks
WestSystem Epoxies
Poor - Excellent
(depends on application)
Poor - Excellent
(depends on application)
Above Water Bonding and Fairing
When applying any of these goopy products, use plenty of paper towels, a large trash bag, plenty of mixing cups and sticks for the epoxies and lots of thick nitrile gloves. Lay out blue painter’s tape on the edges and immediately around the job, for at least several inches, right up to the crack you’re sealing, on both sides. This way, you can mush the sealant inside effectively, watching to see it completely fills the void and squishes out well, wipe all the excess off and remove the tape, while the sealant is wet, leaving a clean, neat installation. I use a clean hand/dirty hand approach, trying to keep one hand clean enough to touch something not goopy. When I get both hands dirty, I change both gloves. This greatly reduces spreading my mess.

The hole is taped and I've wetted the wood with epoxy.

I've added the bedding compound.

I've pressed the slide in, displacing the bedding compound.

Wiping and pulling up the tape leaves a neat installation, without smears