Of all the leaks aboard a boat, the hull/deck joint is probably the most persistent, hardest to find and hardest to fix of all. By the vary nature of the joint, water can travel great distances before appearing as a leak below deck. There are no magic bullets to fix these leaks but here are some tools
Types of hull/Deck Joints
Before looking at possible repairs, let’s look at the kinds of joint you are likely to find.
Box Joint: In a box type joint the deck molding has a downward flange, like the lid of a shoebox, which fits down over the hull.
Outside Flanges: both the deck and hull have a flange on the outer edge. These flanges are usually bolted, screwed or riveted together and covered with a vinyl or aluminum trim strip or rub rail
Flange and Lid: In this joint, the hull has an inward facing flange. The deck is a flat lid bolted or, screwed to the top of this flange.
“H” Joint: The dreaded “H” joint. An aluminum “H” channel is used to connect the deck molding to the hull molding. The hull and deck are secured with either screws, bolts or pop rivets. DO NOT TRY TO REMOVE THE “H” CHANNEL! The hull and deck were assembled in a jig. If you remove the channel, both will be unsupported and floppy. Reassembly will be like putting toothpaste back in to a tube.
There are more types of hull/deck joints out there but they are variations on these themes
Other Complications: There are other things associated with the hull/deck joint that may give you fits. Some examples are wooden toe rails, aluminum toe rails, rub rails, chocks and other fittings bolted to the joint. In some installations, the interior of the joint may be glassed over with several layers of glass cloth, mat and resin. This makes accessing the inside of the joint practically impossible.
Once you have an idea of the type of hull/deck joint you have, you can formulate a repair strategy. In some extreme cases, doing nothing while keeping all interior items protected from leaks may be an acceptable solution.
You will often see boats with globs of marine silicone sealant smeared along the hull/deck joint. This almost universally doesn’t work for long, if at all. Silicone isn’t a good adhesive so doesn’t bond very well. It looks terrible and ruins the gelcoat finish for any further painting. In addition, it usually doesn’t penetrate the crack or openings that are doing the leaking.
If you want to try a simple solution, use something called “Captain Tolley’s Creeping Crack Cure”. It is a very thin liquid that will penetrate fine cracks by capillary action and cure to form a flexible waterproof seal. Several applications may be necessary to totally seal a joint. Do it slowly, don’t try to seal everything in one go, you may find it’s just running down below. No guarantees this will successfully cure your leaks so try it at your own risk.
Box joints usually have the space between the outside edge of the hull and the inside of the flange filled with a sealant of some sort. This is usually a dependable type of joint, but may leak if the sealant fails, and water splashes up against the joint.
Reef out the old sealant and replace with new. While I don’t usually recommend 3M5200 above the waterline, hull deck joints are an exception. To do a neater job of applying the sealant, mask off the hull with blue tape so you don’t smear the sealant all over. Make sure to fill the upper recesses of the joint well and leave no voids or bubbles in the sealant.
Outside flanges and flange and lid joints are usually bolted or screwed together with sealant between. When this sealant breaks down, leaks start. For a long-term solution, the fasteners should be removed and the old sealant cleaned out. This means unfastening part of the joint at a time and prying the joint apart enough to clean out the old sealant. This may require unbolting the toe rail or removing the rub rail.
The two parts must then be very carefully pried apart so you don’t fracture the fiberglass. The joint can be held open with small wood wedges. Work your way around the boat, doing a small section at a time. Then replace the sealant and refasten. Didn’t say it was going to be easy!
Pay careful attention when you reinstall the toe rails and other hardware, making sure to properly bed the fasteners in sealant. Nothing is more frustration than going through this process and still having the joint leak. You did rebed all the other deck hardware to eliminate them as a source of leaks, right?
Remember not to turn a fastener once the sealant has cured. Tighten the nut from below or leave it alone. Twisting the fastener in the cured sealant breaks the sealant bond you just so carefully established.
The “H” joint is one of the hardest to fix. You can’t really take it apart and expect to get it back together without a great deal of difficulty. Most joints of this type are fastened with pop rivets. Each fastener, as well as the top deck and bottom hull joint, are possible sources of leaks. Often the inside is also glassed over, making access from the inside impossible.
First step is to eliminate any leaks from the pop rivets. If they are still tight, you can try sealing the head of each one with 3M5200. If they are loose, you can carefully drill them out and replace them. Make sure you use aluminum pop rivets with closed ends. The closed ends eliminate another potential source of leaks. Dab each rivet with sealant as you replace it.
The outside top and bottom edges are another potential source of leaks in this type of joint. You can try “Captain Tolleys” if you feel lucky. Otherwise, clean the sealant out of the seam as well as you can. Use a thin blade to scrape out as much of the old sealant as you can. The edges of the fiberglass deck and hull won’t be uniform, so the gap will vary from place to place.
Mask off the edges of the channel and the deck or hull with blue tape. Then force sealant into the seam with as much force as you can. Clean off the excess sealant for a smooth seam. Wish yourself luck.