Tracking down a leak on Hunter 410

[JebLostInSpace]

I have a 1998 Hunter 410 with a leak that so far has defied all attempts at tracing it. Any suggestions welcome on how to identify the source of this leak so that I can work out how to address it.

I discovered the leak while working on the genset in the aft starboard lazarette. There is a piece of plywood there that separates the port and starboard lazarettes. It was tabbed to the aft side of the fiberglass rudder tube with fiberglass, as well as to the floor of the lazarette. This piece of plywood was completely saturated with water, and water was seeping over the top of the fiberglass tab and making its way to the bilge.

We hauled out and started cutting away the rotten wood and fiberglass. As we cut away, we discovered that the plywood was not simply sitting on the floor of the lazarette, as we initially believed. The plywood actually extended through the floor of the laz and down to the hull. Between the floor of the laz and the actual hull is a gap of about a half inch or so. the void between the hull and the floor of the laz was apparently filled with water, which then worked its way through the plywood, into the laz, and finally to the bilge.

We figured there were two likely sources for the water to get into the void. Either the somewhat infamous hull/deck joint on the stern at the swim platform, or somewhere along the fiberglass rudder tube.

Starting with the hull/deck joint in the stern, we removed the rub rail. It looked mostly good underneath, but over on port side near the waterline there was some damage to what looked like weather stripping that lined the joint. We repaired that, removed a bunch of old sealants anywhere we could, and repacked everything with 4200. Then we packed the rub rail with silicone caulk and replaced it. Finally we put a bead of 5200 down on the outside of the rub rail both top and bottom.

On the rudder tube side, it's probably worth noting that the H410 rudder tube is constructed a little differently than most that I found diagrams for online. We have a fiberglass tube that extends well above the waterline. In fact, it extends all the way to the cockpit floor. The rudder post and bearings live inside this fiberglass tube and I don't have any way to inspect any of that without removing the rudder entirely.

What we did with the rudder tube was just cut away enough of the laz floor to see the base of the tube where the tube meets the hull. We couldn't visually see any problem, but we painted the area with fiberglass resin.

Imagining that we had sorted both possible sources of the leak, we splashed. We waited to replace the plywood that we had removed so that we could monitor for the leak. Sure enough, within half an hour of splashing we had water back in the void between the laz floor and the hull.

We've tried dropping dye in the water at the rub rail, but haven't seen any get inside. We've tried drying it out with diapers to see which areas get wet first, but the leak is fast enough that we can't ever get it dry enough to see. We've tried hauling the boat down to one side or the other, but can't detect any difference in the speed of the leak. We're stumped! Any ideas, however crazy, to help us figure out where the water is coming from will be greatly appreciated.