I installed an ST40 thru-hull in my bow where the old knot-meter thru-hull used to be. Same size hole and all. Backing block was completely dry and rot free so I left it alone and didn't replace it. I used 4200 to seal the thing in there. Now, the hole is located at the 'vee' of the hull and the instructions say "hand tighten only". Well, I hand tightened alright, but like .. hand TIGHTENED. Apparently I'm a lot stronger than whoever wrote the instructions ... I'll get to that later.
As soon as the boat was floating I went in and had a look at all of the thru-hulls .. all but one, of course, the new knot meter! I had also replaced a thru-hull for the galley drain located just behind the companionway steps this winter and last year I replaced three others. No problems with any of those. I'm blaming a stressful launch for my failure to check on that thru-hull. Anyway, it was leaking. We only discovered while underway half way back to the club. I didn't want to go back because I swore I'de never go back there after my experience with haul-out and launch this year, it's a terrible place. I figured if I have to haul the boat I'll do it somewhere else and there are other places. The leak wasn't sinking the boat or anything but I noticed something odd .. a previous owner had glassed over the limber hole from the bow section to the bilge. So, the bilge pump wasn't going and the bilge water wasn't rising so that's another reason I didn't notice. I'm on a mooring so I took the boat to the shared dock to survey the situation. The leak was pretty bad. The bow had about a foot of water in it after 45 minutes in the water, not good. I bailed all the water out to observe the leak and it was surely coming from the housing, NOT from between the hull and thru-hull fitting .. rather, from between the thru-hull fitting (housing) and transducer. Ok I thought, it's either the housing or the ducer .. I put in the blank, same leak. Ok, so it's the housing, WTF. I messed with it until I got the leak slowed down to a trickle and left it alone. Yes both o-rings were present on both the transducer and blank plug, I had also lubed them with the provided silicon lubricant. This housing keep in mind, is the newer design with the flapper valve, whole thing is plastic. I drove back to check on it again later that evening and the water ingress wasn't too bad. I bailed the half bucket of water out and called it a night. The next morning I took the day off work and started making calls to local marinas pricing out a haul out and calling the chandleries to find a replacement thru-hull. No one had what I needed in stock and everyone was busy with launch, they could make time for me, but it was going to be expensive! I weighed my options and decided to try the cheapest, lowest risk option first .. bigger o-rings. This was recommended to me by an employee at one of the chandleries and he pretty much saved the day. I happened to have the EXACT o-ring on board! One of them anyway, the larger upper one .. I had purchased a pack of replacement o-rings for the fuel fill, there were three o-rings in the package so I had two left, just enough for both the plug and ducer! This slowed the ingress down to a trickle, barely noticable. After 45 minutes there was maybe two tablespoons of water in the bow. Ok, now we're making progress. I went to home depot and bought a pack of assorted o-rings but none of them were the right diameter or thickness ... I also purchased yellow plumbers tape. I put a few wraps around the ducer UNDER where the lower o-ring is located and slipped the o-ring over the layers of plumbers tape. LEAK STOPPED. Just about anyway, if I 'jiggle' the transducer I can get it to leak but if i leave it alone the area remains dry. My theory is that I overtightened the thru-hull fitting causing it to deform due to it's location on the curved part of the hull. The hole is flat mind you and the flange fits in just fine but I don't know .. that's my theory. I know that these plastic thru-hulls are prone to cracking when over-tightened... I don't think mine is cracked because the larger o-rings have stopped the leak. So finally my questions: 1. is my thinking ok? Is it possible that the housing IS in fact cracked? I can't see any cracks. 2. how critical is a boat haul? If the housing isn't cracked what are the chances that it COULD crack while in some chop or something? I imagine the hull is pretty rigid up there at the vee of the bow, it's thick and not a large unsupported area. My gut tells me to haul the boat and replace the thru-hull with raymarine's bronze version but thats' going to be a very expensive exercise. The original knot meter fitting was plastic as well and that lasted 32 years. Thanks all. Steve Suhana, C&C 32 Toronto
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