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How to Find a Leak in a Boat

In this video I go through the various techniques of finding a leak in a boat and talk about how to repair a leaking boat.

Dangar Marine

About Dangar Marine

MarineEngine.com proudly sponsors the Dangar Marine YouTube Channel. Our friend Dangar Stu draws on his experience as a commercial boat skipper and mechanic to make some great how-to videos. Dangar Marine videos cover a range of relevant topics in outboard repair, boat repair/upgrades and seamanship. Subscribe on YouTube to be notified when new videos are released.

How to Find a Leak in a Boat – Video Transcript

Hey there Dangar Stu here, today's video is about finding a leak in a boat and it's proudly sponsored by marine engine.com.

So the boat in question today is the mighty Green Machine. It's been taking on water for a while. The other day the battery went flat so I think the bilge pump has been working overtime on the float switch so it's time to sort that out. Now there's a few ways you could find leaks in a boat, one of the really common ways is just to fill the boat with water and see what's going on. The big downside to that is water weighs a lot. If the hole is low in the boat great - you can fill it with ten centimeters of water and if you find it- fantastic. If the leak is higher up than that I really don't recommend filling a boat to the gunnels because that's like a cubic meter of water and weighs a ton. Your trailer won't handle it, the hull won't handle it so just be careful. Only ever really look for leaks low down if you are using water. Depending on the style of the boat you can also use air the same way we do these vacuum and pressure tests in say outboard gearboxes. If it's got a second sort of watertight compartment like maybe a small sailing dinghy you can tape it up, pressurize it and then just start spraying around looking for where that is leaking out.

Another good option is light- go at night in a dark space and have somebody on the inside with a light. You could be on the outside and just look for pinholes. I actually did that ages ago before I primed the hull, painted and did the anti foul and everything on the green machine. Sometimes if the hole is sort of across, is leaking around a double skin or something you won't see light straight through. You might see a sort of reflected light, but light tends to be best for straight through pinholes. The next thing I'd say about finding leaks in a boat is you're always going to have a couple of prime candidates. You don't always have to inspect every inch of the hull unless you've had a hard grounding or something like that. If you have you probably spotted it straight away. Obviously if you're in the boat on the water and you see the water coming straight in, you know where it's coming from.

So the good option really is to dry the boat completely, so really get it bone-dry. Launch it and even with a couple of sets of eyes just start looking around seeing if you can find where it's coming in. Because the green machine lives in the water all the time and the float switch on the bilge pump won't pick up that last couple of centimeters of water the bilge always has water in it which means I don't really get to see clearly where it's coming in unless I was to dry and then relaunch it. That would be a good option for me, to dry it completely out and launch it to see if we can see where it was coming in. If I can't then I got to put it back on the trailer and bring it up. So I'm going to try a few different techniques first when it comes to leaks. There's always some prime suspects pretty much any time something goes through the hull. So there's the bung itself that's leaking then there's the actual fitting the threaded part of the bung that may not be sealed into the hull properly as well. There's the bolts that go through the motor and mount it to the hull and any other sort of outlet for a bilge pump or maybe where a transducer is screwed on.

Anytime there's something that goes through the hull I'd be checking their first. You might as well look at those prime candidates and rule them out before you start looking for something a little bit harder to find like a crack in the hull or a pinhole. Because this video is about finding a leak not so much about repairing it, it applies across the board whether it's a fiberglass boat an aluminum boat or a timber boat. What I'll do now is I'm going to fill the boat up to make it really clear that any drip I find is actually water that's come from inside the boat as opposed to maybe moisture that's wicking out from the anti foul because the anti foul has lifted in places and moisture does get trapped beneath it. So I'm going to put a liter of antifreeze coolant in the bilge just to dye the water. There's lots of things you can use, there's actually a few specialty sort of leak finding dies but we've got truckloads of this stuff here so that's what I'm going to use. The other thing about coolant like this is as well as being bright green is it's designed to go in aluminum radiators and it's actually designed to protect these metals so I think it's quite good for this purpose.

I can't say I'm particularly bilge proud but this is what it looks like so you can see there's always a couple of centimeters of water in here. Anyway one of my prime suspects is the bung here. Not leaking where the bung itself goes in but leaking where it's sealed around here. The last time I took this out the casing actually rotated a bit. I put a bit more Sikaflex on it but I don't have a huge amount of confidence in that. Having said that, there is a reasonable amount of water up to the bottom of it so we'll see. I'll let the hose run for a while I'm only going to fill it to the bottom of the bilge. I'm only going to fill it about three inches deep just so I keep the weight down. This is a pretty strong trailer but I'm not going to push my luck. So the bilge isn't filled up very far at all but it's now up over the bottom and then down here the bung is quite dry. There's no water coming out of here at all so obviously that Sikaflex I added on after it moved last time is doing its job under here. There is a drip and it's coming from up here that looks like some sort of old hatch so I'm going to strip this anti foul off and have a look at what's under there. This is what I was talking about for wear depending on how much prep you did in some places the paint will bubble and that'll trap the water underneath but that'll be clear water not green water.

This anti fowl has actually been on for a year now so it's time to redo it anyway. There's a few other patches where the anti foul has been peeling off but no other leaks. The next thing to do is just drain all that water back out now before I start looking at that hole. I don't want it dripping the whole time. We've got a disposal tub that I'll be putting it all in but if you're just going to be dumping on your lawn or whatever then make sure you use some sort of non-toxic dye! To be honest this is actually dripping so fast I probably could have used just plain water. I got my trusty wire brush and a paint scraper I've going to clean that section. Cleaning up this area I can see it's an old patch that has been welded on and from what I can see this edge here is cracked. What I'd really like to do is just weld along here again. I'm going to try and just run another bead of weld down here to seal it up.

Before I go and weld it up I want to talk a little bit about rivet patches. I did a video on how to do these riveted patches a while ago and I caught a fair bit of flack over it. I think people just sort of don't believe it works. I've got one on the green machine now and it's not leaking at all. It's on the chine where it had worn really thin and every attempt to weld it even with lower amp TIG welding just melted and made the hole bigger. In the end I thought do I weld on a patch or do I just rivet one on and put some Sikaflex on it. I riveted and it's never leaked since. I put a little bend in it it's about two or three mil aluminum, and then it's just got six rivets holding it on and then a whole bead of Sikaflex between it and the hole to seal around the hole. The patch overlaps by at least two or three centimeters and that's still just completely watertight, it just doesn't leak a drop anymore. In that original video I used blind rivets which means it gets used from one side. It's an open rivet the mandrel pulls through, ie you can do them from one side, a closed rivet probably is a better choice for doing these patches. I did all of these using an open rivet. These patches are a really good way to seal a larger hole you could try some brazing you can try welding, obviously the one we've got here that's leaking was welded. If you've got a really small pinhole you can't just weld it you know a little bit.

Whatever is good, if you don't have that equipment or that skill set all you really need to do if it's a pin hole is drill it out to the size of the rive, drop the rivet in some polyurethane sealer like Sikaflex, put it in the hole and close it up. That hole is not going to leak. In this case though the crack I have is not in a good place to put a patch on and it's not a pin hole so I can't just put a rivet in. I'm kind of going to have to weld it. When I sit at my bench with some nice little new bits of aluminum and practice I think "I can do this". When I hang upside down under a boat that's 40 years old I think "I can't do this to save my life"! It isn't pretty but I don't think it's going to leak. I managed to get reasonably good bonding between the hull and this patch that's now completely filled up so as I clean as it is I think it's going to be completely watertight. And ready now to re prime and re anti foul over the top when I do the whole hull again. Of course this may not be the only leak. You know finding a leak is great, but I didn't fill the boat up that far so there may be leaks higher up but I'm pretty confident that's one of the major leaks so I'll put the boat back in the water. I'll see how dry it stays and if it needs to go through the process again.

Now I do know that I focus a lot on aluminum boats, it's what I own obviously and it's what's predominately used in our area. These boats live in the water. They spend a lot of time sort of running up on beaches, against rocks, oysters and up against wharves - conditions where fiberglass boats just don't fare very well. Having said that though I do know a guy who does professional fiberglass boat repairs so I'm going to try and get him in on a video at some stage we can go through some fiberglass techniques as well. I think this leak is pretty much fixed now so I'll wrap this video up here. I think the technique of putting water In a boat is a pretty good one. Just bear in mind that water weighs a lot and you can damage the hull or the trailer if you put too much in. Alright, well take care and I'll catch you next week. See ya!