leak below the waterline ....

[Cloona]

my marine ply Debutante is has a small leak (about two inches of water per tide) below the waterline ...

i wonder if anyone has any tips on the best way of finding where the water is coming from .....

and any tips on fixing it ...

 

[cliffordpope]

Having just put my wooden boat into the water for the first time for 24 years I know about hanging head down in the bilges looking for leaks!

The first stage is to pump it all out and then watch carefully to see which section of the boat seems to be filling first. You may need to isolate sections temporarily with rags, mopping quickly to try and determine flow origin.
When you have narrowed it down, hopefully to a specific location rather than an entire seam, it is usually pretty easy to confirm, because you can mop the trickle and it will instantly reappear.

I'm assuming that a plywood boat is unlikely to take up over time, unlike traditional construction. So your options are either to pull the boat out and work on a repair or filling from the outside, now you know the precise location, or else to attempt some kind of temporary bodge while afloat.
If you can reach the spot, either from the dinghy or in the water, there are fillers that you can splodge over even under water. Stockholm Tar mixed with lime or polyfilla to the consistency of soft soap works quite well, and also tells you immediately whether you have hit the spot.
People say it is possible sometimes to do this with a brush on the end of a stick, but I'm a bit doubtfull there.

It may even be possible to stop or stem the flow from the inside, depending on what sort of leak it is. If coming up a crack or joint for example you could try wedging in a bit of rag, perhaps soaked in grease or tar or paint, using a screwdriver blade.

 

[Tranona]

Assuming it is not coming in through leaky fittings, then the favourite would be at the seams between the bottom panels and the keel/hog or chines. Take up all the floorboards and bunk tops so that you can see the seams and watch for where the water seeps in.

As to repairs it depends on how these seams were made. Sometimes they are glued in which case the glue line may have failed. Or they may be screwed down and bedded in a flexible sealer. In both cases the failure may be accompanied by local rot/delamination and or screws corroding or pulling out. Best to get it fixed quickly because it is likely to deteriorate at an alarmimg rate.

 

[Peterduck]

With plywood construction you have the great benefit of only having a few seams to consider when looking for a leak. The keel-bottom seam is the likeliest villain, as it is usually under the greatest stress, followed by the chine seams. You could try allowing the water to build up a little, and when the boast dries out at low tide, get under and see where the water is dripping from. Where it goes in, it will also come out. This would not work for a chine seam, as the level of water needed inside the boat to show a leak will cause too much stress on the hull, making things worse.
Peter.

 

[Spuddy]

Others have pointed out the likeliest spots. If observation doesn't find the leak then scattering talc powder about the bilge might show up the track of any trickles.

 

[oldharry]

Done this several tim4es with elderly plywood boats: get all the floorboards out, locker tops etc so that as much of the hull is exposed. Now comes the hard work, dry the bilges right out, fisnhing off with a sponge to remove all traces of water.

Now sit and watch the lowest point in the bilges carefully. Within 5 minutes the water will be creeping back. Follow it back, using the sponge to remove it so as to be sure it has newly entred the boat.

The snag is that m/ply boats have lots of corners where water will stand and slowly seep down. so you have to be sure it is actually coming in and not just running down from somewhere.

The other point is that if the leak is on a seam you will never seal it properly from inside - you have to repair it from outside, otherwise within days you will finbd your bilges filling again!