...so we are a few months further. I moved and don´t live near the sea anymore, so in future in-land lake trips, the boat is likely to suffer less wave tension.
To test leaking in the boat I use the method descibed elsewhere in the fora:viewtopic.php?f=71&t=46579&hilit=leak+test+air+pump&start=0
Now the interesting observation: when you pressurize the hull like that, the crack opens up a millimeter or so! I imagine this basically mimics pressure fluctuations when the boats gets hit by waves or any mechanical pressure on the hull. It´s like large weak pressure on the hull gets levered into very strong, localized forces near the crack. this might be what caused the crack in the first place, and also makes it very difficult to repair. My last glueing attempt with cyanoacrylate (=super) glue looked very solid, but as soon as I go on the water the crack returned and I started collecting water again.
So this time, after further internet study I decided to plastic weld the crack, even though here in Brazil I cannot get the proper gear. I learned that basically any polyethylene resin (e.g. from certain plastic bottles) will do as a welding material. Then all you need is a way to heat polyethylene above its metling point (120 0C) but below it burning temperature. For that purpose I will modify my normal (metal) welding rod with some wire on the tip, so it will become hot but not as hot as its normal working temperature.
Then I will try to weld the crack while my little air pump is blowing air in the hull, so the crack is in the "open" position while welding. This way i hope to reduce the effect of the crack being opened later on the water by external pressure.
Anybody feels like giving me a heads up before i will go ahead? Again, sending the hull back and forth across the globe doesn´t seem to be an option...