Aluminium boats with floors

[Old Scaley]

Just thought I would pass on some advice re maintenance of tinnies with floors.
For a while now I have been getting water into the boat. I was blaming the screw in plastic bungs which were weeping a tiny bit despite me changing the housing, trying new bungs and applying plumbers tape (definitely coming from the thread).

Last time I had the boat on the water when @ellicat and I were at the Pin, it took on a lot more water than you could expect from leaky bungs. Next day I filled it up with more water than I usually used for testing the bungs and found a pinhole in one of the strakes. 
So I bit the bullet and pulled up the floor for the first time in 17 and a half years to see if there were any other holes before taking it for repairs. I have started once before to pull up the floor but the first screw was so tight I decided not to bother. This time I persevered with what seemed like a hundred screws. Actually they all came out surprisingly easy except the one I had tried years ago. 
The flotation underfloor consists of a mix of big and small pieces of polystyrene. There are a sh:tload of them so I am not looking forward to playing Tetris to get them all back in. Didn’t find any metal things like swivels or hooks but lots of mud from years of crabbing. Took to it with the pressure washer to give it a good clean. 
Now it was clean, so I went out at night and ran a powerful work light over it and found a second pinhole. 
Long story short, I took it to Aquamaster Boats at Hillcrest. Picked it up this morning and the guys had found more holes (6 in total, I think), all welded up and tested with floor level water. The advice from the owner was to pull the floor up a couple of times a year and give it a good clean. Apparently the salt will react with the ally if left there long enough. Hopefully I am good to go for a few years yet but thought I would pass on this advice for anyone else with a tinny with a floor. 
Bug shout out to Tom and the crew at Aquamaster. Very friendly and professional and prices are reasonable. 

[mangajack]

Every 4th run in the slat I leave the bungs in and give it a decent flush internally with Salty Captain and about 500 litres of fresh water......take it for a tow around the block to slosh the water about then drain and dry. Pulled the floors up after 2 years and zero signs of corrosion inside the hull except for there the screws that hold the floor down are.

Every washdown is with salty captain or salt away or similar. I wash after every salt run.

[Huxstang]

Thats good advice MJ.    I have a sealed ally floor and 4mm plate but I think I will once a year put her nose down and give an internal flush.  I have had water in the bilge from the transom inspection hatch seal being unseated, only found out when I was fishing around the bung and found a bit of rubber (turned out to be the O ring).

BCF have a sale on 5l of engine flush at the moment as well….$87 instead of $120. (Not Salty Captain but another brand).

[MattInOz]

When I bought mine I had something similar... always taking on water but never distressing amounts. Like you I assumed it was the bungs but to be certain I took the floor out and put it in the river so I could see where it was leaking. I was surprised to find it was coming in from the starboard strake about half way up the length. Took it to my local aluminium welder and he found an 8" long crack in the weld at the strake, which opened up to about 15" as soon as he put the TIG on it. You never know what 20 years of stresses will do to a hull. Had it welded up along with a few other minor repairs (think the previous owner had a drill fetish, I had over a dozen old mounting holes welded up and there's still a few left!) and it's been a solid boat ever since. It's still 20+ years old though so I'm fairly paranoid and frequently check the bilge (I put a hatch in the floor above the bilge pump so I could do so any time). Like a couple of the other guys here have said I flush the interior every 5-10 outings too... that not only cleans the interior, it also flushes and lets me test the bilge pump. I'm still considering putting in a secondary bilge pump as a backup... can't be too careful.