Breathers / Drain holes

The hatch covers currently available, with rubber gaskets and the cover concealing all the fastenings, are airtight if well installed and maintained. The ones we used to get in the 70s, on the other hand, leaked like sieves.

From memory my old moth (magnum 9.9 I think) had a piece of thin rubber membrane glued over a reasonably large breather hole, so it would move in and out like a diaphragm but stay waterproof.

I may have this completely wrong, but it's what I assumed it was for!

Originally posted by iGRF Anyone ever considered/experimented with Goretex breather valves?

and what if any advantage  would they offer over  small holes  or a  u-bend  on a larger  breather?

Boats can very easily leak without a breather.  Cold water and a warm day when you seal the hatches creates a partial vaccum as already described.  Additonally as you move abround the deck it flexes very very slightly and guess what you can pump air out and .. water in.  This can happen anyway, which is why sometimes it is very difficuly to find some leaks when off the water but you are loading the dice if the sealed hull is already likely to be at -ve pressure.  

So always have a breather .... btw some of the most common causes of leaking are 'O' ring seals around hatches and drain bungs.  Consider replacing the 'O' rings just occasionally ! 

Johnjack,