Ramona said..
The trouble with most antifouls is they can vary from batch to batch and then there is the environmental conditions. I'm a fan of Altex No5 but this last lot lasted only 3 or 4 months before I had shell. Jutson alloy 50 here has Altex sea barrier 3000 and is only 60 yards away seems to be much more effective.
I have always used Altex No 5. I have always applied two coats but last year I did a bit of reading and found that Altex recommend three coats when applied with a roller.
Now that wouldn't work with my Top Hat so in April 2015 I applied four coats to use up two tins.
The results have been interesting. I have only a few Barnacles [10mm x 20mm] on the leading edge of the bow.
I applied about 6 coats to the prop and it still has about 33% of its area FREE of barnacles, consider that its done some 1000 plus miles during my trip north and back.
So, I think most of us do not apply enough anti-fouling for it to be effective!