Summer - and worse again if you keep the boat in a marina with little water flow where the water temp tends to rise more markedly.
While the off season is the trad time to antifoul, on my boats (which usually stayed in the water 12 months/year I looked for a lift out/in time in May/June/July when deals could be done on cranes, weather was generally more pleasant for task and the boat would get greater benefit from the clean bottom and fresh paint. Also handy to be able to check out state of anodes after a winter on shorepower.
Agree with Kim. My boat also in the water all year, and I usually antifoul between tides, so wait untill the weather is reasonably pleasant .... usually around May. Boat is then in prime condition for the important part of the season.
Mine doesn't. It swings round in the tide and over 14 days gets equal exposure on both sides. Maybe you're in a river or on a fore-aft (is that a swinging mooring /forums/images/graemlins/smile.gif )
Mine was the same on a half tide mooring in Chi: one side was worse than the other. Worked it out eventually. It always grounded on an ebb tide facing aproximately NE, so that the starboard side was exposed to the sunlight each day, and was drying out more thoroughly in the sun so was staying cleaner.
Also the barnacles spawning season is approximately May to September, and most barnacle fouling only occurs at those times of year - i.e. the sailing season!
Yes both warm water and sunshine make it grow. However i have heard it claimed that a shallow clean white sand bottom will reflect so much light that it makes fouling worse also. perhaps not a problem in the land of the big tides though. regards ole will. PS welcome Kim to the boat owner mad people and I thought you were just a computer nutter. And while talking to Kim thanks for a great forum. It gives me a lot of pleasure in retirement. will