I have a friend who sailed round the world using only the rain water that fell on his coachroof - and his coachroof is quite a bit smaller than mine.
But why bother? I have 2 x 150 litre tanks. On a recent trip from the Azores to the UK that took eight days, I used just 25 litres of water, and that included using fresh water for cooking and to wash up. So I guess I could eke out my water for 300/25 x 8 = 96 days. However washing was minimal (I was sailing alone), and I did no laundry.
It would be nice to be able to take a shower. And to wash clothes in fresh water. And to rinse the salt out of clothes that become salty (otherwise they never properly dry).
I could do all those things using rain water. And to replenish my drinking water, I could pour the collected rain water through a filter - they make them so fine these days that they filter out even things as small as viruses. Rain water fed directly to the main tanks would be silly - I have no desire to drink water contaminated with seagull poop.
It seems a no-brainer. The alternative is to carry lots of water (300 kgs for full tanks), and be reliant on shore supplies (which often means using a marina, which I intend to avoid as much as possible). Or to install a water maker. That's expensive, to buy and to maintain. And it needs a lot of power, so it needs lots of solar panels or a generator. To rely on the sun or a generator, and a water maker for a supply of water seems less secure than relying on rain.
It seemed a good opportunity to fit the catching system - I've just removed all the car body filler that had been used to fair the boat, and refilled using epoxy filler (car body filler absorbs water - a pin-prick through the paint results in filler absorbing water and eventually causing blister and then more holes in the paint). After fairing in the coachroof, I used a hot glue gun to attach to 15mm plastic plumbing pipe to the bottom of the coachroof. I then used epoxy filler to fill the space under the pipe, and create a gutter. I used release mould on the pipe - but this wasn't necessary. Epoxy doesn't stick to the pipe anyway.
After pulling the pipe away, I glassed over the filler underneath and on top.
The red and blue colour in the photo is epoxy filler - I added red or blue builder chalk to each mix of filler, to make it easier to see what I was doing when it came to sanding t smooth. (Currently the boat looks very badly bruised!).
I left a gap where I want the water to run out when I am not collecting, and drilled a hole through the deck just uphill from the gap. In the deck hole, I epoxied a long plastic pipe insert (standard plumbing part, 40p!) which is the right size for push-fit plumbing fittings. The pipe comes out behind a bulkhead in the kitchen, so it will be easy to attach some plastic pipe to the inlet leading to a tap which I'll set at the height of my water containers.
Update: I'm anchored in Turks and Caicos, and water costs $2/gallon. There are no taps anywhere but in marinas, which are $50 a night. We're living on rainwater that arrives in occasional squalls. Crap from seagulls seems an English problem, or northern European anyway. I can't remember how far south we had to go before bird shit wasn't a problem. Anyway, south of Portugal and in all of the Caribbean, all we need to do is let the first water wash dust and salt off the roof, then collect the rest. It is a lot of work though, carrying the containers up to the deck to syphon them into the tanks. Despite my initial reservations, I think it would be much better to plumb the rain catching system directly to the tanks, with a diverter tap for discarding the first water.