I don’t think the 30 Classic has the flap valve that pops up to stem the flow as Bob suggests – water shoots upwards with quite a spurt but there is little pressure and you can stop it by just putting your hand over the tube. The secret, as Allister says, is to be quick with the changeover by having the blank ready in your other hand. You may get a few pints of water in before you can put the blank in and I used to minimise the clean up by keeping some wine bottle corks loose in the bilge. I would push these corks into the limber holes in the box section floor supports beforehand to make sure that all the water was kept in one section of the bilge so that it was easy to sponge out afterwards. Don’t forget to remove the corks when you’ve finished – the holes are there to let the water flow to the bilge pump when you have a real emergency (not that I ever had cause to use the bilge pump!)
Alan ex LARUS