I'm trying to find a plastic cap to go over my 4" exhaust tail pipes to stop water going up them when I launch my Donzi 38ZRC from a hydrohoist. Can anyone help??

Do you have Home Depot or Lowes over there? You can get a plastic cap that inserts into the exhaust tip, and tightens with a wingnut to compress a rubber 'washer' that will hold the caps in for as long as you want or until you start the engine. I have been looking online for a pic, but with no luck.

They look like this:

http://216.110.169.94/cherne_plastic...pipe_plugs.htm

Or get a set of salisbury (sp) flappers and glue them shut. slid them one, no need for clamps if your just using them for launching, until the first time you fire one with it on that is, LOL