Very insane stuff... There is a video of those guys and those very special boats (self righting, high powered etc) but even so I wouldn't like to try it !!! And they have lost a crewman or two !!! The video is worth watching..
Cheers
Geoff
Hi
One product I found to work was CLR clear (try your supermarket). As soon as it has removed them, Rinse well with water...worked great for me, and cheap...
Geoff