those calcs are for whatever you want to put in, but yes, your cat will see thousands of lb load on the boards, every bit of it! What sort of breaking strength do you have for your mainsheet blocks? Is it in the hundreds or the thousands?


Here's a simple diagram of the forces involved on your main (and I don't want to get into the Bernoulli argument, thank you very much, it's obvious that some of us aren't nearly at that level of understanding....):




Now's a simple diagram of the forces on your board:



What do you think? It's really all about Fnet=ma (Thank you Mr Tilley for teaching me that.)

Look, here's a simplified explanation website:
http://www.physclips.unsw.edu.au/jw/sailing.html

Here's a bit about how blocks work:
http://www.phys.unsw.edu.au/~jw/blocks.html

And a bit of light reading:
http://www.sailboat-technology.com/links/online_articles.php