This is a pylon bolt:
It's what holds the trampoline frame on to the hulls via the pylons (the aluminum post that sticks up out of the hulls). They're pretty big - 1/2" diam by about 5" long.
The rivets used in constructing Hobie Cats are "blind" or "pop" rivets. They are set cold.
This link will show you what they look like and how they are set: http://school.mech.uwa.edu.au/~nscott/How_to_do_stuff/hand_tools/
The ones used on the corner castings are 3/16" diameter, 1/2" long, stainless steel or Monel. You can buy them one at a time from a Hobie Dealer or order them online (larger quantities) from any number of places. They are expensive in small quantities.
The proper type are not
sold at home improvement stores (Lowe's, Home Depot). DO NOT
be tempted to use aluminum rivets. They aren't strong enough.
Rivet tools - 3/16" stainless pop rivets are hard to pull/pop. While the rivet tools sold at Home D and Lowe's will work, your hands will wish you had something better.
This is similar to what I bought nearly 30 years ago:
It will pull 1/4" stainless rivets all day long and you won't even raise a sweat. It was expensive and worth it.
Another alternative tool:
This one is sold by Annapolis Performance Sailing and costs about $220.
If all you ever plan to use the tool for is the 6 rivets in the corner casting, then buy the heaviest duty tool you can at a local hardware store, pull the rivets (wear gloves to minimize the pain) and throw it away when you're done (the stainless rivets will ruin it).