Kees,

My idea is to use thermals to climb (much)faster like gliders do.
Is this a stupid idea?
If my idea is not that stupid how do I find thermals and how to use
them properly?


It's a great idea to have that in your bag of tricks. OTOH, consider
first that on a day with 300-feet-thermals, you might also end up in a
downdraft that you can't outclimb, so your safest option might be to
stay on the ground. Also, I take it you are leaning properly AT FULL
POWER in those high density altitude situations to get the most out of
your engine?

That said, just wait until you feel (and possibly see on the VSI) a
strong updraft, then start tight turns, preferably to the side were the
updraft lifted one of your wings (that's were it will be stronger). The
goal here is to circle the center of the updraft. Anything below 25 to
30 degrees bank will probably result in circles that are too wide. You
should be at Vy, but consider stall speed increases with bank angle.

Stay safe!

--
Thomas Borchert (EDDH)