I've never tried this, but I know people that swear by it.

If you're too tired to pull yourself up with upper body strength, use your legs (and hands and brains)...

Take one of the sheets, and tie a large loop in it (like 2 or more feet across). Tie this to the crossbar, and use it like a ladder step. Basically, you're making a foothold so you can use your (much) stronger leg muscles to get up onto the boat.

Hope this helps.

Mike