I added about 65 pounds of glass weight to the bottom of my boat when I built it. I used a mixture of woven roven and layers of 12 ounce cloth with polyester resin. I have bounced down many rivers in low water and have taken blows in the soleduc and calawah and upper Kalama, but haven't had to do any repairs to the boat in 8 years. I even ran into a jerks "boat trap" where angle irons had been driven into a rapid and that was my worse hit ever, and I really should patch the two sides up forward but even that didn't manage to puncture all the way thru the glass. I glassed the inside of my boat with epoxy and that has required periodic maintenance. If I did it again I'd just go all polyester with extra cloth. The cloth is where the strength comes from.