:The gunwalls are 8 feet.... My zip tyes for my forms are not holding well....
o you have the same probblem on longer

Wanted to address a couple of things that you may have figured out by now. 8' board is going to be stiffer and harder to bend than a longer board. Now that I have seen your photos and realize that this is a kids boats, I wonder if you couldn't make them thinner and still be strong enough. It would make the bend much easier. Zip tie are not strong enough because you are fighting a lot of force bending that board.

Secondly, watch the grain. The straighter the better and the less it 'runs' out to the edges the better. It will split along the grain.