I am not sure about how trust loads affect on crack shafts, but I do know that in many cases were the crank shaft has failed it was because of a bad shaft itself. The way airboats use the prop to push and the the boat turns at the rear does put alot of stress right on the crank and many times engines are put together with used parts and nothing gets checked properly. Just like a buddy of mine, who built his first boat, flipped it broke the prop , then fixed his metal works bought a new prop, then lost a rudder while running hard in the water he lost control and ended up flipping his boat again prop hit the water and broke 2 blades. He tore the motor completely down rebuilt it fixed his stuff again then got it going and it did not run for 2hrs and the crank broke, he currently has given up the idea of airboating, what a pity, he never checked or changed his crank after all of the impacts.