Some of the larger weed eaters have a centrifical clutch built int he
motor, so starting as well as stopping would be much better.

at least you wouldn't have to have it full throttle slapping ans
splashing the water when you drop the prop in. and when you let off the
throttle, the prop would free spin and you could still idle.

BTW, most of the weed eaters you CAN reverse the tube on the engine. I
like the idea of straightening it out though. and having a direct line
of travel for the prop.