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A beat up prop will DEFINATELY slip, and "Ventilate" (blow a hole of air around itself), prop's are funny, sometimes the can have a lot of damage and still run pretty well, sometimes a slight nick can cause real problem's grabbing the water, best to replace it, the prop on a boat act's like the tires on a car, worn down tires (even a high performance car) will make it perform poorly.
mw