Re: No power in reserve
motor does not have a shear pin, it has splines on the propshaft. throttle cable adjustment may get you a little speed in reverse, but most are just for manovering the dock. reverse is not the brake, do not approach something faster than you are willing to hit it.
depending on your hull and motor set up, it could have just ventilated, and lost grip. it also could be a spun prop hub.
but if it ran good the rest of the day, i would say ventalation.