Not being an airplane mechanic and confining my skills to engines that remain on terra firma, or sometimes water, I am not overly familiar with how the propellor is mounted to the shaft. It appears that it fits onto a splined shaft. I would presume that it is then held in place with a nut of some type, similar to a crankshaft pulley on a car engine. If that is the case, would there possibly be a dissimilarity in the metals used which would allow the nut to corrode away? I dive alot and I know that in shallow waters the wave action can really move stuff about. My thought is that the nut holding the prop to the shaft may have corroded away, allowing the prop to come off when the entire assembly was rolled about by the surf. After separating, there isn't alot of surface area for the surf to act on with the prop, allowing it to drop to the ocean floor. The engine itself now has a lot less surface area compared to mass and would require heavier surf action in order to move it. I think this would help explain the current condition of the parts. Prop attached, the whole assembly is rocked back and forth and rolled about, dislodging cylinders, etc. Prop detached, the prop lays flat and the engine moves much less, if at all, allowing the two parts to remain close to each other. Separation took place a fair time ago judging by the coral build-up on the prop hub. I'm curious as to how far the engine assembly made it from the airframe. My $.02 for the day. LTM- John