Sandy,
This appears to be a case of stress corrosion caused by an impact to the blade on the trailing edge. You can see where the damage started at the very edge (smooth and mechanically deformed) then granular propagation for the next inch or so then the stress fracture in the remaining root area. These are distinctive in the picture of your blade root area below. I suspect this blade was impacted then underwent a case of stress corrosion cracking and eventual failure. If you look at the root edge of the forward propeller blade, something also impacted that root area; it is mechanically deformed. Seems these blades do not have a lot of cross sectional area at the root; consequently, if I had them on my boat they would be pulled every time the boat was out of the water and looked at by someone qualified. I would also say that in looking at the other metallic structure in the picture that there may be some chloride stress corrosion cracking in other areas of the propeller; this is usually indicative of spider web looking cracks.
This appears to be a case of stress corrosion caused by an impact to the blade on the trailing edge. You can see where the damage started at the very edge (smooth and mechanically deformed) then granular propagation for the next inch or so then the stress fracture in the remaining root area. These are distinctive in the picture of your blade root area below. I suspect this blade was impacted then underwent a case of stress corrosion cracking and eventual failure. If you look at the root edge of the forward propeller blade, something also impacted that root area; it is mechanically deformed. Seems these blades do not have a lot of cross sectional area at the root; consequently, if I had them on my boat they would be pulled every time the boat was out of the water and looked at by someone qualified. I would also say that in looking at the other metallic structure in the picture that there may be some chloride stress corrosion cracking in other areas of the propeller; this is usually indicative of spider web looking cracks.
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