Many thanks for the replies.
I have already come to the conclusion that I will look into getting a new one made in Aluminium – Trevor my cheque for the drawing is already on it’s way.
The only problem still is which grade to use. If I have this right the original made from Hiduminium? had an ultimate tensile strength of 61k to 72.5k psi. 5086 or 6061 marine grades are around 45k psi. How much of the original specification was down to the material being available and was it over engineered? I really don’t want to find out the hard way.
7075 (used in aircraft industry) has the strength but, from what I can find out, is not so happy in a marine environment. Though as Trevor says, the key is to prevent salt water reaching the aluminium in the first place and the original has managed 50 years with inferior sealants to those available now.
I will go and see what the engineering firm, I used to have the prop shaft reworked, in the New Year have to say – though Crewe engineering firms are more used to working with bits for trains than boats.