Thanks, I saw that one for sale.
The main reason that I want to re-use the transducer is that in my class of boat the existing one was glassed in, through the hull. Even if I dug it out, many new units are much bigger in diameter and this would not be particularly elegant in the existing position, which is right forward in the vee of the hull.
Smart ass answer is to check the tech specs in both the manuals to see if they both operate at the same frequency.
Unfortunately I cannot see the frequency mentioned in the ST 40 manual.
Practical answer is that they probably operate at the same frequency so try it.
Yes, thanks for that. The chief candidate is listed as duel frequency 50/200khz, my existing is a nominal 150KHz.
It also has a Co-ax cable/plug whereas the Raymarine has a screened cable and two wires with tags.
If there are no further definitive suggestions I shall have to contact Raymarine, tho I shall do so not expecting much.