Transducer in hull fitting,rapeseed oil ok?

That's it.It specifies castor oil but I ai'nt got none

Provided it does not "go off". Castor oil seems to last forever but I'm not sure if cooking oils in general do.

Castor oil is available in small bottles .. not as cheap but at under £2 for a small bottle not budget busting.

Any fluid that is not hazardous; does not attack the hull, the transducer or the mountng; does not evaporate, freeze, smell or deteriorate over time will be satisfactory
 
Baby Oil seems to work ok and makes the locker smell more pleasant !

And you can use it to smarten up your mast and boom.

Castrol R engine oil contains castor oil (presumably that's where the name Castrol comes from) but I don't know in what proportion. Maybe you can scrounge some from a motorcycling mate.
 
I have successfully used water in an open topped tube epoxied to the hull( really ).

Hence the annual and unfamiliar nautical expression, 'I'm just going to water the transducer'
 
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