I have the same situation except the blank is in the fitting. I think VDO Sumlogs have seen their day.

When I have her on the hard next I will be taking the whole fitting out and either fitting a skin fitting with sea cock as a suction valve for a deck wash pump or just blanking it off completely.

They are only a plastic fitting and getting pretty old now so I would be reluctant to go mucking around with it while floating. If you are going to play with it make sure you have a hammer and correct size wooden bungs handy. You could precipitate an early visit to the hard stand.

Way to go buddy.

A gusher in the cabin is not what one wants at any time. I have pulled out the blank on previous boats and had the transducer handy to insert but there is always a moment of doubt.

Probably two boat dollars Dr Rog But I'm doing it some time in the next few months . I just didn't like having holes entering my cabin and the sun and weather beating down on my instrument's Plus Ill make a face plate for the tack tick to make it removable off the instrument pod and could possible make a spot for it inside the cabin if needed.
Its all a case of which bit to replace first Dr Rog you know excaltly what I mean