Trolling motor help

I have an old Mariner trolling motor that is working fine but I don't know if I'm supposed to lube it at all. There is an allen screw in the bullet just behind the prop and when I unscrew it, there isn't anything in there that I can see. Is that for putting grease into or is that not necessary on an electric motor???

I'm on a tight budget so I need to make this work as long as I can.

Preventive mantainence.

Any answers would be greatly appreciated.

                           Thanks.