You are referring to the stuffing box. (As you said, the screw tube) It is generally a brass tube which extends into the hull at various angles depending upon the model.
You will note the stuffing box has an oil tube, this is an important part as it allows you to put grease inside the stuffing box without having to remove it from the vessel. The grease not only lubricates the shaft but also keeps water from coming into your vessel.
Many members on this website have had discussions about the type of grease, lubricant to use, I use a blue marine grease, bought at the hobby store, the tube lasts many years.
If you could post an image of the barge you are building it will help us provide the readers with information from which we could better provide suggestions. Remember, this website is used by people from various continents, so when you say “barge” it will conjure up various images depending upon where they live.
Hope this has been helpful
Ron, from Canada