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.
Images
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https://www.glen-l.com/designs/special/special-images/modelx1-notes.gif)

https://www.harbormodels.com/shaftstuffbox.html)

I think that you are referring to the stuffing box, one of many terms to describe the tube supporting the shaft where it transits the hull. There are numerous ways to seal the "stuffing box/shaft tube". I have used these three. Many of my 1/48th hulls require a shallow angle for the prop shaft, some are even even with the keel. I do not have leakage.
Perhaps the easiest method is to make the tube with bearings in it, they should all have bearings to reduce the drag on the shaft. Adding an oil or grease tuve to the "box" allows sealant(oil of grease) to be added when needed. it also provides lubrication, although so does the water.
A second method is to use a manufactured(or homemade if you are up for it) "box" that has an o-ring in it. The o-ring seals the shaft, without too much drag, while te water lubricates the o-ring. These work well also.
Third system I'll mention is one I use on my submarine.Cup seals, miniature versions of oil seals found on your car's crankshaft, work very well. They can be harder to find, but are/were available in fittings to silicon to the "box" or they can be fitted in a homemade arrangement. ONE Drawback, they need to be exposed to water all the time to keep them cool and not burn up the fine sealing lip.
I think choice 1 is your best and easiet option. A homemade tube is easy to make with a couple sizes of brass tubing soldered together. Add a grease tube to it and you should have very little or no leakage.
There are many ways to achieve what you need. Hopefully others will chime in a swell.

Owing to the shallow draft of the barge I would angle it slightly to give a more watertight drive.
If you can get a suitable length propshaft and tube you would be able to have the motor further forward, so only a very slight angle.
Not forgetting to add nylon washers at both ends of prop tube and adjust for minimum endfloat, normally about 5 thou. This will also help to stop water ingress.
Post some pictures and I'm sure that you will get more help.
Cheers Colin.