Just watched your fuel harvesting tutorial, so added a request about tugs, curious how you got your fuel stations to Saturn.
The easy math for any ship is: speed = [50000 x power]/[total tonnage]. In words, an engine power of 10% of the displacement of a ship means a speed of 5000 km/s.
10% = 5000km/s,
5% = 2500km/s,
1% = 500km/s
etc.
There is one snag: You can easily factory-build space stations so large that your tug will not be able to overtake celestial objects, which have their own orbital speeds around the star. For example, Steve's tug in the default game has an engine power of 4500. If you try to tow a 5 million ton orbital habitat, you will achieve a speed of 44km/s, which is not enough to overtake and reach Mercury. Your tug will chase the planet forever. Other systems might have faster planets still.