Hi there guys,
I’m a first year maritime engineering student and for this weekend I got an assignment with me.
Next week our group has to build a small scale replica situation to test out what happens when a ship encounters a ghost fishing line, and they decided I should do the math for scaling it down 
We’re planning to build a 1:100 model of the ship and fishing gear, and for our studies it of course is most important to see what happens to the ship. (rather than the fishing lines)
Data we got, to use for the ship:
size LxWxD (m): 120×20x8
Power: 7200hp
rpm: 200
tonnage: 8000GMT
trial speed: 17kt
The size of the ship is easily scalable, though of course it will not be possible to put a 72 hp engine on this tiny wooden boat, nor will it be able to weigh 80 tons.
Of course I could scale both these factors, though in that case I also should scale down the fishing net’s strength proportionally, right?
In short; how on earth do I scale down this situation as accurately as possible??
Cheers and thanks a lot!
If you scale the lengths by a ratio of 1:100, then the mass will scale by the cube of that, or a ratio of 1:100000.
Since boats displace a volume of water as they push through it, I would expect the power requirement to reduce as the cube as well (ratio: 1:1000000).
If you scale down the line’s dimensions by 1:100, then the cross-sectional area will scale down by the square (1:10000). I think that means the strength of the line scales by 1:10000. That’s probably appropriate since the strength of the ship probably also depends upon the cross-sectional area of its members. Plus, the force that the line will apply times the distance the ship will travel (the former goes as the square, the latter goes directly), when multiplied, yield the work necessary to snap the line, and thus is scaled by 1:100000, in line with the reduction in power driving the ship. So it seems to all work out.