Net Register Tonnage (NRT) & Why Size Matters...

I am attempting to get a 'feel' for how a single measure of a vessel might relate to a living, breathing ship in order that I can better-picture of the Jessie, Crest & Lazy Louis Kerr.

All I have to go on for each vessel is a single number, the rather cumbersome 'Net Register Tonnage' (NRT) which is really a figure for how much cargo-space a ship has.

My starting point are two ships that have been seen in Stornoway recently. My main reason for this is that I saw the Westward Ho last year and hence have a reasonably good appreciation of her proportions.
Please see: http://direcleit.blogspot.com/2010/03/westward-ho-at-stornoway.html
Unfortunately I missed the Reaper on her tour of the Western Isles.

Westward Ho 1894
34.7 NRT
Length 77ft
Beam 22.8 ft
Draught 10.6 ft
LxBxD/NRT = 77x22.8x10.6/34.7 = 536
L/B = 77/22.8 = 3.37

Reaper 1901
29.04 NRT
Length 70' 4”
Beam 20' 4”
Draught 8' 8”
LBD/NRT = 70.33x20.33x8.67/29.04 = 427
L/B = 3.46

I had been hoping for a simple relationship between the dimensions of each ship and her cargo space but this wasn't to be. I repeated this for a few earlier ships but the results merely proved the case.

Undaunted, and bolstered somewhat by noting that the Length to Beam ratios of most vessels are in the range of 3.3-3.4:1, I decided to create a couple of hypothetical ships:

Hypothetical
30 NRT
L 60
B 18
D 9
LBD = 60x18x9 = 9720
L/B = 3.33

Hypothetical One and a Sixth
47.6 NRT
L 70
B 21
D 10.5
LBD = 70x21x10.5 = 15435
L/B = 3.33
15435/9720 = 1.59

As you can see, I spent a considerable period of time and creative energy in naming these vessels!

The point is that if we take the good ship 'Hypothetical' and expand her by merely one-sixth in all three dimensions we create a similar sister-ship with a massively increased volume.

In fact, we move from the capacity of the 'Jessie' to that of the 'Crest' and Lady Louisa Kerr'.

Although these calculations are wholly imaginary and utilise a plethora of simplifying assumptions, they demonstrate that all three ships were very likely to have been of a similar order of magnitude as Westward Ho and Reaper.

NB The manner in which NRT is calculated varies across space and time and I have good reason to believe that the 1894 Westward Ho's 34.7NRT is actually much closer to the 1863 Crest's 47NRT than those two figures appear to suggest.