Thursday, June 14, 2018

Rope Reckoning

In comments to the last post, G.B. Veras pointed to a nice web page on the economics of medieval rigging (MIT History Dept.), and asked how much rope costs in my games. The short answer to that is that by default, I just use the stock Original D&D equipment list, read as silver pieces, so: 1 sp for 50'. For things either not on the list or clearly broken, then I look to outside source like the Medieval Price List (or whatever) and convert with the interpretation that 1 sp = 1 groat (that is: 1 shilling = 3 sp). This usually comes out closer to the D&D equipment list than you'd normally expect.

Let's use G.B. Veras' cited MIT page as an exercise.
  • The page in question says that over three years or, 4,327.3 kg (9540 lbs) of rope were sold out of Bridgeport at a cost of 134 pounds sterling (including 20% transport cost).
  • We must convert that weight into a standard length, which is predicated on the standard width of our rope. This site on climbing says that something a little under 10mm (0.39 inches) is common. Then we can find that Home Depot sells a brand of manila rope that is 3/8" (0.38 inches wide), 1200 ft., at a weight of 45 pounds. (Side note: This is rated for a load limit up to 243 pounds.) 
  • 1200 ft/45 pounds is proportional to about 50 feet/2 pounds. Dividing this into the Bridgeport figure, we get 9540/2 = 4770 lengths of 50' rope. Dividing that into the price figure we get 134/4770 = 0.028 pound sterling. Then 0.028 × 20 = 0.56 shillings. Then 0.56 × 3 = 1.69 silver pieces. And if we back out the stated shipping cost, then we get 1.69 × 0.8 = 1.35 sp. So once again I'd say that's "close enough" to the book D&D prices, if we read them in terms of silver pieces/groat coinage (namely: 1 in this example).
  • A side issue is that the weight of the example 50' rope is only 2 pounds, which is less than half the weight currently specified in the OED rules supplement (specifically: 1/3 stone, that is, 14 × 1/3 = 4.6 pounds; whereas the AD&D DMG specifies 75 coins = 7.5 pounds). Presumably that's okay in OED if we assume the number is inflated for overall bulk/awkwardness in carrying it.
Thanks to G.B. Veras for the data and the interesting exercise!

6 comments:

  1. Nice analysis!

    Note that a modern 10m rope is nylon, extremely strong compared to natural fiber, and could support a metric ton at least. We would call them magical in D&D!

    Your cited rope with a load limit of 243 pounds would not be very useful except for tying loads. Just bouncing on a rope, as a rule of thumb, give a peak weight of 2-3 times the static weight of a person. So to support climbing, it would need a load limit of at least 500 pounds I would think.

    So, the weight of a practical rope is going to be a lot more than the 2# you use here and so closer to the encumbrance charts.

    So I guess the encumbrance charts are pretty close even if the cost is not high enough.

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    1. Cool, good point! Although I actually have a rule that if a rope takes a sharp jolt like that there's a 1-in-20 chance it breaks (my players hate that rule).

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    2. At least now we can show that the rule has a basis in the reality of the properties bog standard medieval rope. They need some of that magic/nylon/elven rope mentioned elsewhere.
      Actually, quality rope might be a good "mundane" treasure.

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  2. I'm flattered. Thanks, Delta!

    I agreed with Matthew above. Remaking my calculations, I think I used a rope of at least 1 inch in my price table which makes the price five times higher and the load limit of around a ton.

    My final price is 50 coins for 100 feet or roughly 12sp in your price list.

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  3. Good stuff. Now complete the trilogy and post about the grappling hook. IIRC, not really used when climbing mountains as shown in films, but useful for climbing trees and walls or boarding ships.

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