Question re Venarive module

jwatson19's picture

On page 57 the module says: "The load capacity of a river-boat is roughly equal to the square of its length (in feet) multiplied by 50lbs. Thus a common river-boat of about 30ft in length can
carry around 45,000lbs (22.5 tons†)."
The Pilot's Almanac in Shipwrigtht 8 gives the following claculatiion for a 30 foot Talbar: 30*.36*.30/100 for 11.69 tons.
Which method is correct?

rbs's picture

The two formulas are not calculating the same thing.

The Venarive case gives a result in weight, since it explicitly states a multiplying factor in pounds.

The Almanac case, gives an answer in tuns (you have written tons), which is a unit of dry volume. If I understand the Almanac formula correctly, this "tun" is equivalent to "gross register tonnage" (GRT), which is the internal volume of a ship in cubic feet, divided by 100.

To compare the results, you'd have to consider what exactly is being transported. Venarive gives on page 69 that one tun of grain has a weight of 1920 pounds, or very close to one (short) ton.

However, if the cargo were liquid, the weight would be greater. For liquid water, one GRT of volume would be about 3.0 short tons.

But plainly one could not fill the entire cargo volume of the craft with liquid. Assume that it were in casks and that about 1/3 of the volume capacity ended up unused (there's also a factor of around 10% for converting units of dry volume to liquid volume) then a river boat with a volume capacity of 11.69 GRT would be able to carry about 11.69 * 2 ≅ 23 short tons of wine, beer, etc.

In the end, I would have to say that the statement in Venarive while not necessarily incorrect, needs further qualification. The statement in Almanac would be more correct but leaves it to the user to work out exactly how much cargo can be fit into that space.

jwatson19's picture

Opps I did mean tuns being a cask holding 550 gallons of liquid. I've done some number crunching on this and the actual volume of the cask is something like 73.79... cubic feet but casks being round the space it would occupy is close to 100 cubic feet and the load capacity of of a ship would be how many casks you could get in the hold.

Balesir's picture

Hi, A tun cask actually carries 200 gallons - if filled with water it weighs ~2,000lb or one (short) ton, plus the weight of the actual barrel. The figures in both Pilot's Almanac and Venarive look to be estimates; PA works on a rule of thumb that gross volume in feet divided by 100 gives the number of tuns that can be carried when draft = 60% of hull depth, Venarive gives the (even more approximate) length squared times a factor (50lbs). The Venarive weight looks OK for a vessel loaded to the gunnels (i.e. so that the river water is almost lapping over the sides) - maybe this is borderline acceptable for river transport? The Plimsoll line is not a recognised concept in Venarive, so boat owners might get away with considerable liberties, here, if they were lucky...

If we consider that 60% of the gross volume (i.e. length x beam x hull depth) of a ship is taken up by the hull itself, to allow for the curvature of the hull and the bow and stern narrowing, and we further stipulate that the loading takes up only 60% of the total possible displacement, then the actual hull volume of each 100 cubic feet of gross volume is 36 cubic feet. Water to fill 36 cubic feet would weigh 2,242 lbs - or about the weight of a tun cask full of water, allowing 242 lbs for the cask itself. On this basis, the Pilot's Almanac figures look pretty good, to me - but bear in mind that overloading is possible, since we have assumed only 60% of the total displacement is used.

jwatson19's picture

Actully we both got the capacity of a tun cask wrong. Just looked it up in my hard copy dictionary where 252 gallons (954 liters) is given as the usually capacity of a tun cask. Since one liter of water weighs one killogram 954 liters would weigh 2103 pounds which is close to a long ton. If you add in the weight of the cast you would be real close to a long ton. This could be a cask with a volume of 33.69 cubit feet. this would give a cask size of of about 3.78 feet in diameter by about 3 feet high.

Balesir's picture

252 (US) gallons is a US tun, yes - but since the first HârnWorld set and EH5 the Hârnic tun has been 200 gallons at 10 lbs of water per gallon (i.e. UK gallons). It's a bit of a mix, but it works OK! References: the prices article in EH5, first edition HârnWorld and the latest "Innkeeper" article from "the other side"...

Subdivisions are the "pipe" at 100 gallons and the "hogshead" at 50 gallons. FWIW.

At any rate, it still looks fair enough with the PA numbers ;-)