Designer's Notes: GURPS Low-Tech

Designer's Notes: GURPS Low-Tech

A Tour of the Workshop

by William H. Stoddard

GURPS Low-Tech, like a medieval cathedral, was the work of many hands. It received one of the most extensive playtests of any GURPS title; many sections were virtually completely rewritten as a result of playtest discussion. A number of playtesters contributed valuable additional material, notably the character sketches by Jason Morningstar and David Morgan-Mar. And like a cathedral, its building involved a great deal of scaffolding that was taken down when it was done, leaving no visible trace in the finished product.

Like the other Tech volumes, Low-Tech was intended to stand on its own, so that anyone who owned the GURPS Basic Set and Compendium I could use it. The published volume makes almost no reference to GURPS Vehicles. But the vehicles and artillery weapons in Low-Tech needed to be compatible with GURPS Vehicles -- and that meant building all them them using the Vehicles rules, to make sure it could be done. These designs are the largest body of "hidden text" in Low-Tech. So here is a look at how they were created.

Boats and Ships

From the beginning of civilization up to at least 1800 A.D., large ships were the most sophisticated products of human technology. GURPS Low-Tech presents statistics for over 30 such ships, as well as a variety of boats and rafts. Most of the design process was a straightforward application of GURPS Vehicles, but a few additional rules were needed to cover options not considered there.

A ship at TL1-3, or a boat at TL0-3, consists of one or more propulsion systems (paddles, oars, poles, or sails); space for the crew to man them and for the officers and passengers; space for cargo; and a hull, which in terms of GURPS Vehicles includes a frame, a modest amount of armor (usually wooden), and waterproofing. Warships also have weapons systems, such as rams, catapults, or Greek fire siphons. The volume of the hull has to be sufficient to hold everything else.

So far as propulsion is concerned, GURPS Vehicles provides statistics for oars and sails, but not for paddles or poles. Paddling uses the strength of only the upper body; based on this, a paddling position can be estimated to take up the same volume as a rowing position, 10 cf, but to have half the weight, cost, and thrust: 5 lbs., $10, and 0.25 times the ST of the paddler, in pounds of thrust. Poling is normally done while standing on the deck, and thus requires deck room (9 sf per man) but no space within the hull of the craft. For convenience, the weight and cost of the pole can be included as part of the crewman's personal gear and disregarded. Its effective thrust can be taken as 0.5 times the ST of the poler, the same as for rowing, and more efficient than paddling; but note than poles can only be used in shallow water.

Sails are extensively discussed in GURPS Vehicles, which divides them into three types: square-rigged, fore-and-aft, and full-rigged. There were actually many other types of sails, but their differences in performance are often fairly subtle. GURPS Vehicles classifies the lateen-rigged sails of Arabian ships as a variant on fore-and-aft rigging; the exotic sail designs of Polynesian seagoing canoes can also be placed in this category, based on their ability to beat upwind.

Crew requirements depend mainly on the system of propulsion and the number of watches. Paddled, poled, and oared craft need very large crews; however, up through TL2 they almost always spent their nights ashore, so they needed no accommodations and no space for a second watch. Sailing ships needed far smaller crews to man their sails (see p. VE75). However, sailing ships often spent nights at sea and carried two watches, doubling the crew size. In either case, a few more crew could be added as officers: a lookout, a coxswain, a steersman, and a captain, for example. Late TL3 and early TL4 voyages of exploration carried even larger crews, often by a further doubling, to allow for the risks of long voyages. Sailing ships, starting at late TL2, had galleys and a few cabins, but the crew typically still slept on deck, or in rowed ships such as Viking longship on their rowing benches.

The number of oarsmen can be estimated from the length of the ship, or vice versa. The dimensions of the human body require roughly 3 feet of length between one oarsman and the next. In addition, part of the ship's length fore and aft cannot accommodate oarsmen; the length of a ship's side needs to be at least 5 feet per oarsman seated along it, and slightly higher in some craft such as canoes. It's possible to fit in more oarsmen, either by stacking them above and below, in two or three levels, or by seating them side by side, or sometimes both. The Chinese yichuan must have had some such arrangement, for example; its 79 feet could have held no more than 16 oarsmen per side, or 32 total, but its complement was 48, so they were probably seated two to a bench on each side for a total of 12 benches on each side.

Estimating cargo space was even more complex. GURPS Vehicles allows 20 lbs. of cargo per cf for generic cargo. For specialized bulk carriers, an estimate of 50 lbs. per cf is reasonable for such cargos as grain, oil, or wine (allowing for space between jars of oil or wine). For ships with a stated burden, the weight can be divided by one of these numbers to obtain a cargo volume. If the burden of the ship is unknown, the cargo volume can be estimated by multiplying the length, beam, and draft of the ship and dividing by 4. (If the draft is unknown, set it at half the beam; if the beam is unknown, base it on the ship's lines -- 1/3 to 1/5 the length for mediocre lines, 1/5 to 1/7 for average lines, 1/8 for fine lines. Most cargo ships had mediocre lines.) If the ship carries open cargo, rather than having a hold, only half the total volume of cargo is considered to be within the hull; that is, double the effective cargo space.

Adding up the space for rowing or paddling positions, the crew and passenger space, and the cargo space gives the volume inside the ship's hull. After adjustment for the ship's lines (see p. VE16), this gives the surface area of the ship (see p. VE18). And at this point things get complicated.

GURPS Vehicles distinguishes a vehicle's structure and armor -- though both have weight and cost proportional to surface area -- and specifies that to be sealed, a vehicle must have armor (which may not be open frame armor) over its entire surface. This seems to imply construction like that of Age of Sail ships, where structure represents the internal frame of the ship and armor represents its hull, or outer skin. But ancient shipwrights seldom followed this model. Typical ships were built hull first, with hull members fastened directly to each other rather than to a frame; partial frames were added afterward for extra strength. Small boats might have no frame at all; dugout canoes were simply carved from logs and all one piece.

For most of these cases, the GURPS Vehicles definitions can be forced to fit by assuming that "structure" includes the fasteners that hold the hull together. A craft with only cord fasteners, such as an Arabian sewn boat, has a super-light frame; a craft with clamps, nails, dowels, or the like has an extra-light frame. Adding internal bracing such as ribs increases frame strength from super-light to extra-light or from extra light to light. Most frames are considered to be of standard materials; boats sewn with coarse rope have cheap materials, while the elaborate joinery of triremes counts as expensive materials. Wooden boats usually have standard wood armor, though tropical hardwoods count as expensive wood armor. Bark and leather and basketry are all nonrigid armor.

Dugout canoes are a different case. They actually have no frames at all. Instead, the same weight of material is counted both as structure and as armor. First determine the base weight for 1 sf of structure of the appropriate cost class and TL. Then determine the actual hull weight per sf from DR and type of material, and divide this by the base weight to get a percentage. Multiply the surface area of the craft by 1.5 and then by this percentage to get the HP. This double benefit should only be allowed when the entire craft is carved or shaped from one piece of rigid material. A similar process could be used for the clay tub; however, to reflect the brittleness of its material (in effect, low-tech ablative armor), it is considered as having 0 HP -- any damage that gets through its DR causes hull failure.


A different approach is needed to design rafts. GURPS Vehicles assumes that a vehicle is, in effect, a hollow box, with components, cargo, and crew in the hollow interior. But a raft has no interior. In effect, it is a top deck (p. VE94) with no body underneath. Trying to determine a volume in cf and work out its design from that involves arbitrary assumptions and leads to unrealistic results. Instead, raft characteristics should be computed directly from area in sf.

A raft is typically a number of logs fastened together in some way. If the fastening is cords, reeds, or withies (lengths of willow), treat this as super-light structure, with a 0.1 weight multiplier (2 lbs. per sf at TL0 1). If it is wooden crossbeams, treat this as extra-light structure, with a 0.25 weight multiplier (5 lbs. per sf at TL0-1). The weight of the logs themselves can be found as follows. A squared-off log with a 1-ft. cross section has a volume of 1 cf per foot of length; p. VE88 gives 30 lbs. as the appropriate weight. If the log is cylindrical, the weight is reduced to 78.5%, or 23.5 lbs. per foot of length. Multiply this by the length of a log and by the number of logs to get the total weight of the wood.

If the thickness of the logs is reduced to 6 in., the weight per log is reduced to 5.9 lbs. per foot of length, a factor of 4; but since the logs are only half as wide, twice as many are needed for the same width, and the overall reduction is by a factor of 2. In general, if the raft area stays the same, any reduction in thickness produces a proportionate reduction in weight and cost for the wood. The DR and HP for each section of a square log can be taken from p. B125; for cylindrical logs, multiply by 78.5% again.

Rafts can be built of other materials, such as the reeds the Egyptians favored. Reed bundles are half again as thick as wood of the same weight; for that weight, they have the same HP but 50% as much DR.

How much can a raft support? Flotation capacity has nothing to do with the volume of the raft, but is based on the relative weights of the raft material and water. Since 30 lbs. of wood occupy 1 cf, they displace 1 cf of water, which weighs 62.5 lbs. So if the piece of wood is carrying 32.5 lbs. of added weight, it exactly equals the water it displaces and the water will be level with the top of the raft. If it is carrying half as much added weight, the water will be roughly three-fourths of the way up the side of the rafts, and so on. Similarly, reed bundles weigh 20 lbs. per cf, allowing them to support 42.5 lbs of added weight; in compensation, they have a lower DR.

Building a Trireme

The ancient ship on which the most detailed information is available is the trireme. Its construction has been debated for centuries. Unfortunately, no actual triremes survive, even as wrecks, but the sheds that housed them have been found in Athens, and there are detailed inventories of various pieces of material and equipment that went into them. Based on this information, an actual trireme was built and its performance tested several years ago. Given all this, the trireme is the ideal ship to illustrate ship construction rules for ancient and medieval times. (See the GURPS Low-Tech bibliography for sources of more information.)

A trireme is a long, narrow ship; the hull is 120' long and 12' wide. It carries oarsmen on three levels: down inside the hull, rowing through oarports (27 on each side); along the side at the usual height (27 on each side); and in a superstructure overhanging the side (31 on each side). The ship also carries 5 officers, 11 sailors, and 14 marines. The superstructures are 90' long and 6' wide, of which 2' extends past the side of the main hull, making the total structure 16' wide. (They are traditionally called "outriggers," but they are nothing like the structures on Polynesian canoes.) There is a 25' foremast and a 35' mainmast; typically only one is used at a time, and in battle the ship relies on the oars.

Given the basic requirement for 3' front-to-back space per oarsman, the superstructures can just barely hold the top level of oarsmen. The total ship length is a bit short of the 5' overall length per oarsmen for the bottom two rows. The entire design is organized to fit in the absolute maximum of rowing power. The purpose of all this power is to drive a ram through the water at the highest attainable speed, allowing the trireme to incapacitate enemy ships.

Using the tables in GURPS Vehicles, the lowest 54 oarsmen need 540 cf for their equipment and 1080 cf for cramped seats, all in the body. The next 54 oarsmen need the same space for equipment, but 1620 cf for normal seats; of this, 810 cf is in the body (in effect, they occupy normal exposed seats); the other 810 cf in the superstructures. The top 62 oarsmen are entirely in the superstructures, where they take up 620 cf for equipment and 1,860 cf for normal seats. The body also has space for five roomy exposed seats for the officers, 100 cf, and 180 cf of stores. This totals 3,250 cf in the body (increased to 4,225 cf by fine lines) and 1,645 cf in each superstructure. This produces areas of 1,570 sf for the body and 840 sf for each superstructure. The body has a light, expensive frame (1,178 HP) and expensive wood armor (DR 5); the superstructures have extra-light, expensive frames (315 HP) and expensive wood armor (DR 2), open-frame except on top, supplemented by leather curtains along the sides (DR 4) to screen the oarsmen from arrows.

The two masts, if both in use, carry square-rigged sails with 720 sf of total area. The ship is waterproofed, and the front is a massive ram. The crew includes 30 officers, sailors, and marines, in addition to the 170 oarsmen. The trireme carries a bare minimum of supplies and is not designed to spend a night at sea; its crew expect to beach it at night and launch it in the morning. In short, it is more like a fighter airplane than a warship of the Age of Sail.

Empty weight is 48,000 lbs; crew and provisions raise this to 88,360 lbs. A size of 7,515 cf gives a +6 modifier to hit. Top speed works out to 11 mph under oars, or 13 mph in full sail with a favorable wind; in practice the ship relies on oars in battle. Draft is 3.9' when fully loaded; that is, this is a very light ship. This covers most of the key points in vehicular design, and shows that even a remarkably sophisticated watercraft at TL2 can be worked out fairly quickly by a GM who wants a full GURPS Vehicles design.

At this point, it's useful to do a check of the volume against the dimensions. The hull cross section is nearly triangular (historians compare it to a wine glass), and thus has roughly half the area of a square cross-section; it also tapers to prow and stern, so its average cross section is roughly half its maximum cross section. Dividing hull volume of 3,2500 cf by length of 120' gives average cross section 27 sf, or maximum cross section 54 sf. A triangle with base 12' (the maximum beam) and height 9' has area 60 sf. This seems close enough to suggest that the design is reasonably accurate.

Mechanical Artillery

After the era of the trireme, warships shifted to relying primarily on mechanical artillery, and such weapons were also used in land warfare, especially during sieges. Ships up through the early Roman Empire carried both of the two main types of artillery then in use, the bolt-throwing scorpion and the stone-throwing ballista.

The scorpion was a relatively light weapon; a typical weight for one carried on a ship was 110 lbs. From the design rules in GURPS Vehicles (pp. VE97-100), this indicates a ST 37 weapon (37 x 37 x 0.8 for torsion x 0.1 = 109.5); thrusting damage for this ST is 4d, increased by 1d to 5d. Cost can be computed to be $5,020, rounded for convenience to $5,000.

The ballista was a heavier weapon. Exact weights are not available, but ballistas were rated by the weight of the stones they hurled, typical weights being 1 talent (about 60 lbs.) or one-half, one-third, or one-fourth talent. Ships carried light ballistas firing 15-lb. stones. This indicates a weapon of ST 150, which would weigh 4,500 lbs. Swinging damage for such a weapon is 16d. Cost for such a weapon is $13,800.

Thus, a Roman quinquereme with 9 scorpions and 2 ballistas would have $72,600 worth of armaments. This makes up more than half its total cost of $135,000.

The Fire Lance

The single largest change in the weapons lists for GURPS Low Tech was in the specifications for the fire lance. The version in GURPS China is effectively a primitive one-shot musket. But the actual description gives a different impression: a combustion chamber mounted on the end of a long pole with its exhaust directed at the enemy. Damage from such an attack would be comparable to that from a large, hot fire (1d-1). The exhaust would have no great range; rocket exhaust damage is quartered for each 2 yards distance from the engine (p. VE163), which would effectively reduce damage to 0 after 2 yards. Like a rocket, a fire lance could be expected to burn for several seconds, allowing multiple attempts to catch a foe in the exhaust.

The basic technique for wielding a fire lance involved swinging a long, heavy pole about; this amounts to Polearm skill. Assuming a 3-yard pole, the effective range would be 3-5 yards. A fringe benefit of this redesign was that the fire lance became significantly more exotic -- in effect, a nonmagical flaming melee weapon.

Nearly all of this analysis is invisible in the published book, which is as it should be. But some readers will want to see "the little man behind the curtain." This article is for them.

Article publication date: March 16, 2001

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