[LT] Portcullises

[Anthony]

Originally Posted by Dunadin777

Everyone keeps invoking the Extra-heavy encumbrance weight limit, which is wrong. The rules specifically state that X-Hvy encumbrance is more than a person can normally lift--loading one object into a pack at a time, you could carry 10xBL, but there's no way you could realistically lift that much in the first place.

That's pretty well irrelevant, since we've got a lifting mechanism, and the required power output is fairly trivial (and BL*8 in 4 seconds, assuming that's lift to 6', is about BL*5 ft-lb per second, or BL*15 lb at 1'/3s').

[Dunadin777]

Originally Posted by Anthony

That's pretty well irrelevant, since we've got a lifting mechanism, and the required power output is fairly trivial (and BL*8 in 4 seconds, assuming that's lift to 6', is about BL*5 ft-lb per second, or BL*15 lb at 1'/3s').

The whole point of comparison is the superiority of the mechanism versus a group of manual lifters, so lifting weights are the only way to compare them, not using the encumbrance values. Besides, if the mechanism requires 8 people to work at its standard value, it is clearly a non-trivial power requirement. And, since it takes two seconds to ready something off the ground, I think it is assumed that a GURPS lift refers to lifting something from shoulder height to full extension, which is much closer to 2' than 6'

With that assumption, you could say that the two handed full lift is 5xBL at 1'/2 seconds, but as I mentioned above, it is doubtful that the entire 8-man team is acting at the breaking point of their strength to operate the machine at its usual efficiency.

[Anthony]

Originally Posted by Dunadin777

The whole point of comparison is the superiority of the mechanism versus a group of manual lifters.

Huh? The point of the comparison is to get a realistic baseline for lifting X pounds.

Originally Posted by Dunadin777

Besides, if the mechanism requires 8 people to work at its standard value, it is clearly a non-trivial power requirement.

Well, the stats for the mechanism are outright wrong unless the weight of the portcullis is wrong.

[Dunadin777]

Alright, so now I'm home and have access to my books, and it looks like everyone was missing a key line.

Originally Posted by Low-Tech, page 119

A cross-hatched grate – with additional horizontal bars – has double weight and 25% more HP.

So that indicates that the numbers and weights above are describing a grate that is composed of bars all in one direction--vertical-only or horizontal-only. That's a pretty big point of misunderstanding, since I think we're all envisioning a typical iron lattice portcullis.

Also, looking at the grate DRs, they all match up to bronze or iron's DR in the Basic Set B559. The Light grate's DR corresponds to .5" diameter, Heavy grate to 1" diameter, and Vault to 2" diameter iron.

Given that, we can reverse engineer what the authors envisioned for the grate's composition. Let's figure it for a light grate first:

Iron density=7.8 g/cc or .28 pounds/cubic inch
half-inch bar's cross-section: (.25^2)*pi=.2 square inches
So with the Light grate weighing 7 pounds, we know we have 7/.28=25 cubic inches of iron. If we divide by the cross-section, we know there's 25/.2=125 inches of bar length.

We only need bars of 38 inches, so we have a total of 125/38=3.29 bars over each section, which allows us to place them at about one per foot.

So far, it looks like Low-Tech's math is holding up after all. Now let's look at a Vault grate--what we can assume is the heaviest sort of simple portcullis. We'll make it 30 sections, like the book, but make it square for simplicity--17.32' square. The table in the book says that's 3,000 pounds. Each section has 37 HP and DR 24. Now let's try to picture what this thing's construction is like if we assume 2" iron bars:

Iron Density= .28 pounds/cubic inch
Bar's cross-section: (1^2)*pi=3.14 square inches
Portcullis weighs 3,000 pounds. 3,000/.28=10,714 cubic inches of volume. That's 10,714/3.14=3,412 inches of bar length.
Each bar is 17.32*12=208 inches in length. That gives us 16.4 bars across the 17 feet-wide grate, or one every foot or so.

If we doubled the weight, we could've gotten the typical lattice-style portcullis we all envision, with 2" bars crossing about every foot in either direction.

It looks to me like the grate weight stats are dead on if you assume something less spectacular than the overly chunky wood portcullis one normally recalls from medieval movies. After all, having a portcullis composed of more than 2" bars seems unnecessarily excessive unless in a high fantasy campaign where such things are going to be unrealistically exaggerated.

[Figleaf23]

Eight men to operate a mechanism for lifting a 900lb grate is only slightly less off that 8 men to operate a mechanism for lifting a 450 lb grate.

Is there a medieval engineer in the house? What's the efficiency of pulleyed lifting systems at TL3?

[Figleaf23]

Here we go -- someone who enjoys math should be all over this!

[Anthony]

Originally Posted by Figleaf23

Here we go -- someone who enjoys math should be all over this!

Doesn't tell us much about the efficiency of medieval block and tackle, assuming that's even what they used -- a simple windlass is probably better suited to the task of lifting a gate.

[Bruno]

Originally Posted by Figleaf23

Eight men to operate a mechanism for lifting a 900lb grate is only slightly less off that 8 men to operate a mechanism for lifting a 450 lb grate.

I'm finding that a rather extreme statement, but then folks basing calculations off of maximum lift or maximum carry are also striking me as presuming rather extreme levels of resistance.

If my soldiers can only go around the winch/drag the rope at about a yard a second (which is what maximum encumbrance gets us) that gate isn't going up very fast at all - if there's any benefit from pully ratios, the gate is going up in something like 1/4 the rate of land-based travel (Depending on the ratio) - a 10' gate going up at 1/4 yard per second with some guy standing over the men with a whip and yelling HEAVE is not how I envision this sort of mechanism working. That's hauling an extremely large seige engine, not lifting up your own dang portcullis. :/

Shooting for a felt weight-per-man in the ~60 lbs range (like the usual cited maximum military marching load) seems like a far more sensible target - they can slog along at a reasonable clip and won't be exhausted at the end of it.

The mechanism has to lift the gate and the weight of the cables/ropes. A reasonably simple pulley mechanism is pretty efficient but requires significantly longer ropes (more weight and much more room to work in) or a winch, which introduces more significant mechanical efficiency problems if I've got this right.

I'm not an engineer, and the only one I can easily reach is an electrical engineer, but I'll see if I can dig up someone who can eyeball it.

[Anthony]

Originally Posted by Bruno

The mechanism has to lift the gate and the weight of the cables/ropes. A reasonably simple pulley mechanism is pretty efficient but requires significantly longer ropes (more weight and much more room to work in) or a winch, which introduces more significant mechanical efficiency problems if I've got this right.

A windlass located above the gate requires 1-2x the height of the gate in rope or chain and has a mechanical advantage equal to the ratio of axle diameter to crank diameter. Mechanical efficiency depends mostly on the quality of your axles, but your rope/chain is only passing through one or two axles, depending on how it's configured. If we want to convert 1 yard per second (walking speed) to 1 foot per 3 seconds, that's a mechanical advantage of 9, so one person at 60 lb force can handle 450 lb with a 20% increase for friction; a 900 lb grate would require two people. Depending on just how bad medieval axles are, you might need more people or more force.