[LT] Portcullises
 

Hello,

I'm loving Low-Tech. I'd read it for the info alone, even if I weren't a GURPSer.

However, I wonder if there is a bit of a problem, perhaps an erratum, regarding portcullises on page LT119. The example suggests that a mechanism to lift a 450lb. grate requires 8 men to operate. But 8 average men would have a combined XHvy-encumbrance lifting capacity of 1600lbs, so what would be the point of the mechanism?

Re: [LT] Portcullises
 

It lets them lift it higher than they can reach? And of course they don't have to squeeze in 4 on each side of it, blocking the gate with their bodies, to do it. But yeah, that's a pretty inefficient mechanism.

Re: [LT] Portcullises
 

Drawing raw ropes over a rough beam would be more efficient than this mechanism!

Re: [LT] Portcullises
 

Strictly speaking, how much they can lift is a function of their weight, not their strength.

Re: [LT] Portcullises
 

The depends on the mechanism, but, yes, it's true of a pure conversion of the force to a downward pull. But that points up even more the questionablity of needing 8 men to draw up 450 lbs.

Re: [LT] Portcullises
 

I was referring to raw rope(s) over a beam that you mentioned.

Re: [LT] Portcullises
 

450 lbs sounds a bit light.

Re: [LT] Portcullises
 

I feel like that sentence about the lifting mechanism is referencing text that no longer exists. In any case, a mechanism that allows lifting a 450 lb object at 1'/3s is doing about 200W of work, and with good axles could be done by one person.

Re: [LT] Portcullises
 

Sounds like someone dropped a factor somewhere...

15 lbs for a 10 foot square grate of iron also seems a factor too light.

If we were looking at a small portcullis bars which were say 2" on a side, we could calculate the weight of each 15' bar at:

15' = 180" = 450 cm
2" = 5 cm

5x5x450 = 11250cc x (Iron at 7.80 g/cc) = 87,750 grams

So each 15' bar would weigh 87.75 kg, or 193 lbs.

Each 10' bar at:

20' = 240" = 610 cm
2" = 5 cm

5x5x610 = 15250cc x (Iron at 7.80 g/cc) = 119,000 grams

So each 20' bar would weigh 119 kg, or 262 lbs.

If all the bars are 1' apart, and the gate is 20' wide, we'd have 21 bars which are 15' high, and leaving sharp ends we'd have 15 bars which are 20' long, that'd add up to:

(21x193) + (15x262) = 7,980 lbs ~ 4 tons


Having 8 men raise a portcullis which weighs 4 tons isn't unreasonable...

(Someone please check my math if you will.)

Re: [LT] Portcullises
 

Yes, I know. The fact that a system that permits the crew to use weight rather than ST (3 average men weigh 450, a fourth would overcome the fricton of the rope) would be so simple makes it even stranger that anyone would build a mechanism that needs double the men.