06-26-2011, 09:50 AM | #1 |
Banned
Join Date: Apr 2008
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[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? |
06-26-2011, 10:00 AM | #2 | |
Join Date: Jun 2006
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Re: [LT] Portcullises
Quote:
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-- MA Lloyd |
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06-26-2011, 10:11 AM | #3 |
Banned
Join Date: Apr 2008
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Re: [LT] Portcullises
Drawing raw ropes over a rough beam would be more efficient than this mechanism!
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06-26-2011, 10:22 AM | #4 |
GURPS FAQ Keeper
Join Date: Mar 2006
Location: Kyïv, Ukraine
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Re: [LT] Portcullises
Strictly speaking, how much they can lift is a function of their weight, not their strength.
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06-26-2011, 11:46 AM | #5 |
Banned
Join Date: Apr 2008
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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.
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06-26-2011, 12:02 PM | #6 |
GURPS FAQ Keeper
Join Date: Mar 2006
Location: Kyïv, Ukraine
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Re: [LT] Portcullises
I was referring to raw rope(s) over a beam that you mentioned.
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06-26-2011, 02:11 PM | #7 |
Join Date: Nov 2009
Location: GMT-5
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Re: [LT] Portcullises
450 lbs sounds a bit light.
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06-26-2011, 03:15 PM | #8 |
Join Date: Feb 2005
Location: Berkeley, CA
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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.
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06-26-2011, 04:33 PM | #9 | |
Join Date: Jan 2005
Location: Stuttgart, Germany
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Re: [LT] Portcullises
Quote:
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.) |
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06-26-2011, 08:47 PM | #10 |
Banned
Join Date: Apr 2008
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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.
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