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Plumb bob design thought

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Strictly Surveying

Last Post by Landbutcher464MHz 8 months ago

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bill93

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Someone recently gave me a 32 0z plumb bob, which does not have a typical shape; It is basically a cylinder on top of a short inverted cone. The shape got me thinking.

Heavy plumbs are typically needed where it is important to minimize wind disturbance, no? So you need a point that can be seen for alignment, and the rest of the shape should optimize wind resistance. I'm assuming radial symmetry, so ignore Plumb Barbara.

Wind resistance at low velocities depends primarily on cross section area viewed from the wind source. For a plumb bob of given height, area is twice radius times height, and weight is proportional to radius squared times height. The largest weight to area ratio comes with the larger radius.

So that argues the best plumb bob in wind is very fat with the shape just allowing you to see the point. Much of the weight is hiding from the wind.

Why are so many plumb bobs long and skinny?

Posted : January 7, 2026 1:41 pm

Norman_Oklahoma

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Posted by: @bill93

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Why are so many plumb bobs long and skinny?

It probably has to do with being able to see the point while you stand over it, which I always found to be tough enough with the common skinny cone shape.

Posted : January 7, 2026 2:04 pm

lurker

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How would you know where the plumb bob point is if it were fat? You would need a second person to tell you when it was over the point.

Posted : January 7, 2026 2:38 pm

Jerry Attrick

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I was taught that if you used a fat bob that one would refer to it as a Peach Billy.

JA, PLS SoCal

Posted : January 7, 2026 8:27 pm

Moe Shetty

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if the wind picks up, use a wind box on the windward side of the plumb bob

Posted : January 8, 2026 7:35 am

Tom Bushelman

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Posted by: @bill93

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so ignore Plumb Barbara.

Hard to ignore Barbara. She's a hottie.

Posted : January 8, 2026 12:30 pm

CV-Nevada

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@tom-bushelman But as cold as brass. 😉

Posted : January 9, 2026 10:13 am

CV-Nevada

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In my collection of oddities, I have a plum bob that has a mercury filled core. I have never used it.

Posted : January 9, 2026 10:15 am

Landbutcher464MHz

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Posted by: @cv

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In my collection of oddities, I have a plum bob that has a mercury filled core. I have never used it.

Supposedly that mercury core will dampen the swinging making it stay steady over the point. The basic technique we used was to lightly tap the bob on a flat surface close to the pin (tapping escutcheon pins will make ol'bob wobble like a limp dick) until it stayed steady then move over the pin.

Posted : January 9, 2026 11:22 am

bill93

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