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Block size calculations


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Somewhere I have seen a set of formulae which can be used to determine the dimensions of a block based on the diameter of a rope. For example in 18th century merchant ships the thickness of a sheave is 1.1 times the diameter of the rope and the diameter of that sheave is 6 times the diameter of the rope. I seem to remember the swallow (or the gap into which the sheave fits) is 1/16" greater then the thickness of the sheave.

 

For the life of me I cannot find or remember the other parts to determine the length and width of the block or the thickness of the cheeks or the thickness of the intermediate pieces between sheave in double, triple or fourfold blocks.

 

Can anyone help or point me in the right direction. Thanks in advance.

 

Dave

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Is this what you are looking for?

 

From "Elements of Mastmaking Sailmaking & Rigging" by David Steel, 1794

 

Block Length is 8 times the width of swallow.

Block Breadth is 6 times the width of swallow.

Block thickness is 1/2 the length of the block.

Width of Swallow is 1/16" plus 1.1 times the rope diameter.

Sheave thickness is 1.1 times the rope diameter.

Sheave diameter is 5 times the sheave thickness.

Pin diameter is the thickness of the sheave.

Pin hole diameter is 1/10 less than the diameter of the pin.

Double Block thickness at partition is 1/6 less than the breadth of the swallow width.

 

Bob

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Lees gives similar dimensions.  Pick your medicine

 

Common single block

Width of the sheave was 1/10 more than the diameter of the rope, the diameter of the sheave was four times the width of the sheave.  The width of the sheave hole was one sixteenth more than the width of the sheave and the length of the sheave hole was one and one third the diameter of the sheave.  The length of the shell was eight times the width of the sheave hole; the width was two fifths the length of the shell; the breadth was six times the width of the sheave; and the position of the pin was eleven twenty- firsts from the top of the block.  The edges of the sheave hole were chamfered off and the shell slightly rounded. The pin was made of wood on small blocks and iron or brass on larger blocks.    

 

For common double or triple blocks they were made in the same proportions as the single blocks except in the width of the shell.  This was increased by the separating piece or pieces between the sheaves which were five sixths the width of the sheaves.  James Lees; The Masting and Rigging of English Ships of War page 164

Allan

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  • 3 years later...

So, based on the above formula, if I am using Chucks blocks and ropes I can multiply the rope diameter by 8 and go with the closest available block. For instance if I have a scale rope that is 0.025"D multiplying that by 8 gives me 0.20." my closest choices are 3/16" and 1/4." 1/4" would be easier to rig but 3/16" would probably look better if I have understand this correctly.

Sam 

Current Build Constructo Enterprise

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