Blocks and Tackle and How Are They Measured

[Kurt Konrath]

OK, looking for help from some more knowledgeable members than me.

I have had this question many times and now see others struggling with it.

So when we say a block is single, double or triple, that is not hard to understand, but when they say a 2mm block, is that height, or width, and if so, how is it measured, from top to bottom of block body or from size of the wheel inside the block?

Any help or guidance welcome for us novice builders.

 

[Y.T.]

2mm block is height from top to bottom. Measure with caliper.

 

[AllanKP69]

I agree with YT. I am no where near my books for a while so cannot get at the details, but I am pretty sure it was standard operating procedure to use ratios on the three dimensions of the block and the sheaves based on the circumference of the rope.
Round numbers
The length of the block was about 3X the rope circumference
Sheave diameter 2/3 the length of the block
Block Width: Approximately 2/3 the length of the block
Block thickness (single blocks) approximately 1/2 the length

The hard part for me was finding the rope circumference until about 25 years ago. The most complete source I have ever found to date are the formulas from main mast to ropes in James Lees' The Masting and Rigging book. There are other sources, but not as complete as Lees' work considering the changes that took place over about 200 years. His book is for English ships from the mid 17th century to the mid 19th century so may not be applicable for other nations.
Hope this helps.
Allan

 

[Kurt Konrath]

So then a 2mm block is measured by the external height of the block.

That is what I thought, and hopefully this will help others who have the same questions.

Thanks all for helping to clarify for sure how this is done.