How to Calculate Sheave Size

Contributor

By Will Charpentier
eHow Contributing Writer

Article Rating: (0 Ratings)

If you need a block and tackle to move something aboard your boat, or if you need to whip up a vang to steady the gaff on your mast, you need to know what size block and, more particularly, what size sheave to use so that the rope riven through the block won't jam or kink because it's too stiff to move a block that's too small. A quick calculation will solve the problem; while it may be done in your head, because of less common sizes of woven lines, a calculator may come in handy.

Difficulty: Moderate

Instructions

Things You'll Need:

Measuring tape
Calculator

Step 1

Measure the diameter of the line to be riven through the sheave with your measuring tape. If you have a natural or synthetic fiber line, or a wire rope, measure across the largest diameter of the line or rope.
Step 2

Multiply the diameter of the line by 20 with your calculator. If the diameter of the line is 2 in., then multiply 2 x 20; the result is 40.
Step 3

Measure the diameter of the sheave you plan to use. In Chapter 20, "Rigging," of the Marine Crewman's Handbook, Army Field Manual 55-501 confirms the common practice of seamen over hundreds of years, saying that "the diameter of a sheave should be at least 20 times the diameter of the line to be riven through the sheave."
Step 4

Select a sheave of the appropriate diameter. For the above example, the sheave would have a 40 in. diameter.