Deck General · Blocks & Purchases
Friction in a block and tackle system reduces the actual mechanical advantage. When estimating working efficiency, a rule of thumb deducts approximately what percentage per sheave?
- A2%
- B5%
- C10%✓ Correct
- D20%
Explanation
A common rule of thumb is a 10% friction loss per sheave (some references use 10-15% per part at the moving block). For a 4-part purchase with 4 sheaves, total efficiency drops to roughly 65% of theoretical MA. This means a 4:1 tackle effectively delivers about 2.6:1 practical advantage.
Authority: Deck Seamanship (USCG deck topic)
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