While less commonplace than in the age of sail, mariners are expected to be versed in the names, how they are rove, and advantages of several primary tackles.

**1 â€“ Whip** rove to disadvantage, 1:1**2 â€“ Gun tackle** â€¦ disadvantage, 2:1**3 â€“ Single luff** â€¦ disadvantage, 3:1**4 â€“ Twofold purchase** â€¦ disadvantage, 4:1**5 â€“ Double luff** â€¦ disadvantage, 5:1**6 â€“ Threefold purchase** â€¦ disadvantage, 6:1**7 â€“ Runner** rove to advantage, 2:1**8 â€“ Gun tackle** â€¦ advantage, 3:1**9 â€“ Luff tackle** â€¦ advantage, 4:1**10 â€“ Twofold purchase **â€¦ advantage, 5:1**11 Double luff tackle **â€¦ advantage, 6:1**12 â€“ Threefold purchase** â€¦ advantage, 7:1

In its simplest form, disregarding friction would require 10 pounds of force for a threefold purchase to raise a 70 pound object. The formula would be:

Weight of Object / Mechanical Advantage = Force Required to Lift

When we wish a more accurate estimation of the force required to lift an object we estimate 10% friction on each sheave, although the actual amount will vary depending on the level of attention given to greasing and maintaining the blocks. The formula is thus:

Weight + (Weight x Number of Sheaves x 0.10) / Mechanical Advantage = Force Required to Lift

If we wanted to know the force required to lift a 1,000 lbs. Weight using a twofold purchase, rove to advantage, we have everything we need to calculate the force required to lift. A twofold purchase, rove to advantage, has a 5:1 ratio. We know it has 4 sheaves.

1000 + (1000 x 4 x 0.10) / 5 = 1400

Login

Accessing this course requires a login. Please enter your credentials below!