Block & Tackle Calculations

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.

Common Tackle Names

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

Calculating Force to Lift

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

Calculating Friction

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