Two single blocks, the upper, a tail block, made fast. Power gain is double less friction. With just the top block it would be a single whip tackle with no power advantage.

Whilst discussing the forces involved in running gear the other day, the subject returned many times to the force one could expect a crew to produce. One direction of force we all agreed on was pulling down on a line which would be the person's body weight when their feet left the ground. Forces in other directions produced a variety of answers tempered by the unknowns such as size of rope, wet or dry deck and pure muscle power.

Reading a friend's copy of The Admiralty's Manual of Seamanship Volume III, I came upon the following under the chapter dealing with Extempore Handling of Heavy Weights.

It is of interest to owners of old boats because the information relates to strop or iron bound, plain bearing blocks and more relevant than the information provided by modern manufacturers.

It is also a pleasure to read text written in precise Queen's English, a dying art it seems.

'Manpower.

The force a man can be expected to exert depends chiefly on whether the effort required is momentary or sustained over a period, whether it is made quickly or slowly, and on the manner in which the force is to be applied. It also depends to a certain extent on the number of men employed, the larger the body of men the less will be the individual effort.

In parties of up to four men, and for short periods only, each man can be expected to work as follows:-

(i) exert a standing haul of from 120 to 130 lb. (e.g., on the fall of a tackle) provided he has a good foothold.

(ii) exert a stationary push of 100 lb. provided he has a good foothold.

(iii) bear down on a lever with the force of from 70 lb. to his full weight.

(iv) lift up on a lever at knee level with a force of 150 lb.

(v) carry on his shoulders a weight of 130 lb.

Remarks on Tackles.

The blocks of tackles used for general purposes must be kept as light as possible, otherwise the tackles would be too cumbersome to handle. This limitation precludes the use of the heavy, specially made blocks of a ship's derrick, which have their sheaves mounted on ball bearings, and in which friction is reduced to less than 5% of the load. The friction set up in a well-made block of a general purpose tackle maintained in first class condition will amount to one-tenth of the load for each sheave in the block, but in block less well maintained or manufactured this friction will amount to one-eighth of the load.

Up to a certain number the more sheaves there are in a tackle the greater will be the gain in mechanical advantage (force exerted to weight lifted), but thereafter any additional sheaves will reduce the gain in mechanical advantage due to the friction set up by them. Furthermore, the greater the number of sheaves in a tackle the heavier will be it's blocks and the more cumbersome will it be to handle.

For these reasons a three-fold tackle is usually the highest ratio tackle used for general purposes, and in many cases it is better to use a combination of two lesser-ratio tackles than one of high ratio. For example a gun tackle clapped on to a runner gives a higher mechanical advantage for a lesser velocity ratio (length of fall hauled in to distance load moved) than a three-fold tackle rigged to advantage (i.e. with the hauling part leading from the moving block). When using a combination of tackles, however, the velocity ratio must be taken into account if the fleeting space is limited and speed of operating is of importance. A luff upon luff, for example, provides about twice the mechanical advantage of a three-fold tackle rigged to advantage, but the velocity ratio of the former is more than twice that of the latter.

Leading blocks or rollers to guide the hauling part of the fall of a tackle to a convenient position or to clear of an obstruction should be used sparingly, because the friction in each leading block or roller will reduce the mechanical advantage of the tackle by an amount proportional to the angle through which the fall is deflected. The friction caused by deflecting the fall through angles of 90 and 45 degrees will amount respectively to roughly half and a quarter of the friction set up in a sheave of the tackle.

Tackles used for hauling should, whenever possible, be rigged to advantage. Hoisting tackles, however, must be rigged to disadvantage, unless a combination is used, because of the necessity of leading the hauling part downwards from the upper, or standing, block.

The blocks of a tackle are usually stronger than it's fall, and in tackles rove with cordage falls, and using a factor of safety of six, the safe working load of a tackle is that of it's fall.

If, however, the factor of safety is reduced, then the safe working load of the tackle may depend on the strength of it's standing block. In tackles rove with wire rope falls their safe working loads may be determined not by the safe working load of the fall, but by the strength of their standing blocks even when using a factor of safety of six. If, therefore, there is any doubt about the safe working load of a tackle the strength of the blocks should be compared with the safe working load of the fall, and the lower amount be taken as the safe working load.

Two single blocks. If the upper block moves while the lower block is fixed the tackle is said to be rove to advantage.

A double and a single hook block. If the top block is a rope tail block that can be clapped onto another rope with a rolling hitch then it is called a Handy Billy or Jigger.

Two double blocks. If the tackle is rigged for hauling; the bottom block is fixed, the tackle is rove to advantage and the velocity ratio rises from 4:1 to 5:1.

Two single blocks and a hook. Power ratio 3 to 1. If the hook is fixed and the top block moves then it is a Whip on Whip tackle with a 4:1 ratio. More fleeting required for the same power as a double purchase but cheaper and more efficient.

THREE-FOLD PURCHASE

If rove as drawn it tends to twist and friction increases. Much more balanced if rove with the hauling and standing parts in the middle sheaves.

n

Runner - hauling

Luff tackle - hoisting

Spanish Burton - hoisting

Gun-tackle - hauling

Double whip upon double whip - hoisting

Two-fold tackle - hoisting

Luff tackle - hauling

Runner upon runner - hauling

Three-and-two tackle - hoisting

Two-fold tackle - hauling

Three-fold tackle - hoisting

Luff (disadvan') upon double whip - hoist'g

Three-and-two tackle - hauling

Four-and-three tackle - hoisting

Gun-tackle upon double whip - hoisting

Three-fold tackle - hauling

Four-fold tackle - hoisting

Gun-tackle upon runner - hauling

Four-and-three tackle - hauling

Four-fold tackle - hauling

Luff (advantage) upon double whip - hoisting

Luff upon luff (both disadvantage) - hoisting

Luff upon runner - hauling

Gun-tackle upon gun-tackle - hauling

Luff (advan'ge) upon luff (disadv'ge) hoist'g

Luff upon gun-tackle - hauling

Luff upon luff (both advantage) - hauling

1

3

2

2

4

4

3

2

5

4

6

5

5

7

4

6

8

3

7

8

5

6

4

4

6

5

6

2

3

3

3

4

4

4

4

5

5

6

6

6

7

6

7

8

6

8

9

8

9

8

9

12

12

16

1.82

2.30

2.50

2.50

2.79

2.86

3.08

3.31

3.33

3.57

3.75

3.84

4.00

4.12

4.17

4.37

4.44

4.55

4.70

5.00

5.14

5.29

5.61

6.25

7.08

7.70

9.49

2

3

3

3

4

4

4

4

5

5

6

6

6

7

6

7

8

6

8

9

8

9

8

9

12

12

16

1.78

2.18

2.40

2.40

2.56

2.67

2.91

3.17

3.08

3.33

3.43

3.49

3.69

3.73

3.84

4.00

4.00

4.27

4.27

4.50

4.66

4.78

5.16

5.76

6.34

6.98

8.47

DOUBLE SPANISH BURTON

Two examples shown left. The top tackle uses three single blocks while the bottom diagram uses two singles and a double. The dis-advantage of these tackles is the large fleeting required as the whip block comes down and meets the lower block going up.