Seamanship: Formulas On How To Find The Breaking Stress Of Chains, And Also Strength Of Hooks And Shackles, And Strength Of Spar

The Breaking Stress of Chain is given by the following formulas:

Grade I = 20D2 (tons) / 600

Grade II = 30D2 (tons) / 600

Grade III = 43D2 (tons) / 600

How to find the Strength of the Hooks and Shackles:

Hooks are not durable and strong compared to block and shackles, they are invariably weak and often times give way under strains so implicating heavy weights.
Therefore, great care should be exercise carefully in using its hook preferable by an iron links to avoid hook to open cut heavy works; shackles are fitting to the blocks in place of hooks, and are very

Here is the Formula:

P = 2/3 D2 (tons)

where D = is the diameter at the back

Shackles:

P = 3D2 (tons)

where D = is the diameter at the side

Problem Number 1
A weight is to be lifted with a hook having a diameter of 3/4 of an inch measured at the back of the hook.
Find its Safe Working Load (SWL)

Here is how to solve it:

P = 2/3 D2

P = 2/3 x 3/4 x 3/4

P = 2/3 x 9/16

P = 18/48

P = 0.375 tons

Therefore, the safe working load of a 3/4 of an inch diameter hook is 0.375 tons.

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Problem Number 2
A weight is to be lifted with a shackle instead of a hook; the diameter measured at the side is two inches.
Find the Safe Working Load.

Here is the formula and how to solve:

P = 3D2

P = 3 x 2 x 2

P = 12 tons

Therefore, the Safe Working Load of a 2 inches diameter shackle is 12 tons.

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How to find the strength of the Spar to resist compression

Here is the Formula:

T = 4R4 / L2

Where:

T = safe Trust in tons

R = Radius in spar in inches

L = Length of spar in feet

Note:
The multiplier

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Problem Number 1

A Spar made of lawaan wood in 20 feet long having a diameter of 13 inches.
Find its Safe Thrust in tons.

The radius of 13 inches diameter of wood is 6.5 inches.

Formula and Solution:

T = 4R4 / L2

T = 4(6.5)4 / (20) 2

T = 4(6.5x6.5x6.5x6.5) / (20x20)

T = 4 (1,785.0625) / 400

T = 7,140.25 / 400

T = 17.85 tons

Therefore, the Safe Trust of lawaan wood with a diameter of 13 inches, 20 ft in length is 17.85 tons.