Seamanship: How To Find The Strength Of Ropes (examples And Solving) And Also Wire Ropes

Example Number 1
Given a 40 millimeter diameter of Polypropylene Ropes estimate its breaking stress and safe working load.

We’re using 3D2 because the rope given above is a Polypropylene type of rope.

BS = 3D2 / 300

BS = 3x40x40 / 300

BS = 4,800 / 300

BS = 16

Therefore, the Breaking Stress of that Polypropylene type of rope is about 16 tons

Now, for the SWL or Safe Working Load, here is how to solve:

SWL = BS / 6

SWL = 16 tons / 6

SWL = 2.7

Therefore, the Safe Working Load of that Polypropylene type of rope is estimated 2.7 tons.

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Example Number 2
Find

We’re using 2D2 because the rope given above is a Manila type of rope, and here is the formula and how to solve:

2D2 / 300 x 1/6 = 2 tons of load

D2 = 300 x 6 x 2 tons of load / 2

D2 = 3,600 / 2

D2 = 1,800

D = 42.43 mm (square root of 1,800)

Therefore, for a load of 2 tons, 42.43 mm is the smallest suitable for Manila type of rope.

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WIRE ROPES

The breaking stress of a flexible steel Wire Rope is given approximate by the following formula:

First Formula: 6 x 12 = 15D2 (tons) / 500

or

Second Formula: 6 x 24 = 20D2 (tons) / 500

or

Third Formula: 6 x37 = 21D2 (tons) / 500

Where:

D = is the diameter of the rope in millimeter, the safe working load maybe taken as one sixth (1/6) of the breaking stress.

/ = divided by sign

500 = divided by 500 as constant in formula for ropes made out of wires.

Example:

Find the Breaking Stress (BS) and Safe Working Load (SWL) of a 6x24, 24mm wire ropes.

Here is how to solve:

According to the problem above, that wire rope is a 24 mm in diameter; therefore, we will use the Second

BS = 20D2 / 500

BS = 20 x 24 x 24 / 500

BS = 11,520 / 500

BS = 23.04 tons

Therefore, the Breaking Stress of a 6x24, 24 mm wire rope is 23.04 tons.

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Now let’s solve for the Safe Working Load of that 23.04 tons wire rope and here is the process of solving:

SWL = BS / 6

SWL = 23.04 / 6

SWL = 3.84 tons

Therefore, the safe working load for 23.04 tons of wire rope is 3.84 tons.

Where:

BS = Breaking Stress

SWL = Safe Working Load

6 = divided by 6 as constant to the wire rope safe working load formula