Cordage & Rope - Pride Marine

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Unless specified (or sold in spools), line is sold by the foot.
If you need more than 25 feet, simply add any amount to your shopping cart, and modify it later when you proceed to checkout!

PROPER LOADING OF ROPE

The following information may help you decide the right size of line to purchase. It can also be helpful determining the load placed on other hardware on your boat, such as shackles, blocks, padeyes and eye straps.

The breaking strength of rope is based on direct pull along a single length of rope. When slings, using two or more legs to carry the load are properly employed, the rope's safe working load is substantially increased. However, as illustrated below, the load factor on each kilogram of the sling is greatly increased as the sling angle becomes smaller.

Therefore the use of slings requires certain precautions as well as a knowledge of safe working loads permissible. For best results, sling angles should never be more than 90-degrees, and rarely less than 45-degrees. Remember to regularly inspect splices and all fittings! Consider using rub strakes if the rope will be in contact with surfaces that may chafe!

HOW TO COMPUTE SAFE WORKING LOADS ON ROPE

Use the following equation when rope breaking strength and sling angles are known:

Example: To find safe working load for two-leg sling (2:1 purchase) of 3/8" Polyester (breaking strength 4081kg) at an angle of 60-degrees (sine = 0.866)

Multiply breaking strength of rope by number of legs in the sling (1 & 2 in diagram).
Multiply above result by half of the cosine of the sling's inside angle (this gives you approx max load. Note the 60-degree angle reduces the strength on the rope by approx 13%)
Multiply by appropriate percentage of safety factor to find safe working load of the rope (this is almost universally accepted as 20%)
Therefore, the safe working load is 1413.66 kg!

HOW TO COMPUTE THEORETICAL LOAD ON YOUR SYSTEM

Use the following equation to determine how much load will be placed on the rope when setting up a purchase system or running your lines through fairleads, blocks or other hardware:

Example: Load of 1000 kgs in a two-leg sling (2:1 purchase) running through a sheave at an angle of 60-degrees.

Divide overall load (1000) by number of lines (2) to determine load on each line.
Divide individual load (500) by the sine of the angle (sine of 60-degrees = 0.866)
Therefore, the theoretical load on the line will be 575 kg!
Seek out of a line with a safe working load of this amount. Using the 20% safety factor (see example on left), a suitable rope would have a breaking strength of 2875 kg (575 kg / 0.2)

5/16" Polyester or 1/4" Spectra would do the job in this example.

Other angles produce different results (see below):

Angle-to-Sine Conversion Chart:

You can use a scientific calculator to determine sine. Simply punch in the angle (as a whole number, eg 40) and press the "sine" function (answer 0.642).

Angle	Cosine	Sine
5	0.996	0.087
10	0.985	0.173
20	0.9397	0.342
30	0.866	0.500
40	0.766	0.642
50	0.6428	0.766
60	0.5	0.866
70	0.342	0.939
80	0.1736	0.985
90	0	1.000

Still need a scientific calculator? If you're running Windows, you probably have one! Just go to "Start", "All Programs", "Accessories", then choose "Calculator".
The calculator's default setting is "Standard", so if you don't see the SIN/COS/TAN functions, click on "View", then "Scientific".