These are the Light Duty Dock Springs. 

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The springs are sourced from an SUV with a constant rate 600 lbs. per inch compression rating.  There is nothing light duty about them.  They are just not as beefy as the other two sizes offered. 

 

If your dock is located in protected water, not exposed to a wide expanse of water (and the wind that comes with that), a single slip one story dock or swim platform and not a huge double decker monster dock, these will do a fine job.