What size of anchor line should I get?

Going to replace my anchor rode this season. How do I calculate the size of anchor line I should get?

Comments

  • I hope this helps. I've read that the general rule is 1/8 inch of rope for every 9 feet of boat length. I have a 28 foot jet boat, I use an 18 lb anchor and 1/2" nylon rope.

  • @LifeIsGood Your windlass should tell you the maximum size of rope and chain that it can handle. If you don't have a windlass here are some images that may help you in figuring out your anchoring system.


  • I copied this from a West Marine Advisor article about sizing your anchor rode.

    "What is your boat's displacement and windage?

    When recommending anchor rodes we generally use the following guidelines:

    • Heavy or high windage boats should use 1/8" of diameter for every 8' of boat length
    • "Normal" boats can use 1/8" diameter for every 9' of boat length
    • Lightweight or low windage boats can use 1/8" of diameter for every 10' of boat length
    • BBB chain should be half the line diameter (1/2" nylon line would be matched to 1/4" galvanized chain)
    • Use shackles one size larger than the chain (1/4" chain would use 5/16" shackles)

    In general the load on an anchor line varies with the square of the LOA of the boat. A high windage, heavy displacement boat such as a trawler or fishing boat will require heavier anchor rode than an ultra-light racing sailboat of the same LOA. As a general guide, for winds up to 30 knots, we recommend the following anchor line and chain diameters, using three-strand, high quality line. This table assumes an 8:1 working load ratio."


  • The usual approach to say so and so much scope is needed is only a very rough shot at answering this question. A chain works best in deeper water when it is not hanging almost horizontally between bow and anchor. In shallow water is fails completely, and you need a very good snubber to compensate for that. Otherwise you can have enormous loads in strong waves and gusts...

    I have created a free web-based tool to play around with different anchoring scenarios, vessel types, swell, wind, quality of snubber etc. Perhaps this will help you to get a better feeling at what is needed for you. One of the results is the anchor load, and this will help you determine the correct size of rope and chain...

    https://anchorchaincalculator.com

    The vessel velocity at anchor is usually rather small... 0.1 - 0.4 kn or so. In a gale it can become much larger, of course. In Basic Mode the snubber is defined in qualitative terms, like poor or good, etc. Play around with this and see what snubber stretch fits best with what you observe for your snubber. In Expert Mode you can define the snubber more precisely. The tool needs to know the snubber stretch at 8 Beaufort (40.2 kn) wind. Western Marine always provides the elasticity of their ropes at a certain working load. Use the rule of three to work out what snubber stretch at 8 Beaufort this results in. Described in more detail in the documentation.

    Ah, and you can switch the tool to use feet and pound with the buttons at the top right... ;)

    Hope this helps. Mathias

  • @Mathias This is awesome! Did you build this tool yourself? Very impressive and this will be very beneficial for future viewers. Thanks for the contribution!

  • @Project406 Thanks! It actually was a collaborative family effort: My son did the web page, I did the maths engine behind it, and my wife and daughter did the video tutorial... :)

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