three hulls, two people, one trip around the world…

List of available digests of the anchor chain catenary for different types of boats

Here is a list of digests that focus on the effect of dynamic anchoring for vessels of various size. As a point of reference, an Etap 39 has been quite accurately measured to have an Aeff = 7 m2. For our Neel 51 trimaran I am currently using Aeff = 20 m2 as a rough estimate, but this is subject to change. As of version 3, these digests also include diagrams showing the effect of snubbers / bridles.

As a guidance, for a mono hull Robert Smith did a lot of measurements, resulting in this rough mapping as a function of the vessel’s length (LOA):

8 m LOA -> Aeff = 4.5 qm

9 m LOA -> Aeff = 5.7 qm

10 m LOA -> Aeff = 7.1 qm

11 m LOA -> Aeff = 8.6 qm

12 m LOA -> Aeff = 10.2 qm

13 m LOA -> Aeff = 12.0 qm

14 m LOA -> Aeff = 13.9 qm

15 m LOA -> Aeff = 16.0 qm

16 m LOA -> Aeff = 18.2 qm

17 m LOA -> Aeff = 20.5 qm

18 m LOA -> Aeff = 23.0 qm

19 m LOA -> Aeff = 25.6 qm

If you cannot find your scenario in this list, drop me a line and I can perhaps create it, time permitting. If you like mathematics, then perhaps you can still find the correct digest by applying scaling laws. As briefly discussed in a footnote in the digest, all formulas depend only on the ratios of Aeff/m, F/m, ΔE/m, where m is the mass of the chain in water per meter, F is the force at the anchor or at the bow, and ΔE is the energy burst induced by the swell. Hence, a digest for Aeff = 10 and m = 2 is the same one as, e.g., for Aeff = 20 and m = 4. You need to keep in mind that F and ΔE are scaled as well, of course.

Please note: The formulas can also be used when it is current and not wind pulling at the vessel. Or a combination of both. The Aeff would be different, though, and depend on the shape of the vessel underneath the waterline. Another measurement of Aeff… 😉

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