Category Archives: Anchor Rode Windage Loads

Wind Loads on the Anchor Rode

The “rode” is that line or chain, that is permanently secured to a vessel, usually at the bow, and reaches from a structural part of the vessel to the shank of the vessel’s anchor; the term includes all in-line connecting devices (splices, shackles, swivels, toggles, thimbles, seizings, whippings, etc).  All forces experienced by a vessel at anchor will be transferred from the vessel to the seabed through the deck hardware, rode and anchor.  A failure of any component of this system can result in a fatal loss of the vessel.

For Sanctuary, I calculated the wind loads that would be experienced under a range of wind conditions by a Monk 36 while at anchor.  This analysis probably also applies to other classic trawlers with similar LOA and physical profiles; i.e., Albin, Grand Banks, CHB, Marine Trader, other Taiwanese, etc.  The numbers are based on well understood and long accepted science.  These calculations are consistent with the numbers in Earl Hinz’ book and with the tables presented in American Boat and yacht Council (ABYC) Standard H-40, “Anchoring, Mooring and Strong Points,” 2008 (the current edition of that standard; it’s probably up for review in 2013).

Sanctuary’s bow elevation presents a surface area, at zero degrees of yaw to the direction of the wind, of 165 ft2 with the Flybridge enclosure in place, and 143 ft2 without the flybridge enclosure.  The Monk 36 (and other boats) tends to “sail,” or “horse,” back-and-forth at anchor, and routinely reachs a yaw angle of +/- 30° to the wind, often more.  The surface area elevation for Sanctuary, at a 30° yaw angle to the wind, is 214 ft2 with the enclosure and 180 ft2 without.  The table below shows the results of the calculations:

Monk 36′ Values
Wind Speed: 10 Knots 20 Knots 30 Knots 40 Knots 50 Knots 60 Knots 70 Knots 80 Knots 90 Knots 100 Knots
                     
drag force:
(0º Angle of Yaw)
(Includes area of FB Enclosure)
55.9 223.5 503.0 894.2 1397.2 2012.0 2738.5 3576.8 4526.9 5588.7
drag force:
(0º Angle of Yaw)
(w/o area of FB Enclosure)
48.4 193.7 435.9 775.0 1210.9 1743.7 2373.4 3099.9 3923.3 4843.6
drag force:
(30º ~ Angle of Yaw)
(Includes area of FB Enclosure)
72.5 289.9 652.4 1159.7 1812.1 2609.4 3551.7 4639.0 5871.2 7248.4
drag force:
(30º ~ Angle of Yaw)
(w/o area of FB Enclosure)
61.0 243.9 548.7 975.5 1524.2 2194.9 2987.4 3902.0 4938.4 6096.8

Note that this table describes only wind speed loading.  There is no component in the above numbers for the additional contribution of heave/surge due to sea state.  Hinz (Ref. 1) and others suggest that heave/surge can as much as double these numbers.  Hinz suggests that the displacement-to-length ratio (D/L ratio) of the specific vessel is the key contributing factor to rode loads for sea state induced heave and surge.  Live-aboards and long range cruisers have emergency gear, spares, tools, stores, big battery banks, genset, fuel, water, ballast, etc., so their D/L ration is higher than nominal; i.e., the boat is heavy.

At 20 knots, rode is only loaded to 300 (289.9) pounds of stretching force.  Doubling the wind speed to 40 knots would increase the rode loading by 4 times, to 1160 pounds.  And increasing the wind speed up to 60 knots result in rode loading of 2610 pounds.  At that point, rode loading exceeds the rated Working Load Limit (WLL) for 5/16″ BBB chain.  At a wind speed of 80 knots, rode loading increases to 4600 pounds, and exceeds the WLL for 5/16″ HT chain.

In consideration of the foregoing, analysis, I suggest some questions:

  1. In order to protect your family and your boat, what scope would you want deployed in 30 knot winds?
  2. At 40 knots, do you really believe you’d still have useful elasticity in the rode from the remaining catenary in an all-chain rode?
  3. Do summer thunderstorms contain 60 ~ 80 knot down-bursts?  Is considering that possibility really so unrealistic?
  4. At what point would you prepare for the possibility of higher winds?
  5. If you’re in a crowded a fair weather anchorage where a winter cold front is going to pass through in – say – 48 hours, what action(s) would be appropriate?

As for me, aboard Sanctuary, I would try to move to a more protected spot and I would put out all the rode I have, regardless of scope.  It would certainly be at least 10:1.

Reference:

1)  The Complete Book of Anchoring and Mooring, Second Edition, Earl Hinz, Cornell Maritime Press, 2001.

Related Posts:

1)  https://gilwellbear.wordpress.com/category/boat-technical-topics/anchoring/anchoring-reference-reading/

2)  https://gilwellbear.wordpress.com/category/boat-technical-topics/equipment-topics/anchor-and-ground-tackle/

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