The Statistics of Anchor Dragging
Statistics & math word problems! Both can strike fear into full-grown adults. To a gambler in Vegas: they give an important edge, to a college sophomore: fear and dread of a ‘nightmare’ class. To sailors, the words may never be spoken.
While in Marigot Bay in St Martin earlier this year we went to bed looking towards the headland of Sandy Ground channel and awoke to find a 40 foot sloop just behind us on the rocks bordering that channel. There were 30 knot winds through the night. Someone dragged anchor and their yacht sank. It was under water for a couple days and then was raised. Not a fun weekend for that sailor. Maybe some anchoring statistics would have helped.
With that in mind, and just in case you missed those math word problems: an anchoring statistics word problem:
On a 5 year global sail around the world where you are crossing open ocean 15% of the time, on the hard one week a year, traveling off the boat a six weeks a year, and setting anchor on average once a day the remaining time, how certain must you be of every hook you set to have a less than 50% chance of dragging anchor once in those five years?
And we all thought sailing was fun!
So here goes some anchoring statistics:
- Total days on adventure: 5 years is 5 x 365 days /year = 1825 days. (we will leave leap year out of this) A total of 1825 days for adventure: awesome!
Now for all the days we will not be at anchor:
- 15% on open water = 0.15 x 1825 days = 273.75 or 274 days not anchoring because of passage: we love passage.
- A week a year on the hard for a total of 5 weeks on the hard = 5 x 7 = 35 days: not so fun, but we love a happy boat.
- Six weeks a year travel away from boat = 5 years * 6 weeks/year x 7 days a week = 210 days: we love road trips.
- Total # days not anchoring = 274 + 35 + 210 = 519 days the boat’s not anchored.
- Total # days anchoring (on Average) = 1825 – 519 = 1306 days at anchor, lets call it 1,300 anchor-days.
- So that is 1300 times anchoring (on average) in our 5 year adventure. Some days we anchor 3-4 times in a day, other times we are on anchor for several days at a time. We will average this out to setting anchor once a day for 1300 days.
- So for us to have a 50% chance of dragging once out of 1300 anchoring events over the 5 years we are looking for dragging .5 times in 1300. percentage chance =100 x ( 1-.5/1300 ) = 99.965% certainty of each anchor. To ensure a less than 50% chance of dragging (dragging .5 times during our 1300 attempts) we would need to have less than a four hundreds of a percent (.04%) chance of dragging on each anchor. That is a 99.96 % certainty that we will not drag!
This highlights the importance of making sure EVERY anchoring is secure or that you are on or watching the boat: it only takes once to end up with a boat sinking, as our unfortunate neighbor in Marigot Bay found out.
Most of the dragging I have seen appears to be from changes in wind speed and swell size and insufficient scope for the bottom type. Customarily, when I am in an unprotected anchorage, I ask the boats anchored closest to us how much chain/rode they have out. After that, I watch new arrivals and see what they are doing. A week ago I saw a boat anchor near us in a crowded harbor in the Aeolian islands north of Sicily. Later that night it dragged anchor and got very close to us in the middle of the night. Lesson: move if you’re feeling like the risk level is RED.
Our general considerations for dragging are:
- Scope of anchor chain/rode,
- Anchor size & type
- Chain weight and length and rode length
- Your anchoring technique: backing down, visual inspection after set, willingness to reset until right, etc.
- Depth of bottom,
- Bottom type (and your anchor types ability in that bottom type),
- Clocking (spinning) at anchor,
- Your ground rig’s ability to deal with clocking,
- Changes in wind/direction,
- Changes in swell size/period/direction,
- Your vessel’s wind profile and ability to take swell at various angles
- Did you actually see if it was set?
- Did you back down hard enough?
- Did you check all the ground tackle linkages? (broken or disconnected chain / rode / shackle can be a drag too.)
- Did you offer enough rum to Neptune in your last conversation with him?
FYI: been decades since my statistics classes, so please allow for a fair margin in error in any of the math.
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