The Offshore Voyaging Reference Site

One Anchor or Two?

So the question is: Should you use one anchor or two, and if two, in what circumstances?


Login to continue reading (scroll down)

More Articles From Online Book: Anchoring Made Easy:

  1. Introduction
  2. 4 Vital Anchor Selection Criteria and a Review of SPADE
  3. SARCA Excel Anchor—A Real World Test
  4. SPADE, SARCA Excel, or Some Other Anchor?
  5. Rocna Resetting Failures and Evaluation of Vulcan and Mantus
  6. Some Thoughts On The Ultra Anchor, Roll Bars and Swivels
  7. Specifying Primary Anchor Size
  8. Kedge (Secondary Anchor)—Recommended Type and Size
  9. Third Anchors, Storm Anchors and Spare Anchors
  10. Anchor Tests—The Good, The Bad, and The Downright Silly
  11. Making Anchor Tests More Meaningful
  12. We Love The Way Our Anchor Drags 
  13. Things to Know About Anchor Chain
  14. Selecting a Chain Grade
  15. Anchor Chain Catenary, When it Matters and When it Doesn’t
  16. Anchoring—Snubbers
  17. Anchor Rode Questions and Answers
  18. Q&A: Hybrid Rope And Chain Anchor Rodes
  19. Anchor Swivels, Just Say No
  20. A Windlass That Makes The Grade
  21. The Perfect Anchor Roller
  22. Install A Wash-down Pump—And Save Money!
  23. Anchoring—Kellets
  24. Anchoring—Chain: Stoppers, Termination and Marking
  25. 20 Tips To Get Anchored and Stay Anchored
  26. Choosing an Anchorage
  27. Choosing a Spot
  28. 15 Steps To Getting Securely Anchored
  29. One Anchor or Two?
  30. Two Anchors Done Right
  31. It’s Often Better to Anchor Than Pick Up a Mooring
  32. Yawing at Anchor, The Theory and The Solution
  33. Yawing at The Anchor, an Alternative Cure
  34. How To Use An Anchor Trip Line
  35. ShoreFasts—Part 1, When to Use Them
  36. ShoreFasts—Part 2, Example Setups Plus Tips and Tricks
  37. ShoreFasts—Part 3, The Gear
  38. Gale And Storm Preparation, At Anchor Or On A Mooring
  39. Storm Preparation, All Chain On Deck
128 Comments
Oldest
Newest
Inline Feedbacks
View all comments
mark boden

I agree with everything you’ve written John,

Chris

John,

Glad to see the fall didn’t affect your mind.

In years (far) past we have used Bahamian and tandem arrangements. The Bahamian nearly cost us the boat, and the tandem saved it in Hurricane Fredrick ’79. In each case we would have liked a single anchor solution. We have moved to an oversized Spade this season. Only time will tell if this was wise.

Stan

Hi John:
I need to set up a shorefast on my boat. How have you sized your shorefast, what material is it made of and how long is it.

Thanks
Stan

Shane

Hi John,

Our first season, after a few weekends of anchoring, my wife agreed she preferred being on the hook. Once I received that green light, a 10kg Rocna (w/chain) went on the bow of our 3200 lb. boat, as I knew the first time dragging would likely be our last time anchoring. We usually manage a pretty restful sleep.

Matt Marsh

“Bow and stern…. How much more are the loads?…. I would hazard a guess that they are at least three times greater if the wind is blowing from the beam.”

Sounds a bit low…. let’s check. Knowing Morgans Cloud‘s dimensions, I’d peg her projected area as about 3.75 times greater when viewed from abeam as when viewed from ahead.

From the photos you’ve shared with us, I’d guess her drag coefficients beam-on are about 2-3x larger (above waterline) and ~5x larger (below waterline) than their head-on counterparts.

So, as a very rough first guess using a metric boatload of crude approximations, you might be looking at (3.75 * ~4) = 15 times the forces if she is held beam-on, versus being held bow-on.

I think I’ll reserve that stern anchor for kedging off, thank you very much….

Matt Marsh

I hadn’t thought of it either, John, until you brought it up.

That a boat beam-on presents 3-4x more area to the wind and waves is, to most people, obvious.

That her drag coefficients are so much larger beam-on than bow-on should not be surprising- from the bow, she’s a streamlined body; from the side, she’s nearly a flat plate.

Combine the two, and you get some scary numbers. For shorefasts, I suspect it’s not as bad, because you (a) know the prevailing wind/wave direction, and (b) often leave enough slack that she can re-align herself by 30-45 degrees if she needs to.

Martin Spaans

The calculation of drag-coefficient and area resulting in an overall increase of approximately 15 x seems a bit over the top to me (especially the drag-coefficient part).
However, an other engineering aspect will also play its role. It is the working components (vectors) of the forces in the two lines. The working components shall equal the forces exerted by beam-wind and beam-current where these vectors are only (very) small relative to the line tensions. This is the case when the lines are tight. The working vectors become more effective when the lines are less tight and allow for better angles to act against the wind and tide.
Having said all this I would agree with both of you that the increase in line tension is unexpectedly big and that such positioning could end in unexpected failures where breaking of the fore rode is probably the worst.

Matt Marsh

Cd for a streamlined body with a blunt stern, like a sailboat hull end-on, is in the range 0.04 to 0.1 by projected area.

Cd for a beam-on hull is hard to figure exactly but, approximating from primitive geometric forms, should be confined to the range 0.3 to 0.8 by projected area.

Cd for the superstructure is on the order of 0.7 in either direction, but wind load on a trunk cabin and dodger is tiny compared to the forces of the water, waves and wind on the hull itself.

Projected area beam-on is, for a monohull, at least 3 times more than projected area head-on.

I didn’t believe it at first either, until I checked the Cd figures against several different tables. If you hold a boat like John’s at bow and stern so that it’s beam-on to everything, you’ll get about 15x the force as if you let it swing bow-on. That’s just the total force on the boat- not the tension on the rodes, which as Martin points out will depend on the exact vectors involved.