One Anchor or Two?
128 CommentsReading Time: 6 minutes
Please Share a Link:
More Articles From Online Book: Anchoring Made Easy:
- Introduction
- 4 Vital Anchor Selection Criteria and a Review of SPADE
- SARCA Excel Anchor—A Real World Test
- SPADE, SARCA Excel, or Some Other Anchor?
- Rocna Resetting Failures and Evaluation of Vulcan and Mantus
- Some Thoughts On The Ultra Anchor, Roll Bars and Swivels
- Specifying Primary Anchor Size
- Kedge (Secondary Anchor)—Recommended Type and Size
- Third Anchors, Storm Anchors and Spare Anchors
- Anchor Tests—The Good, The Bad, and The Downright Silly
- Making Anchor Tests More Meaningful
- We Love The Way Our Anchor Drags
- Things to Know About Anchor Chain
- Selecting a Chain Grade
- Anchor Chain Catenary, When it Matters and When it Doesn’t
- Anchoring—Snubbers
- Anchor Rode Questions and Answers
- Q&A: Hybrid Rope And Chain Anchor Rodes
- Anchor Swivels, Just Say No
- A Windlass That Makes The Grade
- The Perfect Anchor Roller
- Install A Wash-down Pump—And Save Money!
- Anchoring—Kellets
- Anchoring—Chain: Stoppers, Termination and Marking
- 20 Tips To Get Anchored and Stay Anchored
- Choosing an Anchorage
- Choosing a Spot
- 15 Steps To Getting Securely Anchored
- One Anchor or Two?
- Two Anchors Done Right
- It’s Often Better to Anchor Than Pick Up a Mooring
- Yawing at Anchor, The Theory and The Solution
- Yawing at The Anchor, an Alternative Cure
- How To Use An Anchor Trip Line
- ShoreFasts—Part 1, When to Use Them
- ShoreFasts—Part 2, Example Setups Plus Tips and Tricks
- ShoreFasts—Part 3, The Gear
- Gale And Storm Preparation, At Anchor Or On A Mooring
- Storm Preparation, All Chain On Deck
I agree with everything you’ve written John,
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.
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
Hi Stan,
I have a piece on our shore fast system coming some time this winter.
Until then, you can get a lot of information by entering “shore fast” into the search box next to the menu.
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.
“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….
Hi Matt,
Wow, 15 times! I knew the problem was bad, but I had no idea how bad. Then when you consider that wind pressure goes up with the square of speed the whole thing gets very scary.
Your calculation really makes you think about using shore fasts too.
Thanks so much. I will change the post to reflect this new ( to me) information.
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.
Hi Matt,
Actually, in most cases where shorefasts are used they are kept really tight to stop the boat surging about and then coming up hard. Also, shorefasts are usually used in small coves where the wind gusts from a multitude of different directions. So I think the problem of high loads is at least as bad. On the bright side, if the small cove selected is tree surrounded the wind and gusting is dramatically reduced. This may be why shorefasts are much loved and used by those who cruise in Patagonia, but not so much by those of us who confine our cruises to the north where there are no trees.
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.
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.