JimB Sail newsletter

Once every few months we'll email you about site additions and cruising news - unless you unsubscribe, of course!

Subscribe to JimB Sail newsletter feed

Anchoring and Mooring

Introduction

When spending more time on board, you'll want to leave the boat at anchor while you spend time ashore. In tidal waters, the stream may reverse while you're away. In any waters, wind direction may change, or wind strength may increase.
 
Most discussions about anchoring revolve around "which anchor is best". Criteria being "holding power", "scope needed", "setting ability", "tolerance to changes in direction of pull". These discussions usually assume you use a single rode, and often ignore snatch loads.
 

An alternative, in challenging conditions, is to consider using two rodes. 

Two Rodes

Why?  Laying two rodes ensures that each anchor only has a force applied to it which is approximately in line with the direction the anchor was set. This removes the possibility that a tide change, a wind change, or "sailing" from side to side at anchor will upset a single anchor An upset anchor has to re-set itself, with some risk of clogging or fouling.
 
In non-tidal waters, one rode is often replaced by a line (or lines) ashore or to a quayside..
 

Running Moor/Tidal Moor

In a tidal moor, a first anchor is set up tide, a second is set down tide. The two rodes are lashed together, and the lashing point dropped below keel level. As tides turn, the up tide anchor usually takes the strain. In strong wind over tide, maybe the down tide anchor takes the strain. Mostly, tidal moors are in estuaries, with reasonable wind shelter, so strong sidewinds are unlikely.
 
A running moor (also Bahamian moor) also deals with wind across tide. Same principle; drop one up tide, the other down tide, dig 'em in. This time allow enough slack between anchors to leave a 120 degrees splay if wind is on the beam. A 120 degrees splay shares the rode stress between anchors, without either anchor being pulled above the stress of a single rode (which would occur with a wider splay angle). Yes, directions of pull will change with wind direction changes. Most anchors can tolerate this without upset.
 

Forked Moor

The forked moor is used to improve anchoring performance in stronger winds from a reliable direction. It reduces the tendency of most sailboats to "sail" from side in winds over 25kts or so.
 
With a single anchor, the swing sector is likely alternately to pull the anchor with a 90 degree change in direction at each snatch. This has three effects:
  1. The bigger the swing, the higher the snatch load, and the closer you'll get to exceeding the anchor's holding limit.
  2. With each change in pull direction, the anchor works slightly down-wind.
  3. Each direction change may un-set the anchor.
The forked moor overcomes these disadvantages. First, the sideways swing is much reduced by the splay. Second, this means snatch loads are much reduced. And thirdly, the snatch loads are taken "in line" by the current windward anchor, so there's much less re-alignment taking place.
 

Reducing Inertia Forces

When boats are checked whilst roaming around their scope, inertia forces are many times greater than just wind drag. This becomes obvious if you watch your single rode in a strong wind.
 
Depending on the boat, from 50% to 75% of the time, the rode will lie 30 degrees or more below the horizontal (my definition of slack). While slack, the boat is accelerating laterally, storing velocity energy from the wind. Then the rode lifts and reverses that acceleration over a similar or even shorter period. This implies 2 to 4 times the wind drag force (average!) is being forced on the rode. Time your own boat . . .
 
Chain. Tradition has it that chain catenary (plus chums, maybe) spread that snatch over time. However, catenary forces are asymptotic, slow to have effect, peaking very rapidly.
 
Nylon. With linear stress/strain behaviour, nylon allows 5% to 10% stretch, up to 4m for a typical 40m rode. Watch your boat when it's on a forked moor with one nylon rode (OK, with a few metres of chain close to the anchor), and a similar length plain chain rode. Note how long the reversals take between slack and slack. And how long slack lasts if you want the whole story.
 
A Bit of Both. Of course, the optimum is to add stretch to your chain rode with a 20m length of light nylon snubber line. 20m - to gain 2m of spring. And light, because you're going to limit its stretch by only letting out 2m of chain slack. How light? Well, if the chain regularly hits limits, it's too light!
 

Choosing Anchor Types

Nothing like sticking one's neck out on this subject. Here follow my opinions after many years of chartering many boats with many different types of anchor - in addition to cruising my own boat.

Old style Anchors. Hinged plough anchors (CQR type) and claw anchors (Bruce type) used to be very common, but compared to others, they need careful anchoring techniques, set unreliably, and often have to be re-set.

Flat anchors (Danforth, Britany, Fortress, Admiralty pattern) are light for their holding power (especially the aluminium alloy Fortress), need a lot of scope, set reasonably easily, but don’t tolerate changes in the direction of pull. However, they are very suitable as stern kedges when on a quay, or as a second anchor on a forked moor.

New Generation Anchors. Tests show that a range of more modern designs (Spade, Bugel, Rocna, Manson, Sword, Delta and many others) set more reliably (some on shorter scopes), tolerate wind shifts better, and provide better holding power than older generation anchors. The tests also have difficulty proving significant differences between these modern anchors, which gives rise to many arguments about which is better. This is a pointless argument, because if you can't reliably differentiate, it proves they're all all good enough.

Conclusions. Check the prices of new generation anchors, check which will fit your hawse. Changes to your hawse may be more expensive than changing the anchor! And, for deployment convenience, use a flat anchor as a second anchor, preferably light weight. Deploy old CQR anchors as chums if you don't want to throw them away . . .

See also http://www.jimbsail.info/mediterranean/mooring

Last checked: Apr 2016


 

To save yourself money (and help pay site costs) buy discounted charts and pilot books through our Book Store

Alternatively, send me the price of a beer or two through PayPal!