Friday, 6 February 2009

Rocna Anchoring 'Knowledge Base' for Sailors/Sailboats

Anchoring your sailboat successfully in many new and strange anchorages and in all kinds of weather can be one of the most traumatic and frustrating exercises in your cruising calendar.

It therefore pays to have first rate gear and a good knowledge of how to go about it in a safe and seamanlike manner.

In my post last year about anchoring (see my blog 'Delta Anchor Real Life Test' 28 May 2008) we looked at deploying a new Delta anchor for the first time. Included was a comprehensive test on fourteen anchors, ranging from traditional Danforth, CQR styles to the more modern types now available.

One of these was the Kiwi built Rocna anchor which rated very highly in the survey.

Rocna naturally, have their own website at http://www.rocna.com/ and included in it now they have what they call an anchoring 'Knowledge base'. This is well worth a read and you can view it on http://www.rocna.com/kb/Main_Page (underscore between Main and Page).

There you will find a host of anchoring knowledge which will have you drooling and impatient to get out on the water and put it to good use. Even the 'Old Hands' will find interesting information contained in it. Just reading the section on 'scope' v 'catenary' and looking at the graph is enlightening.

The following is an extract from that section:

Limits of catenary:
The problem with catenary is that it does not really provide that much assistance. While the force required to pull a length of chain bar tight is extremely high, the force required to straighten out most of its curve is actually relatively low. So we must then examine the anchor's requirements, what it is capable of handling at any given angle of pull and for any given seabed, and how this compares to the forces that can be expected to eliminate most of the catenary curve from the rode.

In the West Marine testing in 2006, anchors around the 15 kg (33 lb) size were tested, with a rode consisting of 1" (25mm) nylon rope with a 20' (6m) leader of 5/16" (8mm) chain. The Rocna was the top performing anchor – Yachting Monthly reported "The Rocna was a powerful, impressive performer in our tests, recording instant sets at multiple 5,000 lb maximum (or near max) pulls at 5:1 scope." The behavior of the rode at these limits of performance were described by SAIL magazine, who commented that "the anchor's resistance produces whirlpools of turbulence [from the test vessel's propellers]... and a bar-tight cable."

So, it is clear that modern anchors are very efficient. Some, particularly the Rocna, also endure high pull angles fairly well, even in poor holding seabeds. This means that the range of force vectors that the anchor can handle is quite wide, and it turns out that the majority of rode make-ups (at least those that are practical) lose most of their catenary curve well before the anchor is likely to be troubled. Beyond this point (once the rode is effectively straightened), the weight of the chain makes absolutely no difference to what the anchor will do, and the sole factor of import is the geometrical scope.

What does this mean?
The practical consequence of this is that it is not necessary to carry heavy chain merely for the sake of it. Rather, chain can and should be as light as possible, subject to strength requirements. Many boats could loose a large amount of weight by swapping to a lighter but stronger chain, and then investing part of that weight back into a larger anchor. Performance (holding power) of the system is thus substantially improved, while total weight is actually lowered. Read more in the chain section.

This extract reproduced courtesy Rocna Anchors.

You can read more about anchoring adventures in the Atlantic, Caribbean and Pacific Islands in my ebook 'Voyage of the Little Ship 'Tere Moana' downloadable from my website http:/www.sailboat2adventure.com


1 comment:

Craig Smith said...

You may be interested in Peter Smith's article on scope and catenary, at his own website (Peter is the Rocna designer). There are more computer simulations plus a fairly impressive photograph which illustrates catenary (or the lack of) in high winds. Let me know what you think.
www.petersmith.net.nz/boat-anchors/catenary.php