Yet another anchor thread - size this time.

snowleopard

snowleopard

Well-Known Member
Joined
16 May 2001
Messages
33,645
Location
Oxford
Visit site
One of the trickiest aspects of choosing an anchor seems to be deciding what size to get. Lots of people - manufacturers, chandlers and independents - publish tables of what size anchor to use on a range of boat sizes so how do you decide?

Of course if you have a heavy displacement boat you can always go up a couple of sizes to be sure but for the lightweight brigade a 40 kg anchor and 100m of 12mm chain is out of the question.

I did a bit of research and came up with size recommendations from a selection of websites. I used the manufacturer's sites where available. Here is what I found as recommendations for a 40 footer:

Delta 16 kg
Spade 20 kg
Rocna 20 kg
CQR 16 kg
Fortress 6.8 kg (Aluminium)
Manson Supreme 35/45 lbs (16/20 kg)
Bruce-type 16 kg

Particularly notable was that the tables for the CQR and Delta didn't suggest going up to the 20 kg size until 45'+

I would have expected that the pattern would have been reversed with the older designs like Bruce and CQR with reportedly lower holding power needing to be heavier.

Now we've all seen posts along the lines of 'my 300% oversize CQR has never dragged' but how about those who haven't 'gone one size bigger'? For my cat, Spade recommend 30 kg ('one size bigger for multihulls'). But I have anchored in some pretty boisterous conditions over the years and never dragged with my 16kg Delta (except for a couple of occasions in liquid mud where I switched to a Fortress).

So - why do the manufacturers of the new anchors demonstrate their confidence by specifying larger sizes? For those who have bought anchors that didn't come with the boat, how did you decide what size to get? Anyone got links to an independent and comprehensive sizing table that covers a wide range of makes?

There - got to the end of the post without saying 'size matters' once. Oops.
 
When I replaced our draggy 35lb CQR a few years back I researched the alternatives and settled on a aluminium Spade. On a visit to Jersey I spoke to the importer about my plans and he was horrified about hanging Adonnante on an aluminium anchor, insisting it would bend. His recommendation was for a 20kg steel Spade which I took. We have used it over the last 3 years on 20m of chain and 80 of rope, 100+ nights at anchor, without any dragging or deployment problems. The anchor and chain is stored and deployed from the main beam, weight reduction is accomplished by carrying wine in boxes and insisting that crew diet before sailing.

Peter.
 
All the "recommendations" from manufacturers should be considered as nothing more than their interpretation of what you need. How much science goes into their calculations is open to debate.

This website http://alain.fraysse.free.fr/sail/rode/forces/forces.htm is probably the most elaborate I have ever seen, and goes into a lot of detail on how changing conditions and environment can make a huge difference to your needs.

What I found most interesting was that, according to his calculations, mixed chain andrope is WAY better than chain only.

I would be interested to hear from others who are more able to understand and assess his science than me. A-level physics was as far as I got and that is clearly way short of the mark for some of his stuff!
 
snowleopard said:
I did a bit of research and came up with size recommendations from a selection of websites. I used the manufacturer's sites where available. Here is what I found as recommendations for a 40 footer:
Click to expand...
You must be very careful - in general you are not comparing like-for-like. Without knowing to what standard the sizing is done, you have no idea what the basis is. Is the sizing worked out for particular wind speeds, storm conditions or a light lunch time stop, or is it just some vague rule-of-thumb with no real thought at all? With the older anchors it tends to be more the latter, frankly.

snowleopard said:
I would have expected that the pattern would have been reversed with the older designs like Bruce and CQR with reportedly lower holding power needing to be heavier.
Click to expand...
If they used the same criteria, that would be the case - but it would look quite ridiculous; if Rocna used perfectly objective testing data and made sizing recommendations for the CQR, the sizes would be almost double that of the Rocna. For the Delta, about 50% higher.

snowleopard said:
So - why do the manufacturers of the new anchors demonstrate their confidence by specifying larger sizes?
Click to expand...
Spade was/is influenced by French regulations. Well I'm not sure about that but it's what Poiraud said at one point.

Rocna are very conservative, their criteria is generally 50 knots wind in poor/moderate holding. That's far in excess of most manufacturers. In this case it's just the result of a background in world / high latitudes cruising on the part of the designer/founder that has informed Rocna's ethos. We don't want people needing to go with the old mantra "go one size up". Sizing can be done with more rationality than that.

Quite a bit more detail here:
www.rocna.com/kb/Rocna_sizing_recommendations

With larger anchors, much about 100 kg, classification society rules or other regulations can kick in and bring a degree of standardization, which frankly is nice.
 
SolentBoy said:
This website http://alain.fraysse.free.fr/sail/rode/forces/forces.htm is probably the most elaborate I have ever seen, and goes into a lot of detail on how changing conditions and environment can make a huge difference to your needs.

What I found most interesting was that, according to his calculations, mixed chain andrope is WAY better than chain only.

I would be interested to hear from others who are more able to understand and assess his science than me. A-level physics was as far as I got and that is clearly way short of the mark for some of his stuff!
Click to expand...
It's great, with a few small flaws which don't matter much in the scheme of things. His data for forces on boats, extracted from the old ABYC tables, is way over-estimated for just wind - but about correct for peak forces if there's much fetch/surge. He slightly biases the case for rope in its favor without properly considering the situation in repeated dynamic cycles, but it's good enough. Lastly there is no consideration of how the picture changes with scale, along the spectrum from tiny boat to large ship (it alters radically), but it's spot on for let's say a 30 or 40 footer.

A bit more here, using some of Alain's input:
www.petersmith.net.nz/boat-anchors/catenary.php

To inform anchor sizing though you also require objective testing data that's not exaggerated by the manufacturer. You will tend to find that there is a tendency to list low sizing recommendations... it makes the anchor look better (and cheaper).
 
craigsmith said:
With larger anchors, much about 100 kg, classification society rules or other regulations can kick in and bring a degree of standardization, which frankly is nice.
Click to expand...

With representatives from three of the major anchor teams on this thread - why do you not agree a criteria for standardisation so that the public actually has a chance of comparison.

But please do it somewhere else!
 
I took into account manufacturers recommendations but also my knowledge of health and safety practice (don't laugh - I need it for my job!) which suggests that the maximum safe load for a non-deformable object carried by an adult male not held tight against the body and not lifted above waist height is about 25 kg (55lb).

Since I need to carry the anchor on my own occasionally and 25 kg was the recommendation for a 40 ft cat, everything seemed to come together beautifully! :)

Richard
 
Adonnante said:
When I replaced our draggy 35lb CQR a few years back I researched the alternatives and settled on a aluminium Spade. On a visit to Jersey I spoke to the importer about my plans and he was horrified about hanging Adonnante on an aluminium anchor, insisting it would bend. His recommendation was for a 20kg steel Spade which I took. We have used it over the last 3 years on 20m of chain and 80 of rope, 100+ nights at anchor, without any dragging or deployment problems. The anchor and chain is stored and deployed from the main beam, weight reduction is accomplished by carrying wine in boxes and insisting that crew diet before sailing.

Peter.
Click to expand...
We went for an aluminium Spade and used it for 10yrs+ (about 1000 nights+) with no bending/dragging problems at all. With our 25' boat we went for a Spade size for over 25' but in Ally so we gained in anchor size without the increase in weight.
Malcolm
 
In very severe conditions ( as shown in Craigsmiths link) there is very little catenary and the holding limit is mainly dependent on the angle to the seabed that the anchor will tolerate before starting to pull out. This is controlled by the scope of the anchor rode and of course the design of the anchor. With the much maligned Bruce design, the limiting angle is about 8degrees which requires a 10times depth anchor rode. More modern anchors may have a higher break -out angle and I would be interested to know if this is a number that Rocna ( or anybody else )publishes.
 
There's no magic number for any anchor, it depends entirely on the seabed make-up and the depth of the anchor's set. Higher holding power will correlate to greater toleration of low scopes; once the catenary is eliminated, the anchor will orient to the elevated pull, and its holding power is dictated by how well it's set (informing the characteristics of the 'wedge' of substrate it has between it and the seabed surface) and how much resistance it can put up.

The only sure thing is that the angle is best minimized, with any anchor.
 
When I purchased my 27 footer it had a 10kg KQR piece of rubbish as bower anchor which I quickly changed to a 16kg delta which was the best I could afford at the time. For a 27footer this might be over kill but I sleep well at night.
 
craigsmith said:
. His data for forces on boats, extracted from the old ABYC tables, is way over-estimated for just wind - but about correct for peak forces if there's much fetch/surge. He slightly biases the case for rope in its favor without properly considering the situation in repeated dynamic cycles, but it's good enough. Lastly there is no consideration of how the picture changes with scale,
Click to expand...

Care to elaborate on that? Some figures and the appropriate calculations would be useful.
 
Stop this discussion immediately!

You are all wasting your time, in a year or so's time there will be nowhere you can anchor!
 
You must log in or register to reply here.

Share this page

Top