Anchors - Bigger is Better ?

[DumnMad]

'We are convinced that the increase in anchor holding as size increases is far more than linear.'

Makes sense to me for ground where heavier means diggier. Not so convinced if its sitting on hardpan.
Unlike beach tests the storm test keeps jerking the anchor for hours on end and with partly liquified soil around it, a well designed heavier anchor will dig deeper and deeper. For sandy silty soil double the depth gives roughly 4 times the holding. Anchors with large surface per kg will have more friction per load to restrict its digging ability.
No rules however since no two anchorages will be exactly the same.(Soil grades & depth, wind, shelter, waves, current etc.)

[JonJo]

[QUOTE=DumnMad;1222405]'We are convinced that the increase in anchor holding as size increases is far more than linear.'

a well designed heavier anchor will dig deeper and deeper.
Anchors with large surface per kg will have more friction per load to restrict its digging ability.

Unquote

What you seem to suggest is that Rocna, Supreme, Fortress, Mantus, Super SARCA which are arguably all godd designs but all aim to maximise surface area (rather than weight) so have a high SA:Wt ratio will not perform as well as the equally good designs of Spade and Excel - which both have weighted toes?

Jonathan

[DumnMad]

No. A bigger Rocna has less surface to weight than a smaller one.
As far as I know, in these designs surface area goes up to dimension square, weight goes up by dimension cubed.

[JonJo]

Originally Posted by DumnMad

No. A bigger Rocna has less surface to weight than a smaller one.
As far as I know, in these designs surface area goes up to dimension square, weight goes up by dimension cubed.

Sorry, I misunderstood - you mean for a given design as 'size' increases the gravitational effect(?) - caused by jiggling (or continuous, on/off, tugging), will allow the anchor to set more deeply.

This would suggest that if Rocna, Supreme wanted to improve their designs they might consider thickening up the fluke plate to get more weight into it? And/or a steel Danforth should work better than an alloy Fortress and a steel Spade work better than an alloy Spade (this latter would contradict what the manufacturers say of their steel and alloy anchors viz - identical holding capacity).

Jonathan

[DumnMad]

The better designers are not silly, they know conditions vary and I presume have chosen a balanced approach. Probably chosen a compromise with a reasonable sized area which is good in some ground and high weight per surface which is better for digging in. Alloy may be the best in soft mud but not so good in the hard stuff.
I think its not a matter of improving the design so much as cruisers learning how to use the equipment.

[Snowpetrel]

Originally Posted by JonJo

'We are convinced that the increase in anchor holding as size increases is far more than linear.'

Now there are some engineers on this thread - surely someone can come up with a sensible technical reason for this. This, to me, lacks logic - there must be a technical reason. Knox suggests holding capacity is linear, at the other extreme Vryhof suggest holding capacity is linear (both by weight as a proxy for surface area).

Carefull. weight (m^3) is not a proxy for surface area (m^2) alone. more like surface area*depth of set, which gives the correct units (m^3) Surface area being m^2 and depth set being m (m^2*m = m^3)

I recall there has been a similar comment of the Bruce anchor (that it worked better in larger sizes), I think based on the genuine model but perhaps true of the myriad of copies, that it worked 'better' at over 45kg (I'd heard variously 50kg and 75kg for the Bruce).

It's all interesting, and slightly confusing. An old approximate formula for anchor holding was K*blade area*depth of set^2. K is a fudge factor to account for bottom density and anchor design. This gave the anchor holding increasing as meters^4 or weight^(4/3) which sort of agrees with the results from here

A later formula gives K*blade area*depth of set (this being the same as m*m*m or m^3)as an approximate formula. And this is much closer to the vryhof and bruce standard formulas, with holding increasing proportionally to weight^1 (weight being the same as m*m*m or m^3), or in the case of vryhof they give it as K*weight^0.92 or thereabouts.

one explanation for the discrepancy is that the first formula (k*area*depth^2) is for anchors that are not fully embeded or that do not imbed deeply and have a hump over the anchor. The second is true for deeply set anchors.

The actual maths for anchor holding is much more complex and includes internal and external angles of friction and all sorts of other stuff that is way beyond me. So we can only consider these as very crude and theoretically incorrect approximations, but they do have the benefit of being better approximations that either weight alone or surface area alone.

I think the deeper complexities are the reason why results and scaling vary so much at times. I really think the depth of set is the big missing part in what we are assessing when we look at anchor design. It is true to say the anchor with the larger surface area will have a greater holding power. But only if both anchors are set to the same depth in the same bottom type. If you factor in that say (for arguements sake) that a sarca excel may set deeper than a rocna because it is not hindered by the roll bar this extra depth of set may more than make up for the fact that it has slightly less blade area.

I am pretty comfortable to say that normally anchor holding power varies roughly proportionally to weight^n (K*weight^n)and n varys between about 0.7 to 1.4 with a good average being 0.92 and a good approximate and convenient value being 1, ie proportionally with weight. but this is based on what I have read not actual tests I have done. I would love to get my hands on some real data to see what comes out.

I would love to know what data has been used here to get K*Weight^1.4. Part of me wonders if the results are due to some of the smaller anchors not setting properly due to a hard bottom. this would skew the result in favour of bigger anchors, but without data I can't really make any meaningful comment.

[Snowpetrel]

Originally Posted by DumnMad

Unlike beach tests the storm test keeps jerking the anchor for hours on end and with partly liquified soil around it, a well designed heavier anchor will dig deeper and deeper. For sandy silty soil double the depth gives roughly 4 times the holding. Anchors with large surface per kg will have more friction per load to restrict its digging ability.

Makes sense. And fits with the SA*depth^2 formula. Call it a weight embedment anchor rather than a drag embedment anchor.

I take it the "sandy silty soil double the depth gives roughly 4 times the holding" is a standard soil mechanics rule of thumb? This seems to explain some stuff but not vryhofs unless it's more like smoothed circumference*depth of set^2

Cheers

[Kettlewell]

I have read some texts on offshore oil rig anchors that indicated holding power is not a linear function of weight, but drops off as weight increases. In other words, heavier anchors of the same design become gradually less efficient. What I wonder is if within the very limited range of weights we're discussing for most normal-sized pleasure boats the relationship is basically linear due to our focusing on a very small portion of a much larger curve. There may be some increase or decrease in efficiency, and it may vary depending on the design of the anchor, but it is so slight as to be not consequential. We aren't trying to fine tune our holding power so we have just enough, but no more.

[noelex 77]

The scale effects for some anchors are quite noticeable.
The Bruce for example works much better as a large anchor (on a correspondingly large boat)

The scale effects for most other anchors are not as great, but there is still often an improvement in setting, and particularly, in reliability for larger anchors.

Obstructions such as weed roots are much less of a hindrance to a large anchor. These factors difficult to mathematically model, but are noticeable in the real world.

[JonJo]

Originally Posted by noelex 77

Obstructions such as weed roots are much less of a hindrance to a large anchor. These factors difficult to mathematically model, but are noticeable in the real world.

I'm not sure I follow this.

The ability to cut through weed root will be contingent on the power of the engine or the windage (again call it power) of the yacht.

The only way a large anchor will cut through weed more effectively than a small one will be if the power to weight (or SA) is higher than for another smaller yacht with smaller anchor. Effectively a small anchor will cut through weed more easily for a given weight and yacht power (that does not mean it will be a better anchor (being small it will lack capacity) but provided it engages with the seabed it will cut weed more effectively. This is why Luke's enjoy support (though they do not cut through but push between the stalks).

I liken it to cutting turf with a shovel or spade, given the individual only has their own weight (as power) then its easier to cut turf with a small spade than a larger one.

Jonathan

[noelex 77]

You are assuming the "power" of the yacht is translated to cutting through the weed roots. This is not the case.

Originally Posted by JonJo

but provided it engages with the seabed it will cut weed more effectively.

This is the important point. A small anchor will not engage the seabed in thick weed.
A small anchor has a greater tendency to sit on top of the weeds. The yachts "power" is translated to pulling the the anchor horizontally.

It is easy to see the effects diving. Small anchors struggle to engage the underling substrate in thicker weed. Often the boat is held by the grip on the weed, rather than the soil.
Adding more "power" (say in the form of a strong wind) frequently causes the small anchor to drag rather than dig in.

In weedy locations a large anchor will set better than a small one.

[Kettlewell]

I suspect that usually a larger version of the same anchor pattern will penetrate weed better, but when comparing anchors of the same weight but different designs there may be marked differences in weed performance. And, many weedy bottoms are not uniform, so the person with the lighter anchor may have no problem getting a hold because the anchor happens to drop in the right spot. And, then there is weed and there is WEED! I've always had a difficult time getting anything to hold properly in Black Sound on Green Turtle, for example. The grass in there is thick.

[JonJo]

I think Kettlewell's comment is valid - that there is weed and WEED. However I am yet to convinced that a run of production 35' yacht will be unable to set a 25kg anchor in weed yet be able to set a 40kg anchor, of the same design, in the same weed bed. In order to penetrate the anchor needs load or power and in this case the load and power is finite. That load and power is imposed over a smaller cross sectional area for the 25kg anchor but a bigger cross sectional area for the larger model. In the same way a Luke penetrates more easily (with its very small volume toe) I fail to see why the same logic is not extended to the smaller anchor (of the 35' yacht). If we wish to extend this discussion to a 35' motor boat with factorially higher power I would agree there is not an issue, the bigger anchor will set better because the MoBo has 'excess' power. Most people who sail are stuck in the 30'-45' range and engines were auxiliary, so small. It was (is?) not uncommon for a 35' yacht to have a 20hp or 30hp engine and in fact having such power base to set even a modern anchor of 40kg, on the basis that the 25kg is an historically accepted size, in a hard seabed looks a big ask.

Jonathan

[Kettlewell]

By the way, my 45 lb. Bulwagga is a great weed anchor. In fact, it was specifically designed to deal with the weed in Bulwagga Bay on Lake Champlain. It also did very well in the hard-packed sand bottoms found some places in the Caribbean, where I think the double sharp points create a clawing action on the bottom. The shank is short and made from round bar stock, and I imagine something else on the anchor would break long before you could bend the shank. It's too bad they are no longer made.

[cal40john]

Originally Posted by nigel1

Made a couple of video's last trip featuring various types of rig anchors, if your interested, enjoy

http://vimeo.com/78979406


http://vimeo.com/78368272

This is a private video, you have no permission to watch it.