Rocna sizing recommendations
This page is an overview of the philosophy and rationale behind our official anchor sizing, which is intended to provide an anchor adequate for use in most all conditions.
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Rocna sizing
Our sizing is considerably more conservative than all other manufacturers and must not be compared on a like-for-like basis.
The Rocna sizing chart
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Some content referred to in this section is located at the main Rocna Anchors website. To avoid duplication, this content is not repeated here. |
Our sizing chart is a simple two-dimensional table, with vessel lengths down the left and displacement ranges in metric tonnes along each row (1 t = 1,000 kg = 2,205 lb). To choose the correctly sized Rocna, the vessel's length (overall) selects the correct row, along which is found the appropriate displacement range (choose the heaviest, i.e. the right-most range, without exceeding the vessel's loaded displacement). The resulting column then provides the recommended Rocna model.
The sizing chart is found on our main website.
Multihulls
Our chart is intended for monohulls. In general, an anchor for a multihull should be up to 50% larger than that for a monohull of the same LOA. Commonly this means going to the next higher recommendation (one size up), depending on how close the vessel is to the upper limit of a particular size range.
Vessels larger and/or heavier than covered by the Rocna chart
Those with boats for which the Rocna 110 (243 lb) is too small (according to our recommendations) will find that our sizing chart does not extend to a large enough range. For more on sizing anchors for these vessels, please refer to classification requirements below.
The rationale behind our sizing
Peter, the Rocna designer, comes from a background of world cruising, and we tend to consider gear inadequate if it is not suitable for extreme environments. By this we mean the anchoring scenarios found in high latitudes northern Europe, Greenland, southern New Zealand, Patagonia, Antarctica, et cetera.
Our sizing is conservative, intended to provide an anchor adequate for use in all conditions most boaters would ever endure. We base our calculations on 50 knots wind, associated surge, and soft moderate holding bottoms into which it is assumed the anchor has set. Adequate scope is assumed. This is far in excess of most manufacturers.
Windage and resulting forces is judged based on typical vessel profiles according to LOA and displacement.
Note that tidal flow generally does not generate a hugely significant amount of force and in most areas can be all but disregarded. On a typical 10m (33') yacht, it takes a 6 knot current to generate about as much force as a 20 knot breeze on the same boat.
Naturally there are many variables involved, and in many situations an adequately sized Rocna will easily handle far worse winds than 50 knots. There are others where even a Rocna will not hold well. However, our aim is to consider a realistic poor case scenario as the basis for our recommendations.
Sizing secondary anchors
The factors which apply to the primary anchor may also apply to any secondary anchors, such as stern anchors, although usually it is undesirable to carry multiple anchors of the same weight as the primary. Typically our advice is for the secondary anchor(s) to be one size smaller than the primary, depending on the intended usage and how often multiple anchor rigs are deployed.
Other manufacturers' recommendations
There is a tendency amongst anchor manufacturers to recommend sizes smaller than those really required. They invariably suggest very optimistic sizes for impractically light conditions. For example, a certain world-renowned name-brand advises the use of a 20 kg (44 lb) anchor on a heavy cruising yacht of LOA up to 14.8m (49'). This recommendation is far too light when the tested performance of this particular anchor is considered in conjunction with the forces this boat size may be expected to generate. Another brand states that a working anchor should hold "up to 30 knots of wind". In our view, an upper limit of 30 knots wind would make the anchor a very light-weight temporary hook which has no place as the primary bower on any vessel.
The tendency to this misleading advice exists simply because it is thought to make the anchor in question look better in the eyes of the customer – either better performing than the competition, or cheaper, or both. Additionally, a manufacturer producing a number of different 'styles' of anchors, commonly copies, may feel the need to 'position' a particular design in such a way so as to rank it 'correctly'. This distorted logic results in compounded errors and unrealistic sizing.
Over-sizing
Many Rocna customers, in improving their anchor type by switching to a Rocna from an older anchor which they have learned to be unreliable, make doubly sure about their upgrade by also increasing the weight of their anchor. We see this tendency a lot, and try to discourage it. As above, our official sizing is very conservative, and in many cases it's a case of "don't over-size – we already did that for you!"
On a weight-for-weight basis, the Rocna represents a very significant step up from most other types, and doubling the size (for example) could inadvertently cause serious problems with retrieval and other handling issues.
Going over our sizing recommendations could in some cases be justified, for example by the requirement to use very low scopes (short rodes). In this case, a higher angle of pull will be applied to the anchor, necessitating a higher element of dead-weight in the anchor to resist this. As a real world example, Steve Dashew uses a Rocna 110 on his 84' powerboat Wind Horse. Our recommendation for this boat is a Rocna 70. However, Dashew routinely anchors in tight and non-ideal anchorages, and reports his Rocna perfectly secure at scope as low as 2:1.
Dead weight can be required in other circumstances, such as when anchoring on a sea-bed into which the anchor cannot possibly be set (e.g. solid rock, or a very thin layer of sand over coral pan, etc). The anchor then depends solely on its weight in order to generate friction against the bottom.
Matching to other anchoring equipment
On a weight-for-weight basis, a Rocna is capable of sustaining far greater forces than many other anchor types. If the anchor on a vessel is being upgraded, this implies that the other anchoring tackle onboard may also need to be upgraded, if it was matched to the old anchor.
This may be an opportunity to switch an anchoring system designed with old philosophy to a more modern and more efficient set-up, for example saving weight out of the rode by using high tensile chain.
Sizing for equitable holding performance
The Rocna out-performs all other anchor types in most real world scenarios. This includes straight-line pull performance. A benefit of this is that when specifying an anchor to handle any given force, a Rocna may be smaller than another type. Conversely, another type will need to be larger in order to offer comparable holding power.
The actual differences depend obviously on the anchor type, the seabed, and the scope of the rode.
Nb.: Such sole consideration of resistance for a straight line pull ignores the fact that in the real world, most failures and draggings occur in different scenarios. Under this false assumption, it may be concluded that a certain size anchor of another type could replace a Rocna – but this would likely be incorrect for general purpose use, as the Rocna will better handle the combination of actualities thrown at it in the real world, and it could take a still larger alternative anchor to provide equitable performance.
Classification requirements
Those looking at a Rocna anchor larger than the Rocna 110 (243 lb) will notice that our sizing chart does not encompass these models. The reason is partly because, with vessels of the size these anchors are intended for, it becomes too difficult to recommend a size based on only two factors. Furthermore, most vessels of this size are built to a classification standard, or need to adhere to local regulations, both of which may speak to the anchor sizing. If this is the case, those rules should be consulted – normally a formula is a provided which takes as its factors a multiple number of attributes of the vessel.
Society rules involve the usage of an "Equipment Number" (EN) formula, which takes as input factors a number of key dimensions of the vessel. A size for a standard stockless anchor is then dictated by a table according to the vessel's EN. An HHP classified anchor may be 75% the listed requisite size, while an SHHP anchor such as the Rocna may be smaller by fully 50%. Because the SHHP holding power standard is in fact 400% that of a standard stockless pattern (not 200%), this means that the society rule for sizing an SHHP anchor is in fact far more conservative than that for the default stockless types.
In turn this means that the SHHP Rocna may be one third smaller (by mass) than any HHP classified anchor such as all old generation types including the CQR plow (and copies), the Bruce claw (and copies), the Danforth, and the Delta plow (and copies), while still meeting classification requisites.
For classified anchors in the size range for which we do provide recommendations, it is the case that our sizing is at least as conservative, and invariably more conservative, than the requirements of all classification, coding, and other standards.