Regular steel coated in zinc by way of a galvanizing process is the typical and most economical material used for anchor construction.
The quality of both steel and the galvanizing process can vary dramatically. Steels vary in both strength, mild through to high tensile, and metallurgical quality. The construction process (casting, forging, or casting) can also have a large impact on quality.
The fluke and roll-bar components of the Rocna make use of geometry to build strength beyond that provided by high quality steels, with carefully chosen folds and reinforcement shaping in the required surfaces. The fluke has a single fold in its shape, down the middle, which means that the narrowing tip maintains the presence of this fold (a simple high radius curve would see the tip be relatively flat and weak). The skids reinforce the heel of the fluke, which is thinner than the tip in order to assist the weight balance of the anchor. The roll-bar then locks the skids and the heel of the fluke together, creating a massively strong structure.
The shank on any anchor is a common failure point, normally bending when a high lateral load is applied (for instance, when the anchor fouls on a submarine obstacle and is jammed). For this reason, the shank on the Rocna is a high tensile steel. This adds to the price of the anchor, but compromising this strength is not something we would entertain.
The zinc coating to prevent the steel from rusting should be added by a process called hot dip galvanizing. Other processes exist but are inferior and will result in less durable finishings.
Rocna steel anchors are hot dipped galvanized to a specification exceeding all industry standards.
If the most economical option is not required, stainless steel presents a practical option which is more aesthetically pleasing than galvanized anchors. It also benefits from not requiring the re-galvanizing of steel anchors which tend to wear out their zinc after a number of years of regular usage.
Stainless steel however is very expensive. Good quality raw material is much more costly than regular steel, and the finishing process required adds further to the price.
You should expect a stainless steel anchor to cost at least 3-5 times that of the galvanized version. If it is cheap, the "get what you pay for" factor will kick in. There are many very cheap cast stainless anchors, typically copies of more reputable brands, but the quality of the steel used in these anchors means they should be condemned and avoided at all costs.
Stainless steels vary in grade just like regular steel. The high costs mean that manufacturers have all the more incentive to make use of the cheaper weak grades, and unfortunately this is commonplace, meaning that stainless anchors are generally weaker than their galvanized counterparts.
Many stainless steel anchors are built entirely of 316L stainless. This grade is "marine" stainless, and has fair corrosion qualities for use in the marine environment. However, it does not have much tensile strength (measuring around 280 MPa depending on sheet thickness), a grade which means it is weaker than mild steel. A shank on a stainless anchor built from 316L is likely to be woefully under-strength and unacceptable against any reasonable design criteria - certainly that of Rocna.
All stainless steel Rocna anchors have a high tensile shank. This is costly and occasionally difficult to procure. However, like the galvanized anchor, we refuse to compromise this important component of the anchor.
There are several options for the finish on a stainless anchor. Electropolishing is an automated and low cost process which brings out a pleasant shine on the anchor, but leaves an uneven surface and does not achieve a mirror finish. Hand polishing is the only way to attain the ultimate mirror finish look that is desirable, and this process involves a high number of man hours and thus is costly. The result however when installed on the bow of a boat is considered by many to be priceless.
Like any stainless fittings on a boat, maintaining a perfect aesthetic finish and shine will require regular hand polishing.
There are a number of aluminium anchors available. In general, aluminium offers higher strength for the same weight, but also higher volume (e.g. aluminium plate must be thicker in order to retain the same strength as steel, although it is nonetheless lighter). An aluminium anchor following the same geometry of its steel counterpart will not have the same strength, particularly if the latter uses high tensile steels.
Aluminium has an advantage over steel in that it does not require a protective coating such as zinc (galvanizing), and is cheaper than stainless steel.
However, anchors typically benefit from weight, especially in the tip area, and of the anchors which are available in both steel and aluminium, the steel versions perform demonstrably better. The Spade is a good example, with the geometry and weight balance of both types being identical, but feedback on poor setting performance of the aluminium version is much more common than for the steel version.
The Rocna is not available in aluminium
The Rocna is envisioned as the ultimate general purpose anchor, a large feature of which is not just performance but strength and durability.
We have not produced an alloy version primarily because we feel it would be a compromise on this. The current design would have to be modified substantially in order to facilitate alloy production, and any changes would represent a compromise. For example, an aluminium shank would not be able to possess the same tensile strength as the steel we use without being significantly thicker, which would then affect setting performance in hard sea-beds.
Aluminium is softer than steel, and wears and deforms at the important cutting edges of the anchor, so performance reduces with use. "High strength" hardened alloys have reduced corrosion resistance, and any aluminium anchor submersed in water for long periods has serious corrosion (cathodic) implications.
Furthermore, setting performance in sand and weed/grass would be adversely affected by the lighter tip weight.
We have avoided alloy construction per se for these reasons.
There are some very few anchors available in titanium. In general, this metal has advantages and disadvantages similar to aluminium, but all to a greater degree. It is very expensive even if procurement is practical, and not normally an economical option.