Bow roller assembly design
This section is intended for those designing a new vessel or bow roller assembly, or those willing to heavily modify an existing set-up should it be considered inadequate. It provides ideas and concepts with a mind toward creating the 'ideal' anchor roller set-up, particularly for a Rocna.
Concepts and illustrations below are only recommendations. Please note that we do not provide design, drawing, or CAD services. We can however assist you or your nominated designer with any queries, including the provision of patterns and plans for the anchor size(s) required.
Comments on prefabricated roller units are also provided.
The ideal roller for a Rocna
The ideal set-up for a Rocna involves orienting the anchor in order it will self-launch, and come home without fuss, and then be stowed securely on the bow. A number of elements are involved.
A single roller would ideally match the radius of the inside curve of the anchor's shank. A smaller roller is also fine, as the shape of the Rocna's shank is designed to prevent vertical motion so long as it is pulled aft securely. So long as the anchor is restrained fore-and-aft, it will not be able to move at all in any dimension. However, the larger roller will better deal with lateral rocking, and also present better behavior of both the anchor and chain during deployment and retrieval (the windlass will also be more greatly stressed when pulling chain over a small roller). If matching to a particular Rocna, we suggest that the roller is as large as possible up to a maximum diameter equaling double the radius of the inside curve of the anchor's shank, as outlined in the below table.
|Size||Rocna Original||Rocna Fisherman|
|4 (9 lb)||40 mm||40 mm|
|6 (13 lb)||50 mm||50 mm|
|10 (22 lb)||75 mm||-|
|15 (33 lb)||90 mm||-|
|20 (44 lb)||100 mm||-|
|25 (55 lb)||100 mm||-|
|33 (73 lb)||100 mm||-|
|40 (88 lb)||110 mm||-|
|55 (121 lb)||130 mm||-|
|70 (154 lb)||150 mm||-|
|110 (243 lb)||150 mm||-|
Dual rollers can be employed in some cases, and a secondary roller aft of the primary can assist with self-launching and retrieval behavior; and guidance of the chain down to a main roller hung below the deck-line. If matching to a particular Rocna, we suggest as above that the forward roller is as large as possible, and that the secondary roller - which may be smaller - is positioned back roughly one third of the distance to the end of the shank. They would be vertically positioned so that the aft upper line of the shank is horizontal when the anchor is home.
Roller sizing is discussed above. The profile of the roller should ideally be grooved such that it accommodates the chain, with a central groove for vertically aligned links, and a wider space for the alternate horizontal links. This will provide ideal behavior from the chain during deployment and retrieval, and help avoid twisting and similar problems.
The sides of the roller outside of the chain grooves are ideally cambered to form a concave profile, to guide the anchor, chain, and rope, to the center of the roller.
Design and materials
A suggested ideal material for a roller is aluminium, from which the above profile can easily be milled. Aluminium is lightweight, soft and forgiving on the chain and anchor, but nonetheless very durable and will not wear or damage as plastics will. Bronze is also very good, but heavy. Alternatively, a UV stable nylon plastic is suitable. However, plastic is nothing like as durable as metal, and such a roller can be expected to endure only a few years of hard usage.
A stainless steel pin (either 316, or high tensile 2205 or equivalent if ultimate weight savings are desired) combined with a plastic sleeve (a suitable plastic bearing material such as oil or molybdenum impregnated nylon, ferobestos, or similar) makes an ideal axle for the roller. It is best to design the roller assembly that this pin is not permanent, so that the roller can be removed and replaced in the future if necessary.
Cheek plates and fabrication
The cheek plates holding the roller(s) in position need to serve a few other functions for the ideal set-up. For one, they can be designed to hold the anchor securely in place and eliminate all motion including rocking.
If the roller assembly is being designed for a particular Rocna model, ideally the cheek plates would be extended downward to meet the upper surfaces of the anchor fluke when it is home. With the anchor held back in place, this locks the anchor in all dimensions. This also removes some of the onus from the anchor retainment system with regard to resisting shock loads on the anchor from solid water when at sea.
Cheek plates should project down some way past the roller, and their lower forward extremities should be cranked outward in the order of 45 degrees. This flaring will guide the chain onto the roller without damage when the boat is yawing such that the roller is not correctly aligned perpendicular with the chain. It also helps stop the chain rotating on the roller and creating a twist between the chain wheel on the windlass and roller.
The cheek plates should be of adequate height to contain the anchor when it comes home on the roller(s). The case scenario of an anchor returning upside-down or sideways must be thought through, and the design should be such that the anchor is unlikely to roll or jump out of the roller assembly entirely. Straps, or inverted U plates, can be installed to entirely eliminate this problem but they introduce their own issues. Permanent enclosures are a nuisance as they prevent easy removal of the anchor from the deck when not at dock, particularly with larger sizes.
Aluminium, perhaps anodized, again is an ideal choice for roller cheek plates, as it is lightweight and 'soft', meaning it will not damage galvanizing or stainless steel if the plates meet the anchor. 5083 alloy, excellent in the marine environment, in a medium temper for strength, would be ideal. 6082 (HP30) is also fine but may require thicker plates. Any grade should be hard anodized, silver or in the color of choice, if it is to maintain a neat appearance and endure contact with stainless steel components such as pins.
Polished stainless steel (316L) may be preferable, in which case it would be best to cap the cheek plates where they meet the anchor, in order to provide a proper mating surface. Caps machined from aluminium would be the premium option, bolted onto the stainless. If the aluminium is not anodized the two metals must be separated by nylon or similar.
There are a number of methods by which the anchor can be securely held back in place on the roller. Please note that we advise against the use of through-shank retaining pins.
For more, please consult the Anchor retainment section.
Prefabricated roller assemblies
Prefab rollers tend to be lightweight and not ideal for a premium anchor set-up, particularly for larger boats and anchors (around the Rocna 20 or higher). They are however the norm on smaller boats and can be perfectly acceptable.
We are not able to recommend specific brands or designs to suit any given Rocna model, as the choice of a particular prefab roller really has more to do with the boat and its bow design than the anchor. The Rocna will fit, and work (self-deploy and self-stow), on any "normal" roller. Thereafter, the closer the unit is to the "ideal" as outlined above, the better.
Units advertised as being compatible with the Delta, in addition to the Rocna, are likely to accept a Rocna in a way close to their design intentions. The Delta, although a fixed shank plow and a totally different design, has an interior shank and fluke profile which is similar to that of the Rocna, meaning that roller behavior and fit tolerances are comparable.