MarsKeel™ has produced cast keel fins and bulbs for such boats as 6.5m Mini to TP 52's and Maxis Racing Yachts. The material range for the production of the fins goes from Nickel Aluminum Bronze to Iron to Stainless Steel.

This combination of processes allows the customer to get a more complex fin shapes and surfaces then can be produced through fabrication.

There are conditions of the casting process that have to be understood. Both the builder and the designer must recognize and accept the dimensional tolerances, or limitations to casting metal keel fins, the generally offered tolerance is +/- 0.125” for bend, warp and twist. These possible movements occur during the cooling process of the metal, not unlike the shrinkage of lead as it cools. Some metals can be straightened or relaxed to remove the movement, some, e.g. Cast iron can not be straightened. This by no means that the movement will always happen during the fin casting it is simply a possibility. 

The production process of a cast metal fin begins with the production of the patterns. Each custom pattern made from the customer offered drawings. The patterns have to incorporate the appropriate amount of shrinkage for the requested material as well as the required machine tolerance for any and all machined surfaces on the fin, on such areas as the mounting surface for the bulb and the hull. 

Other then any required surfacing on the fin any required fastener points, e.g. upper and lower keel bolt threaded holes, machined in place.

From this point the fin can be sent the finishing rooms for final fairing and painting. This process takes a very skilled touch and attention to detail. The actual process is the same as the finishing of a fabricated fin.

There is another completion path for the cast fin. The cast fin can be just a rough blank to be fully surfaced. If the fin is to be machined additional material have to be added to the part right from the design and pattern stage of production, a minimum of 0.25” is required. The complete machining will ensure that the surfaces are as exact as the offered IGES file. The concern over possible movement of the casting during cooling is removed. This does require the casting of a slightly larger amount of material, reflected in the cost of the part.

After the fin is complete there are also a couple of paths that it can take to match up with the required bulb. The bulb can be cast separately and mechanically attached to the fin or the bulb can be integrally cast onto the fin. The integrally casting process simplifies the casting of the bulb, no cores or holes are to be cast. It also obviously removes the necessity for attachment to the fin. There is a limit to the physical size of the fin and bulb that can be integrally cast together. The limit is based on the ability to ship the finished part. Can the keel be placed on the truck bed or in the container? 

There are also some considerations made for the process during the design and pattern production. A below lead armature is required to hold the bulb.

The finishing process for the fully CNC machined fin is very much simpler then that of the cast non-machined part. This is because the milled part is completely fair, straight with a surface of 5/1000” scallops. The finishing process is the following,

• The part is washed clean of all cutting oil.
• The fin is primed with self-etching primer.
• There is a build up of epoxy based barrier coat, Interprotect 2000E, applied.
• The entire surface is sanded smooth, to approx. 320 grit.

Once the fin is faired it is sent to the shipping department for packaging and transportation to the customer.