5.2 Moulded fins

A moulded fin is produced with the help of sandwich technology and a female mould. This technique also brings about advantages and disadvantages.

The epoxy resin used should be of top quality; it should stand vibrations and sun heat; it should offer excellent adhesion to the enforcement fabrics and little shrinkage. Resins of minor quality do not stand the vibrations for a longer period of time, endurance is drastically reduced and therefore the fins can break easily.

Enforcement fabrics should also be of top quality. They should be able to stand permanent vibrations, they should have a low volumetric weight and high tensile strength for stiff composite constructions.

A sandwich construction is a structural panel consisting of two relatively thin, dense, high strength sheets of structural material with their faces bonded to and separated by a core. The structural material has to cope with the tensile and pressure forces. The core serves as a distance holder, maintains the fin profile and has to cope with the shearing forces.

The structural material can be unidirectional and/or multi-directional carbon inlays. The carbon fibres consist of microscopically thin and parallel single filaments. This parallel position makes it possible to meet all the lines of force in the laminate. In this way, the construction can be adapted to the stress which leads to less weight and higher durability. In addition the flexibility in terms of twist and flex can be controlled more precisely. For the construction of an extremely thin fin, high modular carbon can be used.

The core material has to be quite robust which is met by using epoxy resin.

The process of production starts with the construction of the female mould. The two symmetric halves can be made from enforced resin, aluminium or steel.

The inside of the female mould has to be covered with a release agent. Then the fibre inlays and the resin can be applied.

In the case of injection technique, the fibre inlays will be put into the mould, then the mould will be sealed. Resin will be pressed into the mould and air will be sucked out.

The other possibility is to laminate by hand. For the construction of the carbon laminate, the carbon inlays are put into the female mould in a defined angle. Then they are soaked with the resin-hardener mixture and de-aerated. All layers are laminated “wet-in-wet”. Record of the angle of the carbon inlays has to be kept for reproducibility.

The next step involves the hardening of the resin. Depending on the type of resin, it will be necessary to temper the resin at a higher temperature.

After demoulding, the fin has to be deburred, inserts will have to be built in and the surface will be finished.

A good moulded fin will have the desired profile plus the desired twist and flex. By using carbon inlays, the fin responsiveness will be high and the weight will be low. Its performance will be excellent; and as long as there is no damage it is extremely durable. All the desired parameters can be influenced by the producer so they can truly be custom made.

The only disadvantages are that the process of production is laborious as most of the work is performed by hand as well as the costs of the materials. The sailor will have to pay a higher price for a delicate high tech product.

 

DAMAGES AND REPAIR

Fin damages mostly occur on the leading edge or on the tip. These damages always cause a small lag of speed potential, the lift of the fin is worsened especially with a wider angle of attack, planing and acceleration are not at their best any more. Spin out is to be expected more frequently.

The leading edge is responsible for the development of the flow which is crucial for the fin function. Therefore, all damages on this part of the fin should be repaired properly. You may either use epoxy filler or pure epoxy resin.

Damages to the fin tip can be ground away and the fibres can be sealed with epoxy resin or car varnish. In case of a heavy damage, you even may cut off the tip parallel to the base without much of a disadvantage.

If the trailing edge is damaged, repair is mostly a bit more difficult because of its thin section. All projecting parts should be ground away and – if possible – the edge should be square.

 

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BUYING A FIN

If you know which type and size of fin you want to buy and if you have made up your mind for either a moulded fin or G10, there are still a few details that you should think about.

Check the leading edge of the fin with your fingertip, there must not be the slightest damage. Even if the edge is rather sharp there must be some nose radius to be felt or seen. A completely sharp edge has to be taken away with fine grinding paper. Let them do it in the shop and then check again.

There must not be any asymmetries in the fin profile, no dents or other irregularities. Try to get some information about the relative thickness of the fin and the section profile, these are essential features.

The trailing edge should neither be razor-sharp nor rounded. The fin should end with a clean square edge.

If you are not an expert it might be difficult for you to judge the flexibility in terms of the flex and twist. If you don`t trust the shop ask an experienced friend.

The position of the fin blade in the base has to be symmetric, the chord of the fin section has to correspond with the centreline of the board.

The base has to fit tightly into the box, the upper edge of the base should be on the same level as the underside of the board. Therefore take your board along for buying a fin. After you have fixed the fin in the box try to bend the blade sideways, the base should not move.

Quite often it is necessary to adapt the fin base to the box of the board as the different producers are using boxes which deviate slightly from each other. Fitting in fins with Tuttle or Deep Tuttle base is a delicate job as both sides of the base are parallel to each other. Never use force on the screws to pull the fin into the socket. Grinding paper, adhesive tape and lots of patience might be necessary to get a good result.