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How to CNC machine rudders and daggerboards

CNC-Service Herget creates custom rudders for racing yachts

DeskProto user CNC-Service Reiner Herget in Germany has specialized in building custom rudders and daggerboards: exactly conform the client's specifications. For racing sailors such rudder or daggerboard may well make the difference between winning and losing, which is illustrated by the prizes won by his clients.
This gallery page describes the process of creating a rudder for a German "Jollenkreuzer" boat".

Line-drawing of a Jollenkreuzer boat.
Sketch design of a 16 m2 Jollenkreuzer.

The Jollenkreuzer (to be translated as 'dinghy cruiser') is a German class of sailing boats equipped with a small cabin. Several sub-classes are present, divided by their sail surface (from 15 m2 to 30 m2). The boat does not have a keel, instead a long daggerboard is present, that can be retracted in shallow waters. The rudder reaches very low too, it can pivot backwards to retract.

screenshot of the airfoil design program
Special CAD software for designing airfoils, called WINGS 32.

Perfect geometries for this application are symmetrical wing profiles (air-foils), for instance the so called NACA-Profiles, specified by the National Advisory Committee for Aeronautics (USA). This may seem strange, as this is about boats, not airplanes. Still the same principles apply to both air-flow and water-flow. A flat rudder will start slowing the boat down at a small helm angle, a foil rudder won't. The foil shape will also create extra drag (lift), to make it even more effective. It will be clear that only symmetrical NACA profiles can be used.

The screenshot above shows the CAD software that Herget uses:
WINGS 32, by Vacanti Yaght Design in the USA.

Reiner Herget tells: "I normally use the NACA0010 profile as a start. This means a profile thickness of 10%: a profile of 300 mm will then be 30 mm thick. However in DeskProto I can scale per axis: when I scale that same profile to 40 mm thick the result in fact will be conform NACA0013. And for the daggerboards: as these are much longer (500-600 mm) the scaling will result in a profile conform NACA0006."

DeskProto screenshot, with and without toolpaths.
The rudder in DeskProto: without an with toolpaths.

The screenshot above show the rudder in DeskProto: first without toolpaths, then the roughing toolpaths and finally the finishing toolpaths.

The geometry transformation options that DeskProto offers make it the ideal CAM software for Herget: it is not needed to design each new part from scratch, instead an existing design can be quickly transformed, by non-uniform scaling over X, Y and Z. Using DeskProto it is possible to create CNC toolpaths within a few minutes.

The CNC milling machine has been custom-made for this application.

The machine- requirements for this application are rather unusual, so Herr Herget built his own machine. It is a custom design for this application: working area 3500 x 1000 x 140 mm. Clicking on the image above will start a video that shows the machine in action.

Most of the time the toolpaths are generated parallel to the Y-axis. The cutter most used is a flat cutter of 8 mm diameter, with the stepover set to D/3.

The result in wood, just after machining.
The plywood kernel just after machining.

Reiner Herget states: "The only way to manufacture high-end rudders and boards conform custom specifications is to first CNC machine a kernel, and then cover that kernel with glass or carbon fibre reinforced polyester."

Kernel material can be wood or rigid foam. The kernel for this rudder has been machined in ceiba plywood (the ceiba tree is also called "silk-cotton tree". Herget also uses composite wood: boards made of laminated solid bars, mostly Abachi or Mahogany wood.

Rudder on an open boat, on land.
The rudder, mounted on a boat. Total depth 110 cm (in vertical position).

Next step in the production process is adding the outer skin of glass fiber reinforced epoxy. For this rudder Herget applied three layers of fiber at 45 degree angles end two layers straight. The epoxy then is added and cured in a plastic bag under vacuum (Vacuum bagging).

A Jollenkreuzer boat sailing
A Jollenkreuzer in its natural habitat.

This 16er Jollenkreuzer „Sunrise“ is owned and sailed by Reiner Herget and his wife. The photo has been taken while sailing on Lake Neusiedl in Austria.