Monday, May 31, 2010

Making a rudder, part 4: designing gudgeons

With my rudder blade is almost ready for the water, so time to think about it can connect to the boat. To hang it from Briongloid's stern, I need to make a pair of gudgeons; these will allow it to pivot about a vertical axis, and need enough strength to prevent it either from being swept astern or from bobbing up out of the water (the Corecell interior being much less dense than water, the finished rudder is going to be quite buoyant, even with much of the length out of above the surface). Also, we are now talking about a moving part, and so friction is a problem: the gudgeon needs to pivot about a stainless steel pin without appreciable wear to either itself or the pin. Two separate mechanical challenges for our gudgeons: what materials should we use?

Figure 1: Gudgeons (yellow) allow a transom-hung rudder (blue) to pivot about a vertical axis (red)

To give the parts the necessary strength (tensile strength being what they need most), I have decided to imitate the gudgeons that held the old rudder, which were made in stainless steel and worked fairly well. However, I can't work in stainless steel (no skills, no tools, no materials), so I chose a different material, more impervious to corrosion and with even more impressive tensile strength: carbon fibre (weight for weight, the median sample of carbon fibre has six times the tensile strength of stainless steel [Source: Wolfram Alpha]. This can be bought as a woven fabric, just like fibreglass, and worked with the same simple tools - epoxy resin, a pair of scissors, etc (Kevlar needs special shears, but ordinary scissors will do for "mere" carbon fibre). Strength problem: solved.

So much for strength: what about friction? This is a moving part, a type of bearing, and Mr. Friction is not our friend. The pivoting action of the gudgeon around the steel pin is going to cause wear if I just use epoxy-impregnated carbon fibre. For the core of the bearing, a different material is required - strong, but also slippy. After some research, I found the solution on the excellent blog of a Canadian who is building "Raven", a nippy trimaran from Ian Farrier's F22 design. This gentleman used a plastic PET-P, Polyethlyene Terypthalate, also sold as Ertalye, in just the same situation. This particular wonder material shrugs off water, is nice and slippy, and and also pretty strong. With a little research, the amateur builder can buy small quantities in rod or sheet form. Just the thing for the core of our bearing.

Figure 2: Gudgeon cores (carbon fibre ties not pictures)

Figure 2 above illustrates the core of the gudgeons, with the long base of the triangle being mounted on the leading edge of the rudder itself. Layers of carbon fibre cloth around the outside bind the core to the rudder in a continuous loop for maximum strength. The hardwood insert is there just to save some epoxy (cut from the ruined old rudder, it saves me a few pence worth of epoxy and provides similar strength). The hole in the centre of the PET-P cylinder is where the pintle goes, so the gudgeon can fulfil its pivoting function.

Coming in the next post: making and mounting the gudgeons

Previous posts in the series:
Designing a rudder, part 1
Designing a rudder, part 2
Making a rudder, part 1