By Barney Harris 6701 & 8011
Many Albacores were originally delivered with laminated mahogany centreboards. These coated with a varnish and delivered with the boat. If these boards were not periodically looked after they would often fall apart when the glue joints between laminations failed, or when the wood rotted from near constant exposure to water. I have assisted several Albacore owners with older wood rudders and centreboards to repair the damage caused by neglect. Here is the process we followed.
First, sand off the existing varnish – this will be easy as the older varnish is often flaking off anyway! Most of the old wood centreboards’ trailing edges were too fat – which caused vibration when planning. The vibration is caused by vortex shedding on the foil’s trailing edge, the frequency of which is close to the resonant frequency of the board. The way to eliminate this vibration is to increase the frequency by making the trailing edge thinner. This has the added benefit of reducing drag. This can be accomplished while removing the old varnish by carefully sanding the last 3 or 4 inches of the trailing edge to remove material and bring it to a sharp ending. It is a good idea to make sure the trailing edge is straight when sighted from one end or the other. If it is warped, block the wood to bend it back into shape – the wood can withstand a surprising amount of bending with out breaking, however, if you hear cracking noises – stop!
Next make sure the glue bonds between laminated sections are sound. Grip the board with your hands and try to pull it apart – I have seen two rudders which came apart in my hands this year. If the seams are split, but they hold together, excavate the split with a hacksaw blade and fill them with West epoxy resin with milled cotton fibres.
Now, fill and fair the bare wood board with epoxy resin mixed with light fairing compound such as West 410 Microlite. Sand and repeat until the shape is smooth and fair. If you really want to get serious, make some templates for your foil using the NACA 00XX family. A NACA 009 seems to work well for centreboards. Using these templates either build up or remove material until the foil matches the template. Note that the template should be adjusted for the thickness of fibreglass and paint which will be applied.
The next step is to coat the board with fibreglass. Clamp the board by its handle in a vise with the leading edge facing up. Gently heat the board for a day or so with spot lights to elevate its temp about 10 deg above ambient. Remove the lights and paint the board with epoxy resin. Allow the board to cool while the epoxy is applied. Do not heat it after applying epoxy – heating is a sure way to create bubbles! Next, lay one layer of 2 to 4 ounce fibreglass cloth over the leading edge trimming so it hangs about 1 inch below the trailing edge. Wet out the fibreglass with resin, and remove all bubbles and voids. Apply second layer of fibreglass cloth to head area & wet out. Allow this to cure for several hours.
When the epoxy is soft but not tacky, slice off the fibreglass cloth hanging below the trailing edge with a razor. After full cure (2 days or so) gently heat to around 120 deg f for about 30 min to drive all resin to full cure and prevent amine blush production. Amine blush results in a waxy substance on the surface of epoxy repairs. Nothing will stick to this, and it must be removed before applying any coating. Do this by wet sanding with 220 grit. According to the west epoxy dude, the amine blush is water soluble, so wet sanding ensures that all the amine blush is removed and ensures a good paint bond.
After a thorough cleaning and drying, we primed each foil with three coats of Sterling (http://www.detcomarine.com/) two part linear polyurethane primer. We applied each coat within a few hours of the last, so no sanding between coats was required. After several days of curing we dry sanded the board with 360 grit. After wiping with a tack rag and cleaning with acetone, the surface was practically perfect.
For final coating, we used a two part linear polyurethane enamel from Sterling. While this paint is somewhat expensive (over $100 per quart) and is somewhat poisonous, it really flows nicely. I found that a near perfect finish can be obtained with a foam brush. Succeeding coats of linear polyurethane can be applied within 24 or so hours without sanding. We allowed several days for a full cure and wet sanded each foil with 600 grit. The results were very good. It takes a bit of work, but the final product looks like a brand new centerboard.