First Mission

Set target mast bend figures. Selected these Hit and Wilke ones representative of what the top guys are using.

Download a copy of the mast bend graphs here (right click and use the "Save as...." option).

Simply copy a worksheet and insert your own mast bend offsets (12kg at the tip method) in the yellow boxes. Graph and calculations will auto update.

Off particular interest are the theoretical intermediate offsets. Notice how stiff sideways the mast bends are low down. Fast downwind.

Design (or decision making) Time

Older wing mast such as the Southern and Willits were in principle round in section, relying on the carbon layup on the sides to provide stiffness sideways. Unfortunately this layup created a flat layer in the fore/aft plane and increased fore/aft stiffness to a much higher degree then desired.

New wing masts such as the Wilke and Hit have the carbon radiating from the front of the mast in a V shape to enable softer fore/aft bends while the square aft face of the mast provides the all important sideways stiffness without impeding fore/aft softness. Very similar to a "D" shaped mast once prototyped by Clifton Webb and Marten Spars.

How to replicate?

We are going to do the following and see what happens.

The two halves cleaned up and the track removed from the lower end of the mast.  U shaped insert manufactured (one layer of 400 gram triple axis fibreglass cloth sandwiched between two layers of 400 gram double bias carbon). Not too stiff to create a hard spot but strong enough to join the mast section and keep in alignment.

Sail batten cut to length.  Batten runs from the heel and slides 400mm up under the remaining sail track.  Back face of the mast sanded to create a flat area for the sail batten to seat on.

All aligned and joined up to turn two into one. When fabricating in fibreglass or carbon fibre, staight epoxy resin and glue powder is not an option, far too brittle a joint.

Instead you must use a rubber reinforced, high peel strength, high impact epoxy glue product like HPR5 from Adhesive Technologies Ltd.

It is black, it is messy, it is sticky but oh boy it good stuff.

The sail batten dips slightly where it runs under the remaining sail track, this will allow the repair patch to fully encircle the mast and sail batten.

The new sail track will be padded up as required to run straight.

Batten was held in place while the glue set by simply drilling a small hole through the sail track and using a bit of wire to pull the batten flush up under the existing sail track.

Next step will be to scarf the sides of the join in the mast and layup a patch. Add the new soft sail track followed by the all important bend test and sanding or adfding carbon as required.

Finn Wing Mast Fix -- Part 2 is here.