Fixing A Broken Finn Wing Mast

This mast was a good one, having been twice used to win the New Zealand Nationals Finn Championship by previous owners. Really liked this Southern Spars built wing mast but a slow reaction to a massive wind gust and she ended up in two pieces.

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One minute sailing along reaching to the bottom mark, the next a massive wind gust caught me. Tried easing the sheet and steering down to the mark, split second later, boat upside down, bow pointing back to the wing mark and me entangled in the mainsheet under the upturned boat.

Fellow competitors reckon the wind picked up the boat, spun it around and slapped the mast on the water so hard the mast snapped and rolled the boat over without even slowing down. Luckily the sail did not rip and the boat suffered no damage what so ever.

Can we fix it? Lets try.

First Mission

Set target mast bend figures. Selected these Hit and Wilke Mast Bend Numbers as representative of what the top guys are using.

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Download an MS Excel copy of the mast bend graphs (right click and use the "Save as...." option).

Simply copy a worksheet and insert your own mast bend offsets (12kg at the tip method) into 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, on the sides, in the fore/aft plane and increased fore/aft stiffness, to a much higher degree then desired.

How to replicate the bends of the new wing masts?

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

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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.

30x5mm Fibreglass 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.

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All aligned and joined up to turn two into one. When fabricating in fibreglass or carbon fibre, straight 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 HPR25 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.

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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 adding carbon as required. Continued in

Finn Wing Mast Fix -- Part Two.


For a free quotation on yacht and spar repairs- contact us directly using the information at the top.

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