I had some very scary experiences with 6.0NA mast inversion that only happened occasionally but took me quite a while to figure out. I dropped out of one Down the Bay race because this was happening and I couldn't make it stop without slowing way down.

Once I discussed it with some really good sailors and they suggested looking for a couple of things I could suddenly see what was making it happen.

Scenario:

1. I ran very little spreader rake.

2. I ran a lot of diamond tension, about 850 lbs on the outers.

3. I had a 1994 sail #141.

4. My mast would start to pump and invert in meadium to heavy air and heavy chop headed upwind. Typically I had the main sheeted very hard, yet I hadn't started to apply downhaul past the power position.

5. The mast would start to pump, invert, and make some very scary noises as I crashed into waves. When it would pop through the spreaders in mid-inversion cycle the compression loads would have momentarily been much higher and could have contributed to the type of failure you describe.


Understanding and solving the problem.

It turned out that there were a couple of other factors involved.

1. That series sail forced you (me) to run the traveler car fairly far back even when sailing upwind in a decent breeze or the bottom of the sail would invert

2. It liked a really tight mainsheet.

3. What was happening was when the boat would slow down crashing into a wave, the momentum of the sail and rig itself would keep the rig going. The mast would rotate very briefly past 90 degrees (although it was very hard to notice if you weren't queued up to look for it). Once the mast goes past 90 degrees, with the main sheeted hard, you are actually pulling forward (from the mast's perspective) on the top of the mast with the highly loaded mainsheet. The top of the mast pulling forward would act as a lever with the shrouds and forstays attaching at the fulcrum, forcing the middle of the mast backwards, inverting the mast through the diamonds.

As the mast recoiled from the "bounce" induced by crashing the chop, it would swing back to a more appropriate orientation, all systems would return to pulling the "right" way on things and it would bounce back through the spreaders into it's normal pre-bend shape with a loud metallic "boing".

A complete cycle of normal to inverted back to normal would take less than a second and would normally only happen every 3 to 10 seconds depending on the sea state. Backing of the throttle would fix the problem but didn't help me understand the problem.

The answer is 2 fold:

1. Apply backside mast rotation control at the front beam to prevent the mast from over-rotating in these conditions.

2. Add a little more downhaul to give the mast more support down the back preventing inversion.