Post Mortem

Gortobot v3 had a good start, but the sail control started having problems after about the third day. The boat gets orders as to in what position (in degrees) the sail should be held. If everything is working, it will report back (along with GPS, voltage, etc.) that the sail is within a few degrees of that ordered position.

It started sending data that showed two things: the sail was drifting around and not within a few degrees of the ordered position. And, the mast motor was running for more than 10 seconds without moving the sail more a few degrees.

The boat was fairly uncontrollable but every now and then it seemed like the mast motor was having some effect. It also seemed that the sail was still upright since the boat would be going across the wind.

On GBv3's final night at sea, it headed towards the southern shore of Nantucket. Via college networks, Facebook and Radio America (okay, not that one) we found someone on the island who went down to the beach with the exact position and fetched the boat from the surf. Within about 2 hours of landing, GB was safe!

So what happened? Still not 100% sure since the final throes onto the beach were probably violent. The mast was broken but I think it must have broken on the landing, and not on day 3. The sail looked too good to have been dragged over the side for 8 days and it wouldn't explain how the boat sailed so well at times.

Inside the mast gearbox I found seawater. That also could have happened after the landing, since the mast was torqued hard enough to bend the frame on which the enclosure was mounted. Also, the motor still works, and I wouldn't think it would work after a week of being wet.

Looking carefully at the gears I can see what might explain the odd behavior. The motor has a small pinion gear (brass) that contacts the larger mast gear (aluminum) and the contact was no longer solid. The alignment was off enough that the teeth of the pinion just barely touched the aluminum gear. Furthermore, the pinion has a little set screw that sticks out above the teeth. That set screw was contacting the aluminum gear once per revolution and moving the mast! So, very slowly, the mast was getting moved.

It doesn't look like the sail position sensor was slipping, so I still need to investigate that. If it is still reporting accurately, then maybe the sail was broken after all.

I'll get a closer look later, but for now, here are pictures: