Kaimusailing

The message, unexpected, was about the Sailomat and the replacement servo oar we fabricated. Although it seems like a simple task now, after having made the oar, designing the replacement took a while. There was no existing oar to copy and only line drawings in the user's manual. Measurements of the existing Sailomat were used to scale the drawings of the servo oar. Apparently the manufacturer changed the design over the years it was built, modifying the sweep back angle. The angle of the oar shaft is fixed, but the oar itself can have subtle differences in angle relative the the centerline of the shaft. This could be done to make a foil that is balanced, i.e, low effort to rotate the oar, when the boat is on course and the Sailomat is near the center of its operating range; and unbalanced when the Sailomat is operating near its limit and the servo oar has swung out to the side. At that point it is beneficial to have the oar resisting further excursion to the side, even though the wind vane controlling the oar is attempting to push it further. The sweep back angle of the oar helps to bring it back to the centerline due to water flow and if it is unbalanced, i.e., requiring high effort to twist the oar, it will tend to further resist the control of the air foil. The balanced to unbalanced transition when the oar is swung out is accomplished by making the sweep back of the oar greater than the sweep back of its shaft. The area of the lower part of the oar is aft of the axis of the shaft, the upper part is more forward. When the oar is centered and not really doing much, it is a balanced foil and it takes little force to twist it so that it can react to the water flow, which forces it to swing out. As it swings out, resistance from the water flow keeps it from swinging out right to the stops. Also the upper part of the oar is now out of the water and only the lower part can react to the water flow. Because it requires more force to twist against the water flow it is a second kind of inverse feedback to keep the Sailomat from just flopping from full rudder to one side, then slamming all the way back. There is a controlled resistance that builds up. In our fabricated oar the sweep back angle is adjustable by a breakaway bolt that also serves as a "fuse" in case the oar hits something solid enough to break it. The breakaway bolt is nylon.
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The second topic was dismasting. The messages and conversations were generally looking for first hand information, since the Rainmaker people are mum about their dismasting. What could I say about my disaster. I would say that rigging failure does happen on catamarans and one factor is the sudden shock loadings that can cause metal fatigue and snap fittings. The other is the realatively high stiffness of the catamaran configuration which can result in high rigging loads when the hulls do not heel over and release wind pressure. It would be interesting to find out if Rainmaker actually had been driven into a wave by a rogue trough. That is when you are coming off a steep wave crest and the trough ahead is much deeper than all the others, as much deeper as a rogue wave is higher than all the others. The boat then plummets downward and hits the next wave at a steeper downward angle and at a higher speed. Then it is green water that sweeps the boat. The extreme forces on the rig include the boat suddenly stopping, any green water that swamps the rig, and apparent wind suddenly increasing due to the boat slowing. This can result in pitchpoling. In my case it was not wind pressure or giant wave action. The rig should have been plenty strong enough to resist the conditions and had done so many times previously. Perhaps the dynema lashings needed replacement, certainly they will be new when the new mast is stepped.
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I ran across a discussion of the Gunboat dismasting issue and here is a quote: "As for why dismastings occur (a naive question, but...), masts, standing rigging, and the various rigging components are 3rd party designed and supplied equipment. Gunboat doesn't pretend to know more about mast engineering and construction than their chosen designers and suppliers. Gunboat is no more to blame for a dismasting than they are when a watermaker breaks or a winch breaks. Marstom built the rig for Phaedo, and Hall the rig for Rainmaker. Swans dismast, Oysters dismast. It happens. A lot. I'm sure there'll be some analysis in house, but there are any number of failures that could result in a downed rig. Furler failure, shroud terminal, lashing chafe, tang, etc. 70 kts will find the "fuse" pretty quickly."
Unfortunately I didn't save the URL from where I copied this quote.
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I ran across an excellent blog of a voyage of a 40 year old Wharram catamaran cutter similar in size to Kaimu here: https://offshorepiggy.wordpress.com/ And an earlier epub of the same boat: https://offshorepiggy.files.wordpress.com/2014/11/stick_to_the_plans.pdf
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A trip down to the docks revealed a frozen waterscape. The 40 knot winds overnight had blown the water out of the West shore of the bay where we are and the single digit temperatures refroze the inlet.