1657hrs 30 Jan 2005 SYD time 49’29”S 177’08”E Map Ref 35
Windvane artistry – part one:
While her geriatric crew are busy consulting with assorted Doctors and Other Medical Persons and Alphonse is having a tantrum, Berrimilla is steered by an elegant stainless steel device called a Fleming 401 windvane self steering assembly made by Kevin Fleming in Adelaide (hope I’ve got that right Kev).
There are photos on the website (not showing the aerofoil, i think) and a link to kevin’s website in the “preparations” document if you need to know what it looks like. Basically consists of an aerofoil moving through the air above the boat and a big paddle moving through the water underneath it, linked to eachother and to the tiller. The aerofoil is designed to rock sideways (sort of across the boat) and it can be rotated around its vertical axis. The paddle can be rotated around its vertical axis and it also rocks sideways. Sounds complicated but it’s really astonishingly simple. As the aerofoil rocks. it rotates the paddle in the boat’s wake, which moves the paddle sideways and exerts force on the tiller.
There are three concepts involved – apparent wind, the servo principle and balance. The aerofoil ‘feels’ the apparent wind, or, more specifically, changes to the apparent wind, activates the servo (the paddle) which moves the tiller to correct the deviation that caused the change to the apparent wind. Before setting the device the crew are required to abstain from consultation for long enough to trim the boat to achieve balance.
Some simplistic definitions – apparent wind is the combination of the actual (true) wind that is blowing and the wind caused by the boat’s speed through the water. The wind indicator at the masthead always points into the apparent wind.
The servo principle is the use of a small force to initiate a much greater force to do work. Any engineers or physicists out there who want to refine this, please feel free. And balance is when all the forces acting on a boat are in sync and complement eachother – again, oversimplified, but all that’s needed here. When Berrimilla is balanced, she steers herself and Stainless Kev really just limits the bigger wanders caused by the waves. Its a bit like getting an aircraft on the step – minimal need for use of control surfaces therefore minimal drag. It feels good and it goes that tiny bit faster.
Part two tomorrow or whenever I can get it together.
Image for the day – sparkling sunshine, Berri hooning along on a broad reach at 7+kts, a few whitecaps. Big white soft downy feather blowing – sort of rolling – along the surface of the water and keeping up with us for a couple of minutes. Lovely.
And four big albatrosses sitting on the water around a large piece of seaweed. There have been lots of birds sitting on thw water today, including another large albatross that had to waddle out of our way. They seem to be about as big as a goose in the body, but the wings are something else.