rob2
Well-Known Member
The new copy of PBO landed on the doormat and as I work nights I knew that not much would get done today, then. I was fascinated, as I always am, by the article about a cruise to the Faeroes using a Hebridean windvane. I spent quite some time with a magnifying glass, figuring out how the device worked but I found the text not only unhelpful in this respect but downright misleading. The author claims that the horizontal vane and associated power ratio is a unique feature of the Hebridean design. Well that is obviously b###s and probably means that the author, despite having built the unit doesn't fully understand how it works. Checking with the guarded remarks on the designer's website it all becomes clear - and quite ingenious it is too.
In many ways there is nothing unusual about the basic design and linkage arrangement until you look into the angles that both the turret mount and servo blade form with the vertical. If these were in fact vertical, the assembly would demonstrate why feedback is required to avoid simply snaking along a course with even deviations either side of that desired. The setting of the air vane pivot on an angled lever means that as the whole assembly rotates with the pendulum movement, the angle of the vane's pivot axis aligns with the wind direction as the pendulum reaches its maximum design travel, the servo blade's rudder therefore straightening out at maximum swing. All very ingenious and delightfully simple, but in the conventional sense the linkage is reversed to achieve this relationship, so the tiller lines have to cross to achieve the correct sense of output (tiller input).
I wonder whether PBO's editorial staff did any work on the text - It isn't unusual for a piece of text to fail to communicate the message, so an editor may often tweak the language and maybe the author failed to realise the new meaning it gave? I can well understand that the designer, having designed and prototyped the Hebridean, which is offered as a kit of metal parts (just add the wood), and offering it at a very reasonable price, he would not wish to give the plans away too. I hope my comments don't offend, but the claim made is plain wrong - most windvanes nowadays are horizontal axis servo pendulums and share the power ratios of the type. It's the feedback mechanism that differs in this design.
Rob.
In many ways there is nothing unusual about the basic design and linkage arrangement until you look into the angles that both the turret mount and servo blade form with the vertical. If these were in fact vertical, the assembly would demonstrate why feedback is required to avoid simply snaking along a course with even deviations either side of that desired. The setting of the air vane pivot on an angled lever means that as the whole assembly rotates with the pendulum movement, the angle of the vane's pivot axis aligns with the wind direction as the pendulum reaches its maximum design travel, the servo blade's rudder therefore straightening out at maximum swing. All very ingenious and delightfully simple, but in the conventional sense the linkage is reversed to achieve this relationship, so the tiller lines have to cross to achieve the correct sense of output (tiller input).
I wonder whether PBO's editorial staff did any work on the text - It isn't unusual for a piece of text to fail to communicate the message, so an editor may often tweak the language and maybe the author failed to realise the new meaning it gave? I can well understand that the designer, having designed and prototyped the Hebridean, which is offered as a kit of metal parts (just add the wood), and offering it at a very reasonable price, he would not wish to give the plans away too. I hope my comments don't offend, but the claim made is plain wrong - most windvanes nowadays are horizontal axis servo pendulums and share the power ratios of the type. It's the feedback mechanism that differs in this design.
Rob.
