Raymarine only uses a single calibration speed.
So to be close to accurate you need multiple calibration points (speed) to capture the non-linear aspect.
The sensor is non-linear, and differs from port and starboard. My STW instrument has never been even close to accurate, and at least with raymarine, I don't think accuracy from that instrument is even possible. OpenCPN has a data recorder that will record instrument data and create one, but it still requires a lot of manual tweaking, as SOG from the GPS isn't very stable and so there are a lot of erroneous data. I have struggle to create a real working polar. That polar needs to be hand created, I don't think those are available anywhere. The more sophisticated routing(racing) software also adds a polar for waves/swell. In the ocean with a 2-3 meter swell, forget it - they are just wrong. On SF bay, when the water is completely flat, with a consistent and steady wind, the ORA polars are pretty close-but it's super rare to have 12-15 kts of wind and no chop. That at least is based on my boat in imperfect sea state instead of a computer model. The algorithm predictwind uses with just a few wind angles at 15kts does a better job than I was able to achieve with these polar in the real world. True, You need _something_ in order to run the routing in various routing programs, but given that the weather isn't going to be exactly what is predicted, that the polars are for _perfect_ sailing on a _perfect_ boat, all of the errors seem to cancel out. I purchased polars from ORA several years ago.įrankly, after using them preparing for and racing in the Pacific Cup, then cruising thereafter, they are not terribly useful.
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