This works in the same way as all of their plugins-it seems there is not a Mac version.
Install the PC version of the NMEA Converter plugin to OpenCPN. There are geometrical computations of this effect, which can be applied within the wind instruments themselves (high end instruments) or by the navigation program itself for more common instruments that do not make this correction.) In other words, if you have your boat side tied to the end of the dock and read AWS and AWA, and then use a halyard to pull the boat over 20º or so, how much does AWA and AWS change. (Looking ahead, however, the main issue is not just correcting for the boat track vs boat heading, but rather correcting the measured apparent wind angle (AWA) and apparent wind speed (AWS) reported by the instruments for the effect of the heel of instruments relative to vertical.
The latter is what we care about for weather work, in which case it is called the "true wind," but performance sailors have used the term "true wind" for many decades as being relative to the water, and it not worth arguing the terminology. Then we can ask our navigation program to take this into account when figuring True wind, relative to the centerline of the boat, and Ground Wind, relative to the fixed earth. With this approximation (using K=12.4), a vessel moving at 5.7 kts, with a heel of 23º will slip downwind by 8.8º. Leeway = K*Heel/BSP^2, where K is a constant between 9 and 13, depending on the boat. The typical solution is to approximate the leeway from the measured Heel angle, which can be measured accurately.
The topic of this post is how to use that plugin for an extension of that idea to create the needed sentences to test TWA computations.Īlthough leeway can in principle be measured, it remains an elusive number. It was actually designed to read in one sentence and then create another one by redefining the fields in it.
This plugin was not intended for this application, but with a couple extra steps it does it nicely. The method described here uses the OpenCPN plugin called NMEA Converter. Once you have a source of the sentences you want, you can apply them to any program for testing and learning how they make specific corrections. Our own enav simulator generates the proper NMEA sentences within any user selected ground wind and current, for example, and there are programs that will play back a list of NMEA sentences, but treating the checksums properly can be an issue in some navigation programs when using a list. We come back to this topic in another post for now we just show one way to simulate the instrument signals, which is easy to reproduce for individual study. This correction can be a notable influence on a proper comparison of sailing performance with polar diagrams as well as on accurate optimum routing. OpenCPN's Tactics plugin does a correction for this, as do premier racing programs like Expedition. This is a subtle computation that not many ECS perform. Our main goal was to investigate how various navigation programs treat the effect of leeway on true wind angle (TWA) referenced to the boat's track through the water.