MacSailing.net

MacENC has a potentially very useful SailTimer function. As I understand it, this is licensed from SailTimer Inc and is a significant and valuable feature.

However, I get poor results with it as my Course Made Good diverges significantly from that predicted by SailTimer. I presume this is because my yacht's polars are different from the default values in the polars table built into MacENC SailTimer function.

Now I have tried recording my actual boat speed at various true wind speeds and angles, but this is very tedious and I get 'lumpy' and inconsistent results. I am searching for a way to create a better polar table for my yacht.

The iPolar app for iOS calculates a polar table based on given boat parameters (boat length, sail area etc.), but this is a theoretical table only. I haven't tried it.

SailTimer Inc have an iOS app SailTimer Charts Edition which has a learning mode, whereby it develops a polar table from experienced data. This would be an excellent way of gathering data for an individual yacht, and worth the in-app purchase cost. In my set up I have NMEA2000 feeding into MacENC, which sends NMEA data over TCP/IP to iNavX etc. However, according to SailTimer Inc, their app only accepts NMEA data using UDP. Their solution is to wait for their new wind instrument which will send data via BlueTooth to an updated version of their app. I do not want to get into installing a new and expensive wind instrument atop the mast just to get a polar table.

If MacENC could send NMEA data over UDP that would open up the possibility to use the SailTimer iOS app. Alternatively, MacENC has all the information needed to calculate polars, but this is not a trivial facility, not least because recording has to be selective to ignore inappropriate times - like motoring into wind. Given the pace of MacENC development, I see this as a non-starter.

Given the focus on iNavX development, recording polars there would make sense, and as iOS devices get used in the cockpit this would be the natural place to control when recording happened or not. However, building a polar table from a series of observations would, I guess, be a major development. [I wouldn't want to distract effort away from MacENC either!-]

Has anyone got a solution to this problem, or advice to offer?

Tony

Tony,
One solution is to get a multiplexer that has Wifi output on both TCP/IP and UDP. I use the Miniplex2-Wifi. It accepts up to 4 inputs and has two NMEA outputs as well as USB and Wifi (both TCP/IP and UDP) outputs. So that would provide you with all the data you need for the SailTimer app you mention. Not a cheap solution, but I have used this multiplexer for a year and find that it works well. See ShipModul (http://www.shipmodul.com/en/index.html)
if you are interested (I have no financial interest in the company but am impressed with the support it provides). You might also look at my recent post about calculating the boat speed upwind, relative to earth, not the water as is usually done. Works well.
Happy sailing! Eric

Eric...

Apart from the expense, my data is NMEA 2000, so the unit you mention would not be helpful unless I gatewayed everything back to NMEA 0183 first. This seems an unattractive backward step to me - NMEA 2000 is such an improvement.

thanks for your interest
Tony

Tony - Before reading your reply, I tried out SailTimer C E for my iPad I (no GPS built-in). It would not connect to the UDP output of the Miniplex2-wifi from ShipModul and even after several emails with ST it was not obvious what was wrong. So I stopped pursuing that option for generating polars for my boat.
But, and relevant to MacENC (finally!), it is fairly easy to generate your polars just within MacENC. In the Track panel, you can record SOG, COG, true heading and true wind speed/direction. So just let MacENC record this info at frequent intervals (even 1 Hz if you want) and then sail as many different wind angles and wind speeds as you can. You can collect the data over multiple days/trips. Then save those files as CVS files and combine them all together in an Excel file. You can then sort the data so the rows are the different angles and the columns are the different wind speeds, with the SOG values as the data in the array. Now you can average the SOG's for similar wind directions and speeds. This will generate the polars you need, which can then be entered into the "polar" option in SailTimer in MacENC.

This does require some familiarity with Excel, but is pretty easy, and I would be happy to help if you decide to collect your data. My guess from playing with the SailTimer app is that it would not have been much easier there.

A further advantage for you is that there is no problem using your newer data protocol, NEMA 2000.

Eric

Eric...
Thank you for pointing this out to me. Yes - it looks promising.

Main difficulty will be collecting data only when appropriate - avoiding data while tacking etc. and when the tide is distorting things. (Also I will need to thin densely-recorded data down before using in my published track log.)

I'll give it a go - in fact I have some historic track data I can use to develop the spreadsheet work. But for real data I will need to switch to true wind, which is trivial.

thanks, Tony