NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: digital compass, etc
From: Richard B. Emerson
Date: 2000 Sep 12, 6:35 AM
From: Richard B. Emerson
Date: 2000 Sep 12, 6:35 AM
FWIW, I can confirm that calibrating the fluxgate in an Autohelm (now Raytheon) autopilot is a two step process. Specifically, I have a Raytheon Type 300 course computer and fluxgate kit connected to a Type 2S linear drive. As part of the setup, the computer is put into setup mode (surprise!) and the display is advanced through a number of options including drive type, etc. In the "Swing Compass" page, the boat is taken through two turns at very low speed and, if completed properly, the computer displays a value called deviation. In my case, that just happenes to be "2" although it's unclear if that's two degrees, two oersteds, or two bananas; from the documentation it's best to just view this number as a figure of merit. If it's less than 15, Raytheon says the value is acceptable, if it's above that level, it's time find out what's wrong. Having established the deviation value, the user *can* (not "must") adjust the display to match the ship's compass. Frankly, given the state of some compasses, this is a bit leap of faith. Anyway, the point is, the adjustment isn't listed a requirement. The system also uses latitude and hemisphere (that is, enter a value for latitude and whether the boat is in the northern or southern hemisphere - these are two separate display pages in the process) to assess magnetic dip. Going back to the figure of merit, it took three tries to find the right place for the fluxgate. The first location, located on a bulkhead near the galley gave a value of 45... until we removed the hand vacuum cleaner from the locker (oops!). We couldn't get out of the low to mid 20's so we tried another location a but further forward with better but still not acceptable results. It wasn't until the sensor was placed on a bulkhead outboard of the mast that we got the results given above. The explanation is, I think, instructive. One With The Wind is a Baba 35 (you knew that, right) with a modified full keel. I'd been lead to think the keel was weighted with lead but it's not. The ballast is steel punchings, etc. The initial locations were close to the bulk of the keel's mass and, therefore, magnetic influence from the steel. Moving forward, towards the cutaway forefoot, meant the bulk of the steel was far enough away to reduce its influnce on the fluxgate (or to at least allow the computer to adjust the readings accordingly). Rick S/V One With The Wind, Baba 35