A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Should all electronic navigation devices on a yacht be set to True or Magnetic?
From: Lu Abel
Date: 2009 Nov 21, 21:23 -0800
From: Lu Abel
Date: 2009 Nov 21, 21:23 -0800
David: You correctly identify that for most recreational boats steering is done via a magnetic compass. Despite Peter Smith's bad experience, my experience is that on most recreational boats with a properly located compass, a good compass adjuster can zero out all magnetic deviation or at least reduce it to a degree or two on all headings. (On my own 36' sailboat with its compass mounted on its steering pedestal I have zero deviation). Studies have shown that it's very hard to keep a course to within a few degrees on a small boat, especially on anything but glassy seas. So my experience is that with a properly adjusted compass one can ignore deviation. Tradition -- and "bibles" such as Bowditch -- declare that plotting should be in true. But if I'm going to steer in magnetic, when do I want to go through TVM calculations? When I'm planning a voyage (even if it's only an afternoon's jaunt to a nearby harbor) while at anchor or at the dock, or while underway? I personally would rather do my TVMs in the calm of the former rather than while underway where fatigue, seasickness, weather, or a host of other conditions make it more trying to do TVM calculations. I find that when I'm about to offer Poseidon a gift of my lunch, it's darn hard to remember whether easterly variation is subtracted or added, even after a third of a century of sailing experience. And when I do remember that I need to subtract my easterly variation, it's even harder to do it accurately under those conditions. The largest boating education group in the United States, the US Power Squadrons, switched from teaching true to magnetic plotting about five years ago. Traditionalists were appalled. But their educational leaders showed that both the US Navy and the US Coast Guard teach their small-craft navigators to plot in magnetic. There are also devices available to measure and plot directly in magnetic eliminating the need to even do TVM calculations. Running parallel rules directly to the magnetic rose is one way. I personally use a plotter that I purchased many years ago (and no longer available) which consists of two plastic disks and a small plotting arm all joined by a hollow central pivot. Both disks are calibrated 0-360. The rear disk also has a horizontal/vertical grid on it. One rotates the upper disk by variation (and here I assume I am not crossing oceans but sailing locally where variation is constant) and hold the two disks in place with a bit of Scotch tape. Plop the device down on a chart, place the hollow pivot over a course line (or an object on which one has taken a bearing) and read the magnetic course (or set the pointer to a magnetic bearing) directly from the upper disk. It takes far more time to describe it than it takes in practice to use it. Here's a very similar plotter from Weems and Plath: http://www.landfallnavigation.com/np111.html (for those who are sharp-eyed, you'll note that the landlubber photographer turned the upper disk to almost 180 degrees of variation -- note that its magnetic north arrow is set to about 168 degrees true!) Weems and Plath sells other direct-to-magnetic plotters; the widely sold "Portland Plotter" is another one. The basic goal of any navigator should be the safety of his/her vessel. My personal opinion is that plotting in magnetic when courses are steered using a magnetic compass and bearings taken using a hand-bearing compass does much more to enhance safe navigation than plotting in True and having to constantly do TVM calculations. The need to then compensate for deviation depends on each particular vessel. Many, many vessels have compasses adjusted to zero or nearly zero deviation and can be safely navigated ignoring it. David H. Smith wrote: > When a yacht is first fitted with a GPS, I believe most owners choose to set > it to "Magnetic Heading" so that when comparing the bearing to the steering > compass, only deviation has to be considered. However, in a short time, the > amount of electronic navigation instruments grows and grows. A fluxgate > compass along with an autopilot and electronic wind indicators get fitted. > Then are added a radar, a second GPS chartplotter, a handheld GPS, a PC > chartplotter, an AIS and an AIS display unit. > So eventually we have aboard a yacht almost a dozen navigational instruments > (along with paper charts) capable of being configured to "True" or > "Magnetic" and just two instruments - the steering compass and the handheld > compass not being able to give a "True" bearing readout > The steering compass and handheld compass are now far less important than > they used to be, (but are vital if the power fails or in emergencies). > Information given on charts, in tide tables and other publications are all > in "True" bearings. It seems to me that the traditional way of > recreational sailing yachts having all electronic bearings displayed as > "Magnetic" or "Compass", may not now be the most sensible thing. > I would appreciate advice and opinions. > > David. > New Zealand. > > > > -- NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To unsubscribe, email NavListfirstname.lastname@example.org