Lessons Learned – by Pat Carson
The Lowly Magnetic Compass
I know your boat has one. Some are quite well manufactured and if properly mounted, easy to use. Others are cheap dollar store products that are better suited for use on a bicycle; however do not discount the value of that magnetic compass. I am frequently asked, usually by new boat owners, “why does my boat have an old school compass when I have all this cool electronic navigation equipment?” My answer is almost always to share two of my real-life experiences.
Several years ago, I was hired to bring an 80-something-foot Hatteras from San Francisco to Northern Vancouver Island for the new owners. The boat surveyed well and most of the items called out by the surveyors had either been repaired or deemed not an issue for safe navigation. The two owners wanted to make the voyage and they had some boating experience, so the crew was just myself and another experienced captain. We planned a nonstop voyage travelling at approximately 11 knots and would make our destination without a fuel stop in roughly 100 hours after departure. We knew fog was forecasted for the next several days, and until we made it into Washington there would be patches of dense fog. However, our planned route kept us 30 miles offshore for most of the voyage making the fog pretty much a non-issue. All was going well until we made the turn around Point Reyes and set our new heading for Cape Mendocino. Approaching Bodega Head the autopilot decided to start making hard left turns at somewhat unpredictable intervals, thus making it challenging to hold a course.
The vessel was equipped with a nearly modern GPS chart plotter, an older commercial Furuno RADAR that was performing well and a quality autopilot that you would not be surprised to find on a commercial fishing trawler. Having plenty of time to troubleshoot the system and with the help from the factory tech support located in Vancouver, British Columbia, we concluded that the course computer was no longer functioning properly and we would not have a functioning autopilot for the voyage. After deciding that we would continue the voyage as planned, it was a good time for our crew to practice hand steering using the compass in daylight before the fog rolled in. As with most boating skills, steering by compass is a learned skill that takes practice to become proficient, especially with moderate head seas and a beam wind. When fog settles in, the sun goes down, or both, holding a heading becomes much more difficult but can be accomplished even by the new helmsman.
No, as you would guess, we did not always make the intended straight-line course that uses less fuel and is shorter as some of our crew proved better than others at holding a course. At one point along the Oregon Coast we even had one southbound sailboat on a nearly reciprocal course hail us on the VHF asking our intentions on passing as we seemed to be changing course quite rapidly. However, four days later we made our destination having transited nearly 1,000 miles in varying sea and weather conditions, all without an autopilot and by the time we arrived in Canada the crew was much better at holding a steady course.
My second story goes many years back when a group of friends decided to rent a Cal 36 in Huntington Beach for a long holiday weekend. We drove to Southern California in the morning and arrived at the marina and rental office around 1200. After completing the paperwork and provisioning the boat, we departed LA later than we planned, well after 1600 but still intending to make Catalina Island in the early evening.
The afternoon weather started out great for sailing with moderate winds and calm seas, but then the winds subsided and we had no choice but to start up the auxiliary and motor the last three hours. As the sun set, we turned on the navigation lights, the RADAR and autopilot and settled in for a pleasant pre sunset cruise across the Santa Barbara channel. The cruise was great right up until we started getting low battery warnings on the electronic navigation equipment, and then, in short order all the electronic systems shut down. A bit of investigating found the combination of a heavy battery load, a battery well passed its useful life and an engine alternator that was not alternating was what consequently left us literally in the dark. No worries, we had a high-quality compass to steer and the visibility was still better than one nautical mile. We made our destination of Two Harbors nearly on schedule and missed the entrance by less than half a mile. Now the only issue we had was that the compass light did not work, remember the dead battery, so one of the crew had to hold a flashlight on the compass so that I could read the compass card. When we got to Two Harbors, we did not have a VHF radio to hail the harbormaster for mooring instructions – none of us had thought to bring a handheld VHF just in case.
Despite the vast amount of information that you get from the ship’s chart plotter and RADAR, any experienced skipper will practice using the ship’s compass while navigating. If the compass is installed properly, a quality compass will serve as a simple and reliable tool for holding a course. The design of the marine compass falls into two broad categories, the Flat Card and the Direct Read. Each has its advantages and disadvantages, however we find that most larger vessels are fitted with a Flat Card compass. The card, also referred to as the dial, is the numbered circular piece that has the degrees marked and rotates around a pivot mounted on a jewel movement. Using a jewel, usually sapphire, helps with the smooth rotation of the card.
The Flat Card compass has a horizontal dial, and to use it you look down or across the compass dome and the heading is read on the forward part of the card. These compasses generally have a lubber line, which are the heading references located in the direction that the boat is travelling. These work best when you either stand at the helm or have a chair that positions you high enough to look down on the compass. The flat card design is inherently dampened and therefore more stable is rough conditions. Because you are looking down or across the card it is easy to take bearings relative to your course.
The Direct Read compass allows the helmsman to view the heading on an upright conical dial with the lubber line at the aft portion of the compass and read the heading on the part of the card that is closest to him. For new compass users, the Direct Read is more logical and you do not have to look through the dome to read the heading. The Direct Read compass however has more movement in rough conditions than the Flat Card style.
In almost all marine compass designs the compass bowl and housing are made from brass or a high strength reinforced polymer. They are liquid filled, usually mineral oil, but in some cases alcohol and water to provide dampening of the card movement. Internally the gimbaling system of a quality compass is designed to operate smoothy in all angles of pitch and roll. For the fluid filled compass to function properly, the dome must be completely full with no air bubble. A bubble indicates that fluid has leaked and that the compass must be refilled. A compass can develop a slow leak and may be difficult to spot as the fluid tends to evaporate immediately and leaves no trace.
Installing A Compass
If your vessel does not have a compass, you should strongly consider adding one. Look for a suitable location to mount the compass and then determine, do you want a horizontal flush mounted compass, a surface mounted compass, a binnacle mounted, a bracket mounted or one that you mount on a vertical bulkhead? In all installations to get the best performance of the compass, it is crucial that the lubber line be parallel with the keel and along the centerline of the vessel. As a rule, select a compass with the largest diameter that will fit your helm. The larger the compass, the more stable it is and the easier it is to read. The fluid inside the dome magnifies the actual size of the card, so when comparing compasses from different manufacturers some will refer to the size of the card as the apparent size while others do not.
It is also very important that the location you select is far enough away from any magnetic disturbing devices such as speakers, electronic displays, the steering wheel or even a fire extinguisher. This is referred to as the “compass safe distance.” Remember, the compass works on magnetism and any nearby ferrous metal can cause a magnetic disturbance. Test the location prior to drilling any holes and be sure to operate nearby devices such as wipers or autopilot. Most quality compasses have a compensation system that can correct for a deviation up to 20 degrees or so, but remember that proper compensation may not be possible when the magnetic field is variable such as when the autopilot is engaged, the wipers are on or you are transmitting on your VHF radio. Weems & Plath offers an accessory compensation unit for some of their products, and other manufacturers include compensation magnets in the base of the compass.
Some navigation equipment manufacturers will list their compass safe distance in their installation guide. For example, one navigation system manufacturer suggests that the multifunction display or RADAR units be at least three feet from any magnetic north seeking device. For most of us that is impractical, so in that case expect to adjust the compensation of your compass. Weems & Plath has some products with a compass safe distance listed at 27 cm or approximately 11 inches. That is much more practical for our recreational yachts, but it is still quite a large range of safe distance from different equipment providers. Best to check first prior to committing to a compass location.
Separating the compass from onboard electrical equipment will minimize the magnetic interference, but when that is not possible proper compensation will help but not eliminate the errors. Most marine compasses have some sort of internal compensation magnets to correct for small amounts of deviation. During compass installation the compass is adjusted to remove as much error as possible. However, error cannot be entirely corrected. What is left is known as “deviation”, and is slightly different at any direction the vessel might be headed. The instruction manual that comes with the device will cover the compensation procedure. Keep in mind that compass compensation is an art, and do not expect to get it perfect. Commercial vessels are required to have the ship’s compasses professionally compensated at regular intervals, and the provider will generate a deviation table for the particular installation.
Today there is almost always an app for that. You can download a gaussmeter app for your smartphone that will utilize the magnetic field sensor of the phone to measure magnetic flux density displaying in either Gauss or Tesla. With this app it is easy to measure the magnetic disturbance from all the ship’s devices and assist in finding a suitable compass location. Check out Gauss Meter by Keuwlsoft.
That compass at your helm is more than just a decoration or a device that you think should be in a nautical museum, it is a tool that every mariner should learn to use. Convince yourself. Try to steer your boat using the GPS chart plotter while in restricted visibility. It is not impossible, but extremely difficult since the ship’s speed reader and heading indicator are averaged and delayed. Using the RADAR is easier, especially if you have a current generation scanner that has a scan rate of 60 RPM instead of the more common 20 RPM. But you will find a quality magnetic compass is the easiest. A quality compass will last the life of your boat if you protect it from physical damage and keep up basic maintenance. Compasses suffer from sunlight because UV rays cause seal damage, fading and discoloration, just like your gel coat. Heat is a detriment to your compass too, especially while your boat is uncovered at the dock. The dark canvas covered dash builds up heat quickly and a lesser quality compass will suffer.
Until next month, please keep those letters coming. Even the ones where you slam me for my oversights. If you have a good story to tell, I love a good story. Have good photos of right and wrong, please send them and I will include them in next edition of “is it right or is it wrong.” You can contact me at firstname.lastname@example.org