The Sun Compass Compensation and Deviation Tables The sun compass is a handy device for both setting the compensation magnets on a compass and creating a deviation table. Typically, the compensation magnets, once set at installation do not need to be reset. However, in trailerable boats which are subject to a lot of road vibration, these magnets can move. A sun compass can be constructed by anyone with a little mechanical ability. Building a Sun Compass G Using the Sun Compass Return to the Shooting Star Home Page. G The Sun Compass http://ws1.coopfish.siu.edu/sun_comp.htm [3/2/2004 8:16:00 PM] The Sun Compass Construction Materials A flat plate, either plexiglass, plywood or aluminum, about 10 x 10 inches. It should be thick enough to support a vertical rod. (Plexiglass can be layered in the area where the rod will go to build up thickness. G A straight, thin rod about 8 inches long. Brazing rod and coat hangers may not be stiff enough and can bend. You can use a piece of stainless steel CB or VHF Marine band antenna or check for a long needle at a fabric store. G A compass "card" about 8 inches in diameter. You can get "polar coordinate" graph paper (similar to plotting sheets) at some office supply stores, or you can download a gif file (260K). NOTE: since you will be using the shadow of the rod, the card should be labelled in recipricol bearings -- ie. 90 degrees (East) will be at the 9 o-clock position. It is labelled counter-clockwise. G Varnish and/or spray waterproofing for fabric, and glue (contact cement or similar, and a little epoxy if you want to mount the rod permamently.) G Construction If using a plywood base, sand smooth and varnish. G Apply the glue or cement and afix the card to the base. G After the glue is dry, coat the card with clear varnish or two coats of silicone fabric protector, allowing it to dry between coats. NOTE: The fabric protector technique also works for other porous paper -- it is waterproof so ink usually will not run when it gets wet, but you can still write on the paper in pencil. G Drill a hole the diameter of the rod EXACTLY in the center of the compass card. If possible, use a good drill press to assure the hole is perfectly vertical. If using a thin base, drill the hole all the way through the material. You can then glue a small piece on top, over the hole to build be thickness -- then flip it over and drill through the new material using the hole in the back side as a pilot hole. G Insert rod and optionally epoxy it in. G Using the Sun Compass The Sun Compass Construction http://ws1.coopfish.siu.edu/sun_const.htm [3/2/2004 8:16:10 PM] The Sun Compass Use General Supplies -- A means to temporarily mount the sun compass, such as tape or clamps. G Crew -- You will need at least one helper. Two is better, since one can serve as a look-out for boat traffic and obstructions. G Weather -- Early mornings or late afternoons on sunny days are best for the shadows. Calm seas are also a major benefit. G Mounting -- Optimally, mount the sun compass in view of the boat's skipper where it will be in the sun on all headings. G Time -- Due to movement of the sun, you should try to accomplish each task within 5 minutes or you will have to reset the sun compass. This should not be a problem in compass compensation, since you have to reset the sun compass with each "run." If you cannot do the deviation table within about 5 minutes, reset the sun compass back at the "magnetic north/sun compass north." Don't try to continue where you left off. G Compass Compensation Have the sun compass ready to be locked in position. You may be able to tape a piece of plywood to the seat backs and then clamp the sun compass to the plywood. G Begin by slowly heading due NORTH on the MAGNETIC COMPASS. Rotate the SUN COMPASS so that it too reads due north and clamp/tape it down. G Come about and head due SOUTH. If possibly, have the skipper steer by the SUN COMPASS. Steering by sun compass -- The crew should note the error on the MAGNETIC COMPASS, and using the north/south compensation screw on the magnetic compass remove HALF THE ERROR. H Steering by magnetic compass -- Read the error from the sun compass, tell the skipper what heading half the error would produce, and again quickly remove HALF the error on the magnetic compass. H G Repeat the process. Again head due NORTH and reset the sun compass while on the "new" magnetic north heading. G Now, do the same thing for EAST and WEST -- head due east based on the magnetic compass, fix the sun compass, come about, remove HALF the error, come about, reset the sun compass and repeat. G NOW...go back and check the north/south again to make sure that the east/west corrections didn't affect those setting. If they did, go back and do the north/south AND east/west one more time. G Making a Deviation Table Using The Sun Compass http://ws1.coopfish.siu.edu/sun_use.htm (1 of 3) [3/2/2004 8:16:11 PM] Have available a pre-made table, ready to be filled in. There should be three columns, with the first one filled in starting with 0, 15, 30 etc.; Magnetic Heading Sun Compass Deviation 000 000 000 015 030 045 060 075 090 ... ... 330 345 360 360 000 The skipper will steer by the boat's magnetic compass. There should be one crew member in charge of reading the sun compass, recording the readings, and giving the skipper the next heading. Head the boat due North using the magnetic compass. Rotate the sun compass so that it also reads north and clamp/tape it down. (Deviation at 0-degrees will always be 0.) G The crew in charge of the sun compass will then say "15 degrees." G When the skipper has steadied the boat on a heading of 15 degrees, he should say "Mark." The crew should note the reading from the sun compass, write it down, and then tell the skipper the next heading (30 degrees in this case.) G Repeat at 15-degree intervals around the compass. G Remember, try to do this promptly or you will have to start over. You don't have to be too, too accurate. Most small compasses are only accurate to 2 or 3 degrees. G Once the table is filled in subtract the sun compass reading from the magnetic compass readings. e.g. G Magnetic Heading Sun Compass Deviation Absolute Deviation East/West 000 000 000 000 015 013 002 002 E ... ... ... ... ... ... ... ... 330 334 -004 004 W Using The Sun Compass http://ws1.coopfish.siu.edu/sun_use.htm (2 of 3) [3/2/2004 8:16:11 PM] 345 348 -003 003 W Positive deviation is EASTERLY, while negative is WESTERLY. G Typically, a graphic plot of these will look like a sine wave (an "S" on it's side), with half of the errors being negative. This occurs since the source of the deviation (engine, other ferrous metal) will attract the compass magnet in opposite directions on reciprocal courses. G Now, remember the basic piloting adage Correcting -- Add -- East When one word of the three change, one other word must also change to make things equal. G Using the absolute values colume of the table, you would always add to correct and subtract to uncorrect. HOWEVER, rember that subtracting a negative number is adding its absolute value. G If you had determined from your chart that you wanted to set a course of 340 Magnetic, you will want to UNCORRECT the 340 degress to a Compass course. So...... To uncorrect subtract. Since 340 is about half way betwwen the 334 and 348 on the table above, you will want to subtract a negative 3.5 degrees (=ADD 3.5) to the 340 and steer a course of 343-344. (NOTE: the -3.5 is the midpoint of the -003 and -004 in the example.) Now, just when you thought you were done. Turn on all you normal running light and electronics and make another one to see if their electric current affects your compass. Good Luck Using the Sun Compass Using The Sun Compass http://ws1.coopfish.siu.edu/sun_use.htm (3 of 3) [3/2/2004 8:16:11 PM] This site is dedicated to those individual who freely give their time, expertise and, all too often, their lives to assist and educate the boating public. U.S. Coast Guard Auxiliary Boating Safety Programs Including Boating Safety Classes, Operations, Free Vessel Safety Checks Local Auxiliary Unit Web Sites District, Division and Flotilla Pages via a clickable map. The source for local contacts. The United States Coast Guard Links to U.S. Coast Guard Pages Other Boating Safety Organizatons Including U.S. Power Squadrons, Sea Cadets, Boy Scout Sea Explorers Sources of Safety Information Including State and Federal Regulations, tides, weather. Tips and Tricks Feature articles, "Knot of the Month" Please send suggestions and comments on what information you would like to see on this page to The Shooting Star Boating Safety Page http://ws1.coopfish.siu.edu/INDEX.htm (1 of 2) [3/2/2004 8:16:13 PM] Bruce Tetzlaff. This page, and all original contents, are Copyright (C) 1995, 1996, 1997 Bruce Tetzlaff. Not responsible for lost or stolen links. Internet access provided by the Fisheries Research Laboratory at Southern Illinois University at Carbondale. The Shooting Star Boating Safety Page http://ws1.coopfish.siu.edu/INDEX.htm (2 of 2) [3/2/2004 8:16:13 PM] http://ws1.coopfish.siu.edu/gifs/suncomp.gif http://ws1.coopfish.siu.edu/gifs/suncomp.gif (1 of 2) [3/2/2004 10:12:45 PM] http://ws1.coopfish.siu.edu/gifs/suncomp.gif http://ws1.coopfish.siu.edu/gifs/suncomp.gif (2 of 2) [3/2/2004 10:12:45 PM]