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LF real gimballed compass
Max,
Those compasses are not compasses. They are compass repeaters. The compass is elsewhere on the ship.Please reread my earlier answer below. Another solution to the hand held swinging dilemma, might be to swing the compass positions where you would take your sightings and simply create an offset table for each one. This too is common practice. Steve "max camirand" wrote in message ... On Dec 3, 4:13 pm, "Steve Lusardi" wrote: Max, This compass issue is real and it is expensive. Here are your choices. magnetic and true. Magnetic compasses deliver a magnetic bearing referenced to magnetic north. They come in 2 flavors normal and flux gate. Both are influenced by external magnetic fields even undersea cables, which can be really exciting when sailing on auto pilot. Of the two, the flux gate is the most reliable on a steel boat, as you mount the sender high up on the mast away from local influences and in your case, is the best bang for your buck. These are NOT primary ship compasses. This technology is yacht or workboat. All magnetic compasses must be swung at installation and reswung every year. These can be found used, but if you buy used, plan on sending the unit to a compass house for servicing and certification. They will need it, especially if you buy from an Indian ship breaker's yard. Gimbaled, flat faced compasses are available used, but are rare and they are expensive $500-1k. Most cannot be used to take visual bearings. Hand helds are you best bet there. True bearing compasses come in two flavors gyro and now, satellite. Ships under 500 tons require at least one gyro and a satellite compass as secondary under IMO rules. Ships over 500 tons require two gyro compasses. Gyro systems cost $20k new and the gyrospheres need replacing every 5 or so years at $8-10K each. Satellite compasses operate using phase comparison of the carrier signal and are very accurate, if installed correctly on the centerline. Both deliver a true bearing. Gyro compasses lose accuracy above and below the 80th parallel and suffer speed distortion. Satellite compasses cost between $5k and $8k, run on low power, very reliable, but will lose its bearing under severe storm conditions, however the good commercial ones have internal fluxgates and inertial sensing chips that perform automatic dead reckoning under signal loss conditions. Used gyros are a bad deal because most will have duff gyrospheres, use lots of power and are expensive to service. Used sat compasses are non-existent. The cheapest new is Furuno, but they won't dead reckon. They have two models, one more accurate than the other. Both affordable. All true compasses also require repeaters, some are stepper driven, some are differential and others are NMEA 0183. Rotating repeaters have a max ROT rating. If you have a small boat that can turn quickly, their ROT rating can easily be exceeded. Older stepper repeaters are typically rated around 6 degrees per second and the newer NMEA ones are around 20 degrees per second and electrical resync buttons The older ones are mechanically reset. The rotary NMEA repeaters are $2500 each new and very very rare used. I have a CPlath gyro, a Sperry Marine sat compass, a B&G Flux Gate on the mast and a failsafe CPlath magnetic. I have done it all and own the T shirt. Bought everything second hand and rebuilt them, except for the Sat compass. I bought that new. Steve I'm trying to minimize expense and electricity consumption, so I can't have any sort of compass that puts a constant drain on the batteries, as superior as a fluxgate compass may be. Swinging a magnetic compass isn't a problem. In your experience, have you found handheld compasses to be reliable for taking bearings from a steel boat? I absolutely need magnetic bearings for navigation. My first thought was a flat-faced compass with a direction finder, like the ones we use on ships. The compass will be mounted on a steering pedestal and therefore is fairly far away from the nearest part of the steel hull and deck. The other two options are a handheld bearing compass or a relative bearing direction finder on a fixed card (add relative bearing to compass heading to get compass bearing). I probably should have asked at the same time... I'm also looking for a rotator log. -m |
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