Compass Swinging for yachts,you can do it yourself

An early handheld Garmin GPS 40, a simple tool to hand hold while your checking the yachts compass heading.



The compass card becomes easy to read when the view is through the adjustable mirror on top,this compass also has two C type dry cell batteries in its handle for easy night viewing,it still works!


Left.
A British Admiralty Hand Held Compass,type 06 from World War Two



Above.

This is the Garmin 192c used to check the compass heading on the boat we are using,its not a new model GPS and in fact is far more capable than required to check compass headings,a simple hand held set will do the job just as well.You will need to set the GPS set to 'Magnetic' to be able to confirm your compass headings.



This binacle compass has two navigational instrument heads mounted quite close to the compass face,the units were tested to ensure they do not interfere with the compass headings,to do this the compass must be re swung,one of the instruments is an auto pilot control,when the pilot is engaged it will be very important to ensure no interferance with the compass is caused.



Please note,the following information is as I know things should be done,I accept no liability of any sort should my information be incorrect,it is always best you check sources of information and of course use your own judgement.

A Compass Card that is clear and sharp,making it easy to read,the card will swing around a series of numbers on the compass bowl,basically from 0 to 360 degrees with the prime sectors or quadrants highlighted.



Compass Points,the headings we steer to in segments or degrees




A yachting type compass from C Plath a very well known brand.

Every boat needs a compass if it intends doing coastal or offshore work,its not just a job of taking the new (or old) compass from its box and bolting it down,certain things must first be done,here are a few pointers (no pun intended)the first thing to check is if the unit you have is a northern or southern hemisphere compass? this is due to the pull of the worlds magnetic field being different at each side of the equator,this means the compass card will show a lean or 'pull' on a compass fitted in England but now having traveled to South Africa,normally the card tilt is just a bother but in some cases the card can actually stick,then its a worry,assuming we understand that side of things we can move to the next issue.

Not all yachts and boats are made from the same materials,many today are GRP (glass reinforced plastic) this is non maganetic,as are wood and wood composites and even alloy but should we have a boat made from steel,we have a serious bother with the actual boats hull materials affecting the compass and what it does,I will not go into this now but rather cover the more common points.

Assuming we have a none steel construction boat,we can install the compass in nearly any position we want in the cockpit but remember large steel objects such as an engine below the cockpit sole,or a large gas cylinder in a locker,even an outboard motor close by will possibly upset your compass.Its often best to move around the cockpit with the compass in your hands,checking for any changes in the cards heading as you do so,if it swings for no reason at all,its very possible you have found a magnetic source,normally a compass is safe when it is one meter from such an object but you must check this first.Naturally when loading gear onto your boat at some later stage,you must remember the compass and what will happen to it with a metal container close by,a can of cool drink is easily enough to change the compass heading,so be aware and make sure your crew are too.

So we now have the boat,some tools,fixed and hand held too,what we need now is a calm day with no wind or rough sea state to knock the boat around while you test the boats compass reading against known bearings.In a perfect world we can form a series of routes or headings based on the cardinal marks,north,south,east and west,in a real world we will find some other bearings which are close enough to swing the compass.

Its as easy as anything to pre determine those headings,based on the course you intend to take to a known mark,such as a Lighthouse or large building which is marked on your nabvigational chart,make sure that chart is up to date!The idea is to head for your mark,under power will be best,as you will be changing directions many times,then while holding steady to your course,check the compass with either your hand bearing compass,or if using a GPS the course indicator that it has,you may need to change view pages to find this option.

On each chosen heading make notes on the actual ships compass heading against what you have on your GPS or handheld compass,keep changing to a new heading untill you have the main eight headings,this will be enough for you to do a card indicating what you have found.

to be continued: