Select Page

Article submitted by Mike Rossiter, Certificated Compass Adjuster.

Since the magnetic compass was first used by the Chinese for navigation sometime around 1044, it has become an essential instrument on every boat, yacht & ship.

There are a wide range of compass types, including Magnetic, Gyro, GPS, Astrocompass, Base plate & Card. For our purposes, the most commonly used are:

Magnetic compass

The magnetic compass is the most familiar compass type. It functions as a pointer to “magnetic north” because the magnetized needle at its heart aligns itself with the horizontal component of the Earth’s magnetic field. Most hand bearing compass’s are of this type.

A hand bearing compass allows you to take bearings of distant objects, which you can then transfer to a paper chart to create plot lines. Taking bearings of at least two objects that are 45 or more degrees apart results in intersecting lines on the chart, giving a position fix. To improve accuracy, we recommend taking bearings of three different objects and several sightings of each one and taking an average of the readings. If you find that another vessel stays on the same bearing over a period of time then watch out as you are on a collision course.

Risk-of-collision

Fluxgate compass

The fluxgate compass is used in boats mainly for the purpose of steering.

Fluxgate compasses are typically made with coils of wire that employ electricity to amplify the directional signal. Unlike the traditional magnetic compass which relies on a moving needle that is placed atop the magnet, the fluxgate has no moving parts.

These are typically found in Autopilots and Tiller Pilots like this from Raymarine:

Tillerpilot-IMG_1931

The inbuilt compass is housed in the dome underneath:

 Tillerpilot-IMG_1932

GPS compass

These use satellites in a geo synchronous orbit over the Earth to show the exact location and direction of movement your vessel, as well as your speed over the ground. They determine true North, as opposed to magnetic North, and they are unaffected by variations in the Earth’s magnetic field. Additionally, compared with gyrocompasses, they are much cheaper. Best of all, you probably have one with you, built in to your smart ‘phone, as well as on your boat’s chart plotter.

Mike Rossiter, Certificated Compass Adjuster, has contributed the following insights, looking at an instrument that is all too easy to take for granted – particularly on leisure boats. Here he shares some thoughts on common issues in installing, calibrating and using your compass.

 

  • Selection and installation of the compass

Careful adjustment will not remove all the errors if the compass has not been aligned with the fore and aft line of the boat. With the compass amidships it is easy to line up the lubbers line with the bow of the boat, but if the compass is offset you need to measure the distance of the compass from the fore and aft line.

Then move to the bow and measure the same distance and make a mark. The lubbers line is aligned with that mark (AND NOT the BOW!). Yes, it almost looks as if that has been done on some boats…

The compass on one yacht I visited had been removed while work was being done at the steering console. No lubbers line to indicate the direction the boat is heading… and the compass had been re-installed 180 degrees out of alignment!

P7300366

Typical large yacht’s compass showing the lubbers line

On a GRP boat the hull will not affect the compass but care is needed to avoid placing loudspeakers closer than about 1 metre from the compass. Entertainment speakers are the worst offenders!

Multifunction Chart Displays do not generally cause a problem, although it is good practice to see if a “Compass Safe Distance” is marked on the instrument and keep the compass at least that distance away.

Steel boats produce other problems. Magnetism is induced in the steel and the location of the compass can become very important.

P8140381

This is a modern ship’s standard compass. Instead of older, traditional Kelvin Spheres there are horizontal soft iron plates inside the rectangular boxes. Instead of a round Flinders Bar there are vertical soft iron plates.

You may have seen soft iron spheres or washers mounted either side of the compass. These are used to correct for errors caused by induced magnetism. Without the soft iron it is still possible to make a partial corection but there may be significant errors on NE/SE/SW/NW.

The card of smaller compasses will not have the same directive force as that of a larger compass. The bigger the boat the larger the compass that should be fitted.

  • Adjusting and using the compass

A well found boat will have a ‘Deviation Card’. This is produced after adjustment and records the residual errors or deviations. Deviation is the difference between Magnetic North and the direction the compass needle is pointing.

CNITA_Deviation_Card-1

A deviation card

Because magnetic influences affect the compass needle differently on different headings the deviations will not be the same on every heading. And this explains why I cannot adjust a compass on the bench in my workshop.

The boat has to be taken out on the water and turned to various headings, then I adjust and determine the remaining deviation on several different headings.

One skipper reported a deviation of 45 degrees on a compass that I had previously adjusted. There was no further information but we found that the skipper had left his spectacles hanging up close to the compass….which explained it!

Why do we still need the magnetic compass when we have GPS? Many people do not appreciate that the GPS, when it shows 123 degrees magnetic is showing COG, due to the set of the current and leeway carrying the boat sideways. The actual heading may be completely different.

The fluxgate compass is a very nice piece of kit but it is still detecting the magnetic field in which it is placed. Check the installation to make sure there is no magnetic material close to the fluxgate (and I have even seen one of these mounted with the arrow pointing to the stern … wrong!)

Raymarine EV100/EV200 fluxgate compasses for example are self adjusting, but the menu system on the control head still has an option ‘Align compass to GPS’. DO NOT use this option. You will be working with Course over Ground which is not the same as the boat’s heading. Sometimes errors will appear and the option to ‘Restart the calibration’ (basically restarting the compass) usually helps to eliminate these.

Mike Rossiter

Mike’s early career was in Outdoor Pursuits, and ended with two years work on a sail training vessel in Scotland, UK. (‘Captain Scott’). Mike then started work in the UK Merchant Navy as deckhand on a Royal Fleet Auxiliary (RFA) supply ship. Following an HND in Nautical Science at Plymouth Polytechnic he was employed as Third and Second Officer (Deck) with the RFA. After leaving the RFA in 1991 he worked on container ships, being promoted to Master with Maersk Line in 2000. After retiring in 2010 Mike renewed an interest in the magnetic compass. He was able to gain the necessary experience to take and pass the MCA Compass Adjusters Certificate in 2015. Mike used ‘Kamish’, his Vancouver 28, as accommodation in Hull for 3 months during the workshop time.

P7100289

The UK Maritime & Coastguard Agency requires the overhaul of 9 standard compasses as part of the Adjusters Certificate exam preparation.

If you have an article that you would like to submit for inclusion on our website, please get in touch with Safe Skipper by emailing editor@safe-skipper.com

 

Passage Planning Advice & Safety for skippers

Passage planning helps you to: • Decide where to go • Calculate how long it will take to get there • Avoid bad weather •...

Wooden Hulls – Part 2

It is important to ensure the essential hull maintenance of a wooden boat is done, even if you are paying others to look after your boat for you. The priority is to prevent rot from taking hold. The protective layers of paint and varnish over wood are far more critical than on GRP boats, where the topsides are painted more for cosmetic reasons.

Rig check – how to prevent failure at sea

Regular rig checks prevent the risk of mast and rigging failure at sea. This includes regular rig inspections of the spars, ...

Safety Equipment Checklist for Boats

Safety Equipment Checklist for Boats   Liferaft line attached The liferaft will not work unless the trigger line is...

Boat batteries

Under-sized battery banks are one of the key factors behind power failure at sea, as well as the premature failure of batteries, so make sure that your boat battery measures up to the use you want to put it to.

Fractures, sprains and dislocations at sea

Moving about a boat at sea often results in a few knocks and bruises, but if a crew member has a fall or major bump and is in serious pain, they should be examined and treated accordingly.

Repairing a leaking hull-to-deck joint

If you suspect a hull-to-deck joint has failed, then being absolutely sure where the actual leak is occurring is of prime...

Sailing Boat Rig Care

The rig of a sailing boat is put under huge stresses and strains so it is important for inspections of a yacht’s spars and rigging to be carried out at regular intervals.

Essential yacht tender safety for skippers and crew

Essential yacht tender safety - the dangers inherent in using a dinghy to get ashore from a moored or anchored yacht are all too easily...

Pre-start engine checks

According to the Royal National Lifeboat Institution (RNLI) almost one third of emergency call outs at sea are caused by mechanical failure. Many engine breakdowns are avoidable. The best way to avoid a breakdown is to carry out pre-start checks before heading out to sea.

Stern gear maintenance

The stern gear of a boat needs to be checked carefully when the boat is ashore as this is something that can only be done when it is out of the water. The same applies for any maintenance and repairs that may need doing, so it is best to check it all over as soon after an end of season lift out as possible.

Essential Knots: Clove hitch

Essential Knots: Clove hitch Use: Tying a rope to posts, bollards, rings or a guardrail. Step 1. Make a turn around the object and lay...

Going aground – what to do if it happens to you

When a yacht runs aground, it can be a stressful situation, especially on a falling tide in an exposed position with a swell running. There are several steps you can take to address the situation in order to ensure the safety of your vessel and its occupants. Here are some tips should this happen to you.

Essential Boat Spares for Safety

  Boats Spares Tool kit What you carry in the boats tool kit will be useful for many boat repairs, but you might want...

ColRegs when sailing single handed

  Don’t neglect the Colregs when sailing single handed Sailing single-handed represents several challenges for skippers, not least how to...

The Boatyard Book – a boat owner’s guide to yacht maintenance, repair and refitting

The Boatyard Book is a fully illustrated 224 page practical reference manual that provides advice for boat owners on planning and carrying out annual maintenance, repairs, upgrades and refits of sailing yachts and motorboats, up to 20 metres in length.

Jester Challenge 2022 – Sailing single handed from Plymouth UK to the Azores: Part 10 – The Return Trip

Jester Challenge – A modern experiment in old-fashioned self-reliance, self sufficiency, and personal responsibility. This is the final instalment of a 10-part post where solo sailor, Bernie Branfield, shares his first-hand account of his single-handed, 2022 Jester Challenge, from Plymouth, UK to the Azores, in his 26′ Invicta Mk2, Louisa.

Sterndrive maintenance

Sterndrives are a popular form of propulsion in the powerboat market, but require a fair amount of care and maintenance. The main factors to be aware of are salt water corrosion, lubrication and regular inspection of the bellows, the condition of which is vital to prevent water from entering into the hull.

Essential Knots: Round turn and two half hitches

Essential Knots: Round turn and two half hitches Use: Tying a rope to a pole or a ring. Step 1. Pass the end around the object. Step 2....

Keel maintenance and Repair – Part 2

If you have ever witnessed a boat colliding with a rock or other submerged obstacle you will know that there is an almighty thump and the whole boat shakes and judders. While such hard groundings seldom result in catastrophic keel failure, something has to give and even the sturdiest keels can easily be damaged by such an impact.

Light characteristics – how do navigators identify lights at night?

How do navigators identify the different types of light around our coasts at night and what are their characteristics?Navigating at...

Steel hull maintenance

A steel boat owner’s biggest enemy is corrosion. You don’t have to worry about osmosis or rotting timbers, instead rust is the number one issue that will keep you awake at night.

How to read nautical charts

Whilst most sailors today rely on GPS to find their way at sea, nautical charts are a crucial tool for any sailor to navigate safely and effectively. Here is a reminder of some of the basic steps involved in reading a nautical chart:

Boat decks and superstructure

The deck of a boat is constantly exposed to the elements and should be inspected on an annual basis. Particular attention needs to be given to the overall condition of deck fittings such as the stanchions, cleats and chainplates.

Learn ColRegs: Traffic Separation Schemes

Learn ColRegs Rule 10: Traffic Separation Schemes. (c) A vessel shall, so far as practicable, avoid crossing traffic lanes...