Why doesn't a boat compass point north?

A hiker’s compass has one error to correct for. A boat’s compass has two, and the second one changes every time you turn. It is the reason professional compass adjusters still exist in an age of chartplotters.

Why doesn’t a boat compass point north?

Because two separate errors sit between the needle and true north.

Variation is the Earth’s contribution — the angle between magnetic north and true north where you are. It comes off the chart.

Deviation is your boat’s contribution — the deflection caused by the vessel’s own iron, magnets and electrical gear. It comes off a card you have to make.

Their algebraic sum is compass error. Correct for one and you are still wrong.

What exactly is variation?

The NGA’s Handbook of Magnetic Compass Adjustment defines it as the angular difference between the true meridian — the great circle connecting the geographic poles — and the magnetic meridian, the direction of the lines of magnetic flux. Ritchie’s glossary says the same thing plainly: the horizontal angle between magnetic north and true north at a particular location.

Terminology note, because it causes real confusion: landsmen call this declination and mariners call it variation. NOAA uses declination as the primary term with variation as the alternate; the marine literature does the reverse. Same quantity. If you have come from hiking, this is the word that will trip you up — the land version is covered in do you still need a compass if you have GPS.

You read it off the chart’s compass rose. NOAA explains the two rings: the larger outer circle shows true north, and the smaller inner circle shows magnetic north — the direction a compass needle points. Each rose carries the date, the variation, and the annual change, and NOAA reviews them before a new chart edition. Check the date on the chart, because variation moves. NOAA also notes that where measured values differ from charted values by several degrees, a magnetic disturbance note is printed on the chart.

What is deviation, and why does it change as you turn?

Deviation is the deflection of the compass card to the right or left of the magnetic meridian, caused by the boat.

Here is the part that makes it harder than variation. Your boat’s magnetic field is fixed relative to the hull. The Earth’s is not. As the vessel swings through the compass, the angle between the two changes continuously — the NGA notes that plotting these deviations against compass heading produces sine and cosine curves.

So deviation is not a number. It is a function of heading, and that is why it gets recorded as a table.

Two sources contribute. Permanent magnetism is acquired by the vessel during construction — the NGA describes it as arising through hammering in the Earth’s magnetic field, which is a wonderfully physical explanation for why two identical hulls behave differently. Induced magnetism appears in soft iron and depends on field intensity, the alignment of the iron, and its physical properties and dimensions.

Then there is everything you added afterwards. Ritchie names the culprits: magnets in speakers and microphones, ferrous metals such as steel and iron, and current-carrying devices. A phone in a bracket, a new stereo, a toolbox left on the console — all of it is deviation, and the last one is deviation that changes between trips.

How close can the compass sit to your electronics?

At least two feet, per Ritchie’s own guidance — from engine instruments, radios, bilge vapour indicators, electric gauges and any iron or steel.

That figure is worth taking seriously at the installation stage, because correction has limits. Ritchie states that most of its compasses have internal compensating rods that can correct plus or minus 15 degrees of deviation, and that if a compass has considerable deviation beyond that, the built-in compensators will not be enough. Its installation guidance recommends choosing a location with no more than 20 degrees of deviation on any of the four cardinal points.

Two feet of separation is cheaper than a compass adjuster.

What does swinging the compass mean?

Ritchie’s glossary defines it exactly: the process of placing a vessel on various headings and comparing magnetic compass readings with the corresponding magnetic directions to determine deviation. The NGA describes the same procedure — the ship on an even keel, steadied on each heading.

You then do two things with the result:

1. Compensate what you can, using the compass’s internal magnets. Ritchie’s procedure runs a north-south course adjusting the port/starboard compensator, then an east-west course adjusting the fore-and-aft one, then the reciprocals — removing half the observed error by rotating the compass itself.
2. Record what remains on a deviation card.

	Variation	Deviation
Caused by	The Earth’s field	Your vessel
Changes with	Location, and slowly with time	The boat’s heading
Where you find it	The chart’s compass rose	A card you make by swinging
Can you fix it?	No — you apply it	Partly, by compensation
Same for two boats in one harbour?	Yes	No

Is a deviation card actually required?

For regulated vessels, yes — it is federal law. 33 CFR 164.35 requires an illuminated magnetic steering compass mounted in a binnacle and readable at the main steering stand, and a current magnetic compass deviation table, graph or compass comparison record for the steering compass, in the wheelhouse.

The Coast Guard even publishes the form. CG-2596 is titled simply “Deviation Table” and pairs ship’s head with magnetic deviation at 15-degree intervals from 0° through 345°.

On the interval, Ritchie’s own documents give both 15 and 30 degrees in different places, so every 15 or 30 degrees is the honest answer. Finer is better; what matters is that it exists and is current.

For a recreational boat with no legal obligation, it is still the cheapest navigation improvement available. You are not required to have one. You are also not required to know where you are.

When to hire someone

Ritchie recommends a professional compass adjuster when deviation is large, noting that an adjuster is trained to help locate potential causes of deviation and recommend ways to reduce them. That second half is the value: an adjuster does not just null the error, they find the speaker that is causing it.

Worth doing after any of these:

• A new steel fitting, engine, or major electronics installation
• Moving the compass, or moving anything near it
• A lightning strike
• Buying the boat — assume nothing about the previous owner’s card

That last one is the same principle as an EPIRB registered to a previous owner or an expired CO alarm: equipment that presents as coverage and isn’t.

The thing to take away

The compass is not broken and it is not lying. It is reporting the magnetic field where it is mounted, which includes your boat. Variation you look up; deviation you measure; the two together are what stands between a bearing and a position.

And unlike almost everything else at the helm, it works with the batteries flat.