By Peter Garapick

Deviation. Just the sound of that word makes you think that something fishy is going on; some deviant behaviour is taking place onboard your vessel that you can neither see nor hear. It’s there and if you don’t take heed, it will bite you in the backside and usually when it’s a dark and stormy night. So what is it? It is the effect on your magnetic compass from all those metal parts (i.e., engine block, steel tools, electronic gizmos such as radios and wiring) and various magnets (i.e., speakers, electric motor armatures). It will affect the compass differently depending on each bearing you steer and if you only consider the difference between the True bearing and Magnetic bearing to steer a course, you may not quite get to where you were planning on going.

So how does this all work? As we know, the globe has been dissected with imaginary lines called longitude and latitude that are parallel and perpendicular and thus straight and true. It is relative to these lines that we derive our True bearing. Next, a traditional compass relies on the magnetic field of the earth and uses simple magnets to align a needle or a compass card to North. Since the North Pole is not actually at the top of the Earth and the magnetic field around the globe is inconsistent and can even differ between locations separated just miles down the coast from one another, compass accuracy relative to a true bearing will vary from place to place. This variation results in the Magnetic bearing. But when we realize what all those effects (mentioned above) onboard do by pulling and pushing that compass needle or card this way and that, we end up with a compass not quite showing the Magnetic bearing but rather a deviated bearing. This deviation results in what we call the Compass bearing.

To find the deviation aboard your vessel, you have a few choices. One is to have a certified compass corrector come aboard and swing your compass and add little magnets here or there around the compass. Not an inexpensive event, that is if you can even find such an expert kicking about the marina these days. Next option is to fiddle with the compass yourself; many compasses have minor adjustments that can be made but who knows where you may end up with that approach. The simplest option is to create what is known as a Deviation Card for your boat and this can be done over a couple of hours some calm morning with an extra hand aboard.

To do this, take your local chart and look for a set of ranges or ones that you can create by two conspicuous points and determine their Magnetic bearings. Then set a course between those points, call out your Compass bearing and have your crewmate record it next to the Magnetic bearing you should have been steering. Be sure to reverse your course and record that compass bearing, as well as there is no consistency in deviation. You can continue to use this range and sail across it on certain magnetic bearings and record the compass bearing as you cross the line between the two points or you can find other ranges to line up and steer towards or between. Ultimately, you will end up with a card listing magnetic and the related compass bearings – at a minimum for the 4 cardinal points (N, E, S, and W) and the 4 inter-cardinal points (NE, SE, SW and NW) and, if you are so inclined, for all 32 points of the compass.

This Deviation Card is a very important tool for the navigator and helmsman and should be kept close to the chart table or the helm so that every course, be it the compass, magnetic or true bearing that is chosen can be adjusted for variation and deviation. It is these 5 factors that are very important in steering an accurate course – True bearing, Variation, Magnetic bearing, Deviation and Compass bearing. Knowing how to convert between the three bearing types is crucial and requires some mental calisthenics. I won’t go into this here; there are numerous books, texts one could read and courses to take to get it all straight, but in a nutshell…

The phrase above is a proven formula to convert bearings – TVMDC or CDMVT. It can be remembered by the phrases – True Virtue Makes Dull Company – or – Can Dead Men Vote Twice. Going from a Compass bearing through to a True bearing (CDMVT) is called correcting a bearing; going the opposite way from True to Compass (TVMDC) is called uncorrecting a bearing. The biggest trick is adding or subtracting the degrees of variation and deviation since these two effects (errors) can impact the compass in either a westerly or easterly direction. This is where the mental workout comes in – determine the direction of the error and when correcting a bearing, going from Compass to True (CDMVT), add easterly errors and subtract westerly ones. It’s the reverse if uncorrecting a bearing. Got it? Better sign up for that course this winter!

But remember, a bearing can be called a True, Magnetic or Compass bearing – the direction of the boat doesn’t change; only the name of the bearing does. The key is to be sure to know which one you are using and to be able to convert between the three accurately.

When all is said and done, there is one other important factor – be sure that the lubber line of the compass is true; that is, that the compass is installed inline with the keel of your boat. If it isn’t then perhaps you best stay ashore since on top of variation and deviation you have an additional correction to make and you may never be sure what the result will be! Or then again, maybe you do – the result may be that boaters become landlubbers, at least until they control all those deviants aboard their vessels.


Photo Caption:
Photo 1 - We often suggest upgrading your helm with new navigation equipment like this chart plotter but the current passing near the compass likely has added a new source of deviation.

Destinations

  • Prev
At the 2019 Vancouver International Boat Show I had the pleasure of meeting up with Allyson and ...
Following the harsh impact of Hurricanes Irma and Maria, The British Virgin Islands is making an ...
For the adventurous boater Bunsby Marine Provincial Park is a special place, situated due south of ...
There is good anchoring in Cowichan Bay and nearby, and salt water enough to make any boater happy. ...
We’re gliding through green-blue waters, colours so vivid and bright they hurt your eyes. We’re set ...
The Halifax waterfront has been attracting more and more large yachts in recent years. However, a ...
Ah Canadian simplicity at its finest; small town, big marina. Little Hilton Beach (population ...
Vancouver-based Big Blue Yacht Charters Worldwide owner Emma Murdoch explains that luxury crewed ...
In the 1920s, a small cove in Canoe Bay was used as a shipping point and safe-haven for rum runners ...
Here’s an update from Caroline Swann with some news for the adventurous types who may be heading to ...


The Marina at Blind ChannelOne of my favourite places

By Marianne Scott

Sailing north of Desolation Sound, the Discovery Islands and the Broughton Archipelago offer cruisers a bevy islands with ample anchorages. Tides cause swift currents to run through the islands’ waterways. Few marinas are found in this large, sparsely populated region but one that provides all the services boaters need and especially enjoy is Blind Channel, a marina and resort operated by the Richter family located on Mayne Passage on the east side of West Thurlow Island (50 24. 82N, 125 30. 00).

Read more about the Blind Channel Resort...

 

Lifestyle

  • Prev
At the end of last month, Canadian sailors gathered on the Palma Beach in Palma de Mallorca, Spain ...
In 2019, C-TOW celebrates its 35th anniversary of providing 24/7 “Peace of Mind Boating” for ...
West Vancouver Yacht Club reports that following an independent certification process the Georgia ...
It has been hot in the Abacos this winter. Whoever said this area was cool this time of year must ...
Unfortunately this is not a picture from a boat but was taken on the evening of February 27, 2019 ...
On March 1, Tom Ramshaw of Stoney Lake Yacht Club was honoured with the most prestigious National ...
Vero Beach, aka Velcro Beach, lived up to its reputation again. Our original plan was to be there ...
My husband and I were visiting the Bra d'Or Lake from Newfoundland in our 39 foot Sea Ray ...
After an autumn in Canada, we arrived back in northern Florida at Adamant 1 on January 3rd and with ...
This issue, to kick off 2019, we have an unofficial Photo of the week and this, the unofficial ...

Boat Reviews

  • Prev
ILCA is seeking new builders to complement its existing network of manufacturers, the International ...
Nelson Gilbert (1854-1921) began building canoes in Brockville in the 1890s, a time when the sport ...
I have heard a lot of talk lately about trends in yacht clubs where senior membership is getting ...
To get you in the mood for cruising the Boat Show then launching in spring, here’s a boat that ...
Quite simply, the styles of boats have changed. Where in past years a buyer might have been looking ...
At the boat shows, the Ranger Tugs’ classic tugboat lines always grab the crowds, with the wives ...
Sometimes a great idea requires an encore, and French yacht builder Jeanneau got that with the ...
Tactical Custom Boats announces the sale to a North American client of a custom Tactical 77’ – Fast ...
Bruce Elliott is an inventor. And when he sold the technology he developed to build utility poles ...
One often asks of a winning achievement or a fabulous design, could it have possibly been done ...

Swift Trawler 47By Andy Adams

You might look at the pictures of the new Beneteau Swift Trawler 47 and think that this is not a “performance boat”, but I think it certainly is, and here is why; it can top out at 30 mph to get you from A to B quickly or to beat the weather in, so it’s pretty fast, but it can also loaf along doing 1,250 rpm making 9.3 mph and at that pace, it travels 2.4 miles on a gallon of fuel. That’s great performance in my books!

With a light displacement of almost 28,000 lbs, this is a big boat. In fact, it looks and feels more like a small ship than a big boat.

Read more about the Swift Trawler 47......

 

Beneteau Oceanis 46.1By Andy Adams and John Armstrong

Beneteau Oceanis 46.1When Beneteau introduced their new Oceanis 46.1, they were inspired by the fact that their previous Oceanis 45 was one of Beneteau’s best sellers and the new 46.1 had to be a clearly superior boat. The Oceanis range is about space and comfort for cruising while still delivering strong performance.

The yachting world has now recognized the Oceanis 46.1 as being just such a worthy successor. On January 19th, 2019, the Oceanis 46.1 won the highly regarded title of European Yacht of the Year in the “Family Cruiser” category.

Read More about the Oceanis 46.1......

Marine Products

  • Prev
The Walker Bay Venture 14 claims to be the world’s first luxury Explorer Sport Tender. It is ...
Mercury Marine is pleased to announce the launch of the new MerCruiser V8 6.2L 370hp Jet Ready ...
My history with the Cayenne goes back many years, as I was at the launch of the original vehicle ...
Last month, Mercury Marine has announced the launch of the 400hp Verado outboard engine, the ...
Featuring advanced, intuitive 3D controls, Zipwake Dynamic Trim Control Systems deliver a more ...
Gina de Vere approached me at the Canadian Yachting booth at this year’s Vancouver International ...
A revolutionary “assisted docking” system that provides a glimpse into the future of boating ...
After developing the Figaro Beneteau 3, the first production foiling sailing yacht, Groupe Beneteau ...
You most likely operate your vessel with batteries that are rechargeable. Rechargeable batteries ...
This past decade has been a real up-and-down ride for the companies who make boating equipment. ...