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Calibrating my ship’s compass

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Voyager
Master Marine Consultant

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USA
5231 Posts

Initially Posted - 08/10/2019 : 13:38:44

I use my ships compass when sailing along with my windvane and to determine how best to achieve a close reach at 60° to apparent wind. The true wind is often SW or 225° in the summer months and my harbor entrance is a few degrees off of North-South. So once I clear the harbor buoys I’ll turn to 165° or 170° to take a starboard tack. Then when I’m a few miles out past the rocks I’ll turn to 285° to head up to the next harbor.
I’m pretty sure that my NS deviation is within a few degrees of accuracy however my East-West deviation seems to be off by 20° or more.
My GPS is accurate but my boat doesn’t necessarily point in the exact direction of motion. What’s the best way to check the ships compass? Using a hand held?

Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain — Milford, CT

Edited by - Voyager on 08/10/2019 19:28:27

glivs
Admiral

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USA
822 Posts

Response Posted - 08/11/2019 : 03:20:06

Short of a professional service, as a first step can you locate on your paper charts true or magnetic N-S and E-W runs of 0.5 mi length or so with good land marks you can use to steer a course? That would offer a deviation free option. Otherwise, a handheld as you noted could work but requires you "assume" it is unaffected by any source of deviation.

Gerry & Leslie; Malletts Bay, VT
"Great Escape" 1989 C-25 SR/WK #5972

Edited by - glivs on 08/11/2019 03:23:06

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 08/11/2019 : 08:57:13

The greater error on an E-W course suggests magnetic interference from something on the boat that is aligned with the compass when you're sailing on a N-S course.

One way to check the compass using your GPS is to run under power alone at slack tide (minimal current). Your motor will drive the boat with virtually no side-slip, and your GPS course over the bottom should be just about identical the compass says. The motor tiller should be set so that the rudder tiller is centered when not touched. Then check it on 3-4 different courses.

Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge

Edited by - Stinkpotter on 08/11/2019 09:03:36

bigelowp
Master Marine Consultant

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USA
1736 Posts

Response Posted - 08/11/2019 : 14:52:12

I have a hand held compass that I use to periodically check the installed compass. When I got Limerick all checked out fine. I replaced the compass a few years ago with a new Plastimo unit and calibrated it to the hand held. More recently I have used a GPS which confirms the hand held and installed compass are correct. All has been fine since. I have always made sure that no electronics are not he same side of the companionway as the compass and when winch was installed had it mounted farther forward to be away from the compass.

Peter Bigelow
C-25 TR/FK #2092 Limerick
Rowayton, Ct
Port Captain: Rowayton/Norwalk/Darien CT

Voyager
Master Marine Consultant

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USA
5231 Posts

Response Posted - 08/11/2019 : 16:23:24

Aha! The cooler! I don’t use my cooler for keeping perishable foods cold, I prefer to use my 5-day cooler. What I DO put in there are aluminum and stainless pots and pans. Recently my wife gave me a cast iron frying pan which I “seasoned “ with extra virgin olive oil and sealed in a ziplock bag. I placed it right on top of the pile, about 8” from the compass.
That was it!!!
How I found it was by using the handheld. I waved that sucker all around the vicinity of the ships compass and it deviated quite a bit once I got it behind the compass and near the galley countertop.
Mostly everything else in that area are stainless, plastic or wood. The handheld compass pointed straight toward the soft cast iron like white on rice.
This coming week I’ll set up my NS and EW courses with the pan in the Vee berth to prove my theory. Thanks to all.

Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain — Milford, CT

Edited by - Voyager on 08/11/2019 16:24:51

glivs
Admiral

Members Avatar

USA
822 Posts

Response Posted - 08/11/2019 : 19:26:34

Good find!

Gerry & Leslie; Malletts Bay, VT
"Great Escape" 1989 C-25 SR/WK #5972

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 08/11/2019 : 19:56:15

How ‘bout testing with the pan in your car?