Cost of Compass Swinging

[Reefmagnet]

Originally Posted by Wotname

This will work if you have no drift. Just check the ships head is pointing exactly to the landmark and while maintaing that, check to see if your COG remains poniting to the same landmark. If so, you don't have any drift or leeway. Compass heading should equal ships track (magnetic).

A good way to check but a bit harder to adjust as it is way way easier to adjust on the NS & EW bearing and land marks aren't always so aligned. In fact I'm not sure if I could adjust on non cardinal bearings any more - if I ever could

[Dockhead]

Originally Posted by Robin3

I had a professional compass swing many many years ago when a group of us at the YC got together to share the guy's travel costs and therefore reduce the fees. That man was Hamble based and my previously very usable ( seemingly deviation free) compass was way off on some headings after the swing, so much so I had him back to redo it whereupon I was accused of moving all sorts of metal stuff around on the boat which I hadn't. I was later advised never to have this guy do it 'after lunch' as he liked a tipple or three. I then found a day later when going into Cherbourg on the leading lights it was still way off. I resolved thereafter to do it myself.

I bought a plastic pelorus made IIRC by Davies and put an engraved line filled with black ink line on the pointer of this to act as a shadow pin. I mounted the pelorus ( accuracy fore/aft is essential) on the mainhatch garage. I then took bearings of the sun via the shadow line every time an opportunity arose that was at different ships headings, preferably in a fixed location like tied alongside ( but beware, well away from metal pilings etc) It took a week or two, waiting for opportunities, and sunny days, but I gradually got readings taken on enough headings around the compass to construct a deviation card and create a spreadsheet to give actual ( corrected) CTS on each heading for both magnetic and the then fluxgate compasses. I used time from my GPS and a Casio programmable calculator I had loaded with a celestial almanac program (a PBO magazine series back then gave the program details to put in the casio,with valid almanac data until around 2020 IIRC) This program would give the sun's actual bearings at specific times and dates to compare with those that I obtained on the pelorus. I was very happy with my own DIY swing and could check it fairly quickly every season if needed.

ON another boat however and needing a swing soon after it's purchase, I did a very fast swing by motoring in a tide free area on different headings and comparing GPS COG with compass heading. I did one 'swing' in a well sheltered bay around slack water and another check one in the inner harbour of Cherbourg (outside the marina between it and the inner breakwater, (nowadays too close to the French Navy base and they would most likely chase you away) I later did another confirmation check upriver of the Arzal dam in S Brittany between it and La Roche Bernard on the Vilaine. I did a similar check to calibrate log speed versus GPS SOG

Am I right in thinking there are tide free places in the Baltic you could take advantage of whilsyou are there?

MY DIY swings did show up significant errors on some headings, I remember on 'Easterly headings on one boat it could be as much as 5 degs out, not much over a mile but significant on a very long leg, like 5mls in every 60 mls of easting. I never adjusted the compasses with their inbuilt magnets like the original typsy adjuster did, but preferred to keep the compass in it's default neutrally adjusted state and use my prepared deviation cards for corrections.

BTW The fluxgate compasses previously swung by the circling method and auto correct instructions in the manuals always still had residual errors and I therefore had corrected headings for the autopilots to use printed out in a spreadsheet for those too.

Long winded but I hope understandable!

PS I hope I"M not slandering a currently working Hamble adjuster,but I tell it like it happened. albeit 25 years ago maybe now.

My hat is off, to you sir -- deepest respect. That is exemplary resourcefulness and seamanship indeed.

My $400 quote came from no doubt the same character, so I think I will give it a pass. I will print out and attempt to follow your methods. I will also keep my eye out for a pelorus on FleaBay.

Cheers.

[Robin3]

Originally Posted by Dockhead

My hat is off, to you sir -- deepest respect. That is exemplary resourcefulness and seamanship indeed.

My $400 quote came from no doubt the same character, so I think I will give it a pass. I will print out and attempt to follow your methods. I will also keep my eye out for a pelorus on FleaBay.

Cheers.

If it is indeed the same person I would probably recognise his name if you PM me it, i think it was a strange one. (It wasn't Robinson BTW, the only name Google came up with for me just now .

[El Pinguino]

I've swung mine... I've also adjusted big ship compasses many years ago.
On a 'glass boat your steering compass shouldn't need adjusting... deviation under 5* on any heading is more than adequate... esp as the majority of yachts will only have a compass card graduated to 5* anyway.

I use transits, either man made or natural, put objects in transit right ahead... read compass. You don't have to do the whole 'swing ' at once... you can do it over a cupla weeks if you want.

[Wotname]

Good point about doing over time - never thought hought of that

[rivercrane]

I turned 60 today I think its time to reset my compass to !

[El Pinguino]

Originally Posted by Wotname

Good point about doing over time - never thought of that

You can do it over an even longer time if you want...

First big ship one I did ( factoid... the Master of a UK ship can adjust his own compass but the master of an Australian ship can't) was built in Scotland and had spent most of her life in the N Hemisphere. Once coming out of Hobart and steering Eastish towards Tasman Island the Gyro pooped itself.... ( long story involving the engineer on watch and a sandwich) ... went on to hand-o-matic... found there was about 40* deviation... oh dear.

Decided to adjust the compass but not right then.

As was the norm the adjuster at the builder's yard had just dumped a bloody great slab of soft iron in the Flinders' Bar tube thingo.

So... anyway ... over a period of time I corrected it ( running HK-Oz ) and squared away the Flinders Bar issue when on an E/W course in the Philipines - which is on the magnetic equator.

All helped by having a gyro ( 2/E had been sacked by then) which was always +/- 1* so you could pretty much 'shake down the magnets' anywhere..any time if you knew what you were doing.

I was bright eyed and bushy tailed back then and was lucky in that the man who taught me about magnetism had been taught his stuff by either Kemp or Young who had taken both the mumbo and the jumbo out of the black art... much to the disgust no doubt of people who try and make a living out of charging unsuspecting yachtsmen UKP270 to do a very simple job.....

[meirriba]

Open the following NGA link to full Bowditch navigation book.
Download chapter 6 - compass.
There is full discussion of the process.
Maritime Safety Information

[Wotname]

Originally Posted by rivercrane

I turned 60 today I think its time to reset my compass to !

Happy birthday rivercane. Unusal first post but welcome nevertheless.
Never too old to reset your compass

[Wotname]

Originally Posted by El Pinguino

You can do it over an even longer time if you want...

First big ship one I did ( factoid... the Master of a UK ship can adjust his own compass but the master of an Australian ship can't) was built in Scotland and had spent most of her life in the N Hemisphere. Once coming out of Hobart and steering Eastish towards Tasman Island the Gyro pooped itself.... ( long story involving the engineer on watch and a sandwich) ... went on to hand-o-matic... found there was about 40* deviation... oh dear.

Decided to adjust the compass but not right then.

As was the norm the adjuster at the builder's yard had just dumped a bloody great slab of soft iron in the Flinders' Bar tube thingo.

So... anyway ... over a period of time I corrected it ( running HK-Oz ) and squared away the Flinders Bar issue when on an E/W course in the Philipines - which is on the magnetic equator.

All helped by having a gyro ( 2/E had been sacked by then) which was always +/- 1* so you could pretty much 'shake down the magnets' anywhere..any time if you knew what you were doing.

I was bright eyed and bushy tailed back then and was lucky in that the man who taught me about magnetism had been taught his stuff by either Kemp or Young who had taken both the mumbo and the jumbo out of the black art... much to the disgust no doubt of people who try and make a living out of charging unsuspecting yachtsmen UKP270 to do a very simple job.....

This post is one of the many reasons I read CF - great tale - thanks!

[psneeld]

If you can find a body of open, occasionally currentless water...a GPS swing is more than adequate.

The last compass I installed had the GPS method right in the instructions.

I have done it both ways on small boats...GPS is way more accurate if you know what you are doing and are shorthanded.

I have seen larger vessels and shops done and in the long run...for the average small boat compass and your ability to even steer to a decent compass increment, I have yet to hear how someone can adjust a small boat compass to a higher degree of accuracy.

[carstenb]

Dockhead

If you can find a bit of slack water (difficult in the Channel - but if you wait until you come to the Baltic, it will be much easier.). Now simply set your course to a cardinal point and read the COG off your chart plotter. Compare with your compass (don't forget your deviation and magnetic offset). Now you can see how far off it is.

Continue your way around the rose (don't stop for drinks along the way).

As someone above noted - if it is only a few degrees, I doubt if I would bother since very few people can steer a compass course within a few degrees anyway (especially in any kind of weather).


You can always make a note on your deviation chart and just add/subtract this mentally.

[a64pilot]

While we are on compasses, sometimes the engine or alternator's magnetic field will have an effect, possibly even the auto pilot. I've seen landing lights on aircraft cause a deviation in excess of 10 degrees, and they are way out on the wing.
Easy to check, just watch the compass when you turn big amp items on and off.

[Wotname]

Originally Posted by a64pilot

While we are on compasses, sometimes the engine or alternator's magnetic field will have an effect, possibly even the auto pilot. I've seen landing lights on aircraft cause a deviation in excess of 10 degrees, and they are way out on the wing.
Easy to check, just watch the compass when you turn big amp items on and off.

Ahh yes and don't forget the wiring from the landing light switch may go close to the said compass.

Lucky for boaters, wiring doesn't usually go too close to the mag compass but as A64 suggests, large currents may make up for the distance - in a bad way!

FWIW, if DC wiring has to pass near the compass, tightly twist the pos and neg conductors together to minimize the magnetic field of the wiring.