Sowester Bosun compass deviation adjustment.

[Red]

Hello,
I would like to know how to readjust the compass deviation which on a unit I have is set to 8 degrees west. I do not have the instructions. This is the rotating steering grid type compass with a rotating bevel. There does not seem to be an obvious way of detaching the bevel from the compass, it rotates but when you rotate the bevel the deviation set remains fixed. There a couple of screws in the side of the compass and these adjust the ease of movement of the bevel. Any help appreciated photos attached.
Thank.

 

[johnalison]

I think you mean variation, but I don’t know the compass enough to answer.

 

[sarabande]

Does your model have two small screws labelled N/S and E/W ?

The 360 degree bevel is to make steering easy, by aligning with respect to the N mark on the card. It's not for deviation adjustment.

There is a lot of confusion about variation and deviation, and the two terms are becoming inextricably mixed unless one is an old-fashioned geek.

 

[zoidberg]

The excellent Bosun Grid Compass is a superb instrument for the small-boat sailor. There is no facility for 'offsetting' Local Magnetic Variation ( or 'declination' in some quarters ) to make the compass read True. The 'BEZEL' ring is used to set the required Magnetic Course To Steer against the 'lubber line mark'. When the boat is turned to align the floating North pointer with the painted parallel lines on the glass, the boat is then pointing/heading in the required direction.

The two screws set into the bowl hold the components together. See here:

A Compass for ‘Sandpiper’ – Part 2

 

[cagey]

Fabulous compass, I think they were originally RAF WWII depending on aircraft, pilots or navigators, compass.
The chap who repairs my compasses told me the other day that he is testing the glow in the dark letters and numbers as he has been warned by his insurance company that because of the radioactive properties of some of the paint he has to determine whether they are a risk to health. I’m just passing on what I have been told, I can’t swear to the veracity of the comment.
Keith

 

[AntarcticPilot]

Fabulous compass, I think they were originally RAF WWII depending on aircraft, pilots or navigators, compass.
The chap who repairs my compasses told me the other day that he is testing the glow in the dark letters and numbers as he has been warned by his insurance company that because of the radioactive properties of some of the paint he has to determine whether they are a risk to health. I’m just passing on what I have been told, I can’t swear to the veracity of the comment.
Keith

A WW2 compass might well use radium paint, and that is hazardous. See Luminous paint - Wikipedia

 

[neil_s]

I had one of those - very nice compass. There are no adjuster magnets. You will have to fit a wedge behind the mounting bracket if you have a large fixed offset.

 

[andsarkit]

I still use mine. It is much easier to keep the compass arrow aligned with the grid than to check numbers on a compass dial.
There was one memorable occasion when after being becalmed mid ocean for several hours, when the wind returned I set off 180deg in the wrong direction with the N arrow aligned the wrong way!
Also if you knock it hard and it does a somersault in the gimbals, the grid will have turned 180deg.

 

[srm]

I had a Bosun many years ago. Grid steering makes life easy for tiller steering as you can line up the card arrow and grid from any angle of view. I also had an ex-RAF grid compass of WWII vintage, it was a much more substantial piece of equipment.
However, the Bosun is a very good, cheap and cheerful, basic magnetic compass so no magnets to correct for the vessel's deviation or a way to apply an offset for magnetic variation.
But then anyone with a basic knowledge of navigation would have been able to apply those corrections in pre-GNSS days. Now it seems that such elementary knowledge is regarded as the domain of old fashioned geeks.

 

[zoidberg]

But then anyone with a basic knowledge of navigation would have been able to apply those corrections MENTALLY in pre-GNSS days. Now it seems that such elementary knowledge is regarded as the domain of old fashioned geeks.

'SimpleSums 'R Us'......


BYW, I have TWO Bosun Grid compasses - His 'N Hers - tucked away somewhere. And TWO Yeoman Sports.....
Now that's Double Geek for you!

The rotating bezel on the 'Bosun' compasses, as with others e.g. by Ritchie, suffer from 'sticky' on the undersides of the bezel. One reason offered is that salt water gets in there, dries, and leaves a thin film of salt deposit which gets ground down by movement, but enough remains to narrow the fine gap. It's not easy to remove, but applications of warm fresh water may help - just as it does on sailors' grimy hands.

BTW I'm reminded by my 'Line Manager' that there's no such thing as a daft question, but there sure are daft answers.

 

[Red]

The excellent Bosun Grid Compass is a superb instrument for the small-boat sailor. There is no facility for 'offsetting' Local Magnetic Variation ( or 'declination' in some quarters ) to make the compass read True. The 'BEZEL' ring is used to set the required Magnetic Course To Steer against the 'lubber line mark'. When the boat is turned to align the floating North pointer with the painted parallel lines on the glass, the boat is then pointing/heading in the required direction.

The two screws set into the bowl hold the components together. See here:

A Compass for ‘Sandpiper’ – Part 2

Many thanks for that, I will remove the two screws in the side and rotate the compass so that I can line up North with the white line. At the moment it is 8 degrees off.
Regards

 

[VicS]

Many thanks for that, I will remove the two screws in the side and rotate the compass so that I can line up North with the white line. At the moment it is 8 degrees off.
Regards

The compass must be mounted square to the boats heading, so that the white index mark on the rim of the bowl is in line with the fore and aft line

Index mark ringed in yellow here





The bezel should rotate freely without removing the screws .
If it does not remove the screws lift the compass unit out of the bowl clean off the old grease and re-grease with some silicone grease. Dont over do it though.

Place the compass unit back in the bowl and carefully refit the screws noting that the compass unit will have to be lifted very slightly off the rim of the bowl for the screws to line up with the groove in the compass unit. Dont over tighten them and damage the threads
( My compass has brass screws but I think for some reason n I made those and fitted them in place of the original plastics ones.)

When the compass is away from the boat and any object that might affect it the arrow on the card should point to magnetic north, Now about 1/2 degree east of true north in the UK. ( this difference is called "variation" )

If it does not still do this do this when mounted on the boat there there is something causing "deviation" . Without corrector magnets you will have to try to locate the compass where this is minimised ( it will vary with the boats heading ) then if still significant consturct a "deviation card" by "swinging the compass" and use this to correct the compass reading as well as correcting for "variation_ ( 1/2 degree variation can be ignored )

In use you can either set the bezel to align N with the nark and steer by suing the graduation on the card itself or set the bezel to the " compass " couse required and steer by aligning the pointer on the card with the grid

I've had two of these. I don't remember any instructions apart from what's printed on the box.



Hope this helps. Apologies if a bit condescending.

( notice they call it a "bevel ring rather than "bezel" ring

 

[Red]

Many thanks for your help, the bezel ring rotates freely, as you may be able to see the heading white mark has been altered. Thanks for all the information, I didn’t want to break anything as the compass is like new.

 

[VicS]

Many thanks for your help, the bezel ring rotates freely, as you may be able to see the heading white mark has been altered. Thanks for all the information, I didn’t want to break anything as the compass is like new.

The white index mark I refer to does not seem to exist on your compass or at least is very indistinct .On mine its outside the rotating bezel on a boss that forms one of the gimbal mounting points. Look closely at my second photograph to see what I mean.
Its the mark to set the required compass course on the bezel to when steering using the grid.

Its the lubber lines, and the grid, inside the compass which appear to have been altered. They should correspond to the cardinal points on the bezel ring, with the grid pointing at N, but as you say they have been offset by about 8°.
Perhaps this was done to correct for the "variation", which was about 8° t back in in the 1970s when the compass was new.

The bezel is only clipped to the actual compass capsule with spring clips so it would be possible to re-position the bezel on the capsule so that the lubber lines are in line with the N,S,E and W lines on the bezel and the grid markings align with N.

It will be very difficult to use the compass as it is. If it was mine I would look at repositioning the bezel on the capsule but if you don't want to tackle this yourself contact Barry Philips at BPSC Marine. (He was at one time an occasional contributor to these forums)
BPSC Marine - the one-stop instrument repair of your compass or compasses

 

[VicS]

When I enlarge your photo I think I can just see the index mark I referred to, roughly adjacent to 117° on the bezel



The gimbal rings have been flipped over so the mark is on on the opposite side to where mine is pictured