Deviation table

[alant]

I've never really swung a compass properly - I've compared varying headings with charted bearings to check it's not grossly out - but I'd like to. Am I right in thinking you motored on exactly the cardinal headings on the right hand side of the diagram, then measured the degree figures on the left? How did you measure those? Hand compass bearing to whatever was directly over your bow? Have to say I've never found the conventional practice of checking a steering compass against a hand compass very convincing - why on earth should I assume the hand compass is correct?

I guess there's no point me doing a swing now, as part of my plan for this winter involves rebuilding the binnacle. I'll give it a go in the spring. With the luxury of land all the way round (in Lake Solent or Southampton Water) I think I'll do it by charted bearings from my known position to various distant objects, rather than a second compass.

Pete

I did a deviation check whilst on a rig safetyboat in the Baltic, so few if any landmarks to use & no handbearing compass.
Worked something out, running down all the compass point bearings shown on our GPS display & checking these against 'ships' compass. Got a good 'sinewave' diagram.
Still not sure how accurate, but better than nothing.

 

[oldbilbo]

Am I the last guy on the planet to have heard of Sun Amplitude Tables, and their use in determining compass error/deviation....?

 

[Tempus]

Am I the last guy on the planet to have heard of Sun Amplitude Tables, and their use in determining compass error/deviation....?

You may well be. As I note above, I know nothing about swinging compasses, and yet have owned and sailed cruising boats for 20 years. Which got me thinking...

We used a deviation table on our first boat, but haven't used one for the last 15 years or so since GPS got good and COG Boat Head etc is shown electronically.

Tend to use a GPS compass. When on passage the boat is on Autohelm and I rework the CTS every few hours and just add or remove a few degrees.
When helming we tend to be in sight of land and not that focussed on the compasss.
We use a HB compass for checking leading marks etc, but then steer down on the marks.
But mostly the proper steering compass stays covered - shocking!!

So that begs the wider question: Are you the last guy still to do a deviation chart? - Or am I the freak?

Hard hat on!!!

 

[prv]

Am I the last guy on the planet to have heard of Sun Amplitude Tables, and their use in determining compass error/deviation....?

I have once gone through the process of checking the gyrocompass against the sun on Stavros; it is done on a regular basis. Taking the bearing of the sun was easy enough with the mirror and slit arrangement on the bridge wing repeater, but I found the subsequent paperwork difficult to follow as the second officer walked me through it. I don't know whether those would have been Sun Amplitude tables or something else.

Pete

 

[Twister_Ken]

Y
We used a deviation table on our first boat, but haven't used one for the last 15 years or so since GPS got good and COG Boat Head etc is shown electronically.

Tend to use a GPS compass. When on passage the boat is on Autohelm and I rework the CTS every few hours and just add or remove a few degrees.
!

Reasonable policy, but on a long x-tide leg, you'll probably end up sailing further through the water doing that than calculating a CTS for the entire leg and steering it on the magnetic compass. That's why I wanted a deviation table. On a couple of S-N trips I've found myself a few miles west of where I wanted to be as I closed the IOW. A bit of leeway, probably, and a bit of deviation.

 

[Tempus]

Yes slight misunderstanding, I understand about XTE and steering to a constant heading on a long cross channel, my point is that I rely on the Compass in the Autohelm and instruments, not the main boat compass. I do find myself a few miles out half way across. This is partly because I don't have the bottle to go as far out on the XTE as my calcs say, (usually because as I'm wondering how I'll ever get back to Cherbourg once I'm well to the west of Aldernay.), and partly tidal errors/ leaway which I suspect give a much bigger error than a steering compass on a grp boat.

My view is that the boat steering compass is now becoming a lttle redundant - Vital for a Transatlantic etc, but am I alone in relying more on auto-corrected electronic compasses?

 

[prv]

My view is that the boat steering compass is now becoming a lttle redundant - Vital for a Transatlantic etc, but am I alone in relying more on auto-corrected electronic compasses?

I'm embarrassed to admit that my steering compass currently has no illumination - Ariam's electrical system has been in a bit of an interim lashup state this season to get us out sailing before a thorough overhaul this winter. After the first of this year's Scuttlebutt Cherbourg dinners, we popped round to Barfleur which involved catching the tide at 0330 (following afternoon tide was getting a bit close to yet another forecast gale). As we crashed out of the eastern entrance, I realised that with sizeable waves and no visible compass, it was actually pretty hard to steer in a straight line without any visual references ahead. I guess I hadn't appreciated how much I use the compass at night to stay in a straight line, even if I'm not actually following a course as such.

I did have a backup plan in the shape of the gooseneck torch from my toolbox, which I'd have taped to the binnacle with the business end pointing into the compass and most of the lens taped up to give a dim light, but in fact it was OK steering by the illuminated wind dial (assuming constant wind direction) until we reached a point where Barfleur light gave a visual reference.

Pete