Deviation card

[chappy]

I need to do a deviation card for my boat, what i need to know is a simple way of knowing if the deviation is east or west of the ships head, for example if my boats compass shows 100 deg C but my hand bearing compass shows 98 deg M will this be an easterly or westerly deviation, any simple solutions are most welcome.
Cheers
Dave.

 

[AuntyRinum]

There is no way to create a deviation card by comparing boat compass with handbearing compass as they both suffer from deviation.
The only way to do it is to sail along a transit which will give you a true bearing and then compare it with the bearing shown by the boat compass.

 

[Anonymous]

Deviation (or variation) west, compass best
Deviation (or variation) east, compass least

Is that enough, or would you like me to talk you through it?

 

[chappy]

Lemain can you talk me through what you have said.
Cheers.
Dave.

 

[roger]

use of hand bearing compass

I think its generally accepted that a hand bearing compass positioned as far as possible away from magnetic iron is adequate for making a deviation card. I suggest you try it with the HB compass while hanging onto the backstay (stainless). You could check the bearing of a distant object from various positions on the boat to see if there are systematic differences.

 

[Anonymous]

Sure....The rhyme is good for both variation (i.e. the difference between true north and local magnetic north) and deviation (i.e. the amount by which your compass is being affected by iron and steel on your own boat).

"Variation (or deviation) west, compass best" If your variation (or deviation) is 'west', then your compass must read higher than the required heading by the amount of the variation. So, if variation is 5 west, then to head 090 true, written as 090(T) then you must head magnetic 095 (090 +5) which is written as 095(M) If you also have some compass deviation and your deviation is, say, +3 west then you must head 095 + 3 degrees, making 098 degrees. We call the final heading of 098 'compass', or 098 (C).

Does that help?

Variation (or deviation) east, compass least" is the exact opposite. The rhyme helps you remember whether to add or subtract.

 

[peterb]

Re: use of hand bearing compass

[ QUOTE ]
I think its generally accepted that a hand bearing compass positioned as far as possible away from magnetic iron is adequate for making a deviation card.

[/ QUOTE ]

It's easily checked. Stand in your preferred position (one well away from magnetic material). Use the hbc to take a bearing of a distant object (the further away the better). Keep taking this bearing while you turn the boat in a small circle. Any deviation will cause the bearing to change while you are circling. If the change is more than a degree or so then move to another position and try again.

Incidentally, on many boats it is difficult to look directly along the centre line. Instead, we set up a sight line between an object near the cockpit (such as a deck-mounted winch) and a piece of red tape wrapped round the guard rail at the same distance off the boat's centre line as the winch is. Look at the tape over the top of the winch and you are looking directly ahead. It's not just useful for swinging the compass; it's surprising how many people seem to think that "dead ahead" means in line with the centre of the pulpit, irrespective of where they are in the cockpit.

 

[alant]

The adjustments for both Variation & Deviation are the same.

*(T)--- +W/-E Variation--->*(M)--- +W/-E Deviation--->*(C)
Conversely
*(C)--- -W/+E Deviation--->*(M)--- -W/+E Variation--->*(T)

Everything on a chart is (& should be plotted) in *(T), with the rest following from there.
Steered courses or bearings from a handbearing compass are in *(C).

Hope this helps.
PS Why not write it down & paste near the nav table as an aide memoire.

 

[Anonymous]

[ QUOTE ]
Steered courses or bearings from a handbearing compass are in *(C).

[/ QUOTE ]I disagree about handbearing compasses - you must not apply deviation to HBCs because deviation is a very local thing (it can change just by moving a few inches). You should always use a HBC in a place known to be virtually free from deviation, such as by the pushpit and all readings should be recorded as (M) or (T) if you have applied local variation, but never recorded as (C).

 

[Anonymous]

[ QUOTE ]
for example if my boats compass shows 100 deg C but my hand bearing compass shows 98 deg M will this be an easterly or westerly deviation,

[/ QUOTE ]I think you added this into the question in an edit after I had replied the first time? Anyway, let's work through this one....

The correct magnetic heading is 098 from the HBC and the compass is reading higher by 2 degrees. So the compass is 'best'. Remember the rhyme "Deviation west, compass best" so in this case the deviation is west. On you deviation card you mark 2 degrees west against a heading of 098. Hope that helps?

 

[chappy]

Sorry to say these answers are not telling me what i want to know.
I understand it about how to add & subtract the deviation from the bearing, but what it is i want to know is when i am makeing the deviation card up, the question is when my ships compass reads 100 deg but my hand bearing compass reads 98 deg how will i know if that 2 degrees differance i have got is a westerly or easterly deviation, sorry for acting a bit dumb bot it is wrecking my brain cells at the moment.
Cheers.
Dave.

 

[Anonymous]

I answered that half an hour ago - did you miss it?

 

[chappy]

Lemain, cheers for your persistance i have finaly clicked & got what you are saying.
I shall print the posts so i do not forget, again thanks for all your help.
Cheers.
Dave.

 

[alant]

<disagree about handbearing compasses - you must not apply deviation to HBCs because deviation is a very local thing (it can change just by moving a few inches). You should always use a HBC in a place known to be virtually free from deviation, such as by the pushpit and all readings should be recorded as (M) or (T) if you have applied local variation, but never recorded as (C).>

Agreed, you are correct, deviation cannot be used in HBC, since it will vary according to position from which used & would probably not be known, so couldn't be applied.
I was trying to get over the obvious point that all on a chart is True & that a bearings from a HBC cannot be directly used.

On a different issue, what effect (if any) have you observed in regard to deviation caused by S/S - spectacles, backstays/shrouds/stanchions.

 

[Anonymous]

[ QUOTE ]
On a different issue, what effect (if any) have you observed in regard to deviation caused by S/S - spectacles, backstays/shrouds/stanchions.

[/ QUOTE ]It's interesting you should say that...I have come across a couple of bits of SS that cause my magnetic dropped-nut-picker-upper to stick to it. And some SS nuts that shouldn't be picked up by it! I am about to swing my main compass and I will check the SS tubing (I have a great deal of it by my upper steering position) and the stays, and davits which are being fitted in a few weeks. Fingers crossed. I would hate not to have anywhere to use a HBC, I am usually to be seen with a HBC round my neck when at sea - the best collision avoidance device of all.

 

[Stemar]

[ QUOTE ]
I would hate not to have anywhere to use a HBC, I am usually to be seen with a HBC round my neck when at sea - the best collision avoidance device of all.

[/ QUOTE ]AFAIK, unless you're inside an electromagnet, variable deviation doesn't matter for collision avoidance - all you're interested in is that the relative bearing of that bloody great container ship is /forums/images/graemlins/cool.gif- or isn't /forums/images/graemlins/frown.gif changing. The fact that your compass is 20 degrees out is irrelevant

Or have I got the wrong end of the stick again?

 

[Anonymous]

[quoteAFAIK, unless you're inside an electromagnet, variable deviation doesn't matter for collision avoidance - all you're interested in is that the relative bearing of that bloody great container ship is /forums/images/graemlins/cool.gif- or isn't /forums/images/graemlins/frown.gif changing. The fact that your compass is 20 degrees out is irrelevant .....Or have I got the wrong end of the stick again?

[/ QUOTE ]If you move a compass more than a few centimeters the deviation can change considerably. Unless you can find a magnetically clear area such as the pushpit you cannot rely on a HBC, whether that is to take an absolute bearing or a relative bearing.

 

[VicS]

Re: Deviation and HBC

And if your ships heading changes the deviation of the HBC, if any exists, will change even if used in a the same location.

 

[Hakuna]

Re: Deviation and HBC

As a solution to getting the HBC deviation to virtually nil it has been suggested to put your H/SWMBO in a rubber tender and tow behind your boat. When H/SWMBO sights a straight line through the mast/forestay/backstay s/he calls out the HBC reading which can be compared to the ship's compass.

Never tried it, but sounds interesting. Remember not to take glasses/watch into tender!

 

[jimi]

Re: Deviation and HBC

might be easier to use the gps to get a bearing as a way point on various landmarks point the boat at them and then compare the (magnetic) bearings .. in fact its such a good idea I think I'll do it myself!