Question about Navigation Course

[cliffordnolan]

Re: Very nice and all that ....

The first paragraph of my previous message is flawed - I'm afraid I did not understand what you were thinking afterall.

Cliff.

 

[peterb]

Re: Very nice and all that ....

[ QUOTE ]
The first paragraph of my previous message is flawed - I'm afraid I did not understand what you were thinking afterall.

Cliff.

[/ QUOTE ]

Cliff

I suspect that you did understand what I was getting at. I think it's easier to understand if your frame of reference is the earth, rather than the boat.

The earth has a magnetic field. In the absence of deviation, the compass (assuming it's the old needle type, not a fluxgate) will line up with the earth's field. As you have pointed out, when the boat turns the helmsman will apparently see the compass turning, though actually the compass is still while the boat turns round it.

But if the boat has any ferrous content (as virtually all boats have) then it will have its own magnetic field. This will have a hard steel component and a soft iron component. The hard steel component will give a field which is fixed in magnitude and direction relative to the boat, irrespective of which direction the boat is pointing in. The soft iron component will give a field which varies as the boat turns, because the iron is being magnetised by the earth's field. (Incidentally, because the strength of the earth's field varies with position on the earth's surface, the soft iron component will also change; but the change will be small unless you are doing oceanic sailing, For normal coastal sailing you can forget it.)

The sum of these two components gives the deviating field. The field seen by the compass needle will be the vector sum of the earth's field and the deviating field. For any one alignment of the boat there will be a unique value of the deviating field, and hence there can only be one deviation value.

Stick to a stable frame of reference (the earth) with what should be a constant direction of magnetic field, then look at the behaviour of the boat's field as producing deviation.

It all gets much easier with pencil and paper; maybe we'll have to get together. Where are you?

Happy christmas!!

 

[cliffordnolan]

Re: Very nice and all that ....

Peter, it's more complicated that I had realised! I'll have a think about it and get back to you after Christmas. I am in Ireland. Have a great Christmas, Cliff.

 

[peterb]

Re: Very nice and all that ....

[ QUOTE ]
I am in Ireland.

[/ QUOTE ]

Sorry, Cliff, that's a bit far for a drink and a chat! Happy New Year!

Peter