Any navigators out there?

I mainly sail on Moreton Bay which is shallow in places. Its mostly mud so the odd contact with the dagger board hasn't done any damage. Lately I've been leaving the dagger board out (with the fins) and the rudder not locked down on the basis that whatever side slip results from no dagger board is more than made up for by the direct course I can now sail.

I haven't noticed a lot of side slip but also haven't been out in strong winds yet without the dagger board.

My GPS is set at true readings. I have the Ritchie compass. The variation here between magentic and true should be about 12 deg. I noticed the other day that the variation was much wider than that on some points of sail. I hadn't taken note of it before. It may just be metal near the compass.

Can someone with more navigation experience tell me if it is possible that the extra difference is the amount of side slipping I'm getting? Logic tells me it's not going to show in a variation between the two 'compasses' but it might be a variation in the way I'm reading the compass caused by sideways movement which isn't the same with the GPS because it just gives a bearing without any need to actually 'read' it.

Any thoughts?

Any variation will not be a consequence of side slip or your direction of motion. I suggest the following possible causes:

1. As you have already noted, possibly magnetic material on your boat. Test this by moving a compass around the boat and look for deflection in direction.
2. Latency in your GPS - would only be apparent whilst you are in a turn.
3. Local magnetic anomaly - ore deposits or ship wreck. These will be marked on a current chart.

okz00k has it right. There is a basic element to consider, though, which is orientation. Are both your compass and gps properly oriented the same direction parallel to the centerline of the boat? These are both accurate devices but they are small and getting them oriented to the centerline with the same precision with which they can be read may be a trick, especially if the gps is set up in a movable mounting device. The shape of the plastic at the compass mounting point or unequal pressure on the screws that secure it could produce some variation. None of this would be apparent if you were just using the compass. A good compass and chart will get you anywhere. I'd check the mounting of the gps for proper alignment.

baysailor

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baysailor

This is a good book that will explain how to use the compass properly, RYA Navigation Handbook
Tim Bartlett.

First you should check your GPS settings. Is it on 'true north' or 'magnetic north'? It should be on magnetic north to be the same reading then your compass.There can be a big difference between the two. On some places in the world up to 90°! The true north pole is on a differen location than the magnetic north pole.
See here:http://yakass.net/articles/90-safety/161-north-magnetic-true-a-grid
Another thing to consider is the angel you are sailing with, what I mean is how much your boat leans to the side (ama). Most compases only work well to a number of degrees tilt, then the reading is incorrect, and finally the 'spinning compass rose or needle' touch the bottom and stop the compass spinning.
Hope that helps