Originally Posted by Zubenelgenubi
What method are you using for the compass swing?
Pick a calm day, flat water no tide (easy to find round here). Using the GPS. steer 000. 045, 090, 135, etc. and on each heading note the compass reading on that heading. (The GPS should be set to show courses in True.)
Apply the Variation (from a chart), which in the UK is W'ly so you subtract it from the compass reading, and compare the result with the True heading. The difference is the Deviation of the compass and is plotted on a graph, W'ly if the compass reads more than it should do, E'ly if it reads less. You get a curve, usually S shaped. If the deviation is very bad (20 degs, say), then you may change heading tens of degrees without the compass moving.
There are lots of other ways to do the job, like using a hand bearing compass well away from any magnetic fields on the boat and comparing its reading with the steering compass, but some more traditional methods are impractical on a RIB - taking a bearing of the sun with the steering compass, for example, or using a pelorus.
Now, adjusting the compass to minimise the deviation (and it's important to do so) is an altogether separate black art. My compass came with basic instructions of how to do it (lost) and I managed quite well.
You may of course already know all that.