Buying a compass is an infrequent event.  Most builders will choose one for you and make the choice strictly on price point.  Your compass should be chosen to match the damping of the dial with the turning ability of the boat.  Larger boats turn slower and should be matched to larger compasses with greater damping.  Larger is also easier to read.  Bifocals anyone?  Examine your existing compass or the standard installation on a new boat and imagine spending an hour or two concentrating on that instrument, perhaps when your radar packs it in one foggy day.

There are two types of compass dials in use today.  The conventional flat dial (Actually most are slightly concave for easier reading at the edge.), and the direct reading or convex dial in which the lubber's line is read on the wheel side of the instrument.  The latter is actually an adaptation from the aircraft industry where the compass must fit in a panel mount at the front of the cockpit.  The conventional flat card is easiest and safest to use as you simply turn the wheel toward the number for the desired course.  Direct read dials have usefuness when the only mounting option is high in the field of view of the helmsman.  Otherwise the conventional dial should be fitted.  In any case on a two-station boat both compass dials should be of the same type to avoid confusion when shifting control.  More questions?  Feel free to call Bill Haimes at Island Marine Instrument Co, Inc.  (425)258-4120.