I believe its the same as my Navman 2100 unit.
As Ian says they are only as good as the effort you put in to callibrate them, but now I've done mine it appears to be very consistant.
It sits in the dash OK with just a normal boat cover on.
I couldn't do without it now, but it can become addictive at constant cruising speed adjusting trim, throttle etc to balance speed with fuel flow rate.
Sad I know.
RIBBED For extra pleasure.
Member of the ebay Blue RIB cover club.
Member of the Bombard 380 Aerotec club