Boat speed off

[Brad K]

I have a C97 MFD purchased two years ago. I am on a trip and find Boat speed is out by about a knot. Compared to my datamarine knot log and a Garmin hand held GPS and Navionics on my Iphone 5.
I will be doing 6.0 knots and the MFD says 7.0 knts +- 0.2. After 30 minutes it might agree and be the same.
I am in northern Lake Huron Ontario Canada.
I am using Datum WGS1984
The MFD is using the original software. no upgrades yet.
serial# C97 E700120520293
Using navionics chart that came with unit.
I checked the distance log with the latitude travelled and it is ok.

[Chuck - Raymarine - Moderator]

Welcome to the Raymarine Forum Brad,

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