My furuno depth finder gives me crazy depth readings after i go over 10knots..whats the problem?

There are set by you to feet, metres, 'F or 'C, knots, kph, or mph.From a bit more reading, depth is Feet, distance is Nautical Miles. Temperature you have a choice.

Note: The water temperature graph can be
set on or off (See page 11-4.). The temperature scale is fixed between 20-80 °F (0 to
30°C).

Read the manual
https://www.furunousa.com/-/media/sites/furuno/document_library/documents/manuals/public_manuals/gp1650fdf_operators_manual_version_m.pdf

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I DON'T KNOW.. CONSIDER FRESH BATTERIES.
TEST IT ON IN THE AIR. WITH IT STILL ON, PLACE IT ON A SOLID SURFACE SUCH AS A DOOR FRAME.
THERE SHOULD BE A STUD INDICATOR LIGHT OR SIGNAL. IF YES, IT SHOULD BE GOOD TO GO. MY
STUD FINDERS ARE NOT ADJUSTABLE FOR READING MULTIPLE WALL COVERING DEPTHS.
YOU COULD MAKE UP A TEST BOX, READING A
BARE WOOD PIECE THEN LOOK FOR A READING AS YOU SLIP PIECES OF DRYWALL OR SIMILAR BETWEEN THE FINDER AND THE WOOD. THAT WILL GIVE YOU AN ACCURATE TEST OF DEPTH LIMIT.

Call service engg.

There is a keel offset or waterline offset that can be entered in the menus. Press "MENU" + go to "DRAFT", use right arrow to enter menu, change using up and down, the press "MENU" when done. Note that "DRAGHT" is on PAGE TWO(2)!! of the main menu. John 847-746-0100 technicalmarine.com

You need to set the transducer lower. When at speed, the hull of the ship raises up out of the water and hence your transducer is probably not deep enough at these times to give a proper reading.

check your gain setting try auto gain and try manually gaining back to see if it resolves, if yes, send unit in to furuno, if no, ducer is probably at fault.

I own a 1995 Four Winns and what you need is a Lowrance LSM-1 depth finder unit. It accepts the two pin lead from the transducer and send the depth reading to the dashboard display via the 6-pin connector. There is also a three wire harness (red/blk/wht) that connects to a power supply.

The reason I know all this is my depth finder fried a couple years ago so I removed the Lowrance LSM-1 from the boat in order to find a replacement. Unfortunately, this was an OEM part that is not available from Four Winns or any aftermarket source. Probably your best bet is to look in a boat salvage yard or purchase an aftermarket depth finder.

If the transducer is mounted off of level, an error is introduced due to angle. It is a fixed percent offset, say 10%. so 10 feet reads 11 and 100 reads 110. There is an offset function to read depth below ducer, depth below keel, and depth below waterline, but these are fixed amounts in feet, not a percentage. Look at the transducer when it is operating, boat angle considered.

Check the finder on the bottom of the boat and make sure the cable is ok?