I think you should first check wether the erratic (calculations?) response out from your instrument is a matter of hardware (the wheel down your hull) , or an instalation issue.
First test can be carried out by lifting the speed sensor, (don''t forget to cover the through hull) and checking the readout of the instrument while somebody blows or moves the wheel of the sensor. (Ask this buddy specifically not to blow or move erratically the wheel, if not you will be in square one again
If this is a brand new equipment, it is hardly to believe you are having a hardware problem. Probably as Jeff suggests, you may have a instalation or configuration problem. Following the manual instructions very close, is easy to check the configuration out. I am an absolute asle in this fied, and was able to fix all of the problems arising from an ST 50 Raytheon system.
In addition I wonder whether the 12 v supply of your system is affected by its routing or any other equipment in the same line
Lastly, if nothing said here provides a solution to your problem, which is something I do regret to expect, you may post your question in this electronic forum. (I thought it was sponsored by Raytheon)
<a href="http://www.yachtingmagazine.com/forums/electronics.html"> http://www.yachtingmagazine.com/forums/electronics.html</a>
Don''t give up. An american guy, step the moon 30 years ago, so your issue is a piece of cake.