Terry ...I understand your frustration...

Testing the external antenna on your 788 (in this case) is not to determine accuracy of marking waypoints....(so it doesn't matter that the antenna is at the other end of the boat)...

The test is determine positively that the internal antenna is not performing satisfactorily ...

I would suggest unhooking all Ethernet cables and plug the external antenna into the 788 and see if the speed and direction issues disappear....

If it does, then you have ample positive results to explain your case to HB that an external antenna is the only way to fix this issue of unsatisfactory internal antenna operation...

Rickie