Sorry to post this again, but I have failed to resolve the problem, and have
more clues.

I have a Raymarine ST60 Multi and want to get NMEA data (Wind & Depth) into
the 9 pin serial port on my laptop. If the laptop is not connected to
anything at all (including power supplies) this works fine. If I connect it
to the boats 12v supply via a cigarette type laptop power supply, or an
inverter using the normal laptop power lead I get nothing.
Equally if I connect the USB port via a Serial to USB lead to the GPS and
AIS input I immediately loose the NMEA input.

Kees suggested using an optocoupler which I bought (not cheap) from Active
research which looked as if it should do the job. They are boat people and
supplied me with a 2 way adapter cable for NMEA In and out to serial plug.
Just occasionally with the computer not connected to either external power
supplies or to any other equipment a got a little garbled NMEA through.

I have replicated everything on my son's laptop, a totally different make so
I think we can eliminate the computers. I can connect the laptop through a
cigarette type laptop power supply to an external car battery and again no
problem. If I connect the negative of this battery to the negative of the
ship's battery I immediately loose the signal.
If, with the computer isolated and the NEMA going in correctly via the
serial port I connect the optocoupler input across the NEMA out terminals I
again loose the signal. Its as if there isn't enough power in the NEMA
signal to power anything.

2 other things. If I put a GPS signal into the serial port there are no
problems even when the laptop is connected to the ships power supply again
confirming that the laptops seem OK and that HyperTerminal is configured
correctly.
Along with the equipment mentioned above, I also have a Raymarine S1G course
computer.

Has anyone any further ideas

Thanks,

Phil