Volt Meter

First the gauge test is take your handheld meter check the voltage across the battery. The dash gauge & voltmeter should read the same. Marine voltmeters are not exactly the most accurate devices by are fairly close if working properly. The alternator output voltage running should be between 13.6 vdc. & 15.5 vdc.. Any less and the battery will loose charge. Start the engine with the handheld across the battery you should read between these voltages @idle. You may see some increase up to 2000 rpm. The dash gauge should read close to the same. If the reading on the dash is off check all connections to the gauge including the ground. You might even pull the dash unit and hook it across the battery to see if the readings are close. This eliminates the wiring in the boat when checking the gauge.

What does your in-dash voltmeter read? My 87 reads about 10 volts, but I've probed around and found right where the voltage drop is at on one of the buss bars. I haven't replaced the wire yet but was wondering if anybody else had these symptoms. It also has a weak horn, due to the same wire.

I know for sure the older Mustangs (74 and up), were famous for inadequate wire size from the engine to the dash. They only had current meters in the dash(at least all that Ive seen). This causes any add-on or factory installed volt meters not to be accurate if wired at the dash panel and not at the battery. You may have an alternator that puts out 14.2vdc at the output stud, yet only get 12.8 or so at the dash. This is due to excessive voltage drop in the wiring. The more that is drawn, the more the drop is. Voltage drop in a wire of a certain length and current draw can be calculated. My boat had a factory #10awg hot and ground wire to feed the dash and accessories. I got over 2 volts of drop with the lights and blower on and less than .5 volt drop when shut off. Unlike house wiring where you can draw 30 amps off a of #10 wire, if you get a 5 volt drop- who cares, everything is to spec, but loose 5 volts from 14.2vdc and youre in trouble! If anyone has any wiring/current/voltage/ohm/electrical Questions feel free to ask because I may have questions with fiberglass/gelcoat/ where Im lacking alot of knowledge.   

Skyhawkflyer, usually theres a larger #10 or #8 hot wire coming from the engine where it goes to a busbar in the dash, then split into several fused lines, one being the ignition key. Does the output of the key go to another bussbar? Because if it is connected like that you might get away with adding a simple relay from the main bus bar to key switch bus bar. I need a bit more info on how its wired cause im not real familiar with newer boat wiring methods. Might also want you to try a few more measurements to be sure. let me know..