I agree the alternator output is low - best to check that out at an alternator shop. But the voltage should not be higher at the alternator than at the engine panel. What gauge is the wire? Most factory wiring on older boats is inadequate in gauge and travels a sometimes convaluted route to get to the batteries. I prefer a straight path as there is less chance of problems. I understand the batteries are set up as bank 1 and bank 2. A single group 27 for house loads while away from the dock is pretty minimal on a 30' sailboat unless you are using kerosene lamps. You would be best to increase the size of the house bank in my opinion. If your batteries are the same age I would suggest paralleling them to create a house bank twice the size and purchasing a start battery for the engine side of the system. The best way to charge batteries is not through the main battery switch. If you were to purchase an Echocharge you could wire the alternator (with a heavier gauge wire) directly to the house bank. The Echocharge is a simple 3 wire installation and is a reliable solid state device (available for $100-$125 US). It automatically takes care of the start battery - when it senses a charge voltage is present it allows up to 15 amps to travel to the start battery. The battery switch would determine which battery is used for which purpose. Leave it on house when on the boat (#1), start (#2) to start the engine and off when leaving the boat. If someone were to switch it off when the engine is running there would be no problem as charging wouldn't be interupted. If you have a shore charger with this scenario its output would also go directly to the house bank. I hope this helps. Please inform us of the results.