The details of this problem are not adding up. Way too much hit and miss and partial information. I think you need to go back and analyze the whole problem step by step from the beginning. So:
1. You have two, separated and independent battery systems, one for starting, one for house, correct?
If so, you cannot draw any conclusion about one system based on what happens with the other. So, just because your starter will crank the engine (with or without combiner, battery switch to all or any other factor) does not necessarily tell you anything at all about what is going on with the house system.
2. You purchased two new house batteries and claim they were fully charged, correct? How do you know they were fully charged? Did you use a shore power charger
or engine alternator
? Did you check the output to confirm it is putting out a full charge? Did you check the before and after voltage at the battery with an accurate, digital meter?
I had a similar problem with a new boat and didn't have all my tools with me so used a cheap
analog meter to check the battery. It looked charged and the voltage increased when I turned on the charger
so thought it was the battery and bought a new one. No improvement so went out a bought a new digital volt meter and discovered the batter charger was only putting out about 12.5 V, just barely enough to keep the battery alive, but not anywhere close to enough to give it a full charge. You need to check the voltage of the battery and charger with an accurate volt meter to be sure you are getting a charge. You should see around 13.5-14V at the battery terminals when the charger is connected and charging
the battery. Wouldn't hurt to get a hydrometer as well.
3. If the batteries are fully charged and you have a short that is draining them to 8V or anything like that in a matter of minutes then you will be seeing hot, melted insulation
, smoke and maybe even fire. Absolutely without a doubt. If you are not seeing something really, really, obviously very hot then a short is not the problem (or not the main problem but could be contributing).
4. The starter draws more power than anything else on the boat. Several things about the starter.
a. If it still turns over and cranks the engine then it is not burned out.
b. It will be connected directly or almost directly to the battery with large cables
. It should be easy to track and visually confirm if any of those cables
c. If a battery has enough power to crank the engine then anything connected directly
to that battery should also work unless that thing is broken or shorted. That means directly to the plus and minus terminals on the battery, not through any switches, combiners, grounds, etc.
Try all this, write down the data and results and the exact
conditions on how you measured ( for example: with a digital meter at the + & - battery terminals, or from the engine block ground to switch panel postive, or charger on, or engine running, etc) and get back to us.