GC batteries do not make good starting batteries? Tell that to my twin Lehmans. Recently, I depleted my eight house GC battery bank to 22%. The engines started without any difficulty, any. No separate starting batteries for the Lehmans. I do have two Group 27's dedictated to starting my generators. Yes, I have two on board.
8 golf cart batteries!, would start the engine ok, even depleted. that is a lot of batteries.
I had 2 golf cart batteries, and they did not crank well the engines.
Made me wonder at the time if the starters were going out.
I took the starters apart, could not see anything wrong, took them to a starter repair service, guy looked them over and said they were ok.
Putting in a starting battery, they cranked great.
I had 4 golf cart in 2 banks, but the banks were separate to each engine, and it was poor cranking. That setup was done by a previous owner. If I had 8 golf cart batteries joined together, my engines would also crank great.
And the batteries were not new, had a few years on them. They failed on me all 4 about the same time a few years after I got the boat. Their lifespan was not spectacular, not any longer life than the starting batteries I used afterwards..
When they finally failed, they were swollen up on their sides like other batteries do. So I suppose they sulfated. Most of my starting batteries, the failures are shorted cells, so I would get 10 or 8 volts.
One week the battery started the engine, next week, one battery 10 vdc, another 8 vdc. The 8vdc was paralleled with another good battery and it is ok. The charger ramped up the volts to 15.6v output and they were gassing. The charger knew something was wrong and tried to compensate. And had to clean up some acid in the battery tray.
A good idea is to sprinkle a layer of baking soda in the tray- box to neutralize battery acid.