You might get away with it for awhile, depending on how you use the boat and how diligent you are in setting the switch in the right position.
However, this is far from the most desirable setup.
Much better is to route
all onboard charging
sources -- alternator
, battery charger
, wind generator
, solar panels
, etc. -- directly to the house batteries (the two 8-D's). Then, use a voltage follower device like the Xantrex EchoCharge or the Balmar
DuoCharge to maintain the start battery.
These devices do this automatically...no need to switch anything, and no need to worry about making a mistake (and possibly blowing out the diodes in your alternator
if you pass thru the OFF position while the engine
So-called "battery combiners" or ACR's also would work...they combine the batteries whenever a charging current
is present. I much prefer the voltage followers, though, because they don't "combine"; rather, they sense the presence of a charging current
on the house batteries and bleed off a little to maintain the start battery...automatically. This avoids any possibility of creating huge in-rush currents when you combine, e.g., a deeply discharged large battery bank with a near fully charged start battery.