Hi,

There is no way of defining a voltage that will start a engine, there are to
many factors. Even a fully charged battery will have problems when it and
the engine are below freezing. That said, you can improve your odds in
getting the engine to start.

1-A separate starting battery, or two that are only used to start the
engine.
Wire the boat so that you have to do nothing to use the starting
battery, it is normal.
You will need to have a house bank to run everything else.
2-A solar panel that maintains the starting battery.
Keeping the battery fully charged helps starting the engine and
increases the life of the battery.
One of the new solar Pulse chargers works wonders for battery life, even
brings a marginal battery back.
3-Replace your old starting wires with new tinned welding cables. A lot of
starting problems are the cables.
Cooper wires without tinning corrode, this increases your loss in the
cable, Never use auto style battery cables.
Anchor sells battery cables that are tinned, welding cable is more
flexible and I like it.
Keep the wire runs sort.
4-Wire the system for emergency starting.
A normal battery switch allows you to:
Start the engine on the starting battery
Start the engine on the house battery
Start the engine on both batteries.
More complex is system that connects the house and starting battery in
series, increasing the voltage.
You can purchase the system used on diesel trucks, push a button,
start the engine normally.
You can use knife switches to do it manually.
NOTHING can be connected to starting battery except the starter.

That said, here is a real life system. This system is set up to run a live
aboard sailboat.
Starting batteries-2 group 24 batteries in parallel connected the starter
with common ground on the engine.
Engine wiring picks up voltage at the starter to start the engine and
run the engine instruments.
Factory alternator on the engine is wired to charge the starting
batteries.
House batteries-10 group 24 6 volt deep cycle batters making a 12v 1050AHr
battery pack.
All other loads on the boat are connected to this battery bank.
Charging sources for this bank.
130 Amp alternator on the main engine.
130 Amp alternator on a 1 cylinder diesel engine.
540 watts of solar panels.
300 watts of wind generator.
100 Amp battery charger in the inverter.
Normal battery switch that allows the engine to select either battery bank.
Battery combiner, automatically connects the battery banks when either bank
is above 13.2 volts, I.e. charging.
Each battery bank has a Pulsar on them to increase life. This runs off the
battery and is a small drain but worth it I believe.
Since the house bank is always in use all the charging sources are set up
for it. I have thought to put one of the solar panels on the starting
battery but decided it would be a waste. The solar/wind system keeps the
batteries charged up in normal usage on the mooring, when sailing and
autopilot, radar, etc. are in use then sometimes a engine needs to be used
to catch-up.

Second real life system. A small fishing boat that lives on a mooring.
Sometimes when the owner wanted to fish the battery (only one) was low from
sitting without a charger. The owner also had a bad habit of losing track of
time when fishing and running the single battery down. His fix was to carry
a spare battery and jumper cables. Once it was not enough to start the
system and he spent a cold night on the pacific. The boat was rewired to be
idiot proof.
Twin engine, both engines are wired separate. Each has its own starting
battery, alternator, instruments.
House bank is two 12v deep cycle batteries. All loads are connected to it.
The only connection between the two engines is that both have battery
combiners connected to the house bank. These are wired with switches to
manually connect the batteries, allowing for emergency starting.
There are three solar pulse maintainers, one for each battery bank.
A low voltage cutoff is installed on the house bank to save the batteries if
something is left on.
Since the rewire the owner has been very happy, no problems, no thought,
just go fish which is all he really wanted to do.

Mike