When I use to purchase the 26.5 boats for resale, it came from Hunter supplied with one battery, usally a groupo 24 battery. Depending on usage which of course is the biggest determination, a second battery was added. Most people refer to amp usage. Simply put, you can run interior lights maybe 2-3 nights before recharging the single battery but with two, then you double the nights again depending on how many lights are draining the power source. An inverter will even draw the amperage down faster. . With the group 24 battery which is the smallest battery offered, the next size is the 27 which has more amp. or more stored power. Of course I will speak in layman's terms as many do not know what we are talking about amp hours, battery groups and so forth.
With an electric start engine starting at 8 hp, generally you will draw 18-22 amps , when you are starting the engine with electric start. When the engine is running, then it is recharging the battery or batteries anywhere between 3-6 amps. In the case of a dual battery system, generally there is a battery selector.
When I installed a battery selector, I made sure it was the kind with a quick field disconnect as some do not have that. Why? When the engine is running with electric start, I told my customers never to switch the battery selector as you could possibly destroy the internal alternator within the engine by switching that selector. I have seen it happen but I am not a mechanic; therefore, do not ask me any further on that other than what I said.
For most using outboard engines, I never will suggest anything above 10 hp on the backend of the trailerable sailboats. The transom is not built for the weight. In addition, many of the outboard motor brackets will specify that as well.
I hope this explains why.
Crazy Dave