Sorry for the delay I had not had a chance to draw this out until now..
Here's how to keep the wonderful safety features of the 1/2/BOTH/OFF most boats already have and you can simply add one more ON/OFF and have both a dedicated start battery and complete redundancy..
1 - If the starting battery goes bad simply turn the 1/2/BOTH/OFF to BOTH and flip the ON/OFF to OFF you can now start your engien and do house loads off the house bank yet still 100% isolate the starting battery.
2 - If the House bank goes bad simply flip to position #2 and leave the ON/OFF turned to ON. This will allow powering house and starting loads from the starting battery and allows it to also act as a reserve.
3 - For normal everyday use flip the 1/2/BOTH/OFF to #1 and the ON/OFF to ON..
While a tad more "complicated" than a single ON/OFF, you have two ON's and two OFF's to do it is still very easy compared to most boating things. In an emergency you still retain all the flexibility and isolation you'd want or need.
Here's how to keep the wonderful safety features of the 1/2/BOTH/OFF most boats already have and you can simply add one more ON/OFF and have both a dedicated start battery and complete redundancy..
1 - If the starting battery goes bad simply turn the 1/2/BOTH/OFF to BOTH and flip the ON/OFF to OFF you can now start your engien and do house loads off the house bank yet still 100% isolate the starting battery.
2 - If the House bank goes bad simply flip to position #2 and leave the ON/OFF turned to ON. This will allow powering house and starting loads from the starting battery and allows it to also act as a reserve.
3 - For normal everyday use flip the 1/2/BOTH/OFF to #1 and the ON/OFF to ON..
While a tad more "complicated" than a single ON/OFF, you have two ON's and two OFF's to do it is still very easy compared to most boating things. In an emergency you still retain all the flexibility and isolation you'd want or need.