Originally Posted by scottbr
I have a 3YM30 in our Hunter. Installed a group 27 start battery, 600 CCA, 750 MCA, on an echo charger and 2 Group 27 as house bank on a 1,2, both switch. Typically I leave it on the house bank and start the engine from that bank. I only use the engine start battery if I draw down the house bank too far. It's happened a couple of times, but have the reserve engine battery as back-up.
The 3YM only needs about 400 CCA for a start battery. For the YM series Yanmar moved away from CCA or MCA ratings for batteries I suspect because many boaters use deep cycle batts that are not rated in CCA or MCA in some cases. Yanmar now wants to see a min Ah rating of 64 Ah's for the 3YM30. Your starter, at "max loaded current", is only rated at 250A.
That being said you can just use the house bank to start and your new batt of at least 400CCA for reserve. Might as well go for something in the 600+CCA class as even small group 24's will do this..
Are you wiring a dedicated engine start circuit or using a 1/2/BOTH/OFF switch?