[honest mark]

so my port engine has a very slow rolling start, if i do not use the battery parallel i do not think it will even start, i am the 3rd owner of the boat and have all the service records, the other owners have had similar problems, when i bought boat previous owner had 1 year old batterys  (8) of them, had the slow rolling start so i replaced all batterys, didnt really help much, maybee a little bit, now fast forward 2 years and it's the same thing, my local mechanic does not think it's the batterys, i have checked the batterys, they are clean and tight, if it is not a loose wire is my only option to replace the starter? is there a quick check to see if it is my starter? i have read similar thread subjects about the same problem, i have the cummins mechanic coming out in a couple weeks $$$$$$$$ anyone have a idea what this could be? is there a simple check to see if the starter is working properly?? i have cummins qsm 11 660 hp if this helps, lookin for sum advice or what i should ask the mechanic when he gets to my boat? thanks in advance....