You generally tend to find the altitude info inside a service manual for your engine model…, Here would be the part number you’d need… Maybe try searching the part number online to see if there’s a copy online already. https://www.mercruiserparts.com/827242r02-sm-o-b-6-15-2sTrying to find the correct jets for 6000 to 8000 elevation.
Ran at about that elevation this weekend and hardly any power compared to 1500-2000 elevation that runs at about 18mph.
SN: OT618494
Usuall they drop a couple inches of pitch when they move to a 4 blade prop, check yours...if same pitch or higher than the 3 blade, then you hurt your power output.Also I have a 3 blade prop and a 4 blade. The 4 was in at this high altitude....would the 3 blade be better?