How Altitude Affects Outboard / Sterndrive Prop Sizing

We get calls from customers who after propping their boat at a lower elevation have trouble reaching their RPM when they take the boat to a higher elevation. This is because as altitude increases the air gets thinner and oxygen levels decrease, causing loss of power and RPM.
The general rule for correcting this issue is to drop down 2" of pitch (usually one size) at +4000' and 4" of pitch at +6000'. Changing pitch by these amounts should get you to your proper RPM, or at least give you a close starting point.

Mountain lake image credit: James Wheeler