We do a fair amount of inboard propeller sizings here at The Prop Shop and we are always very insistent that the customer makes sure the numbers they are providing are the actual numbers and not a guess or an approximate value. We aren’t stressing the importance of these actual numbers to be difficult, but rather to make sure the customer gets what they need and are not stuck with a semi-custom, non-returnable item or one that has restock fees (almost all inboard propellers are considered semi-custom since they need to be bored/ keyed, and sometimes pitch changed).

The reason that it is so important is that small changes can have fairly large impacts on sizing. The transmission gear ratio and max rated RPM are the biggest factors. See below for an example of what happens when we change the gear ratio (Table 1) and the max rated RPM (Table 2) while keeping everything else constant.

The example application we will use is a sailboat that can accommodate a 15” diameter propeller with a theoretical hull speed of 7.32kts.

Table 1 – Changing the gear ratio

Table 2 – Changing the Max Rated RPM

A 15x10 might not seem that much different than a 15x12 or a 15x13, but it is the difference between a properly performing/ running engine and one that will not perform well and cause premature wear and tear which shortens the life of the engine and leads to premature engine failure/ replacement.