Prop identification Tips. Most manufacturers mark the propeller with
prop diameter, pitch and shaft diameter. Some times the letter R or L for right or left hand will appear in the marking. A
prop marked 12r7x1 would be a 12 inch diameter right hand 7 pitch prop for a 1” shaft. The markings will vary a bit
between manufacturers the same prop could also be 127R1from a different manufacturer. Folding and feathering props may have
individual blades marked in a similar fashion. By measuring the prop diameter and shaft diameter it is usually easy to figure
out which is the pitch marking on the prop.
Prop diameter is easily measured. Just a tape measurement is all that is needed. Measure
a two blade propeller from tip to tip. A three blade propeller will need to be measured from the center of the propeller to
the tip of one blade and that dimension is doubled to get the diameter. Diameters are usually in inch increments.
Pitch is difficult
to measure without special equipment.
The USA (sae) shaft diameters are usually
¾”, 7/8”, 1”, 1 1/8, and 1 ¼. The shaft can be measured on the boat just behind the prop or close to
where the shaft exits the hull using a rule or caliper. Inside the boat the shaft can usually be measured at the stuffing
box or coupler.
If the prop is not mounted on a shaft the shaft diameter cannot be directly measured because the propeller bore is tapered
and changes sizes. However by measuring the smallest diameter in the bore of the propeller the shaft size can be determined
from the chart below.
Shaft dia smallest bore dia