How can I determine the pitch of my boat's propeller?
I want to identify the pitch of the three-bladed propeller on my boat before ordering a replacement. There are some faint numbers on the hub, but I'm not sure which describe the size or whether I can measure the pitch myself.
Answer
Check the propeller's hub and blade roots for stamped size markings: in a marking such as 15 × 12, the second number normally gives the pitch in inches. If the markings are missing or unreadable, identify the manufacturer's part number or ask a propeller specialist to measure the blades with a pitch gauge.
Check the markings first
Start with the propeller fitted to the boat. Its markings identify what is there; the boat's original specification may describe a different propeller. Photograph any numbers before cleaning, then remove loose fouling and dirt with a nylon brush and a cleaner suitable for the propeller material.
Before handling the propeller, isolate engine starting, remove the ignition key and disconnect shore power if it could energise the propulsion system.
Look in these places:
- Outside the hub: around the central barrel between the blades.
- Hub end faces: markings may be hidden behind the propeller nut, washer or thrust washer.
- Blade roots: particularly on bronze shaft-drive propellers.
- Inside the exhaust barrel: on some outboard and sterndrive propellers.
A marking of 15 × 12 RH usually means 15-inch diameter, 12-inch pitch and right-hand rotation. Pitch is the theoretical forward travel in one revolution with no slip. A 12-inch-pitch propeller does not necessarily move the boat 12 inches through the water each turn.
A longer number may be a casting or part number rather than a size. Record every character, including suffixes, then check it against the manufacturer's catalogue or ask a dealer to identify it.
Diameter, pitch and adjustment settings
Diameter is the size of the circle swept by the blade tips. Pitch describes the blades' helical geometry. Measuring across the propeller will not tell you its pitch.
On a three-bladed propeller, measure from the exact shaft centre to the furthest blade tip and double that distance to establish diameter. Measuring between two blade tips gives the wrong result because they are not directly opposite each other.
Markings vary between manufacturers and production years. Some outboard propellers show pitch separately from the part number; some shaft propellers carry diameter, pitch and rotation together. Check what an isolated number means before treating it as pitch.
With feathering or adjustable-pitch propellers, such as some Max-Prop and Bruntons models, the current setting matters as well as the model. Record the hub identification and consult the instructions for that exact version. An adjustment number or blade-angle setting is not necessarily pitch in inches.