Protective cover fitted over an outboard propeller for storage or transport

How to Measure an Outboard Propeller for a Cover

A propeller cover can help reduce accidental contact with blade edges during storage and transport. Choosing a suitable cover starts with the propeller’s diameter, its front-to-back depth and the clearance around the hub. A storage or transport cover must be removed before the engine is operated.

Before you measure

Stop the engine and follow the engine manufacturer’s procedure for preventing an accidental start before working near the propeller. Keep clear of sharp blade edges. If the propeller is difficult to reach safely, ask your marine dealer to confirm its dimensions.

1. Check the propeller’s size markings

Start with the markings on the propeller or the manufacturer’s specification for its exact part number. Diameter and pitch are different measurements. For example, 13 1/4 × 17 normally identifies a 13.25-inch diameter and a 17-inch pitch. The diameter is approximately 33.7 cm; the pitch is not the cover diameter.

2. Measure diameter using the blade radius

Propeller diameter is the diameter of the circle swept by the blade tips. To check it, measure the radial distance from the propeller shaft’s centerline to the outermost blade tip, perpendicular to the shaft, then double that distance:

Diameter = 2 × radius.

Measure in the plane of rotation, rather than along the curved blade or diagonally from a projecting hub nut. On a three-blade propeller, a straight line between two blade tips is a chord and is shorter than the full diameter. If you cannot establish the shaft centerline accurately, use the manufacturer’s specification or ask a dealer.

Why not wrap a tape around three blade tips?

A tape pulled tightly between separate blade tips follows straight chords across the gaps; it does not trace the circular path swept by the rotating propeller. Do not divide that wrapped length by π to select a cover. The circumference formula only applies to an actual circle, so this shortcut can result in an undersized cover.

3. Check cover depth and hub clearance

Measure the full front-to-back envelope that the cover needs to enclose, parallel to the propeller shaft. Include the blades’ sweep and any hub projection that sits inside the cover. Check the opening and closure against the nearby gearcase and skeg. Compare these measurements with the cover’s size chart and ask whether its dimensions are internal or external; do not assume a universal fit allowance.

Protective cover fitted over an outboard propeller for storage or transport
A protective propeller cover is for storage or transport and must be removed before engine operation.

Before ordering

  • Record diameter and cover depth in the same units as the size chart.
  • Note the propeller part number, blade count and hub shape.
  • Check that the cover can close around the whole propeller without being stretched over blade edges.
  • If the fit is unclear, send your measurements and a photo before ordering.

Explore our outboard propeller covers.

Diameter reference: Mercury Marine: propeller diameter and pitch. Always follow the operating and safety instructions for your engine and cover.

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