Estimate tip-to-aperture clearance and tip clearance as a percent of diameter from the limiting aperture opening and propeller diameter. Optionally solve max diameter for a target clearance fraction. Geometry only — not an OEM prop selector.
This tool helps when you want a prop tip clearance estimate with the assumptions written down. Enter aperture / limiting opening (in), propeller diameter (in), and target tip clearance fraction.
Estimate tip-to-aperture clearance and tip clearance as a percent of diameter from the limiting aperture opening and propeller diameter.
Tip clearance (each side, equal) is half the leftover aperture after the prop diameter. Tip percent is tip clearance divided by diameter. Max diameter for a target fraction f is aperture ÷ (1 + 2f).
tip_clear_in = (aperture − D) / 2
tip_pct = tip_clear_in / D × 100
D_max = aperture / (1 + 2 × target_fraction)
Not OEM diameter sizing (Honda/Mercury/Vicprop charts). Confirm with a prop shop and your hull.
Example inputs from the form defaults — Aperture / limiting opening (in) = 24; Propeller diameter (in) = 18; Target tip clearance fraction = 0.15 — are chosen to be plausible coastal/cruising figures, not your boat’s survey.
Compute with: tip_clear_in = (aperture − D) / 2 tip_pct = tip_clear_in / D × 100 D_max = aperture / (1 + 2 × target_fraction).
When the calculator and your scratch math disagree, fix units first; the prop tip clearance model is only as good as consistent inputs.
Assumptions: inputs for this prop tip clearance estimate match the hints (soundings, nameplate Ah, marked tank capacity, and so on).
The tightest tip-to-structure dimension that limits diameter — often hull above, keel/skeg forward, or rudder aft. Use the smallest limiting opening.
No. Sources cite roughly 10–20% depending on hull and RPM. Enter your target; this is a geometry check, not class approval.
No. It only checks tip-clearance geometry from numbers you measure.