Reconcile one propeller test point with explicit mph units, calculated prop-shaft RPM and neutral apparent-slip output.
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Reconcile one aligned propeller test point with an explicit mph formula, corrected prop-shaft RPM and neutral apparent-slip output.
Align verified pitch, engine RPM, reduction ratio and measured mph.
Record all four values at the same stabilized condition and inspect prop-shaft/theoretical speed.
Can reproduce apparent slip without a knots/mph mismatch.
Keep tachometer, gear, pitch/cup, speed method, load and conditions visible.
Treat negative or unusual output as a record/unit/condition diagnostic and repeat controlled tests.
Does not label apparent slip efficiency or a universal failure.
Avoid a purchase/selection recommendation from one percentage.
Compare controlled manufacturer/dealer evidence for the exact boat/engine/prop combination.
Consults qualified guidance before changing hardware.
Outputs and checklists are planning aids. Review the linked current authorities and the records, terms, instructions, and requirements that apply to your exact situation before a consequential decision.
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Entered engine RPM divided by the engine-to-prop reduction ratio gives prop-shaft RPM; prop RPM × pitch inches ÷ 1,056 gives zero-slip theoretical mph, not knots.
Pitch, RPM, ratio and measured mph must describe the same stable run. Load, trim, current, wind, sea state, tachometer and speed method remain documented outside arithmetic.
Signed apparent slip is reported without efficiency grades. Negative output remains visible because it commonly exposes unit, pitch, ratio, RPM, speed or condition mismatch.
Updated: August 2026
Align verified pitch, engine RPM, manufacturer reduction ratio and measured mph from one stabilized run.
Preserve an apparent negative result and investigate units, pitch/cup, ratio, RPM, speed method, current and test-condition mismatch.
Compare repeated aligned records for the exact boat, engine, gear and propeller configuration with manufacturer or dealer guidance.
Keep measured speed explicitly in statute mph with the 1,056 constant. Convert external readings before entry and document the source unit.
Mercury notes that slip is necessary and not efficiency. Verify the exact setup and conditions, repeat controlled tests and consult qualified manufacturer/dealer guidance before changes.
Reconcile one aligned propeller test record with explicit mph units and calculated prop-shaft RPM. Treat apparent slip as a diagnostic—not efficiency or a hardware recommendation.