The one rule

At wide-open throttle with a normal load, the engine must turn within the WOT rpm range published in the service manual. Everything about prop selection serves that rule. Under-revving lugs the engine (hard on pistons and bearings); over-revving beats it up and wastes power.

The vocabulary

• Diameter: overall blade circle. Mostly fixed by the gearcase; changes are small.
• Pitch: theoretical forward travel per revolution. The main tuning knob — as a rule of thumb, one inch of pitch changes WOT rpm by roughly 150–200 rpm (verify on the water; hulls vary).
• Rake, cup, blade count: fine-tuning for bow lift, grip in turns, and holding water at high trim or in chop.

Field procedure

1. Record the current prop's markings (diameter x pitch, material).
2. Run a WOT test: normal load, trimmed properly, GPS speed and tach reading. A shop tach or diagnostic tool beats an old dash tach — verify the gauge before trusting it.
3. Compare rpm to the manual's WOT window.

• Below the window → drop pitch.
• Above → add pitch.
• Way off with correct pitch → check for a spun hub (paint a reference line across hub and blade; if it shifts under load, the hub slipped), wrong gearcase ratio assumptions, or a slipping/damaged prop.

1. Match the prop to the mission: watersports boats often want lower pitch/more blades for holeshot; offshore hulls may want stern lift; pontoons usually want grip over top speed.

Aluminum vs stainless

Stainless holds thinner, more efficient blades and survives better — but transmits strikes harder into the gearcase. Aluminum is the sacrificial choice for rocky, stump-filled water. Say this out loud to customers; it frames the price difference honestly.

Common mistakes

• Selling speed instead of correct rpm
• Ignoring a damaged prop's effect: bent blades and missing cup change performance and cause vibration that kills seals
• Forgetting the load question — a boat propped light with two people will lug with a full crew and gear

When to walk away

If WOT rpm can't reach the window even with sensible pitch changes, the problem is the engine (compression, fuel delivery, fouled bottom, wrong engine height), not the prop — stop selling props and diagnose.

Safety: all WOT testing with a lanyard on, in open water, and never with the customer's kids aboard as ballast.