Specifiche Elica 13" × 21"

Overview

A 13-inch diameter by 21-inch pitch propeller produces a pitch-to-diameter ratio of 1.62, which falls into the highly-geared / over-pitched bracket. The diameter sets the disc area that converts engine torque into thrust, while the pitch sets the theoretical distance the propeller advances per revolution. At 2,500 RPM through a 1.5:1 gearbox this combination posts a theoretical no-slip speed of 28.8 knots; at the 4,000 RPM upper trace the same prop reaches 46.1 knots, with real-world slip pulling those numbers down by 10–20 percent depending on hull loading and bottom condition.

Specifiche elica

Velocità teorica per giri

Typical applications

The 13"×21" size is most commonly fitted to family runabouts, walkarounds, and 75–150 hp center consoles, where the highly-geared / over-pitched pitch profile matches the high-performance / racing performance window. Boats inside this class generally cruise between 28.8 and 46.1 knots on the speed chart above. High-RPM, high-pitch combinations are characteristic of light, planing hulls and performance fishing rigs. If your boat tops out far below the 46.1-knot theoretical figure, the propeller is over-pitched for your loaded weight and slip will climb toward the 15-percent column; if you over-rev past the engine's WOT range, the propeller is under-pitched and you should step up one or two inches of pitch.

Compared with adjacent sizes

Holding diameter at 13" and dropping pitch by two inches yields a lower top speed but quicker hole-shot and better load-carrying behaviour, which is why 13"×19" propellers are popular for heavy or family-loaded boats. Raising pitch by two inches to 13"×23" trades acceleration for roughly 2.7 knots of additional theoretical speed at the same RPM. Holding pitch at 21" and changing diameter shifts thrust area: a 12"×21" prop spins up easier on small engines, while 14"×21" needs a stiffer driveline but bites harder under load.

• 13"×19"
• 14"×21"

Sizing notes and assumptions

The speed table assumes a 1.5:1 gear reduction and the standard propulsion identity (pitch × RPM) ÷ (gear ratio × 1,215.2) to convert inches-per-minute into knots. Slip estimates of 0 percent (theoretical) and 15 percent (realistic cruising) bracket most clean-bottom planing hulls; expect higher slip on displacement vessels, fouled bottoms, or when towing. Always confirm propeller choice against the engine manufacturer's recommended WOT RPM window — landing inside the band protects the powerhead from lugging or over-revving and is the single biggest factor in long-term engine life.