16" × 21" Propeller Specifications
Pitch ratio 1.31: theoretical speed at RPMs 1,000–4,000
Overview
A 16-inch diameter by 21-inch pitch propeller produces a pitch-to-diameter ratio of 1.31, 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.
Propeller Specs
| Diameter | 16" |
|---|---|
| Pitch | 21" |
| Pitch Ratio | 1.31 |
Theoretical Speed by RPM
| RPM | Speed (0% slip) | Speed (15% slip) |
|---|---|---|
| 1,000 | 11.5 | 9.8 |
| 1,500 | 17.3 | 14.7 |
| 2,000 | 23.0 | 19.6 |
| 2,500 | 28.8 | 24.5 |
| 3,000 | 34.6 | 29.4 |
| 3,500 | 40.3 | 34.3 |
| 4,000 | 46.1 | 39.2 |
Typical applications
The 16"×21" size is most commonly fitted to offshore center consoles, cruisers, and 150–250 hp powered planing hulls, 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 16" and dropping pitch by two inches yields a lower top speed but quicker hole-shot and better load-carrying behaviour, which is why 16"×19" propellers are popular for heavy or family-loaded boats. Raising pitch by two inches to 16"×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 15"×21" prop spins up easier on small engines, while 17"×21" needs a stiffer driveline but bites harder under load.
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.