At the higher end of the RPM scale, the 3-blade propeller will almost always outperform the 4-blade propeller, so you can expect to lose about 5% of the top speed with a 4-blade propeller.
In contrast, a 4-blade propeller provides better low-speed acceleration, stability in rough waters, and a smoother, quieter ride, making it perfect for fishing and heavy boats. Understanding these differences helps you choose the best propeller for your needs, enhancing your boating experience.
Generally a good 4 blade will allow the boat to plane sooner, run on plane at slower speeds, and in twin engines I've noticed they ride better in a beam sea and other conditions most of the time. In the ocean you can usually stay on plane at a slower speed and also helps with ride.
If the prop pitch is too high (higher in inches), the motor may run below the WOT range—called "lugging the engine"—which can also put undue stress on the engine and gearcase components. Moving up or down two inches in pitch will usually change WOT engine speed by approximately 400 RPM.
In general 4-blade or 5-blade props give better hole shot and slow speed/rough water performance because of their increased surface area in contact with the water. 3-blade props typically give better top speed. But again these are for planing hulls at high speed.
Typically when you add blades, you get rid of some of the pitch. The reason for this is that the addition of the extra blade causes increased drag, which, in turn, requires more horsepower to achieve the same rpm. Since the horsepower is limited, the RPMs drop and the speed will tend to drop with it.
Most outboard engines use three or four-blade propellers. Three blades are the most common, as they provide great all-around performance together with superior top-end speed.
Is a 19 or 21-inch prop faster? In theory, a larger diameter at the same pitch and RPM will have the same speed, but greater thrust. So a 21-inch prop won't go faster than a 19-inch. However, the increased pitch will increase the top speeds.
Assuming you have properly sized the prop for mission and power, peak performance is 0.8-0.9 mach as shown in comments and other answers. Aim low if your prop has fat or blunt tips.
Incorrect Pitch: If the pitch is too high, the engine may struggle to reach its optimal RPM, leading to poor acceleration and potential engine strain. Conversely, a pitch that is too low can cause the engine to over-rev, reducing fuel efficiency and increasing the risk of engine damage.
As a rule, a stainless-steel prop of the same pitch and diameter will make your boat go faster — up to 2 to 3 knots, on some boats — than an aluminum prop. Stainless steel is stronger and can be made thinner, which allows more speed. Stainless steel also flexes less and keeps its shape at higher speeds.
Increasing the number of blades on a propeller is one solution to reducing cabin noise. In most installations, increasing the propeller blade count also reduces noise. This is largely due to a reduction in vibration. In a single-engine aircraft, the propeller blade wake beats on the windshield and produces cabin noise.
A propeller with three blades achieves the best balance of smooth operation and minimal drag through the water, which is why a three-blade design is the most popular for general boating.
Performance: If extracting every bit of performance potential from an outboard or sterndrive engine is a priority, go with a stainless steel prop. The thinner blades produce less drag in the water, typically resulting in higher top speeds.
Propeller Design Efficiency Factor Velocity of Flow
Large propeller designs can become less effective operating at the axial velocity. The most efficient designs are those which maintain a pitch to diameter ratio of 1:1.
On September 2, 2017, Steve Hinton Jr, in the modified North American P-51 Mustang Voodoo set the new record of 531.53 mph (855.41 km/h) in the C-1e class (the same weight class Rare Bear would fall into). This record is also the fastest for any propeller driven piston aircraft.
“A propeller overspeed occurs when engine RPM exceeds redline. This can overstress the propeller blades and hub resulting in failures that can break the propeller apart.
Propspeed relies on movement for its effectiveness—the more it moves, the better it performs as marine growth can't get a grip to grow. Propspeed can last up to a year and is relatively non-toxic according to the manufacturer.
The lower the prop pitch, the better your hole-shot. However, this comes at a price: top speed. The lower pitch makes the engine reach maximum rpm at slower speeds. Conversely, a higher pitch will deliver greater top speeds, but slower acceleration.
A lower pitch will give you a better hole shot. Most bass boats run a 25 or 26 pitch prop. Some that carry really heavy loads or extremely large boats might run a 24. Some faster boats run a 27.
When we use the term over-propped or oversized propeller, we are talking about the diameter or pitch being too large for the engine horsepower-to-load ratio. Often both are the case. Warning signs. There are a few warning signs that may indicate an oversized and/or overpitched propeller. Cruising speed at low rpm.
Lowering prop pitch will increase acceleration and thrust. Increasing prop pitch will make the boat go faster (provided the engine has enough power to keep the RPMs in the optimum operating range.