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By Jason Frey
- Choosing the right
propeller is as important as choosing the right motor for your boat.
- Despite how much power your engine produces, if you don't transfer that power into thrust through your prop efficiently, you're robbing yourself of performance.
When selecting a prop, understanding the factors that go into design is enough to save your engine's thrust from being wasted. Propeller geometry directly affects how a boat performs. It can impact fuel efficiency, wear on the drive train, top and bottom end speed and even ride quality.
To reach the highest level of performance, there are a few important prop elements that you need to consider: pitch, diameter, rake, cupping, vent holes and material. Picking the perfect prop for a particular boat and motor is actually fairly easy once you learn a little about them.
PERFECT PITCH
On any propeller, you often see two descriptive figures, for example 14-1/4 x 21. The first number is the diameter and the second is the pitch. Pitch is the theoretical forward movement of a propeller through a solid medium for one revolution. This is, of course, assuming there is no slip.
For instance, a 21 pitch prop will theoretically advance 21 inches per revolution.
When selecting a pitch, it is important that the prop run at the upper range of your engine's top end rpm. If you want your rpm to increase, go down in pitch. If you want it to decrease, go up in pitch. On average, for every 2" of pitch, the rpm will change by 400.
For water sports and extra people on board, you should drop the pitch by 2" to compensate for the added weight and drag on your boat. Remember, pitch from different manufacturers will run at slightly different rpm due to variations in design.
DIAMETER FOR DUMMIES
Propeller diameter is the distance across the imaginary circle a spinning propeller makes. It can be easily determined by measuring the distance from the center of the hub to the tip of one blade and multiplying that number by two.
As diameter increases, propeller slip decreases. Slip is a necessary factor in creating efficient thrust. Too much diameter can result in performance loss due to the fact that your engine no longer has enough power to efficiently turn the propeller at the recommended rpm operating range.
Traditionally, larger boats with more powerful engines require a longer diameter. These larger applications necessitate more blade surface area to push additional water. Likewise, smaller craft need less.
RAKE, CUPPING & VENT HOLES
Many propeller features play a role in overall performance. Rake is the degree that the propeller blade is angled to the hub. Typically, the more higher the rake, the faster the speed and greater the lift. Highperformance props usually have a 20 to 30-degree rake angle.
A curved lip at the trailing edge and/or tip of the blade produces cupping. Cupping helps the propeller acquire a stronger grip in the water for superior holding and higher trim on turns. It also increases efficiency and can result in greater top end speeds when properly designed.
Vent holes are openings behind each blade designed to aid in the hole shot of the propeller. When accelerating from neutral, these holes allow exhaust bubbles to flow through and flood the blades. Now spinning in more turbulent water, the prop gains speed more quickly. At 1,000 rpm, the rapid moving exhaust will bypass the holes and flow through the hub.
ALUMINUM VS. STAINLESS STEEL
Understanding the differences in aluminum and stainless steel construction will help you decide on the right propeller. The main advantage of stainless steel is durability. Stainless props can withstand the damage caused by small rocks, sand and other loose objects in the water.
Unlike aluminum though, steel blades have minimal give. If you hit an object hard enough, there is a possibility of damaging the lower unit. With aluminum the blades will almost always sacrifice themselves before causing any damage.
Editors Note: Jason Frey is the development & engineering coordinator for Turning Point Propellers. For more information, contact Turning Point Propellers at 847-437- 6800 or visit www.turningpointpropellers.com.