Basic Propeller Info
Choosing the Right Propeller_By Neil Mullen
Recent advantages in outboard technology have made choosing the correct propeller for your boat both easier and more difficult... easier because the number of choices has doubled, and harder, for the same reason. Propeller manufacturers have been busy developing new 3-blade and 4-blade products and size ranges of stainless steel propellers to meet a growing number of hull types and horsepower ranges, especially for 4-stroke engines.
Four-stroke engines are designed to run at very specific RPMs, so pitch sizes have become available in 1 inch increments and new designs have appeared, each more tailored to a specific hull type and application. Propellers with higher rake angles and some with more surface area have been developed to maximize the power delivery of the 4-stroke torque curve.
The two most important things to remember in choosing a propeller are that it meets your individual needs for your individual application and that it allows the engine(s) to run within the specified RPM range at full throttle.
What’s important to you... Top Speed, Cruising Speed, Hole Shot, Load Carrying, Slow-Speed Handling, Slow Trolling, Fast Trolling? Two identical boats with identical engines could be propped quite differently, depending on the usage, water conditions, and load. There is no such thing as the best or ideal prop for all applications of a similar nature. Acceleration may be compromised for top speed and fuel economy, and visa versa. Often times, there may be 6 or 8 different props that seem to run about the same, with differences so subtle that any of them could be considered satisfactory by most standards. This just makes it that much harder to make a decision and choose the right prop.
The purpose of this article is not to explain propeller theory or hydrodynamics, but rather to point out the various options in the market place and set some common rules of thumb. This article confines itself to the discussion of 3 and 4 blade props as these are the most common in the real world.
Three Blades or Four?
In general, 3 blade props are the most common. They are available in wide size ranges and cost less than 4 blades. They typically yield a slightly higher top end speed than 4 blades. They are available in a wider variety of designs and offer more left hand rotation pitch options for twin counter-rotating engine applications.
3 or 4 Blades?
Four blades have some features of their own, though. They often provide more lift at the stern which will help accelerate the hull, especially if it is stern heavy. They come out of the hole strong and work well for pulling skiers and water toys. In fishing and offshore boats, they are oftentimes slightly faster than 3 blades at mid-range rpm’s, where coastal anglers most often run their engines. They also deliver slightly better fuel economy at mid-range rpm. Oftentimes, a poor-handling boat will improve by switching to a 4 blade propeller, and more often than not, a 4 blade will run smoother with better balance than the 3 blade equivalent.
Right hand or Left hand Rotation?
A 4 blade propeller will usually have a smaller diameter for the same pitch size of the 3 blade equivalent. This is one reason they spin up quickly and yield good acceleration. The blades are often a bit smaller but offer more total blade area because of the additional blade, so they have more grip on the water. When switching from a 3 blade prop to a 4 blade, you’ll usually need to decrease the pitch by 1 or 2 inches to keep the engine RPM in the same range.
Propellers are sized and described by their diameter and pitch. A propeller listed as a 15 ? x 17 x 3 would indicate a 17 inch pitch, 3 blade propeller having a diameter of 15 ? inches. Pitch is the theoretical distance that the boat will move forward with each revolution of the prop shaft, minus the slippage. The pitch ultimately is responsible for the top speed of the boat, much like the main jet in a carburetor is responsible for the ultimate power and speed of an engine.
The pitch must be matched to the engine’s recommended rpm range for full throttle. For most engines, this top range is about 500 to 1,000 rpm (typically 5,000 to 5,500 for 2-strokes, 5,000 to 6,000 for 4-strokes). A light boat and load will pull a high numerical pitch prop, whereas a heavy boat and load would have to run a smaller numerical pitch to load the engine less and allow the engine to reach recommended full throttle rpm. Keep in mind that most propeller manufacturers design their pitch in a progressive manner, to the point that the actual pitch will vary across the blade surface. Also, keep in mind that different propeller manufacturers each measure their pitch in slightly different ways with different tolerances. This means that two propellers of the same diameter and pitch from two different companies can yield different performance data.
For anglers slow-trolling for species like rockfish and flounder, a propeller with lower pitch (less distance per turn) that still allows the engine to rev to the top of its range will offer lower trolling speeds. It will also push loads easier and make maneuvering around a dock easier. On the other hand, a prop with more pitch that lets the engine turn to the lower end of its range may yield higher top speed.
Hooking-Up with the Water
There are other dynamics that come into play as the boat accelerates to its top speed. When it’s sitting still in the water and the skipper advances the throttle(s), the diameter and surface area of the prop develop the initial static thrust and launch the boat. As the hull gains momentum and speed, the dynamic thrust now is largely influenced by the prop’s ability to connect itself to the water and hook-up without cavitating or ventilating.
Cavitation is loss of hook-up due to the water literally boiling, caused by extreme low pressure near or at the blade surface or blade edge. Ventilation is a loss of hook-up due to the introduction of air or exhaust gases around the propeller. Basic blade design and diameter can affect these problems. If the diameter is too small for instance, it can cause cavitation. If the engine is mounted too high, it can cause ventilation. Both of these phenomena can be minimized by installing the correct prop. Going to a larger diameter or switching to a 4 blade can sometimes accomplish better hook-up. Larger diameter propellers usually yield better maneuverability as they push a larger volume of water on initial rotation, especially at slow speed. They also grab more water for better control when reversing. Matching the diameter and pitch for a given load and application gives the best performance for a specific boat.
Another design concept, called cupping, can also come into play here. Cupping means curling the trailing edge of the blade slightly to better grab water as it comes off of the blade face. This facilitates hook-up, but it can also load the engine more, much as adding pitch does. Different series of props have different amounts of cup in them. It is not uncommon today to have props with cupping added to the tip area of the blade to minimize tip losses and maximize efficiency.
Vented props are available with an exhaust relief hole at the base of each blade. These holes can range from ? inch to approximately 3/8 inches in diameter. They allow exhaust gases to escape around the propeller as it begins to spin up on acceleration. The engine gains rpm more quickly and reaches its ideal power curve sooner to improve overall acceleration. These props, however, do not work well for fishermen who do a lot of slow trolling, as the boat never gains enough speed to leave the ventilated water, causing the prop to catch-and-release, making the boat surge. Vented props also do not work well with cat hulls.
Matching style, blade design, pitch, and diameter is just as important for boat handling and safety as for maximizing cruising speed and fuel efficiencies. Large diameter props with lots of surface area help a boat climb a big wave and allow the operator to maintain good control in offshore conditions with rough water. Again, good-hook up is essential without overloading the engine and prop to the point of causing cavitation.
Original Equipment Considerations
- Determine the year, brand, model and horsepower of your motor.
- Calculate the diameter of the OEM propeller by measuring from the center of the shaft hole to the tip of a blade x 2 (Example: a measurement of 7 inches = a diameter of 14 inches).
- Is the type of material aluminum or stainless steel?
- Type of boat (cruiser, ski, runabout, bass, pontoon)?
- Prop on boat now (brand, model, diameter, pitch, cupping)?
- Engine RPM at wide open throttle (WOT)?
- Speed of boat over water at WOT?
- Normal operating altitude (motors lose 3% of power per 1000 ft.)?