Article by Capt. Steve Katz
Propellers
Propellers or props are the most critical piece of underwater hardware needed to keep your boat running properly. There are many aspects that come into play when selecting and maintaining the proper propeller(s) for your boat.
A propeller is a complex design, often customized for your boat and operating style. Very minute changes in propeller geometry can have major changes in the boat’s performance. A propellers job is to transfer the engines power to the water and create the force that moves the boat both forward and reverse. Any damage to a propeller, no matter how minor, should be fixed immediately to maintain the designed performance and efficiency and to prevent damage to other mechanical components.
While a propeller is usually one solid piece of metal for most pleasure boats, there are many parts of the propeller we need to be familiar with when discussing a propeller.
Hub – This is the solid center of the propeller that has a hole for the shaft and that the propeller blades are attached to. The design varies depending on if the propeller is used on an outboard or inboard engine, but its function is the same. There are various attachment methods of connecting the propeller to the propeller shaft.
Keyway – On a traditional inboard propeller there is a keyway cut into the inside diameter of the hub that matches with a keyway on the shaft. The propeller and shaft are then locked together with the use of a key, long square metal pieces, inserted into the common keyway. Along with the tight taper, the key keeps the propeller from spinning the shaft. The size, shape, material of the key and fit are critical to the performance of the propeller system.
Blades - The propeller blades are the twisted fins that extend outward from the hub. Propellers can have any number of blades, though three or four are the most common in pleasure boating. These blades are where most of the work happens between the engine and the water. Blade design is the main factor of propeller performance.
Blade faces – The side of the blade facing the back of the boat is the blade face. This is the high-pressure side that pushes the water to move the boat forward. The opposite side is called the blade back. This side faces forward and is the low-pressure side of the propeller blade.
Blade Tip and Root - The root of the propeller blade is the area where the blade is joined to the hub. The blade tip is the outermost edge of the propeller blade.
Edges – The leading edge of the blade is the part of the blade that enters the water first and the edge that leaves the water last is the trailing edge.
Rotation – Rotation, left-hand, right-hand, clockwise or counterclockwise is the description of which way the propeller is designed to rotate. All references are envisioned while standing at the back of a boat and looking forward. A clockwise turning propeller that moves a boat forward, while looking at it from the back of the boat, is called a right-hand propeller. A propeller that rotates counterclockwise and moves the boat forward is called a left-hand propeller. There are a lot of propeller elements that can be modified at a propeller shop, but right-hand and left-hand propellers cannot be changed, even if they are turned around! Single engine boats most often use a right-hand propeller. In a twin-engine boat, most often the propellers turn outward with the right-hand propeller on the starboard side and left-hand propeller on the port side. This set-up provides the best propeller efficiency. A twin engine boat with both propellers turning the same direction will have handling problems and is not recommended.
All the above elements come together to create a propellers design.
Selection
The engines power and boats specifications, particularly RPM, size and weight of the boat are critical in selecting the proper propeller for your boat. Engine manufacturers have an operating RPM range that needs to be obtained and maintained for best performance and operation. An undersized or oversized propeller is bad for the engine and boats performance.
When shopping for a propeller, the initial criteria is diameter and pitch, along with a few other tweaking specifications.
The propellers diameter is the distance across the circle formed by the tips of the propeller blades. Diameter is the single most important factor in propeller selection. Generally, the larger the diameter the better the performance, but an oversized or undersized propeller can be detrimental to the operation of the engine and the overall boats performance.
The propellers pitch is the theoretical distance the propeller travels through the water in one revolution. This cannot be measured using a ruler while looking at the propeller. The pitch measurement is a theoretical distance since the propeller may “slip” in the water while turning and not cover the calculated pitch distance.
Other aspects of propeller design and selection include blade rake, blade cup, number of blades and other elements that can be adjusted by a propeller shop to obtain the best propeller for your use.
When it comes time to re-evaluate your current propeller or select a new one, many factors are involved. Fortunately, propeller manufacturers and propeller shops can walk you through the necessary steps to select or design the best propeller for your boat. While an out-of-the-box propeller may work for many applications, those Captains looking for a little more speed, fuel economy, and a smother ride will team up with a custom shop to tweak different parts of the propeller to obtain the best results.
Captain Steve Katz is the owner of Steve’s Marine Service and holds NMEA, AMEI and NMEA2000 certificates along with ABYC Master Technician certification and factory training from many manufacturers. To reach Steve, call 410-231-3191.
