If the motor is powerful enough to turn it in the specified peak RPM range, the 19-inch prop will provide less hole shot but a greater top speed. The trade off is more power needed at the motor. If weight is not a concern, go with the 20-inch prop.
The hub gear on a tricycle engine is usually either 3 or 4 teeth. A 4-speed transmission uses two sets of pins inside the casing to connect each gear pair. A 3-speed transmission has only one set of pins required for both gear pairs.
A 4-speed transmission is easier to build because you do not have to worry about which direction to put the pins into the casing. With a 3-speed transmission, it is important to remember that if you are going from low gear to high gear, the directions of the pins and the layout of the casing should be opposite ones another. For example, if the low gear is on the left side of the transmission and the high gear is on the right side, when you go from low to high you need to insert the pin on the left into the left-hand opening of the casing.
A 5-speed transmission adds a second set of pins inside the casing for each gear pair.
The lower the pitch of the prop, the better your hole-shot. However, there is a cost: top speed. Because of the reduced pitch, the engine can reach its maximum rpm at slower speeds. A higher pitch, on the other hand, will result in faster peak speeds but slower acceleration. A pitch that is too high will also cause more wear and tear on your engine.
The choice between top speed and distance depends on what you value most. A low-pitch prop is useful for short bursts of speed as it allows you to stay under power longer. But it's not suitable for long distances because you won't be able to accelerate hard enough to reach high speeds.
A high-pitch prop is good for long distances because you can reach high speeds quickly, but it's not very useful for short bursts of speed because you need to lift off the throttle to avoid stalling the engine.
There are two types of pitch: polyhedral and elliptical. Polyhedral props have flat surfaces that are perpendicular to their axis of rotation. They're used on aircraft where weight is no object - such as race cars. Elliptical props have curved surfaces that are parallel to their axis of rotation. They're used on aircraft where weight is important - such as commercial airliners.
Both types of prop can be divided into smaller pieces called "stubs".
The 21-inch prop moves 21 inches in a rotation in this illustration, whereas the 13-pitch prop moves just 13 inches. As a result, the 21-pitch prop is doing more work with each spin.
A prop with too much pitch may have a higher peak speed since it goes forward with each spin, but acceleration may be weak and the boat may struggle to get on plan. A lower-pitched prop is easier to accelerate and has more thrust per square inch of surface area. It also floats better when not spinning.
The average speed of a boat is determined by two factors: power and drag. Power is how fast you can push the props down; drag is how much resistance you get from the water and the environment around you. The greater the difference between these two factors, the faster the boat will go. For example, if you double the power of your motor, then the same amount of drag still applies, so your boat will only go half as fast. Increasing power while keeping drag constant will result in a faster boat.
Drag increases due to three main factors: weight, shape, and size. Weight refers to the amount of material that makes up the boat; shape is how the material is used (e.g., flat surfaces or curved surfaces); and size is how big the whole thing is. For example, a heavier, flatter boat will be harder to turn than a lighter, more curved one of the same size. Drag also increases as height increases.
Moving the pitch up or down two inches generally changes the WOT engine speed by 400 RPM.
Aim towards the middle or upper limit of the suggested operating range. Remember that high altitudes restrict engine power, so use a lower pitch to attain the same RPM as at sea level. The performance of the propeller is also affected by engine height.
Stainless props must be examined using the same criteria as bronze props. In general, stainless steel props are constructed from two types of metals: High-carbon stainless steel is extremely robust and resistant to harm. It welds well and is typically repairable unless bent significantly.
Prop pitch is the potential distance forward that a prop would go if spun in a soft solid such as wood. (Photo credit: Mercury Marine)
The larger the diameter of the blade, the greater the potential force it can produce per unit of weight. A blade that is twice as long and wide can be used instead to generate an equivalent amount of power. This is because double the area means double the pushing force.
So for example, a 13-inch blade on a 50-hp motor would be a slightly oversize blade for its motor, while a 9-inch blade on the same motor would be an undersize blade. It all depends on what size blade will best suit your application. Smaller blades are better for fine tuning maneuvers, while larger blades offer greater strength.
There are two types of outboard motors available in commercial fishing applications: single cylinder and three-cylinder. Both have identical parts except for the number of cylinders. The only way to tell them apart is by looking at the oil reservoir on the side of the motor. Three-cylinder motors have separate reservoirs for each side of the engine. Single-cylinder motors have one large reservoir that serves both sides of the engine.