Propeller Basics

Before we begin to list the actual steps that one must take to determine the size of boat props, let us discuss the 2 primary players in the boat prop measuring world. They go by the name of diameter and pitch. These numbers go together like water and boats, marinas and money, and fish and water. The two elements that you always want to be aware of are the diameter of the prop, as well as the pitch of the boat prop. For example, if a boat propeller has a pitch of 24" and a diameter of 10, is would be known as a "10/24" propeller. The best place to find out what kind of boat propeller that you have is to look at the hub of the boat prop. This information is usually written conveniently on the hub of the boat propeller. Unluckily, because of the erosion that has taken place because of water, constant use, and other mitigating circumstances this number may no longer be visible. Also, if the person who owned the boat before you did had a propeller that was unique, this information can be next to impossible to find.

5 Easy Steps

  1. The first thing that you need to do is to measure the distance from the center of the hub to the farthest point that can be found on a specific boat prop blade. This is also called the boat prop radius. Remember that the diameter of the boat prop is double the radius. In other words, if you have a boat prop that is located 8 " from the hub's center to the outermost point from a single propeller blade, you know the diameter must be 16".
  2. 2. Assess the prop pitch by seeing how the boat operates at 50 percent of its speed level. It is a good practice to write down the actual speed and precise number of RPMs which can be determined by looking at the boat's tachometer.
  3. The next factor that we will look at is the prop pitch. The pitch of a boat propeller is calculated using inches. You can figure this number out by determining the distance in inches that one revolution of the boat propeller will cause your boating vehicle to move in the forward direction. These numbers must be rough calculations because of various factors such as, the shape of the hull, the amount of salt in the water, the temperature level of the body of water, and other circumstances that are beyond a boater's control.
  4. Next, a boater must find out what the gear ratio of the boat's power train is by consulting their owner's manual online or in paper format. When figuring out this information you just need to go with the first number of the ratio that is expressed in order to figure out the pitch of the boat propeller.
  5. Finally, you can figure out the pitch of your boat prop by simply dividing the amount of RPMs by the information that was derived from the boat's gear box. Do not forget to use the tachometer to assist you in determining the gear ratio. Once you have the numbers you need, you simply multiply the numbers you have obtained by 1.2 and then divide that figure by the speed of your boat.