Basic boat prop knowledge

                                                       

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How does a boat propeller work?

Every prop has two numbers to describe it. The two numbers are pitch and diameter. The other part of the prop is the hub, the hub is the area where the prop shaft is attached. There are three types of basic props we use on pleasure power boats and they are aluminum, stainless steel and brass. Each of these materials has special issues that are associated with its use in prop construction. Aluminum and stainless are the standard on outboard and inboard/outboard configurations where brass is used on inboard configuration. All three of these materials can be repaired with in limits.
                                           

First we will cover Pitch 
 The first number of two marked on the prop. Pitch describes the angle of the blades. A slang term for a prop is the screw and this is for good reason, like a screw  being turned into wood each turn moves the screw a measured depth in the wood. Pitch determines the amount of forward distance the prop travels for each revolution the prop makes. Pitch is measured in inches so a 15 pitch prop moves 15 inches through the water with every complete rotation. Pitch describes two things in layman's terms, imagine the pitch number describing the ability to gain traction, the lower the number the better the take off from a stand still (hole shot) your boat will be. Pitch also could be considered like a gear arraignment the lower the number the more robust the teeth on the gear would be, in other words there is less chance of skipping teeth when you suddenly increase the rotation(speed and torque) speed.

Pitch affects the top speed the boat can produce, the fuel burn your motor uses, and the pulling or pushing power that the boat can provide when the boat is moving from a standing start (hole shot).

Speed
 This is obvious if the prop will move 15 inches with every rotation and the identical motor and boat has a 19 pitch, the 19 pitch moves 4 inches through the water further than the 15 pitch prop. However this is not an unlimited number as the motor will only provide a maximum number of revolutions of the prop, the pitch becomes limited. You could not have a 40 pitch prop as you would never grab enough water to move at all as the prop would simply act like a blender and air the water and spin with no real effect. This is called cavitation, to much cavitation simply boils the water and burns the edges of the prop. So there are a select number of pitch combinations for each motor and boat combination. The magic is to find the prop which gives you the best hole shot (standing start) and the maximum top speed based on the top rpm that it is safe to run the motor.

The next number is diameter 
This is the distance from tip to tip of the prop. The diameter is also a restricted number, as it is restricted by two other items. The diameter of the prop is restricted by the size of the prop shaft. If you try to turn a large diameter prop on a small prop shaft that prop shaft will snap. The torque that the prop places on the shaft would stress the shaft causing the shaft to twist or snap it some where close to the back of the prop. The other item that affects the size of the prop is the location of the anti cavitation plate, this is the flat plate directly above the prop on your gear case. This plate is designed to be even with the water height that is created from the bottom of the boat when you are on plane. If this plate is too high or too low you will not achieve maximum performance of the motor or the boat. This plate also stops the water from simply flying out above the prop creating the maximum forward thrust that the prop develops. You do not want to have a prop which is so close to the plate or the hull that the water cavitates because it cannot pass through the gap between the prop and the anti cavitation plate. In the case of prop being close to the hull you could if you hit something cut a gash through the hull of the boat. These are the reasons why you are generally restricted to one or possibly two diameters you can use.
                                                               

The last part of the prop we described is the hub, this is the center of the prop were the blades start and the shaft penetrates the prop. There are different types of hubs on power boats, the hub is designed and dictated by the prop shaft and or gear case. in the case of an outboard or conventional inboard /out board the hub of the prop has exhaust gases that pass through this area. This is why when the water is clear you can see a trail of bubbles leaving the center of the prop. The hub of this type of prop is the same size as the gear case with open passages through the hub. There are two styles of these props, the break away hub and the fixed hub. You will only be able to determine the type you have by removing the prop off the shaft. When you remove the prop on the inside of the hub you may find a hard plastic block with a hole and splines for the shaft to pass through it. this block is designed to break away if you impact the prop to reduce the prop damage and the possibility of damaging the gears inside the gear case. The fixed hub does not have this plastic block and therefore if you impact the prop you will surely damage or break off the blades and possibly damage the gear case. A racing gear case on an outboard or inboard /outboard has a solid hub similar to an inboard configuration as there is no exhaust port requirement..

Prop materials
are aluminum, stainless steel and brass. On an inboard or inboard outboard configuration the props are made out of aluminum or stainless steel, why is this? The aluminum prop is a soft material which will easily change shape or break away under impact, this is critical to protect the inner workings of the gear case. The disadvantage to an aluminum prop is that the blades flex under load so the pitch is not a constant as you power up or are pushing the boat through heavy sea conditions. The pitch on an aluminum prop will change over time they are not reliable to always remain the same as they were made, affecting performance and fuel burn. Stainless is the next material, it is stronger and rigid so this material gives you reliable pitch through its life and while you power up. The blades do not move around. This great for performance however if you impact the prop you can damage the gear case workings. The stainless prop is more durable and less apt to be as immediately affected by corrosion. The last material is brass which is what the inboard configuration has, brass is less flexible than aluminum and not as durable as stainless. The perfect material for props that you cannot see. If you impact the blades they will flex and break away to help protect the rest of the running gear. One of the disadvantages to brass that the pitch can change with high hours of use, so it is a maintenance item to have your props tuned on a regular schedule. Doing this will keep the performance you should expect and the fuel burn correct. A prop tune is not expensive and will pay you back in fuel burn.
                                                         

Props come in several ways, there are the number of blades, the style of these blades, and the direction of rotation. There are also right hand and left hand rotation. On small outboard motors you will have two blades on more power full outboards and inboard/out boards you will see three and four blade props, the same as inboard configurations.You also see dual prop configurations on more current boats. The amount of blades can assist in achieving the best performance for a hull design, and weight. A four bladed prop can be more efficient in providing pull out of the hole and something called transom lift providing quicker hull planing capabilities. When we talk about left and right hand props this is the direction of the rotation of the drive shafts. When you have a twin configuration you are looking for the starboard drive or shaft to be turning right hand and the port providing you with left hand rotation there is a reason for this set up. In a single motor single prop the boat has a natural tendency to pull to the right going forward and to the left in reverse, by having a counter or left hand rotation on the left side it counters this natural reaction. Which creates a more manageable boat to operate. Twin with right and left hand rotation also provides transom lift, because the props turn in against each other there is a channel of water which is pushed straight down lifting the hull out of the water. This is taken advantage of the most with performance boats where the distance between the props is very small. If you have a twin that has two right hand drives there is a high chance that the drive on the left has been changed, the boat will never perform to it maximum at all. All left hand props are marked with an L, right hand props are not marked.
                                             

Choosing a Prop for your boat requires some calculations which Mercury or Volvo can provide you with based on your specific configuration. If you still feel that you can get a little more out of a prop then a prop repair person can put an extra half pitch in the prop, and yes this does work, you can get a real gain in a half pitch custom set up. Knowing if you have the right prop is an entirely separate topic to cover. When you read the numbers on your prop a 17x19 is a 17 pitch 19 inch diameter, a 17x19L is a 17 pitch 19 inch diameter left hand or counter rotation prop. These numbers are either on the side of the hub or on the end of the hub. If you are not sure a prop repair person can tell you what you have.

There is a mass amount of science to prop construction, materials, and selections. This article was simply to cover basic information, to create a basic understanding. A duo prop drive has one prop turning right and one turning left, so a single duo prop will not have the natural pull to the left or right. Duo prop provides a more stabile steering and control platform. IPS and the new forward drive are all duo prop configurations. If you need more information on props Mercury is running information this year on this topic.

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