Your Prop: the Most Important Aspect of Boat Performance
In spring, a young man’s fancy turns to thoughts of … propellers? With apologies to Tennyson, more Lake George boaters think of their propellers than they do of love – at least when it comes to getting ready for the season. And what better time to order a new prop, or at least confirm that your present one is just right, than now, when your boat’s out of the water and you can get to her running gear without holding your breath?
Finding the right match between the propeller, engine type and boat size will optimize the following performance factors; improved low-end punch and load carrying capability, getting on plane faster, and increased top end speed.
For safety and efficient performance, it is critical that your engine operates within the RPM range recommended by the manufacturer. Matching the right prop for the load is the most significant factor when choosing a new propeller. If you want to modify your boat’s performance, consider the following information before making your selection.
Diameter is two times the distance from the center of the hub to the tip of the blade. It can also be looked at as the distance across the circle that the propeller would make while rotating. It is the first number listed when describing a propeller.
Pitch is defined as the theoretical forward movement of a propeller during one revolution, assuming there is no “slippage” between the propeller blade and the water. Slip is the difference between actual and theoretical travel of the propeller blades through the water. For most boats, there is slippage and therefore the distance advanced is less than the design pitch. A properly matched prop will actually move forward about 80 to 90 percent of the theoretical pitch. The amount of slippage varies from boat to boat. Pitch is the second number listed in the propeller description.
Rake is the degree that the blades slant forward or backwards in relation to the hub. Rake can affect the flow of water through the propeller and has implications with respect to boat performance. Aft rake helps to trim the bow of the boat upwards which often results in less wetted surface area and therefore higher top end speed. Aft rake props also typically “bite” better on ventilating-type applications. Forward (or negative) rake helps hold the bow of the boat down. This is more common in workboat applications.
A cupped propeller also works very well where the motor can be trimmed so that the propeller is near the surface of the water. Many of today’s propellers incorporate a cup at the trailing edge of the propeller blade. This curved lip on the propeller allows it to get a better bite on the water. This results in reduced ventilation, slipping and allows for a better “hole shot” in many cases. The cup will typically result in higher top end speed on one of these applications.
Ventilation is a situation where surface air or exhaust gasses are drawn into the propeller blades. When this situation occurs, boat speed is lost and engine RPM climbs rapidly. This can result from excessively tight cornering, a motor that is mounted very high on the transom or by over-trimming the engine (or drive unit).
Cavitation (which is often confused with ventilation) is a phenomenon of water vaporizing or “boiling” due to the extreme reduction of pressure on the back of the propeller blade. Many propellers partially cavitate during normal operation but excessive cavitation can result in physical damage to the propeller’s blade surface due to the collapse of microscopic bubbles on the blade.
There may be numerous causes of cavitation such as incorrect matching of propeller style to application, incorrect pitch, physical damage to the blades, etc. Be advised that disturbances in the water flow forward of the propeller (such as a thru-hull transducer) can result in blade damage which appears to be cavitation but is actually due to non-favorable water flow into the propeller.
Thru-hub exhaust and over-hub exhaust propellers are used on boats where exhaust passes out through the rear of the “torpedo” on the lower unit around the propeller shaft. Most outboards and I/Os utilize this type of exhaust.
Thru-hub exhaust propellers consist of a round barrel to which the blades are attached. The exhaust gasses pass through this barrel and out the back without making contact with the blades. This provides a good clean water flow for the blades usually resulting in good acceleration and hole shot.
Over-hub exhaust propellers have the blades attached directly to a smaller tube which fits over the propeller shaft eliminating the larger exhaust barrel. These props are often used for attaining maximum top speeds. On some boats the hole shot can often suffer due to the extreme exhaust flooding that occurs around the prop blades during acceleration.
Non-thru-hub exhaust propellers are used for inboards using shaft driven propellers, sterndrives using thru-hull exhaust and on some outboards that don’t route the exhaust through the lower unit torpedo.
Two propellers spinning the same direction on twin engine boats will also create steering torque. In other words, two right-hand propellers pull the stern hard to the right and the bow to the left. Two opposite-spinning propellers on twin engines eliminate this steering torque because the left-hand propeller balances out the right-hand propeller. This results in better straight-line tracking and helm control at high speed.
Most pleasure boats are factory equipped with aluminum propellers. Aluminum props are relatively inexpensive, easy to repair and under normal conditions can last for years. Stainless steel is more expensive but much stronger and durable than aluminum. If you are looking for better performance than can be provided by your aluminum prop, such as ultimate top speed or better acceleration, a stainless steel prop might be required.
I personally recommend 3-blade propellers for recreational boats with 2, 3, 4, and 6-cylinder outboards and I/O engines. These propellers provide good hole shot and top-speed performance. I recommend 4-blade propellers for bass boats and boats with high-performance hulls running high horsepower engines. Compared to 3-blades, they provide better hole shot performance with less steering torque and less vibration at high speeds.