Ask The Experts | Back to Propeller Basics
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As with many things in life, the basic working principles of something get pushed aside in our minds. This is often due to worrying about work, getting the bills paid, and running the kids to school and practice. Once we make it to the water with our boat we are more than ready to relax our mind and enjoy the down time. One of the many ways our sacred time on the water can be spoiled is by a boat that doesn’t perform to its fullest potential. Often times this can be addressed by just being sure that the thing that transfers all the power of our engine(s) to the water; the propeller, is the right match for our boat and activities.
In order to know whether or not the propeller is the right fit, we start with the two most important questions;
1) What is the size of your existing propeller?
2) Based on your engine owner’s manual, what are the maximum RPMs the motor can achieve at Wide Open Throttle? Then, what RPM’s are you actually getting from your engine(s) when you run them under normal conditions for the on-water fun you want to be enjoying?
- The size consists of the diameter and pitch. This information can often be found stamped somewhere on the propeller. Common locations for this information are either on the “barrel” portion of the prop or on the fore or aft areas of the hub housing. (The hub housing is the area of the prop that holds the hub, which fits over the propeller shaft of your motor). If you cannot find the pitch and diameter stamping (i.e. 14.5 X 19, which would be 14.5 inches in diameter by 19 inches of pitch) look for any markings, such as the propeller part number, on the prop. Often times these markings can be helpful information for our propeller specialist at iboats.com to know.
By being able to supply this information we will be able to tell whether or not your boat has the correct propeller. You may say, “I never run my boat at full throttle, why is this important?” This information is vital to knowing if the motor is operating in a “safe” RPM range. A “safe” RPM range is one that meets the engine manufactures recommended RPM range. The answers of these questions will lead us to several common scenarios. A sampling of the scenarios is as follows;
- This information can be learned by having a little fun! Under a normal load of passengers, toys or fishing equipment, and fuel, push the throttle down as far as it will go and take note of the RPMs the motor is achieving (i.e. 5600 RPM).
A) “My engine manufacturer specifications say my motor should have an operating range of 4400 – 4600 RPM, but the boat can only achieve 4000 RPM at Wide Open Throttle (WOT).”
- To address this situation we would recommend that you reduce the pitch of the propeller to allow the motor the capability of reaching the specified 4400 – 4600 RPM range. For every 2 inches down in pitch you should expect a 200 to 300 RPM increase. In this case we would most likely suggest that the boater reduce pitch of the new propeller by 4 inches.
B) “My engine manufacturer specifications note that my motor should run 5500 – 5800 RPM and the motor is getting up to 6000 RPM @ WOT.”
- In this situation we would recommend that the boater purchase a propeller that is 2 inches larger in pitch. This would drop the engine RPM back down into the manufacturers specifications.
C) “Our boat is reaching the recommended RPM @ WOT, but the boat is really slow to get on plane and/or pull people behind the boat. Sometimes we even have to ask our passengers to move to the front of the boat until we get on plane.”
- This is a classic example of a boat that would be better suited for a 4-blade propeller. 4-blade props help boats get on plane quicker and stay on plane at a lower RPM. This is a great solution for anyone who enjoys a lot of water activities and or carries a lot of passengers. The 4-blade will make this type of boating much more enjoyable and the boat should also be more responsive around the docks and boat ramps. When moving from a 3-blade prop that reaches a good RPM range to a 4-blade propeller be sure to drop down 2 inches of pitch. This will help the engine continue to achieve its recommended RPM range. This will often result in a slight reduction in top end speed, but the boat will perform better in the range at which it spends most of its operating hours.
D) “We were not able to obtain information regarding the prop size and our engine RPMs at Wide Open Throttle.”
- The iboats.com prop specialist will be happy to give their best educated guess, however it is strictly an educated guess. There is not possible way to give any boater a solid suggestion regarding their prop without knowing what size prop the motor is currently operating with AND the RPM @ WOT. A good option in this situation is visit our propeller forum to see if any other boaters have a similar set-up to your boat, and what works best for them.
E) “The engine manufacture states that my motor should run between 5500 – 5800 RPM @ WOT. The boat is currently getting 5800 RPM @ WOT with our 3-blade 21 pitch stainless steel propeller. The top-end speed is good and the boat gets on plane well, but I want more top-end speed and better hole-shot. What prop should I get?”
- At this point you are most likely not going to get much more speed and hole-shot out of your boat by changing the propeller. Iboats.com has a great boats classified section that should have a boat with more horsepower and a more efficient hull design.
These are just a few of the most common scenarios that occur and countless more scenarios exist. Our propeller specialist at iboats.com can help you navigate the challenges of purchasing a new prop. The Propeller Help Guide will also help take you Back to Propeller Basics.
Founded in 1985, SOLAS is now one of the most well known and trusted names in outboard and stern drive propellers. All SOLAS propellers are developed through a state of the art process. Using theoretical calculations, geometry design, modern CAD technologies, prototype testing, pilot production and field tests, SOLAS props are designed to be superior to other propellers. Click here to shop Solas Propellers.