Boater's Top FAQs About Propellers
In the FAQs below you will find useful answers to questions about propellers, propeller repairs and several other aspects of boat propellers that our members get on a daily basis. There’s info on propeller pitch, propeller thrust, cavitation, propeller hubs, shafts, rotation, slip and more. If some of the terms throw you for a loop, check out our Prop Talk section for explanations and definitions or call one of our member shops near you.
Good question. That all depends on the level of damage. Some propellers are so badly damaged that they are no longer repairable, other times it definitely makes sense to visit your friendly propeller pro to repair it. For example… if half the blade is missing entirely, this can’t be repaired because it’s just too much material to replace to bring the propeller back to a good and functional state. Alternatively, a bent or otherwise damaged propeller can be returned to a good working state.
It is recommended that you contact a NMPA member repair facility or a NMPA Certified professional in your area to get the best information on your particular needs.
Yes, if the repair is done correctly!
A NMPA certified repair facility has the knowledge, equipment, ability and years of experience to return your damaged propeller back to factory condition and original specifications. All the technical measurements are taken into consideration when repaired by NMPA professionals.
Locate an NMPA member in your area to discuss your needs.
Correct Prop sizing is attained by a mathematical formula consisting of rated engine RPM at wide open throttle, gear ratio of the lower unit and pitch of the propeller with a GPS speed of the vessel.
Theoretical speed of the boat = (Engine RPM/Gear Ratio) x Pitch / 1056.
If the Theoretical speed of the boat is 64 mph, and the actual GPS speed of 42 mph, then divide the Actual Speed by the Theoretical Speed to determine efficiency. In this scenario 42/64 = .66 or 66% efficiency. This would mean you’re experiencing 34 % slip. 34% slip is way too high or inefficient. Efficiency should be somewhere around 88 to 85 %, or your slip should be between 12 to 15 %. Depending on the boat and your usage this may vary.
Boat manufacturers make recommendations on the right prop for their boats, and propeller manufacturers offer guidance on which propellers work well on which boats and certain scenarios. Check with your NMPA pro for the best advice.
Taking care of your propeller is rather easy, first check it after every trip for any bends in the blade, rough edges, did you feel any vibration during use? If any of those things happen, then it would be a good idea to take it to an authorized NMPA certified shop to have them check it.
After every second or third trip it is recommended that you take the prop and thrust washer off your motor to check for any fishing line that might have wrapped around the base of your prop shaft behind the thrust washer, if not check the monofilament line can cut into your rear lower unit seals. Once a boat’s hauled out of the water, it’s a lot easier to inspect the propellers, prop shafts, seals and zincs. It’s also a good time to get any needed repairs or maintenance done, rather than waiting until the start of next season when everyone else is clamoring to get their boats fixed.
Pitch is the theoretical advancement in one revolution of the propeller. The diameter and pitch is most always stamped on the propeller. Usually on the aft end where the nut attaches. Some propellers have the size stamped between two of the three blades.
It’s the second number that is listed on your propeller, diameter being the first. It represents the theoretical distance the prop will travel in one complete revolution of the prop. If the size is stated as 13.25×17, then 17 would be the pitch of that prop.
The diameter, or first number stamped on the propeller is the distance from the center of the hub to the tip of the blade, then doubled. If the size is stated as 13.25×17, then the 13.25 would be the diameter.
The cup in a prop is the curl at the trailing edge of of each blade. A cupped prop will HOLD the water on the blades better, help the prop to not cavitate, and corner better then a non cupped prop.Cupped props are much more efficient than a non cupped prop. Most props for 30hp motors and above come with cupped props today.
A boat’s fuel consumption or efficiency will be optimized with a propeller that attains wide-open throttle (W.O.T.) RPM that matches the engine manufacturer’s recommended value. If the pitch is too low, the RPM will be too high and potentially damage the engine. If the pitch is too high then the RPM will be lower than optimum and the boat will not perform at it’s most efficient RPM and power range. An NMPA propeller shop can help you to determine the optimum propeller for your application using your W.O.T. performance results.
A boat’s speed may be limited by available horsepower or by a propeller with to low of a pitch. To achieve the best all-around performance it is recommended to use a propeller that attains wide-open throttle (W.O.T.) RPM that matches the engine manufacturer’s value. If the pitch is too low, the RPM will be too high and potentially damage the engine. If the pitch is too high then the RPM will be lower than optimum and the boat will not perform at it’s most efficient RPM and power range. An NMPA propeller shop can help you evaluate your performance data to determine the optimum propeller for your application.
Bronze material has long been the standard for inboard propeller manufacturing because of it’s resistance to corrosion and good machinability. Nibral bronze is a metal alloy made of nickel, bronze and aluminum. Nibral is a stronger material that allows the propeller designs to handle more power as well as allowing thinner cross-sections for more efficient movement through the water. AN NMPA member propeller shop can help you decide which propeller material best suits your needs.
Aluminum propellers are the standard material for outboard and stern-drive applications. Many boats come from the factory equipped with aluminum propellers. Aluminum propellers do have some advantages, as they are relatively inexpensive and suitable for general purpose use. For performance, stainless steel propellers may be a better choice. Stainless steel propellers are more expensive but are far stronger and more durable than aluminum. There is also minimal prop flex even at top speeds with stainless. A NMPA member propeller shop can help you decide which propeller material best suits your needs.