Tip
The maximum reach of a propeller blade from its root at the hub. This is also the point that seperates the leading and trailing edges on a prop.
Back Blade
The negative (suction) side of the blade facing the boat.
Blade Face
The positive (thrust) face of the blade facing away from the boat.
Leading Edge
The blade edge nearest the boat extending from the Hub(7) to the Tip(1). This is the first part of the prop to cut the water and the most likely part of the prop to suffer damage.
Trailing Edge
The blade edge furthest from the boat extending from the Hub(7) to the Tip(1).
Prop Adaptor
Lock's the prop onto the drive shaft and adapts to fit a variety of motors. Mark's Marine caries adaptors for every motor we've encountered and a number we haven't.
Outer Hub
The Outer Hub is the surface the blades attach to. It is connected to the inner gub by a series of ribs.
Inner Hub
Interfaces with the Prop Adaptor(6) to transfer thrust from the drive shaft to the prop. On OEM prop's there is no adaptor and the inner hub locks directly onto the drive shaft.
When working with the largest practical diameter prop, pitch will be your most common tuning tool.
Step1: Test
Run your motor at full throttle with a light load and your normal operating trim. Note the highest RPM you acheive and compare that to the engine manufacture's maximum operating range. Make sure that your engine has been properly broken in before performing this test.
Step2: Evaluate
The best performance is generally acheived when you are within a few hundred RPM of the maximum operating range. Modern motors like RPM's. If your prop is significantly lower or higher than your Maximum RPM's at full throttle then you should probably replace it. Too much operation at low RPM's put's a strain on the pistons, bearings, and crankshaft, while excess RPM's risk catastrophic failure.
Step3: Experement
Decreasing the pitch of the prop will generally bring up the RPM's. Increasing the pitch will generally bring down the RPM's. As a rule, for each 1" change in pitch, you should see a diffrence of 150-200 RPM's at full throttle.
Identifying your prop
A boat's prop is typically desginated with 5 numbers. For example, 15-21-3-92-RH.
- Diameter
- Pitch
- Number of Blades
- Blade area ratio
- Direction of Rotation
Prop Repair
Mark's Marine ships our props out to be repaired by Precision Propellor in Spokane. Props are picked up and dropped off each Thursday and repairs typically take a week. Repair costs usually range from $65-$100.
Aluminum
This is the most common and economical prop material. It is relatively strong and easy to repair, however it is also easily dinged or bent. Because of its flex, it provides a little less performance than stainless props at acceleration and deceleration.
Stainless Steel
Stainless Steel is 5x stronger than aluminum, making it better for resisting dings. Because of its strength it can be cast with a thinner profile than an aluminum prop while still being repairible. The downside to a stainless prop is the cost, which is 2-3x that of an aluminum prop and a slight risk when striking submerged obsticals such as tree stumps that the drive shaft will break before the prop.