Propeller Characteristics - Q&A
Common Boat Prop Questions & Answers
Why do outboards with the same horsepower often need different prop sizes?

The size of the boat propeller needed is largely influenced by the lower unit gear ratio. The stock gearing on outboards causes the shaft to turn slower than the RPM at the powerhead. Gear ratios, such as 12:21, are a measurement of the difference between the number of teeth in the crankshaft gears and the prop shaft gear. So, although the horsepower of two outboards may be identical, lower gear ratios use larger propellers and vice versa.

How can you tell if your engine is running within the manufacturer's recommended RPM range?

To check if your boat motor is performing within the RPM range recommended by the engine manufacturer you will need a tachometer. Many designs and styles of tachometers are available to help measure performance.

Will a different propeller fix torque issues like listing and hard steering?

The propeller is not typically the problem with steering issues. The usual causes are hull irregularities, steering system hook-up, or outboard engine mounting.

Check the boat hull for any distortion that could cause a problem.

Make sure that the steering system has enough adequate sized pulleys, is properly swiveled, and has the right steering cable tension. Engines that use right-hand rotation propellers should have the steering wheel on the starboard (right) side of the boat. The driver's weight offsets the tendency of this side to lift from torque action.

Outboard engines must be mounted in the exact center of the transom and setting level. When a boat is underway the engine tilt should make the prop horizontal, otherwise it can have a pull to the side.

Why does my outboard vibrate excessively when the boat prop hardly looks used?

This is fairly common. Propellers that aren't showing any signs of impact may still have been damaged. Bent or distorted blades will cause the motor to vibrate even if the damage is indiscernible.

Will a different sized prop help me troll better?

Standard sizes, with a standard pitch, typically troll too fast. Throttling down to very low speeds causes the prop to overload, thus allowing the engine to idle faster. Lowering the pitch is always best for trolling. Use the lowest pitch possible for the best trolling results.

Will a different sized prop help me in water skiing?

Original outboard propellers are typically selected knowing the exact boat that it will end up on. So they are pitched a little high to prevent it from exceeding the top RPMs on a light boat. A prop change is usually a big benefit for water sports. The original equipment option tends to give poor speed, acceleration and performance when used on heavier boats or when towing a skier. This poor performance can make it more difficult for skiers to get up. Lowering the pitch or changing to a 4 blade propeller can improve hole shot.

My engine came without a propeller, how do I know if I need a right or left-hand prop?

Stand behind your boat and look at the prop shaft. If it rotates clockwise then you need a right-hand or standard rotation. If it rotates counter-clockwise then you need a left-hand or counter rotation.

Should I change the propeller size on my sterndrive engine?

Boat manufacturers typically run extensive tests on I/O boats to ensure that the propeller size performs well. However, if the boat displacement changes, due to the addition of heavy aftermarket equipment, you should consider a different boat prop. The diameter would remain similar to the original, but with a lower pitch.

Should I change the propeller size if I boat on high elevation lakes?

Gas engines produce less power when they are run at elevations above 3000 ft. Lowering the boat prop pitch will help keep the RPMs within the desired range. However, due to the lower air density, the engine will not have as much horsepower and the boat will have a slower top speed. Boaters who frequent high and low elevation lakes often use two different sized props to accommodate for their boating elevation.

What is electrolysis and how does it affect my propeller?

Electrolysis is caused by two distinct metals being near each other. A stray electrical current, from the boat or another source, causes pitting on the boat prop blades. Sacrificial anodes can help protect the prop, but the best solution is to find the source of the electrical current.

Galvanic corrosion is very similar to electrolysis. However, galvanic corrosion occurs in salt water. The less noble of the two adjacent metals begins to corrode. Reduce the corrosion and pitting on your propeller by using adequate sacrificial anodes on your boat.

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