How They Work

Propellers have two basic dimensions, the diameter and the pitch. These sizes are normally expressed in inches as 13 x 17 or 13-1/2 x 17P. These figures are found on the outside of the propeller hub itself or on the centre bush (where the prop nut is).

In the case of propellers the smaller the diameter and pitch the faster you accelerate however you will have a lower top speed. Conversely, the bigger the diameter and pitch the higher your top speed will be, but your will have less accelerate. Think of this in terms of your car, if you drive around in first gear you are quick off the mark but you lose out on overall speed. Where, if you drive around in third gear you can go a lot faster but it takes longer to get to that speed.

As a general rule for every 2 inch change in diameter or pitch you will change the RPM range by 400. So, if you currently have a 14 x 17 propeller and your max WOT 5500 RPM and you use a 14 x 19 your max RPM will now be 5100. If you used a 14 x 15 your max WOT RPM will now be 5900.

TERMINOLOGY

The Diameter

Is the measurement across the circle formed from the outer edge of the propeller blades, which is expressed in inches.

The Pitch

Is the measurement used to describe the theoretical linier distance the propeller moves after one complete revolution, also expressed in inches. So, if you have a 17 inch pitch on your propeller it will (theoretically – note the definition of slip below) move forward 17 inches for each complete revolution.

Wide Open Throttle (WOT)

There are two key figures here; your minimum and maximum WOT. These figures are important as they determine what your min and max Revolutions Per Minute (RPM) should be when you have the throttle at full power. Your engine manufacture will specify what the RPM range is for each model, don’t be concerned if you are not familiar with these figures as we can let you know what they are or you can check your owners manual.

RPM Range

The diameter and pitch of the Propeller will determine how quickly you accelerate, top speed, fuel economy and the minimum and maximum RPM your engine will reach. Unlike your car, a marine engine has only a forward and reverse gear meaning the size propeller you choose is critical in determining your vessels performance on the water.

Cavitation

Cavitation is the result of a decrease in pressure on the forward side of the propeller that causes the water to vaporize or boil. This results in a loss of traction or thrust causing the engine to over rev.

Ventilation

Ventilation is air from the water surface or exhaust gases being drawn into the propeller blades which causes the propeller to over rev and lose thrust.

Slip

3 or 4 Blades

The majority of propellers are available in 3 or 4 blade configurations, both have advantages unique to each type.

A 3 blade propeller is normally fitted to any new marine engine as standard. They provide reasonable performance levels at reasonable prices.

A 4 blade propeller is used to obtain faster acceleration and allows for lower planning speeds. Your top speed will be reduced a little however your also make significant gains in handling, particularly the ability to turn without cavitation (grip in the water).

Stainless Steel verses Alloy verses Composite

Stainless Steel:       Best performance

                               Strongest propeller available

                               Look great

                               Expensive to purchase and repair

Alloy:                     Good performance

                               Good strength

                              15% tensile strength increase compared to an Alloy prop

                              Easy to repair and very inexpensive

                              Flexibility of pitch sizes to suit any application

                              Life time warranty on the Hub

                              Offers superior protection to your lower unit and prop shaft

DETERIMING YOUR NEEDS

The array of vessels and how we use them has changed considerably in recent years, it is now common to see a centre console tinny tearing around the dam with a tube in tow as it is to see the fibreglass bowrider out cruising the rivers or bay packed with fishing rods.

The flexibility offered by a Piranha propeller means that you can choose a pitch that will suit all of your boating needs, you can even carry a spare set of blades that are a different pitch giving you the opportunity to change them quickly and easily and maintain performance regardless of the application.

In the case of propellers the smaller the diameter and pitch the faster you accelerate however you will have a lower top speed. Conversely, the bigger the diameter and pitch the higher your top speed will be, but your will have less accelerate. Think of this in terms of your car, if you drive around in first gear you are quick off the mark but you lose out on overall speed. Where, if you drive around in third gear you can go a lot faster but it takes longer to get to that speed.

As a general rule for every 2 inch change in diameter or pitch you will change the RPM range by 400. So, if you currently have a 14 x 17 propeller and your max WOT 5500 RPM and you use a 14 x 19 your max RPM will now be 5100. If you used a 14 x 15 your max WOT RPM will now be 5900.

Step 1: Have a look at the size propeller you are currently using. There will be a set of number stamped or embossed on your propeller. They will be located on the hub somewhere or on the front of the thrust shift (where the prop nut is).

Step 2: Note your engine make, model and year. For inboards you will also need to know the type of leg used.

Step 3: Take the boat for a run and take note of what your WOT RPM is at full speed.

Step 4: Consider what you want you are using your boat for; skiing, fishing or cruising and what performance characteristics you would like to change or improve.

Step 5: Give me a call or send me an email with those details and I will be contact with you to discuss the options available.