# How do you size a prop?

4 min read

##### Asked by: Shannon Zoldesy

**Propeller size is expressed with two numbers, diameter and pitch, with diameter always stated first**. Diameter is two times the distance from the center of the hub to the tip of any blade. Smaller prop diameters generally go with smaller engines, or with fast high performing boats.

## How do I determine my prop size?

All propellers can be described in basic terms using a set of two numbers: **diameter x pitch**. Hence a prop with an 18in diameter and 12in pitch is described as 18 x 12. The diameter of the propeller is the circle which the tip of a single blade describes in a complete rotation.

## What do prop size numbers mean?

Two numbers are used in describing the size of a propeller. **One describes the diameter and the other describes the pitch**. Diameter is always the first number listed. A 24 X 20RH prop has a diameter of24″. The “RH” stands for right hand rotation of the propeller.

## How do I choose the right prop for my boat?

**If the RPM is below the recommended operating range, switch to a propeller with lower pitched blades.** **If the RPM is higher, switch to a propeller with higher pitched blades**. Each inch of pitch size will change the RPM by 150-200 RPM. Aim for the midpoint or higher of the recommended operating range.

## Does the diameter of a prop matter?

**Diameter usually increases for propellers used on slower and heavier boats, and decreases for propellers intended for faster boats**. A prop with more diameter has more total blade area, which allows it to handle more power and create more thrust to move a heavy boat.

## How far below boat should prop be?

If you have a short shaft motor, the top most part of the transom and the lower most part of the boat should be about **15 to 16 inches**. For a long shaft, I think 20 to 21 inches is correct.

## Is 3 or 4 blade prop better?

3 or 4 Blades? **A 3 blade propeller usually offers top speed performance while a 4 blade propeller provides maximum thrust and smooth cruising operation**. Four blades have some features of their own, though. They often provide more lift at the stern which will help accelerate the hull, especially if it is stern heavy.

## Will a bigger prop make my boat faster?

**Increasing prop pitch will make the boat go faster** (provided the engine has enough power to keep the RPMs in the optimum operating range. If the engine doesn’t have enough power to run a higher pitch prop, performance suffers across the board and you could easily damage your engine.

## Will a 5 blade prop make my boat faster?

*I gotta admit I think the high-five is my favorite propeller here today it has by far the best acceleration of a pole fantastic mid-range decent top speed.*

## Will a bigger prop mean more speed?

Change Your Propeller

If you are running an aluminum prop, **changing to even a basic stainless steel prop will usually improve top speed**.

## What does Overpropped mean?

Overpropping **results from a prop which has a combination of too great a pitch and/or too great a diameter and/or too many blades**.

## Is a bigger prop better?

**The bigger the prop, the more drag we have**. This goes for the number of blades that the prop has as well. The more blades that the prop has the more drag it will have and the less top end wide open throttle speed we will be getting.

## What prop pitch is best for speed?

**The lower the prop pitch, the better your hole-shot**. However, this comes at a price: top speed. The lower pitch makes the engine reach maximum rpm at slower speeds. Conversely, a higher pitch will deliver greater top speeds, but slower acceleration.

## What is the difference between a 19 and 21 pitch prop?

A simple explanation of prop pitch indicates that a 21-pitch prop, if turning in a thick substance, will push your boat forward 21 inches for every revolution of the blades. Conversely, a 19-pitch propeller would move your boat forward 19 inches.