If you've ever looked at a boat propeller and seen something like **14 x 19 RH**, you've seen prop sizing in action. Those numbers aren't arbitrary. They tell you exactly how that prop will move your boat through the water, and getting them right is the difference between a boat that planes cleanly and hits its top end, and one that bogs down, over-revs, or burns fuel for no reason.
This guide breaks down every measurement and characteristic that defines a propeller, what each one does, and how to read the markings on your own prop. By the end you'll be able to look at any propeller spec and know what it means for your boat.
The Two Numbers That Matter Most: Diameter and Pitch
Every propeller is described first by two numbers, almost always written in this order:
Diameter × Pitch** → for example, 14 × 19
- The first number (14) is the **diameter** in inches.
- The second number (19) is the **pitch** in inches.
So a "14 x 19" prop has a 14-inch diameter and a 19-inch pitch. Everything else—cup, rake, number of blades, material, rotation—refines how the prop behaves, but diameter and pitch are the foundation.
Let's take them one at a time.
Diameter: How Big the Prop Is
Diameter is the distance across the full circle that the blade tips trace as the propeller spins. Picture the prop spinning and drawing an imaginary circle in the water—diameter is the width of that circle, measured in inches.
What diameter does:
- Larger diameter moves more water and delivers more thrust—useful for heavier boats, displacement hulls, and lower-speed pushing power.
- Smaller diameter lets the prop spin faster with less drag, generally suited to lighter, faster boats.
Here's the important part: on most outboard and sterndrive boats, you don't actually choose diameter freely. It's largely dictated by your engine's gearcase and the gap between the prop and the hull or anti-ventilation plate. The manufacturer designs the prop family around the right diameter for that engine, so when you're shopping replacements, diameter is usually fixed and **pitch is the dial you turn** to tune performance.
On inboard boats, diameter is a much bigger part of the sizing equation, because there's more freedom in shaft and clearance design. That's why inboard sizing is handled differently—our inboard propeller sizing program walks through that specifically.
Pitch: The Most Important Number You Can Change
Pitch is the theoretical distance, in inches, that the propeller would move forward in one complete rotation if it were turning through a solid—like a screw through wood. A 19-inch-pitch prop would, in theory, advance 19 inches per turn.
In reality, water isn't solid, so the prop never advances the full pitch distance—the difference is called **slip**, and some slip is completely normal. But the pitch number still tells you how aggressively the prop is "biting" the water.
Think of pitch like the gears on a bike:
- Lower pitch = a lower gear. Easier to spin, faster acceleration, more pulling power (great for watersports, heavy loads, or getting on plane quickly). The trade-off is a lower top speed.
- Higher pitch = a higher gear. Harder to spin, but more distance per revolution, so higher top-end speed. The trade-off is slower acceleration and the risk of lugging the engine if the pitch is too high.
The pitch-to-RPM relationship (this is the key rule)
Pitch is the lever you use to land your engine in its correct wide-open-throttle (WOT) RPM range —the range the manufacturer specifies in your owner's manual. Run outside that range and you either leave performance on the table or risk damaging the engine.
The rule of thumb most riggers use:
Changing pitch by 1 inch changes WOT RPM by roughly 150–200 RPM, in the opposite direction.
- Lowering** pitch by 1 inch → RPM goes up ~150–200.
- Raising** pitch by 1 inch → RPM goes down ~150–200.
So if your engine is over-revving past its WOT range, you go **up** in pitch. If it can't reach the bottom of its WOT range (it's "lugging"), you go **down** in pitch. We'll come back to this in the diagnosis section below.
Cup: The Detail That Quietly Changes Everything
Cup is a slight curl or lip on the trailing edge of the propeller blade. It's subtle—you might not notice it unless you're looking—but it has a real effect.
A cupped blade grips the water better, which:
- Reduces slip and ventilation, especially in turns or when the engine is trimmed up.
- Acts like added pitch — a cup typically behaves like an extra inch or so of pitch, lowering WOT RPM slightly.
- Improves performance on boats running higher trim or surfacing the prop.
This is why two props with the same stamped diameter and pitch can perform differently if one is cupped. When you're comparing or replacing a prop, cup matters, even though it's not always in the headline numbers.
Rake: The Angle of the Blades
Rake is the angle at which the blades lean back (or forward) relative to the hub.
- High rake props tend to hold water better at high trim and help lift the bow—often a plus for lighter, faster boats and for getting the bow up.
- Low rake props can deliver better performance on heavier boats and at lower speeds.
Rake is more of a fine-tuning characteristic than a primary sizing number, but it's part of why prop families exist—manufacturers vary rake, cup, and blade design to suit different hull types.
Number of Blades
Most props you'll encounter are three-blade or four-blade, though two-blade and five-blade props exist for specific applications.
- Three-blade props are the all-around standard—a good balance of top speed, efficiency, and cost.
- Four-blade props generally give better hole shot (acceleration onto plane), smoother ride, and better grip in rough water or at higher trim, often at a small cost to top-end speed. They're popular for watersports, heavier boats, and offshore use.
More blades = more blade area gripping the water, which usually trades a little top speed for better low-end performance and a smoother feel.
Material: Aluminum, Stainless Steel, or Bronze
Material affects durability, performance, and price:
- Aluminum: the most affordable and most common choice for everyday outboard use. Lighter-duty, easy to replace, and great value. Browse aluminum props here.
- Stainless steel: stronger, stiffer, and more durable than aluminum. The thinner, stiffer blades flex less under load, which usually means better performance and higher top speed. The premium choice for performance and longevity. Browse stainless props here.
- Bronze: the standard for inboard propellers, prized for strength and corrosion resistance in continuous-duty marine applications.
Because stainless blades flex less than aluminum; swapping from an aluminum prop to a stainless one of the *same* stamped pitch can effectively feel like slightly more pitch, worth keeping in mind when you switch materials.
Rotation: Right-Hand vs. Left-Hand
Propellers spin in a specific direction, noted as RH (right-hand) or LH (left-hand).
- A right-hand prop rotates clockwise when viewed from behind the boat looking forward. This is the standard for the vast majority of single-engine boats.
- A left-hand prop rotates counter-clockwise.
Why it matters: On **twin-engine boats, the two props usually rotate in opposite directions (counter-rotating) to cancel out steering torque and keep the boat tracking straight. If you run twins, you'll need one RH and one LH prop—ordering two of the same rotation is a common and costly mistake. Always confirm rotation before you buy.
How to Find Your Current Propeller's Size
Before you replace a prop, you need to know what you have. There are three places to look:
1. Stamped on the prop hub or barrel. Most props have the diameter and pitch stamped right on them—look on the hub, the side of the barrel, or between the blades. You may see something like `14x19` or `14 P 19`.
2. On the blade or a hub decal, sometimes alongside a part number and the manufacturer name.
3. By the manufacturer part number.** If the dimensions are worn off but you can read a part number, that number identifies the exact prop. Our OEM propeller matching tool lets you find an exact or equivalent replacement from your current prop's part number.
If you can't find any markings at all, note your engine make, model, and horsepower—that's the starting point for our propeller guide, which narrows down the right props for your setup.
How to Tell If You Have the Wrong Prop
Your propeller is wrong for your boat if your engine can't hit its correct WOT RPM range at full throttle (with a normal load and the boat trimmed properly). Check your owner's manual for that range first—then run the boat at WOT and read the tach.
Engine over-revs (RPM above the top of the range):
Your prop pitch is too low. The engine spins freely but you're losing top speed and risking running past redline. Go up in pitch (remember: +1 inch of pitch ≈ −150–200 RPM).
Engine can't reach the range (RPM below the bottom, "lugging"):
Your prop pitch is too high. The engine is straining and can't rev out, which stresses the powertrain and hurts acceleration. Go down in pitch (−1 inch of pitch ≈ +150–200 RPM).
Engine hits its range correctly but acceleration is sluggish:
Consider a four-blade prop or a slightly different cup/rake to improve hole shot without changing your WOT RPM much.
Putting It All Together: Reading a Full Prop Spec
Now you can decode a complete listing. Take:
Michigan Wheel 14 × 19 RH 3-Blade Stainless, Cupped
- 14: 14-inch diameter
- 19: 19-inch pitch
- RH: right-hand rotation
- 3-Blade: three blades
- Stainless: stainless steel construction
- Cupped: has a cupped trailing edge (effectively behaves like a touch more pitch)
You now know its size, rotation, blade count, material, and that it'll grip a little harder than an uncupped equivalent. That's everything you need to compare it confidently against your current prop.
Frequently Asked Questions
What does "14x19" mean on a propeller?
The first number is the diameter in inches (how wide the prop is) and the second is the pitch in inches (the theoretical forward distance per rotation). So 14x19 is a 14-inch-diameter, 19-inch-pitch propeller.
Does higher pitch mean more speed?
Higher pitch raises potential top-end speed but reduces acceleration and lowers your engine's WOT RPM. It only delivers more speed if your engine can still reach its proper RPM range with that pitch—too much pitch lugs the engine and actually hurts performance.
How much does 1 inch of pitch change RPM?
As a rule of thumb, each inch of pitch changes wide-open-throttle RPM by roughly 150–200, in the opposite direction. More pitch lowers RPM; less pitch raises it.
Should I get a 3-blade or 4-blade prop?
Three-blade props maximize top speed and efficiency. Four-blade props improve acceleration, grip, and smoothness—often better for watersports, heavier loads, and rough water—usually at a small top-speed cost.
Aluminum or stainless steel?
Aluminum is the affordable, easy-to-replace standard for everyday use. Stainless is more durable and performs better thanks to stiffer blades, at a higher price. Stainless is the upgrade choice if you want maximum performance and longevity.
Can I use any prop on my engine?
No. Diameter and rotation are largely dictated by your engine and gearcase, and twin-engine boats need opposite-rotation props. Always match diameter, rotation, and hub/bore to your specific engine, then tune pitch to hit your WOT RPM range.
Find the Right Prop for Your Boat
Now that you can read propeller sizing, finding the right replacement is straightforward:
-Know your engine but not your prop? Start with our Propeller Guide.
-Have your current prop's part number? Use our OEM Propeller Matching tool to find an exact or equivalent replacement.
-Running an inboard? Our Inboard Propeller Sizing program handles the different sizing approach inboards require.
- Ready to shop? Browse outboard propellers, stainless props, and aluminum props.
Still not sure? Call us at 1-800-454-6960 or email sales@propellerdepot.com we'll help you dial in the right prop for your boat