If you own a boat, chances are it has a propeller.
Well, unless you’re tacking, jibing and hoisting sails.
You wouldn’t think there was much to a prop. It’s a piece of metal with a couple of blades on it, right?
Well, no. This seemingly unobtrusive—it does spend most of its time under the water, after all—piece of metal actually has quite a lot to do with the performance and fuel efficiency of your boat.
You might be surprised by all of the different technical terms that go into boat propellers. I know I was. It’s so much more than a spinning piece of metal hanging onto the bottom of your boat’s engine (or a decorative touch on the trailer hitch on the car in front of you in the drive-through).
How to Choose a Boat Propeller That Won’t Drag You Down
Features of Boat Propellers
When you think about all of the words that describe boat propellers—words like slip, rotation, cup and skew—it sounds like something out of a baseball game.
Before you call a ref to intervene, check out these things you should know about choosing the best boat propeller.
The blades are probably the most obvious part of the propeller. They’re the things that spin and propel your boat forward.
They’re also the things you have to watch out for when swimming around the back of the boat while anchored out, but that’s a different topic altogether.
Number of Blades
Two-bladed boat propellers are made for smaller boats and smaller motors, providing quiet, gentle power. These are usually the propellers you’ll find on trolling motors.
Three-bladed propellers are the most common type. They’re usually made out of aluminum and are less expensive to produce. Being that fewer blades means less drag, this allows for faster speeds. However, this can also cause more vibration.
Four-bladed props are also fairly common. They’re usually made of stainless steel, making them strong and durable. Four blades are good for rough conditions and offer great fuel efficiency.
Five and Six Blades
As with most anything, the more parts there are, the more expensive the item is going to be. Five and six-bladed props are the most expensive to produce.
On the upside, more blades equal less vibration. They’re used on boats that need more thrust, but they also create more drag.
Bore is the middle of the prop where it fits into the boat’s shaft. The bore size has to match the boat shaft size.
Cup refers to a curve at the edge of the blades. Just like your coffee cup, it allows the prop to hold water, or coffee as the case may be.
An un-cupped blade would be perfectly flat, and not very good at holding water. Cupped props can enhance speed and add to the blade’s pitch. Pitch is coming up.
You can find the prop’s diameter by measuring from the center hub to the tip of one blade, and then multiply times two. I won’t bring in complicated math here. It’s basically the distance around the outer edges of the blades.
More diameter equals more control and power, but it can also increase drag. Large boats require more diameter, and small boats require less diameter.
Drag is pretty much what it sounds like. Something is “dragging” down, which makes for a slower ride. Drag can be caused by water conditions or, as mentioned above, a prop that has too much diameter for the size of the boat.
Like the pitcher’s mound in our proverbial baseball game, the hub, a.k.a. “barrel,” is the center of the action. It’s the middle part of the prop that the blades spin around as well as where the prop attaches to the shaft of the boat engine.
Most stock props are aluminum, which is less expensive than stainless steel but not necessarily as durable.
It’s fairly easy to repair a slightly-damaged aluminum prop, but if the damage is too dramatic, it can’t be repaired.
Composite props are made out of a plastic polymer substance and are the least expensive to produce. The composite prop’s main job is to protect the inner workings of the boat.
They’re actually designed to break off if they hit something, in order to avoid damage to other parts of the boat.
Pricewise, it’s much less painful to replace a prop than your boat’s whole lower unit. As we all know, there’s no crying in baseball, but there can and will be crying in boating when you have to shell out thousands of dollars to repair that lower unit.
Stainless steel props are very durable, but they’re also the most expensive. They’re meant to provide high performance as they protect the shaft and gears. After-market props are usually stainless steel.
No, it’s not what the guy with the ball does in a baseball game. Well, okay, it is, but not in this particular instance. Pitch refers to the measurement that the prop goes around in one full rotation, but there’s also a hypothetical side to it.
For instance, a 23-inch propeller should move 23 inches in one rotation. Makes sense, right? Now here’s where the hypothetical part comes in. That’s just the distance it could go if all conditions were perfect. Let’s be honest. Are all conditions ever perfect? Factors like cupping and slip play into this, so it might not actually be 23 inches.
When there’s more pitch, the boat goes faster. Likewise, less pitch means less speed. Boat propellers can have controllable pitch or be self-pitching, but I’ll get to that in a minute.
Rake refers to the degree of slant on the blades. They slant to either the front or back of the hub. If the blades are raked, or slanted, toward the front, it can help to increase speed.
Think of it as digging with a shovel. It’s much easier, and faster, to dig out a spot for your petunias with a curved end than a flat piece of metal, isn’t it? Here’s an example (shown right) of a prop with a high angle of rake.
This is an easy one. Rotation refers to the turning of the blades, either left or right. Rarely are there switch hitters in the propeller world.
Much like the wind, slip is something that’s always changing. Slip works in conjunction with pitch, as described above.
It’s the difference between the theoretical distance the blade “could” revolve and how far it actually does revolve… after taking into consideration all of the external factors.
Slip can be affected by water conditions, the wind or even the boat’s weight and design. As the boat slows down, slip increases. It decreases as the boat goes faster.
Types of Boat Propellers
Controllable pitch, also called variable pitch, is when the blades rotate around the axis, much like helicopter blades. Boats like ferries and tugboats use this type of prop as it needs a really big hub to turn the blades.
In fixed pitch props, the blades are just that, fixed in place. They don’t move around the axis. Most boats use fixed propellers.
Skew refers to the shape of the blade. Skewed propeller blades have a slight angle or curve along the side, like a leaf. Skewed prop blades help to reduce cavitation and noise.
The Best Boat Propeller for a Pontoon Boat
- Big, rounded blades that give a more responsive, but slower, ride
- Four blades can be beneficial because of all the surface area
- High diameter
- Low pitch
- Low rake angle
- Low cup
The Best Boat Propeller for a Deck Boat
- Low pitch
- Low rake angle
- Low cup
The Best Boat Propeller for a Ski Boat
Depending on whether you’re pulling water-skiers and wakeboarders or towing tubes, you’ll want a propeller that can get up and go.
- Three or four blades
- Moderate pitch, but higher than a pontoon boat
- Light cupping
- Less diameter for less drag through the water, creating the ability to go faster
Our Choice: Spline Bore LH Propeller (check price on Amazon)
The Best Boat Propeller for a Speed Boat
Whether you own a Scarab or Stinger, have adrenaline issues or you’ve watched one too many episodes of “Miami Vice” (or perhaps all three), if you have a speed boat you want to get there and get there quick.
Besides looking sleek and colorful, speed boats were designed for racing and going fast, and it’s apparent in their propellers.
- High rake
- High pitch
- High cupping
- Specialized stainless steel props with thin blades
Hopefully I’ve covered all the bases with this propeller tutorial.
Now, get in the game and get out on the water!