How does a folding propeller work?

Folding prop or fixed prop? That's the big question.

At Flexofold, we'll always choose a folding propeller, and there are good reasons why. From efficiency to onboard comfort, the advantages are hard to ignore. Are you in doubt about whether a folding propeller is right for your boat?

Let us guide you. 

What is a folding propeller?

A folding propeller gives you benefits in both its active and passive form. Unlike a fixed propeller, where the blades stay open at all times, the blades on a folding propeller fold together, giving you the least amount of drag under sail of any propeller type. And you don't give anything up in return: engine performance stays exactly where it should be.

It all comes down to physics. A folding propeller works via centrifugal force. In its active form — when the engine is running — the engine rotates the hub, and centrifugal force pushes the blades open. This happens already at low RPM, and as long as the engine keeps running, centrifugal force keeps the blades open. Simple mechanics, simple physics.

The same goes for reverse. When shifting into reverse, it may seem like water pressure would force the blades shut. And while that water pressure is real, it's nowhere near as strong as the centrifugal force created once the propeller starts spinning. That force keeps the blades firmly open and easily overcomes any pressure trying to close them.
So yes, a folding propeller handles reverse just fine.

In its passive form — under sail, with the engine off — there's no more centrifugal force to hold the blades open, so water pressure takes over and folds them together, giving the propeller a streamlined shape.

Simple construction

A folding propeller is built on a remarkably simple construction. Simple enough that there's almost nothing that can go wrong. Unlike other propeller types, there are no small gears that need regular lubrication. That matters, because a propeller sits under the boat, out of sight, where it's not exactly convenient to keep an eye on it (and not everyone wants to jump in the water to fix it).

There's real peace of mind in knowing that the simple construction makes it nearly impossible for anything to fail. And if you ever do need to make a change, you won't have to haul out the boat: thanks to the simple design and Loctite®-secured screws, a Flexofold propeller can be mounted underwater.

The only thing that can actually go wrong is incorrect mounting. But because the construction is so simple, it's so easy to mount that even a person without any sailing experience can do it.  

Simple construction calls for solid materials. That's why Flexofold propellers have always been made from ALCARO, a high-quality aluminum bronze alloy known for its strength and durability in marine use.

The pros of a folding prop

No matter what type of sailor you are, a folding propeller can be the right choice for you. You'll enjoy the benefits both when the engine is on and when under sail.

Five advantages stand out in particular:

1.     Reduced drag and higher speed
2.    Increased comfort on board
3.    Driven by the laws of nature 
4.    Increased safety 
5.    Better sailing performance 

Together, these five benefits add up to one thing: a noticeably more comfortable sailing experience. 

Reduced drag and higher speed

With the blades folded together, a folding propeller drags far less through the water than a fixed propeller, where the blades stay out permanently. The smaller surface area is only part of the story. On a Flexofold propeller, the twisted blade shape lets water flow wind around and between the folded blades, instead of being slowed down the way it would be by a fixed prop.

Many of our customers tell us their boat suddenly feels faster after switching to a Flexofold propeller — and it's not just a feeling. Our folding propellers increase sailing speed by 0.5 to 1.5 knots.

That lower drag also means you can sail in lighter wind. A fixed propeller creates enough drag that you need a decent breeze just to pull it through the water and get the boat moving, which means starting the engine earlier than you'd like. A folding propeller needs far less wind to keep pressure in the sails, so you get more hours under sail before the engine comes into play at all.

The hydrodynamic shape also gives a Flexofold propeller superior power, whether you're sailing forward, reversing, or bringing the boat to a stop. In one test, a 63-foot Hanse went from 8 knots to a complete stop in just one boat length. 

Increased comfort on board

Onboard, that lower drag translates into something simple but valuable: quiet. Under sail, a folding propeller makes no noise at all, unlike a fixed propeller, which keeps creating drag unless you leave it in neutral to spin freely with the water flow. That free-spinning trick reduces drag, but it puts wear and tear on the bearings in the engine, gearbox, and saildrive, and you'll hear a growling sound from the rotating shaft and propeller.

With a folding propeller, it's silent under sail, because the propeller folds and therefore doesn't need to rotate the way a fixed prop does. That silence adds real comfort onboard, especially if you enjoy getting away from the noise of everyday life and disappearing into the sounds of nature. 

The laws of nature

A folding propeller isn't mechanically driven. It works through natural centrifugal force and water flow, which also makes it a simple, resource-efficient choice. There's no extra technology to learn: it just works, because it's governed entirely by physics.

Centrifugal force pushes the blades open while the engine runs; water pressure folds them back in the moment the engine stops. It’s simple and super easy for you as a sailor, which is exactly the point. 

Increased safety

A folding propeller makes the sailing experience safer. 

Since the blades are folded under sail, the propeller doesn’t catch old fishing lines, crab pots and marine debris in the sea.  

Unfortunately, the oceans are full of debris like this. Getting a line or net tangled in your propeller can be serious, and in the worst case, costing you both engine power and steering control. That's a real risk, especially in rough weather, when control matters most. If you have a fixed propeller, loose fishing lines and nets can turn into a minefield to navigate around, with a real risk of getting caught up in them.

A Flexofold propeller also converts more horsepower into actual thrust in its active form. The biggest, best engine in the world is useless if the propeller behind it can't get that power into the water. Every sailor can get caught out by bad weather, and it's reassuring to know you have the needed propulsive power to get safely back to the marina, without worrying about debris stopping the engine along the way.

Better sailing performance 

You will experience a better sailing performance with a folding propeller. 

A classic example is when tacking. With a fixed propeller's drag working against you, you lose speed every time the wind drops out of the sails mid-tack, and you have to build it all back up again before the next one.

With the drag from a Flexofold propeller almost eliminated, the boat holds a lot of its speed through the tack instead of stalling out. So if you're quick on the sails, you barely lose momentum. You avoid coming to a complete stop, only to build up speed again. That means you can sail closer to the wind and navigate your way to your destination more precisely, in less time.

A folding propeller is for all sailors

Still wondering if a folding propeller is right for you? In short: if you sail a boat with an inboard engine, it is. Racing sailors, family cruisers, bluewater sailors, and anyone who likes to trim a little extra to beat the boat next to them. Everyone gets the same benefits, just suited to how they sail.

It's also one of the better investments you can make in your boat for relatively little money. If your sails are in good shape but you're considering an upgrade anyway, a folding propeller will get you more performance for far less than a new set of sails costs ($15,000–22,000) — unless your current sails are genuinely worn out, in which case, replace them regardless. For most sailors still running a fixed propeller, switching to a folding one is simply where the money goes furthest.

We guide you

Choosing the right propeller is one of the things we know best at Flexofold. You don't need to figure out on your own whether you need a 2-, 3-, or 4-blade folding propeller. Just give us the details about your boat, and we'll calculate which propeller matches your engine and gear ratio. If more than one option would work, say a 2-blade and a 3-blade, we'll talk it through with you to land on the one that fits how you actually sail.