We have all been there. The wind dies as you sail back to the dock ahead of the kickoff. Or a stiff sea breeze comes in during the summer. You then find yourself motoring straight into the wind with the family onboard on your weekend getaway.
In moments like these, you might ask “Do I have the right propeller”? Typically, it’s something most people forget about– out of sight, out of mind. Sailors convince themselves they’ll only need the propeller occasionally. So why make sure it’s optimized?
The truth is, we sailors use our motors more often than we would like to admit. And it’s important to have the right propeller under the boat. It can make the difference between crashing the boat into the dock and making you look like a pro at the marina.
Importance of the Propeller
In simple terms, a propeller absorbs the horsepower provided by the engine, creating forward thrust. This involves many factors. But this post focuses on the various propeller types for sailboats, looking at the advantages and disadvantages of each type. Hopefully, it will help you navigate the confusing world of propellers.
The engine and gear ratio, along with engine power, are critical for any propeller sizing. When choosing an engine and/or propeller for your boat, consider the effect of the gearbox ratio on the propeller. Most marine diesel engines have a reduction gearbox. That causes the shaft to turn slower than the engine, at a speed more suitable for the propeller. Contact your local propeller shop or dealer to run the calculations optimizing it for your boat.
So let’s take a look at the main type of propellers available to sailors.
Fixed propellers are the most traditional and straightforward option. As the name suggests, the propeller blades are fixed in place. They don’t pivot or fold. Fixed propellers are a common choice for many sailors due to their simplicity and cost-effectiveness.
Advantages of Fixed Propellers
Simplicity: There’s not much to it for moving parts. The propeller can be repaired easily by any propeller shop.
Cost-Effective: Fixed propellers are generally more affordable than their folding or feathering counterparts, making them an attractive option for sailors on a budget or who prioritize economic considerations.
Minimal Maintenance: With fewer moving parts, fixed propellers require minimal maintenance. Just keep the propeller clean and free of growth.
Disadvantages of Fixed Propellers
Increased Drag: One of the main drawbacks of fixed propellers is the increased drag they create when sailing without the engine engaged. A fixed propeller typically slows a sailboat down .85kts to 1.5kts of boat speed, especially in lighter air.
Performance Under Sail: Fixed propellers reduce sailing performance due to the drag and turbulent water flow over the rudder, affecting upwind performance. Sailors who prioritize speed and efficiency under sail might explore alternative options. A fixed-bladed propeller can freewheel and not lock into place, creating a surprisingly huge amount of drag.
Maneuverability: Everyone has been there, trying to reverse into your slip or a dock and the boat starts to move sideways into the next boat.
The blade design on a fixed prop is made to push one direction and that’s forward. When in reverse, the propeller is trying to use its trailing edge and reversed profile, causing it to climb sideways as much as it goes backward.
Folding propellers address the issue of increased drag associated with fixed propellers. Almost all modern folding propellers are geared to keep the blades opening together. The propeller works by using centrifugal force to throw the blades fully open in forward and reverse gears. When under sail the propeller is designed to fold back, creating a streamlined profile, reducing drag, and minimizing the impact on sailing performance. It is the lowest drag of any of the propeller options.
Advantages of Folding Propellers
Reduced Drag: The primary advantage of folding propellers is the ability to fold when sailing. This significantly decreases drag, allowing the boat to move more freely through the water and non-turbulent water over the rudder. The other benefit is the propeller will not spin when the engine is off and under sail, reducing noise and hours on the gearbox.
Improved Speed/Steerage Under Sail: Sailors who prioritize speed when under sail often opt for folding propellers. The reduced drag contributes to the boat accelerating quicker and has better speed/steerage in light winds.
Maneuverability: Folding propellers can enhance a sailboat’s maneuverability as they have a fully shaped blade ie. camber, curve, and twist. Especially in tight spaces or when docking. The blades flip around to a reverse position, so you get minimal prop walk and full reverse thrust. The reduced turbulence over the rudder when sailing allows for better pointing and control. Some folding propellers have a 2nd bigger pitch for motor sailing.
Disadvantages of Folding Propellers
Mechanical Complexity: Folding propellers have more moving parts than fixed propellers but less than feathering, which may lead to increased maintenance requirements.
Cost: Folding propellers are generally more expensive than fixed propellers, both in terms of initial purchase and potential maintenance costs. Sailors need to weigh the benefits against the investment.
Feathering propellers take the low drag concept in another direction. Instead of folding back for reduced drag, when sailing the blades align themselves with the water flow like a flag in the wind. This creates a low drag profile. This can be a great application for boats with small apertures or if the shaft is close to the rudder. The gearing/mechanicals for these propellers are inside a greased housing, eliminating chances for growth/debris.
Advantages of Feathering Propellers
Sailing Performance: Feathering propellers offer low drag when under sail and improved water flow over the rudder compared to fixed but with more drag compared to folding.
Adjustability: Feathering propellers provide the owner with an option to adjust the pitch after installation. Each boat and engine combination is slightly different from the next. Having the ability to dial in your engine’s RPM with the turn of an Allen key is a great feature. This makes an excellent choice for sailors who like to tinker.
Additional Blades: If there is an issue with propeller tip clearance or the wrong engine/gearbox ratio was installed, then the ability to increase the number of blades to add additional blade area can help make up for this as in adding blades from a 3 to 4 blade or 5 blade.
Disadvantages of Feathering Propellers
Higher Cost: Feathering propellers are more expensive than fixed propellers. The advanced engineering and materials used in their construction contribute to the higher price tag.
Maintenance Complexity: Similar to folding propellers, feathering propellers work by the blade gears rotating on a single center gear, leading to increased maintenance complexity. Sailors must be willing to invest time and effort into proper care to regularly grease them.
Blade Shape: Due to the blades needing to feather their blade design/shape is mostly flat which is less efficient than a fully shape-bladed propeller.
Choose Your Best Propeller
Choosing the correct propeller is a difficult and timely decision but should not be one overlooked. Similar to tires on your car where if it starts snowing or the road gets bumpy you want to make sure you can trust what’s under the waterline. All three different types of propellers require owners to weigh out their priorities. Is it budget, performance, or adjustability?
For sailors who prioritize simplicity and cost-effectiveness, fixed propellers might be the ideal choice. Those seeking improved sailing performance and maneuverability may opt for a folding or feathering propeller, despite the higher initial cost and maintenance requirements.
Ultimately, the choice between fixed, folding, and feathering propellers is a personal one, influenced by a sailor’s specific needs and preferences. As technology continues to advance, propeller designs may evolve, offering sailors even more options to enhance their sailing experience.
Brendan Prior is a partner at AB Marine, specializing in helping customers optimize their sailing propulsion. He has been involved in the marine industry, racing, and cruising for over 30 years. These days he is on the hunt for good surf and chasing fish in the summertime off Newport, RI.