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Choosing a Sailboat Propeller

| Engine & Propulsion | July 19, 2015

Sailboat Propellers

 

Standard props supplied with many production boats are just enough to do the job. They will propel the boat under power, but will not give optimum performance across a range of conditions. The reason for this of course is cost. The best propellers cost quite a bit more than a standard or fixed prop.

Choosing a Sailboat Propeller for your boat you would consider factors like;

    • Drag under sail
    • Speed & Power in forward
    • Stopping power in reverse
    • Vibration
    • Maintenance
    • Cost

For the racer you are primarily looking for a low drag propeller, for the cruiser however a small increase in drag is well worth the better handling under power.

Propeller Terminology

    • Propeller hand. A right handed propeller is one that turns clockwise in forward.
    • Propeller Pitch is defined as how far the propeller advances with one full revolution of the shaft. Think of a screw and how far it goes in with one full turn.
    • Number of blades. Two blade props are far less powerful than 3 blades. A two blade prop has only one blade working in clean water while the other blade is in disturbed water from the keel. With a three bladed prop you have two blades working all the time.
    • Propeller Balance; 3 or more blades have better balance than two blades, more metal evenly spread around, which reduces vibration.
    • Propeller clearance; the distance between the blade tip and the hull.
    • Propeller Diameter; The diameter of the prop plays a big role in how much power it produces, but hull shape restricts available space.
    • Blade area; More blades increase the blade area.

Types of Sailboat propellers

    • Fixed
    • Folding
    • Feathering
    • Autoprop

 

 

Fixed blade propellers

 

Fixed blade propellers are the most common found on all types of boat. They perform quite well and are reliable and cheap. In forward the fixed blade prop is very efficient if it is pitched correctly. This applies to all props. The engine manufacturer will recommend the RPM that the engine runs best at. This is the RPMs that the engine runs when the boat is at its cruising speed and so the pitch should be designed to achieve that. If the pitch is too small the engine will run over the recommended RPM and potentially damage the engine. Power in reverse is not what it could be, due to the leading edge becoming the trailing edge. The inefficient blade shape in reverse does not produce the lift as found in forward. The problem with a fixed prop however is the drag. Drag can reduce a sail boats sailing performance by about 1/2 to 1 knot or more. One half knot if you let the prop spin and If you follow the engine manufacturers recommendations and put the shaft in gear the drag could a whole knot and more.
    • Pros; Cheap, Good power in forward, Reliability
    • Cons; Low power in reverse, High Drag, Weed catcher

 

Folding propellers

If you are looking for a propeller for a sail boat and are particularly interested in sailing performance, a folding propeller may be best. Many performance sail boats specify a folding prop as it will produce the least drag under sail, better than a fixed prop by far as mentioned above and slightly better than a feathering prop.

Mechanics

The mechanics of a folding prop are a hub with blades fastened to the hub with a pin. The blades hinge around the pin and close by the water flow folding the blades aft of the hub. The result is a very low drag form. In modern folding props the blades have gears so they open and close in unison. The mechanics of a folding propeller work by using the centrifugal force of the shaft spinning, which opens the blades to the operating position. The blades are fully open when they hit the stop. This is the hard clunk that sometimes can be heard. Folding props come in two blade, three blade and 4 blade options.

Performanc

In forward the folding prop ranks close to the top and some out perform the fixed prop. The good blade shape in forward leads to this high performance. Unfortunately performance in reverse gear is not what could be achieved with other props. In reverse the shaft rotates the other way so the leading edge becomes the trailing edge, hence the propeller shape is not optimum. When in reverse a folding prop takes longer to develop force and It will take longer to stop from full ahead. They also have a harder time opening up in reverse. Therefore reverse power is much reduced Take this into account when you are entering a harbor. With a folding prop its easy to take precautions. Slow down early and if you need reverse put the gear in reverse early to combat the slow bite of the propeller.
    • Pros; Low Drag, Low profile shape does not catch weed
    • Cons; Cannot change pitch (except Gori does have overdrive which is a different pitch), Low power in reverse

 

Folding propeller models include;

    • Flexofold
    • Gori
    • Volvo
    • Varifold
    • Slipstream
    • Martec

Gori folding props; The unique feature about Gori props is that you can change pitch in overdrive. The idea behind overdrive is that you can get to speed with less RPM. To get to overdrive go in reverse and then in forward. Normal forward is attained by starting in neutral with the blades folded.

 

Feathering Propellers

 

Feathering props are one of the best propellers for sailing boats when mixing performance under sail and power. they get their name due to the feathering blades, which align with water flow when in neutral.

Mechanics

Feathering props have blades that rotate 180 degrees so the leading edge is still the leading edge in reverse, which makes them more efficient in reverse over folding and fixed propellers. The images right show the 3 modes. From top to bottom: feathered, forward and reverse. The blades of a feathering prop rotate about the axis perpendicular to the hub. Under sail the gear is left in neutral and the blades are allowed to fall in line with the water flow. When put in forward gear initial turn of the shaft rotates the blades till they meet the stop. The blades are in the forward pitch. By putting the gear in reverse the blades return to neutral and then rotate to the astern pitch. This mechanism produces power quite quickly.

 

Performance

In forward the max prop has around 90-95% of the thrust of the folding or fixed props In reverse however it is the top performer. Despite the cost, the propeller will reward you with great control under power. The best part is the grip of the prop which will give you the best power in reverse and stopping. The blades of the max prop rotate through 180 degrees when going from forward to reverse, so that the leading edge is still the leading edge in reverse. This feature comes into it own when pulling into a slip. The reasons for getting a folding or feathering propeller is to reduce the drag of the prop, while still giving good control and performance under power. I have added either 2 or 3 blade max props to sailboats from 30 ft to 80ft, with great success.
    • Pros; Low Drag, Good power in forward and reverse, Minimal vibration, Adjustable pitch
    • Cons; Blades catch weed etc, Maintenance & installation, Expensive

 

Feathering propeller models include

    • Max Prop
    • Variprop
    • Featherstream
    • Kiwi Prop

Max prop features & benefits video

Another feature of the Max Prop is the ability to adjust the pitch. Part of the assembly requires you set the pitch. The correct pitch is set to load the engine at max RPM. Most engine mechanics can tailor the correct pitch for you.

 

Autoprop

The Autoprop is an odd duck and does not follow the feathering or folding model.

Mechanics

The Autoprop takes a different approach with the three blades operating independently. The AUTOPROP’s distinguishing feature is that each blade revolves 360 degrees. The image show the three positions for the blades. The top image is forward, the middle astern and the bottom in feathered or sailing mode. In forward the propeller rotates to the correct pitch automatically. This results in increased propulsion efficiency which reduces fuel consumption, and extends cruising range. The Autoprop also self pitches in reverse to give you the same thrust in reverse as you would get in forward. This greatly improves stopping power, backing down, and overall maneuverability.

Performance

The test results shown below (Segeln magazine) have the Autoprop at the top of the list when it comes to performance under power. The problem with the Autoprop is its high drag in feathered mode shown left. This is demonstrated in the Tank tests in the Segeln article below. If power is most important and not speed under sail, the Autoprop has a very good following. Autoprop is available as a 2 or 3-blade propeller, in sizes 13″ to 35″ for a power range of 6 to 294 kW.

 

Conclusion & References

If you were unsure of the differences between all the various sailboat props, hopefully we have helped clear the air. There is no single prop that performs the best in all conditions so you need to make some choices when picking the best prop for your boat.
All of the feather/folding propeller options will reduce drag and increase performance between 1/2 and 1 knot over a similar sized and number of blades fixed prop. So for performance sailing and a prop with the least drag a Folding prop like the Flexofold is a good choice. For a low drag prop with good handling under power a feathering prop works well. For just the benefits under power some argue the Autoprop is best.

References These articles show test data of all the prop types in a variety of conditions

 

As for the choice between 2 and 3 blades in either of the folding/feathering types. The difference in drag is something a cruiser will not notice. A racer will but it is just a few %. However the advantages of a 3 blade prop in maneuverability, stopping and the reverse power make the 3 blade the best choice. Three blades however does offer a greater chance of fouling with weed or a pot etc.

Sailboat Propeller Cost comparison

2 blade fixed prop costs around

    • $400 2 blade flexofold costs
    • $1,115 2 blade Max prop costs
    • $1,630 for 16 inch blades.

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