Which one is best for your boat? Emrhys Barrell compares 15 different propellers in a test of speed, thrust, rolling resistance, stopping speed and stern cast.

Free translation of the article , made by the author of "Dnevok".

Test of folding and feathering propellers.

If you don't know what a feathering screw is, watch this video:

What is the difference between a folding and a fixed three-blade propeller? The difference is small if you are comfortable with slowing your boat down by at least half a knot while under sail. This test of fixed, folding and feathering propellers is the most detailed test ever done to the best of our knowledge. A fixed three-blade propeller, with locked rotation, offers the same resistance as a bucket towed astern.

So, what to do if you do not want to sacrifice precious speed? What to choose from the many folding or feathering screws on the market (Europe and USA)? What are the advantages of each and what are the disadvantages?

We tested every propeller we could get our hands on (with the exception of the Volvo, Radice and J-Prop, which came too late for us to test), measured top speed, static force, and - for the first time in such tests - we measured side force ( propwalk, stern throw), appearing during the reverse.

It is this important property that throws the stern of your boat to the side when you try to stop abruptly or reverse when maneuvering in the marina. Three typical propellers were then towed by a test boat to measure their drag, this test shows the propeller's drag under sail.

For many years, long keels have reduced the drag of two-bladed propellers on sailboats (by locking the propeller blades and aligning them with the keel plane), but the advent of the finned keel has again caused a problem, with the added factor of more powerful engines requiring three-bladed propellers. The need to maneuver neatly and accurately in tightly packed sailboats in moorings and crowded harbors has convinced even the owners of many long-keel sailboats to use three-blade propellers.

1970s: First folding screw

Most cruising skippers simply ignored the unwanted drag and loss of speed, but in racing circles this was becoming more and more significant. This led to the development in the 1960s and 70s of propellers whose blades flexed backwards as they sailed, markedly reducing drag. The blades returned to their places under the action of centrifugal force when the engine was started. The earliest folding propellers had blades that moved independently, but this could cause the lower blade to drop under gravity,Therefore, the axis of the blades began to be connected with gears to each other, which guaranteed a fully folded state when sailing.

At the same time, an alternative approach was developed: the feathering screw. Here the blades were mounted at right angles to the shaft, as in a conventional propeller, but they were on pivot hubs. When the engine is running, the blades work like a conventional propeller, and under sail they become parallel to the shaft, thereby reducing the resistance to movement.

One of the advantages of this type of propeller is that it is suitable for many sailboats with fins and long keels, in which the rudder is located immediately behind the propeller, so there is nowhere for a conventional folding propeller to fold - its folded blades will rest against the rudder blade.

In the earliest feathering propellers, the blades remain in the same orientation for forward or aft stroke, just like conventional fixed propellers. In reverse, the hydrodynamic profile works worse, giving less traction than in forward. Some of the newer feathering propellers have 180-degree blades in reverse, which results in the propeller's hydrodynamic profile being equally effective in reverse and forward.

Reduce propeller sail drag by 90-95%

Under sail, the resistance of folding and feathering propellers is negligible compared to standard propellers. Feathering screws create about 5-10% of the resistance of standard screws, sometimes less, while folding screws have almost zero resistance. This gives a significant speed gain for sailing, between half a knot and one knot, with the biggest savings at low speeds.

Inevitably, there are disadvantages. The first is cost: a folding or feathering prop costs two to six times more than a standard prop. The second issue is complexity - the engagement and folding mechanisms are subject to wear and corrosion in salty and sandy environments, resulting in reduced performance and even loss of blades in extreme conditions.

The third issue was the performance when driving under the engine (efficiency). Early folding and feathering propellers produced less thrust than the equivalent standard fixed propeller, especially when reversing, with the ill effects of having the blades unable to turn and you need to bring the boat to a sudden stop to avoid a crash.

Manufacturers now say they have addressed every issue except cost. They claim that the latest generation of their propellers gives good performance under the engine as well as big speed gains under sail. We ran our tests on two cold days in February. To our folding and feathering props we've added a fixed 3-blade prop as a benchmark (to compare) as well as the Axiom prop, a new evolution of standard fixed props, just to see how they are better and compare everything.

Some facts and concepts about screws

To help you understand our test, we'll take a quick look at screw theory and numbers. The four basic concepts that we will look at are used to describe any propeller: diameter, pitch, number of blades, and rotation.

The diameter is twice the distance from the center of the shaft to the tip of the blade. As a general rule, the more powerful your engine, the larger diameter you will need.

Pitch is a measure of how much the propeller will advance in one revolution and therefore how fast it will push your boat through the water for a given engine RPM (rpm). To understand the step, imagine screwing a screw into a block of wood. The angle of the helical thread determines how far it travels for each turn. Likewise, the propeller blades are set at an angle to the shaft. The larger the angle, the larger the step. However, this is only a theoretical step. In practice, since water is not solid, the support will slip througha certain degree and will advance less. The slip value is around 30% for propellers and speeds at which we are testing.

Diameter and pitch are still measured in inches around the world - a quirk of history that would have pleased Henry VIII and Napoleon turned over in his grave. But pitch can also be measured in degrees, especially for feathering props where the pitch can be varied.

The number of blades will vary between two, three or four, or even five for some high speed boats. In practice, more blades will have more power for a given diameter. For many years, sailing boats have used two-blade propellers because they offered the least drag in a boat with a long keel, provided the propeller could be locked in a vertical position. Most fixed propellers today have three blades. Folding or feathering screwshave either two blades for cheapness, or three blades for more power.

The direction of rotation is the direction in which the propeller rotates when you look at it from the stern. Right rotation - when the screw turns clockwise.

The blade area ratio (BAR), sometimes called the disk space ratio, (DAR) is the area of the blades as a percentage of the area of a circle of the same diameter as the propeller. A propeller with a larger BAR will produce more thrust but have more drag. In figures for the propellers of sailing boats, this is about 60%.

Incidentally, the explanation that the propeller only advances because its blades point forward is a convenient way to imagine what's going on, but it's not entirely correct. The blades of a standard propeller are actually cutaway airfoils, like an airplane wing, and move the boat forward because as they spin, they develop lift. This lift is caused by a decrease in pressure on the back of the blade. The faster the rotation, the greater the pressure drop. Once a certain point is reached, the mindThe decrease in pressure causes the water near the blade to evaporate and form bubbles. This is called cavitation and limits the amount of energy a given area of the blade can handle. In addition, as the bubbles collapse, they destroy the metal of the propeller, which leads to the appearance of shells on the back of the blades.

Test

We used a Bénéteau Oceanis 323 courtesy of Sailtime in Lymington. She has a typical keel shape, but atypically has a built-in skeg that supports the propeller shaft. The skeg protects the shaft and propeller from underwater damage, but the disadvantage is increased vibration as the blades pass through the disturbed water flow behind the skeg. This does not matter when installing a three-bladed propeller, a two-bladed propeller will have understated performance. But in general, the differences in power and efficiency of two- and three-bladed propellers willsmall.

Yanmar YM20 engine , 21 hp max at 3600 rpm . The gear ratio is 2.6:1 for forward and 3:1 for reverse . This is a very common combination of motors and gearboxes, so it works well for our test.

We measured the static pull , or Bollard Pull, in forward and reverse, over the entire rev range, using a remote-readable canter borrowed from Diverse Yachts.

We then measured the " stern cast " (lateral thrust) force at full power in reverse. We also compared the pull of a 3 HP outboard. suspended on the transom and the results obtained for our propellers in terms of thrust are approximately equal to the power of this 3 hp. suspension when it is in reverse. It is not surprising that the stern of the boats is thrown sideways due to this effect.

On the water, we measured the speed across the entire rev range up to the maximum. Then we performed an emergency stop with 6 knots . We recorded the time it took to bring the boat to a complete stop from the moment we engaged reverse gear.

The distance the boat will travel to stop will be from 12 m (39 ft) with the best prop to 17.4 m (57 ft) with the worst.

To measure the resistance of all 15 propellers accurately enough to compare them to each other while making allowances for different hull shapes, we would have to build a complex test rig, hire a team of scientists and spend several days in a research lab. Our goal was to demonstrate the difference in resistance caused by different screw types.

We installed a fixed propeller, then a folding propeller and then a feathering propeller, on an outboard motor leg mounted on the transom of a 14ft light boat. We then towed the boat up to 7 knots and measured the difference in drag. We don't claim that this gave us an extreme degree of accuracy, but it was enough to compare them with each other. We then compared this drag to the natural drag of the hull of the Océanis 323, a typical 10m cruising sailboat.

On the question of gravity, Mr. Newton To keep things simple, we have presented our thrust figures in kilograms (kg). Strictly speaking, thrust is a force and should be measured in Newtons - 1 kg is multiplied by the acceleration due to gravity (force of gravity) to get the figure 9.81N. On another planet with a different gravity, our conclusions would be erroneous, but until Elon Musk announced a regatta on the channels of Mars, we will use kilograms.

results

Featherstream three-bladed

Made in England at the time of writing (2009), this three-blade feathering propeller has a bronze hub and stainless steel blades. The pitch of the blades can be adjusted from the outside and can be different for forward and reverse. The blades rotate 180º to paddle the leading edge of the blade and back and forth on demand. As a result, this screw performed well in reverse: second in static force in reverse and third in distance to a complete stop of the boat. However, this screw is only opened for reversing at high speeds.otah, so you have to learn how to squeeze the throttle hard in order for it to start working in reverse.

Max Prop three-bladed

Designed by Massimiliano Bianchi in 1976, the Max Prop was one of the first feathering type propellers. The hub and blades are bronze, the pitch can be set when assembling the propeller on the shaft (or at the factory on request). The blades rotate 180º so that the leading edge of the blades also works in reverse. This prop has the best static reversing force, but it is only in the middle of the list of tested props in terms of boat stopping time. This screw opens up perfectly when reversing.

Max Prop Two-blade

Two-blade version of Max Prop. As expected, this prop creates some vibration on our boat (due to the propeller shaft skeg), however this will not happen on a sailboat with a normal shaft line or a saildrive. This propeller has the highest stern throw among all the tested ones, in all other tests it is approximately in the middle of the ratings.

Kiwi three-lobed

Developed in New Zealand in 2000, this propeller comes in three bladed only. The hub is stainless steel and the blades are Zytel fiberglass. Each blade has two different hydrodynamic profile sections. The propeller has no internal gears, so each blade turns independently of the other blades. The blades do not rotate 180 degrees, so the trailing edge of the blades becomes the leading edge on reverse. Installing this screw on the propeller shaft is one of the easiest - just push it in and tighten the nut. Average values forI reverse thrust and stop distances, but the lowest top speed. This screw is easily switched to reverse.

Autoprop three-blade

Brunton's Autoprop has a completely new approach to implementing feathering propellers, this propeller was first released in 1987. The three blades are connected to each other, moving from completely parallel to the course of the vessel while sailing to deployed at maximum speed. The difference between this propeller is that the degree of rotation of the blades depends on the speed of the propeller shaft.In the opinion of Brunton, this improves efficiency and fuel economy for operation at lower power.Our tests showed that Autoprop reached 6 knots at a speed of rotation of 2100 rpm, comparedwith 2500 rpm for most of the other propellers tested. However, previous drag tests have shown that this comes at the expense of slightly more drag than other feathering props, although still 80% less than a standard non-folding prop. The static force of this propeller is low, however, a good maximum speed and an average value of the distance to a complete stop of the boat.

Autostream three-bladed

The Autostream propeller is designed in Australia and is a 3-blade feathering propeller that has been in production for 20 years. The entire structure is made of stainless steel, and the blades rotate 180º so that the leading edge of the blade works in reverse and forward. Reverse and forward pitch can be adjusted separately without dismantling the propeller. This propeller has been designed to offer little drag when sailing up to 25 knots, making it suitable for fast multihulls. In the test, he showed the fastest stop timeki boat among all propellers, as well as a significantly lower stern cast than the rest, while maintaining good speed when running under the engine.

Variprofile three-bladed

This three-bladed feathering propeller is made in Germany. The blades rotate 180º to match the same leading edge of the blade in reverse and forward, the pitch can be set differently for reverse and forward. Usually the step is set in advance by the buyer, but can be changed on the spot. The blades and hub are bronze, with stainless pins and an anode in the tail of the nut. It has a low speed, low static force in forward and medium in reverse, but can stop the boat quite quickly.

Flexfold three-blade

Made in Denmark at the time of writing (2009), the Flexofold is a bronze folding propeller with stainless steel pins and anode covered by blades. In the test, it had the best forward speed, also the strongest forward static force, while being rather mediocre in reverse. It also has one of the weakest stern casts. At a cruising speed of 6 knots, the motor shaft rotates at 2300 rpm, compared to 2500 rpm of our standard non-folding propeller and most other propellers in testing.

Flexfold two-bladed

A two-blade version of the Flexofold, it's only slightly slower in performance than the three-blade but has better reverse thrust. Again, it had some sort of vibration due to the skeg.

Gori trilobe

Another of the earliest folding propellers, Gori was first produced in Denmark in 1975 in two- and three-blade versions, but the company only recommended a three-blade for our boat. They also make a racing version with two blades, with reduced drag. The hub and blades are bronze with stainless steel pins. a feature is the overdrive function, which changes the pitch of the blades as you progressively open the throttle, theoretically improving economy at low power runs. Tests have shown what producesProp performance in forward and reverse was closer to the bottom of our list, with the stopping distance being the longest, but the stern roll was fairly low.

Slipstream three-blade

Screw from the same Australian company as Autostream. The hub and blades are stainless steel which gives greater strength, allows for thinner blades and eliminates the need for an anode. The bearings are bronze, the gears are bevelled, with two rows each, the company claims this finish better grinds barnacles that might grow on them. Polyethylene thrust washers further improve opening. It did average in the test, though with a comfortable low cruising speed of 2,250 rpm.and 6 knots.

Slipstream two-blade

Similar in design to the three-blade variant, although with slightly worse reverse, the forward travel was still 6 knots at a low 2350 rpm.

Varifold two-blade

Designed in the UK, the blades close tight for low drag yet have a pronounced propeller shape to provide good motor performance. Due to this feature of the blades, you will have to give a sharp throttle to open it. It has a bronze bushing and blades as well as stainless steel pins. This prop showed excellent top speed, but was closer to the bottom of the list in reverse performance.

Axiom three-bladed

Axiom is the dark horse on our list. This non-folding propeller has a revolutionary profile and blade shape and we wanted to include it in our prop test to see what it can do. As you can see in the photo, the blade profile has a rectangular shape, while at the same time it has an S-shaped bend in the middle. Its developers claim that this gives more traction and braking power with a lower wetted area. How did he show himself? The test showed that this propeller has a low stern casting force, good thrust when reversing, but a weak forward stroke.

Propeller drag curves under sail

In the graph above, you can see that at 5 knots, a fixed three-blade propeller with a locked shaft creates almost half the drag that the entire hull of the boat creates. The resistance can be halved by unlocking the shaft, thereby allowing the propeller to rotate. The resistance of the feathering propeller is negligible, and the resistance of the folding propeller is too low to plot this scale.

The hull drag curve for the Océanis 323 was calculated for the YM by the Wolfson group at the University of Southampton using data from the Delft University Systematic Series. Propeller drag curves based on SSPA Maritime Consulting data using Volvo S-drive. This data has been verified with the YM resistance test on water.

Stern casting

Only three propellers gave more stern casting force in the test than the fixed propeller. The Axiom fixed propeller and feathering Autostream performed the best, with almost all folding propellers outperforming other types of propellers.

Max speed

More than half of the node difference between the worst screws and the best ones was obtained. Four folding propellers and one feathering propeller performed better than the standard fixed propeller.

Interestingly, some of the best propellers on this list were two-bladed, although we thought it should be the other way around.

Static traction force - forward stroke

The fastest propellers also tend to have the most static force, and most of the slower ones are among the least powerful. However, only one propeller - the three-bladed flexfold - generated more thrust than a standard fixed propeller. The most powerful propellers produce almost a third more thrust than some of their rivals.

Static traction - reverse

Three propellers produced more thrust than the standard fixed propeller on reverse: two folding propellers and a new kind of Axiom fixed propeller. Almost all feathering screws performed better on reverse than folding props - the difference in effort between the worst and the best is almost 2 times. The 3-bladed Max Prop is nearly twice as powerful in reverse as the 2-bladed Varifold.

Stop time

Axiom's new kind of fixed props won this test, and almost all feathering props were better at stopping the boat than the standard fixed prop. The folding propellers were less efficient, and some of them stopped the boat a third slower than the Axiom. The difference between the best and worst performance was about 3 seconds. This may not seem like much, but in an emergency it can make a big difference.

conclusions

If you want to add a knot to your boat's speed, then installing a folding or feathering prop is a must. And as our test shows, you generally still get the same amount of control and performance with the same power you had with a standard fixed prop. In fact, in some cases, you will get even better performance.

Five test propellers gave more speed than a standard fixed propeller, four of which were folding models. And although the increase in speed is only 0.15 knots, for some it may matter. On the other hand, for reverse travel, feathering propellers have proved to be more suitable, as they have better stopping times and better static force than a standard fixed propeller. The highest values were shown by the Autostream propeller in reverse. Folding propellers have generally not performed as well as a standard fixed propeller, although in generalIn general, their parameters are worse by no more than 10%.

The most interesting were the figures for throwing stern. Ten of our test propellers showed less stern casting force than a standard fixed propeller, the Axiom and Autostream performed best in this test, they showed 30% less lateral stern casting force than a standard three-bladed fixed propeller, this is a significant advantage when you need suddenly stops in a crowded marina.

In terms of drag under sail, our test shows that if you allow the fixed propeller to turn, if the gearbox model allows it (it's much easier for electric motors), it will reduce the drag. But in order to make a real difference, installing a folding prop will give at least 95% less drag than a fixed standard prop, while a feathering prop will give at least 92% less drag - that's still a huge savings.

But these benefits do come at a high price.

The following are prices for 2009 in pounds sterling - I think no one in the CIS is interested in this, so I will not give these figures . It will be enough to point out that folding screws are 2-3 times more expensive than fixed ones, and feathering screws are 5-6 times more expensive than standard fixed ones.

Due to the large differences in cost, performance and specifications of all tested screws, we do not consider it appropriate to recommend any of the reviewed screws for purchase. Tables and graphs give you the information you need at a glance, allowing you to make your own decisions about what is best for you and your boat.

Screw Tips

Some of the screws in the test are very easy to install, others are very difficult. However, while everyone has DIY installation instructions, if you're unsure of your own abilities, it's best to hire a professional to install the screw, both for safety and peace of mind. For our test, we had all the support from reps, so there were no questions about installing them.

Bronze has been the material of choice for screws ever since they were invented. Durable, resistant to salt water corrosion, and easy to cast with a low melting point, as our ancestors found out 4,000 years ago. Recently, stainless steel has appeared. Even more robust, it allows the use of thinner and more efficient blades. It is even more resistant to corrosion and is also harder, therefore less vulnerable to impact. However, it has a much higher melting point, so the production of stainless steel screws is difficult.hotter and more expensive.

Service

No matter which prop you have, it should be checked each time the boat is brought ashore for wear, corrosion, and rotation. Folding and feathering propellers require more maintenance than fixed standard propellers. Some hubs are filled with grease, which should be renewed annually. Some of them have nylon gaskets or bearings which should be checked, especially in silty waters. Most of them have an anode that needs to be checked and replaced if necessary.