The Benefits of Low-profile Propellers

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           With its blades feathered, this Max-Prop propeller creates minimal resistance    

           With its blades feathered, this Max-Prop propeller creates minimal resistance    

The challenge of establishing healthy performance under power without compromising a boat’s sailing ability seemed unsolvable until the first folding and feathering propellers for leisure sailors appeared on the market. Now, the quest for the ideal low-drag propeller constitutes a highly competitive niche industry. Indeed, if there’s any single area in the marine world as competitive as that of sailmaking, it’s the folding and feathering propeller business.

With respect to any of today’s low-drag propellers, however, the benefits in terms of any boat’s sailing characteristics are going to be immediate and readily noticeable. As David Sheppard of the UK’s Bruntons has observed, the “real” competitors out there are fixed propellers. Granted, fixed wheels may be a good deal cheaper, but they seriously compromise the performance of any sailboat on which they are installed—to the point where Sheppard says that if more sailors knew just how much, there would be a lot fewer fixed wheels out there.

In terms of those gains, Lars Østergaard of folding-propeller maker Gori says that depending on wind strength, the average boat equipped with a folding propeller will gain between 0.5 to 1.5 knots of boatspeed under sail, compared to a boat with a fixed-blade wheel. “Typically, the boat with the folding propeller will also be able to aim higher due to the water flow to the rudder,” Østergaard adds. “Many sailors often say, ‘We do not race, we do not need a folding or feathering propeller.’ But once they have seen several equal boats passing them, they will consider optimizing.”

Similarly, Sheppard, whose company manufacturers both the feathering Autoprop and folding Varifold lines, notes that no matter what type you go with, you’ll be reducing propeller resistance under sail by up to 90 percent compared to a fixed wheel; while Fred Hutchison of PYI Inc., makers of the feathering Max-Prop, adds, “When it really makes a difference is in light or medium air. Suddenly you can carry speed through a tack. I tell customers it makes it fun to sail in light air again, because you can.”

  Note how streamlined the four-blade the Max-Prop (top) and three-blade Variprofile (bottom) both become when not engaged   

  Note how streamlined the four-blade the Max-Prop (top) and three-blade Variprofile (bottom) both become when not engaged   

Hutchison also notes that, in contrast to, say, a new set of sails, which will soon lose their edge, any gains you make will not only be immediate, but permanent.

“When the guy with a fixed propeller is forced to turn on the engine, the one with a folding propeller is still able to enjoy his sailboat,” says Flexofold’s Keld Willberg, summing up the difference a typical sailor will experience.

Then there are the benefits to be had when off soundings, with Østergaard and Willberg both noting how a low-profile prop’s efficiencies only become that much greater with the length of any passage: that and how you no longer have to worry about the noise of a turning propeller or the wear on bearings. “Think about the gains when you have an extra knot over 24 hours,” Hutchison says, noting you can figure a 15 percent increase in sailing speed in lighter conditions, in particular. “When you spend more time at sea, it also makes a difference, cutting passage times by days.”

Survey Says!

Of course, such glowing reviews are only to be expected from manufacturers. To get a more objective view, therefore, SAIL teamed up with the World Cruising Club, organizer of such rallies as the ARC, World ARC and Caribbean 1500, to see what its participants think—and the results were no less impressive (see ‘Low-drag Props in Use’ farther below). In fact, approbation of both feathering and folding propellers under sail was nothing less than universal, with a number of respondents saying they wouldn’t think of casting off lines without one.

Our respondents also responded very favorably to questions concerning the performance of their low-profile propellers under power—which is after all the point of putting them on a boat in the first place. Not surprisingly, there was a bit more variance here, which only makes sense given the complexity that is part and parcel of ensuring correct propeller fit for a particular boat (that and the fact that with their moving parts low-profile propellers require a bit more care and maintenance).

  The gearing in the hub of this Gori prop allows the blades to swing through a full 180 degrees (left);  This two-blade Bruntons Varifold is incredibly streamlined under sail (bottom right);  Note the complex curvature of the blades on this three-blade Gori saildrive propeller (top right). 

  The gearing in the hub of this Gori prop allows the blades to swing through a full 180 degrees (left);  This two-blade Bruntons Varifold is incredibly streamlined under sail (bottom right);  Note the complex curvature of the blades on this three-blade Gori saildrive propeller (top right). 

Nonetheless, in light of the overwhelmingly positive responses received in our WCC/SAIL survey, it’s safe to say that hype and marketing aside, the various different feathering and folding propellers now on the market all work just fine. In fact, given how persnickety and/or skeptical many sailors can be with respect to any new bit of gear, the positive response was nothing less than extraordinary. And remember: these are sailors who have logged some serious miles.

Note that over the years, a number of publications and organizations have attempted to create a definitive measure of how the different types perform with respect to one another. However, given the tremendous number of variables involved, there can really be no final word in this area. Prop walk, for example, is also influenced by the down angle of the shaft, while everything from diameter to blade pitch needs to be carefully tailored to a particular engine/transmission combination. Again, it’s vital that any low-drag propeller be properly sized and maintained to function.

With that in mind, what follows is an outline of some of the characteristics of the two types. The idea is that anyone interested in making the switch from a fixed wheel, can use this as a starting point from which to begin querying manufacturers to see what best matches their budget and sailing style. There’s also plenty of debate to found online that sailors can refer to regarding the various types: some of it even well-informed. (Though much of it clearly not!)

Bottom line, the various different types of low-profile propellers out there all have their pros and cons. They are also all fairly pricey, so do your research. Suffice it to say, any sailor would be foolish to make his or her final decision based on a single source of information, this article included.

 The composite hub on this Flexofold prop serves to stave off galvanic corrosion (top left);  Note the unique shape of the feathering blades on this Bruntons Autoprop (top middle);  This three-blade Flexofold boasts an impressively large blade area (top right); Ewol propellers are exquisitely crafted in stainless steel (bottom left);  The feathering Kiwi Prop features blades made of Zytel composite (bottom middle);  This three-bladed feathering model from J-Prop includes easy pitch adjustment (bottom right). 

 The composite hub on this Flexofold prop serves to stave off galvanic corrosion (top left);  Note the unique shape of the feathering blades on this Bruntons Autoprop (top middle);  This three-blade Flexofold boasts an impressively large blade area (top right); Ewol propellers are exquisitely crafted in stainless steel (bottom left);  The feathering Kiwi Prop features blades made of Zytel composite (bottom middle);  This three-bladed feathering model from J-Prop includes easy pitch adjustment (bottom right). 

Folding Propellers

With folding propellers, the two, three or even four blades are hinged so that the force of the water folds them shut when you’re sailing. Then, as soon as your engine is put in gear, either in forward or reverse, centrifugal force and the hydrodynamic shape of the blades causes them to flare out. Although some early models had blades that hinged independently, today’s folding propellers are geared so that they open together and are always similarly oriented—vital to minimizing vibration.

Generally speaking, with any propeller, not just folding models, the more blades, the more power, especially when it comes to punching through rough seas or into headwinds—figure roughly 30 percent per blade. Any propeller, whether it be fixed, folding or feathering, with three or more blades will also be less prone to vibration than a two-blade, especially with a conventional shaft. That said, the truly critical factor in terms of power is diameter. However, diameter is invariably limited on any sailboat because of the size of the propeller aperture and/or the distance between the hull and the shaft: thus the need for more blades.

Other factors to keep in mind when looking at folding propellers include the following:

• With folding props, the blades are carefully curved or have a consistent helical shape, similar to the blades of a fixed propeller, thereby providing greater efficiency and more thrust. In fact, some folding propellers have even been found to provide roughly equivalent or even better forward thrust than a fixed prop.

• Because the blades all need to fold in together, they may be limited in terms of area, which in turn, can limit propeller power, although this can be offset by sophisticated blade design; some of today’s folding props also have surprisingly large blade areas.

• With some models, there can be a slight hesitation in thrust at low rpms as the centrifugal force and blade shape open the blades when shifting into reverse, although accommodating this quickly becomes second nature in practice.

• Because of their length when the blades are folded, some folders won’t fit in a smaller propeller aperture; on the plus side, when folded they are far less likely to catch ropes, seaweed and other debris.

Not surprisingly, there are a variety of different shapes, styles and types on the market. Denmark’s Flexofold, for example, in addition to its sleekly modeled standard folding propellers, offers a two-bladed prop for use with saildrives that includes a composite hub to stave off corrosion. Its propellers also boast impressively large blade areas. Similarly, Bruntons’ Varifold line has been optimized with large, carefully sculpted blades designed to minimize vibration and maximize power, while its two-blade props fold together almost seamlessly when not in use. Yet another option is Autostream’s Slipstream saildrive model, which is fabricated in stainless steel.

Denmark’s Gori, on the other hand, has taken a completely different tack by incorporating blades that don’t just open and close like a clamshell, but rotate through a full 180 degrees. The result is that the blades are oriented in the same direction in both forward and reverse in the interest of maximizing efficiency. The blades are also constructed to work themselves out to diameter at low RPMs, making the switch between forward and reverse exceptionally smooth. Finally, Gori propellers include an “overdrive” feature, whereby you engage the engine in forward at the same time the blades are in the reverse position. This results in the blades spinning “backward,” so to speak, with the regular trailing edge now in the lead giving you 20 percent more pitch, thereby creating additional fuel efficiencies over the long haul.

Feathering Propellers

With respect to feathering propellers, the first obvious difference compared to a folding wheel is that the blades are always extended. However, when you are sailing they are kept on centerline, where their fairly flat shape creates a narrow profile and, by extension, minimal resistance. When you put your transmission in gear, another set of gears inside the hub rotates the blades to a predetermined pitch, allowing them to provide thrust in either forward or reverse. Other salient points include the fact that:

• There typically is much less helical shape, or twist, to a feathering prop’s blade design, which means less efficiency; however, the larger area of the blades helps compensate for this.

• Because the blades rotate 180 degrees when switching from forward to reverse, thereby employing the same leading edge, they offer equally solid performance in both directions.

• Because the overall space requirements are on par with a fixed propeller, feathering propellers are ideal for boats with limited aperture space; however, they will also catch lines, abandoned fishing nets, etc., far more readily than a folding model.

• Feathering propellers are available with up to five blades; that said, most sailors will go with either a two-blade model if theirs is a less powerful engine (less than, say 25hp, in which case two blades are sufficient to translate engine power into thrust) or if their priority is sailing performance (especially racers); or three blades, if they want to get as much as they reasonably can out of a more powerful engine, especially in rougher conditions.

• Feathering propellers are more expensive than their folding counterparts: figure roughly 25 percent more (although prices vary dramatically based diameter, the number of blades and factors such as construction materials).

In terms of the different types of feathering props currently on the market, PYI Inc. touts ease of installation and the way the blade pitch on its “Easy” and “Whisper” Max-Props—a critical factor in performance—can be easily and reliably adjusted by swapping out a simple set of screws. (The job can even be done with the boat still in the water.) Beta Marine’s J Prop line also offers easy pitch adjustment.

Italy’s EWOL, on the other hand, takes pride in the fact that, in addition to top-flight design, its product line is built in stainless steel, as opposed to a bronze alloy, in the interest of resistance to galvanic corrosion and overall durability. Autostream’s “Saildrive” models are also fabricated out of stainless steel, though it builds in bronze as well, while the Kiwi Prop features lightweight composite Zytel blades on a stainless steel hub—making it a true outlier with respect to materials.

In terms of efficiency, Germany’s SPW likes to highlight the carefully engineered blade profile and resulting efficiency of its Variprofile wheel. Its Variprop, on the other hand, boasts an exceptionally short hub, allowing it to fit in the smallest prop apertures.

Finally, there is Bruntons’ feathering Autoprop. In addition to carrying a good deal of curvature, especially for a feathering wheel, the uniquely shaped blades are offset from their pivot points as part of variable pitch function, that causes the propeller to vary its blade angle in response to high winds, choppy seas and other load variables—all with an eye toward maximizing fuel and overall efficiency. This variable-pitch feature also serves to optimize performance when motorsailing. 

Low-drag Props in use

In a survey sent out by the World Cruising Club (worldcruising.com), we asked past rally participants how their low-profile feathering and folding propellers performed, and the response among the 271 who took part was almost universally favorable. Irrespective of type or model, respondents said they experienced very real benefits, both in terms of passage times and boatspeeds—just as advertised.

“It was on the boat when I purchased it, but I wouldn’t change the Autoprop for any other type of propeller,” said Oyster 45 owner Steve Jackson. “It’s expensive, but there is a reason for that. It is exceptionally well made. A proper quality piece of engineering.”

“Very good for both inshore and coastal,” agreed Alerion Express 33 owner Jeff McKinney, adding he’d also had a Gori on his previous boat and liked the performance in that instance as well.

As part of the survey, we also attempted to roughly quantify how participants felt about propellers by asking how significant they found the change in sailing performance. Among those who responded to this part of the questionnaire, two claimed less boatspeed, 17 claimed “no change” in their sailing speed, and the remaining 252 were about equally split between “slightly more” and “significantly more” speed. Considering the variety among these sailors—not to mention that this informal survey took place mainly among cruisers who don’t spend as much time tweaking their rigs—results like these speak volumes.

“The idea of dragging a fixed three-blade prop through the water is just not an option,” said David Burnett, whose Beneteau Oceanis 50 carries a Flexofold, summing up the general consensus in terms of the benefits when sailing.

As for respondents’ views on how their foldering or feather props performed when their engines were engaged, the results were also impressive. With respect to power in forward, for example, only four respondents reported “significantly less” boatspeed, while seven respondents reported “slightly less.” Compare this to 64 reporting “slightly more” boatspeed; 29 reporting “significantly more boatspeed” and 75 reporting “no change.” (Not all respondents answered all questions.)

As for power in reverse, three respondents reported “significantly less” power; 32 reported “slightly less” power; 36 reported “slightly more” power; 56 “significantly more” power; and 54 reported “no change.”

Similarly, with regard to maneuverability, three respondents reported “significantly less;” 15 reported “slightly less;” 51 reported “slightly more;” 51 reported “significantly more;” and 74 reported “no change.”

Beyond that, the overall nature of the comments was also positive: some reported slightly more prop walk; some reported slightly less. None expressed any great disappointment with their feathering or folding propellers, and for many, this was not their first time with these types of props—a good sign.

One thing a number of sailors did note was that feathering and folding propellers do require more attention. Feathering props, in particular, need occasional greasing, and the mechanisms of both types must be kept free of debris or marine growth if they are to function properly. One folding-propeller owner, for example, related how he once had to clear some weeds out of his prop. Another described how the blades on his feathering prop picked up some junk transiting a “not so clean river.” In both cases, cleaning out the mechanisms immediately resolved whatever problems they were experiencing.

Finally, though our sample size was an admittedly limited one, there seemed to be no marked difference in terms of satisfaction with the two basic types: feathering and folding. All the major manufacturers were well represented and no readily discernable trends seemed apparent.

RESOURCES

Autostream (feathering) seahawk.com.au

Bruntons Propellers (folding and feathering) bruntonspropellers.com

Ewol (feathering) ewoltech.it

Flexofold (folding) flexofold.com

Gori Propeller (folding) gori-propeller.com

J Prop (feathering) betamarinenc.com/j-prop

Kiwi Prop (feathering) kiwiprop.us

PYI Inc. (feathering) pyiinc.com

Variprop USA (folding and feathering) varipropusa.com

May 2018

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