Features February 2018 Issue

Folding vs. Feathering Props

With a new engine, Mahina Tiare gets a new streamlined screw.

 

For the past 40 years we’ve sailed an average of 10,000 miles annually between Australia, Alaska, Antarctica and Spitsbergen motoring or motorsailing between 400 and 600 hours, depending on the area – more hours in high latitudes of Antarctica and the Arctic, fewer in the tropical trade winds.

feathering mechanism


Amanda Neal checks the feathering mechanism on the Max-Prop (left). The Max-Prop had excellent stopping power, but needed rebuilding three times in 18 years. The Flexofold (right) was sized to match the new smaller 75-horsepower engine.

Our HR 42 came with a fixed three-blade prop which served us well when pushing bergy bits out of the way in Patagonia, Antarctica and Alaska, but obviously slowed us down in light airs. When we had our current Hallberg-Rassy 46 built in 1996, anticipating better sailing performance, we opted to install a Max-Prop as that was acknowledged as the best option at that time.

Max-Prop has been around since the early 70s and is a proven design, but after 18 years and 190,000 miles of use, we had the prop rebuilt three times, each time costing between $800 and $1,200. The first time the prop was rebuilt by Henley’s, an Auckland prop shop, and the other two times by PYI, the U.S. importer/distributor located near Seattle. I was told that the rebuild included brazing more bronze on to the prop’s inside working surfaces followed by machining the built-up surfaces down to original tolerances. When I enquired what I could do to extend the period of time between rebuilds, PYI said, “More grease!” So for 15 years, we not only greased the prop before launching at the start of our season, but also mid-season going underwater with mask, fins and grease gun to do so.

Flexofold propeller
Photos courtesy of Mahina.com

The low-drag Flexofold propeller folds making it more streamlined and less prone to snagging traplines and jetsam.

The advantages of the Max-Prop are very little drag when sailing as the prop instantly feathers unless the engine is engaged, no freewheeling in neutral, adjustable pitch, and incredible stopping power in reverse. Max-Prop can literally bring a boat from 6 knots to a dead stop in a boat length. This feature has kept us from nailing coral heads at least twice. The ability to adjust the pitch to maximum engine efficiency improves fuel economy and the newer Easy model Max-Props can be adjusted in the water without having to disassemble and reassemble the propeller, which is a substantial improvement over the earlier Classic version. We did encounter (and frequently use to our advantage) fairly substantial propwalk.

The disadvantages are that when the Max-Props feathers, the blades are still open and prone to fouling lines and kelp. During our 2008 Atlantic crossing we had to get in the water with mask and fins three times to remove cargo nets and large 3-inch floating lines from the prop. The Atlantic Ocean has a garbage gyre similar to, but smaller than the Pacific Garbage Patch, and we must have found the edge of it during our 2008 crossing. Last month, we completed the identical route from the Canary Islands to Antigua with only one sighting of trash and favorable winds that allowed us to basically sail the rhumbline. As a funny side note we were once making landfall at Fastnet Rock when a smaller boat passed us under sail. Trim as we might, we couldn’t catch them and only when they were several boat lengths ahead did Amanda think to check astern only to discover a fat turquoise rope trailing behind. The rope, obviously was not ours, was caught on the prop and we wondered how long it had been there as we’d not used the motor for several days.

Flexofold

During a 2015-2016 winter refit in Sweden, after reading the results of two European propeller efficiency tests and after speaking with several owners who had switched from Max-Prop to Flexofold, I had a Flexofold prop installed. Jack Skrydstrup, Flexofold’s Danish manufacturer recommended the size and pitch, and the boatyard installed it without problems.

In comparison the stopping power is not nearly as good as with the Max-Prop, and the prop freewheels when the engine is shut down in neutral. We’ve found that shutting down in slow forward solves the freewheeling issue. Strong on the Flexofold’s plus side are blades that feather aft, instead of extended, so they’re less likely to snag lines or kelp. We notice almost no propwalk in reverse. In addition, cost of ownership is reduced as there doesn’t appear to be a need to rebuild and there are considerably fewer spare parts to carry.

However, the improvements in efficiency are difficult to quantify as we also repowered at the same time, switching from a 95 hp. to 75 hp. engine, but our top speed rose 0.7 knots, and fuel consumption on an identical Atlantic crossing dropped from 1.33 gph to .83 gph.

On the service side, PYI, the U.S. importer/distributor of Max-Prop is renowned for their customer service, doing a superb job of shipping spare parts immediately and quickly handling rebuilds from their shop near Seattle. Flexofold’s Jack Skrydstrup is helpful, and ships quickly, although shipping is factory-direct from Denmark. I’ve only heard one incident of an owner needing parts during European summer vacation period when shipping was delayed slightly.

Amanda and John Neal spend seven months at sea sailing 10,000 miles a year while leading sailing training expeditions. The have more than 500,000 sea miles combined experience.

Comments (20)

Great useful article and insights. I am interested in expanding this information to include,
[APP] Adjustable pitch propellers, and [CPP] Controllable pitch propellers. Especially CPP systems as there is almost never any coverage or information on them here in America. There seems to be a lot more usage of CPP systems in northern Europe and Scandinavian counties in recreational and light commercial vessels. Cost I'm sure is part of the reason for the lack coverage but there still seems to be many benefits for sailboats and power vessels. Matching engine loads/RPM, sea state, wind conditions, lack of transmission to name a few. Downsides, cost, mechanical complexity, suppliers/support.
It may well be that Practical Sailor has done articles on CPP systems and I'm not able to find them. Any help
Thanks

Posted by: Adroid | January 28, 2018 6:36 PM    Report this comment

Explanation of prop walk:
Prop walk is due to the difference in side thrust (a tangential force) between the top of the blade and the bottom of the blade. The magnitude depends on at least the depth of the prop, the magnitude of the forward (or aft) drive force, the RPM, and the blade shape. This depth effect is caused by the static water pressure at the top of the prop being less than the pressure at the bottom and by cavitation or aeration that occurs when a very shallow prop rotates. This also means that the bottom of the prop also produces more thrust than the top of that prop.

Comparing the performance difference in different props with different diameters, pitches, or engines is very nebulous. Choosing a prop diameter and pitch is very complicated and there are analytic methods. For prop selection on big ships Computational Fluid Dynamics may even be used. For most small craft some basic arithmetic provides basic sizing information, and that is tempered with practical experience. The issue of prop walk with different props is usually a secondary consideration and you just get what you get.

Posted by: Captain Ed-79 | January 28, 2018 2:23 PM    Report this comment

We sailed with a Max-Prop for a number of years. The Max-Prop caught lobster pot lines daily and frequently more often when cruising Maine. One day I dove three times to cut the like off my prop and we are very careful to watch and steer around pots as best as we can. We finally switched to Flexofold and never in many more years have we caught a line. We are still very careful to steer around pots or immediately put it in neutral if we think we have run over a pot. I would not recommend cruising Maine with a feathering prop. I highly recommend a folding prop for line infested waters like the coast of Maine.

Posted by: rickrick | January 22, 2018 10:51 AM    Report this comment

After reading many comparisons including an excellent one done by Yachting World (I think) I opted as well for Flexofold. I changed out a fixed 3 blade bronze 21 X 14 that came with my Jeanneau SO 52.2 and I was amazed at immediate increase in speed with the Flexofold. About 1 knot across the RPM range. I now cruise at 8.5 knots at 2700 rpm. The prop seems to make better use of the extra kick of the turbo on my 100hp Yanmar. What I was also looking for was a reduction in propwalk and the Flexofold did not disappoint here either. I can reverse with much more confidence now. Also dealing directly with the factory was efficient and painless for technical advice on dimensions etc and shipping the prop via courier directly to me here in Trinidad.

Posted by: Stella Maris | January 22, 2018 8:34 AM    Report this comment

While your comments re: Maxprop and Flexifold are good, at least considering your personal experience with them, I think you failed to consider a couple of other points of view. And when you publish an article, it needs to address more than just your personal opinion. Your HR, like our Taswell, has a skeg-hung rudder. That, more than the shape of your feathered prop will be the governing factor in catching floating lines etc. But the most important fact is you failed to mention/consider the Autoprop! Don't know why you are so biased against it, as no mention was made. But after having one mounted on our T-43 now for some 17 years, and sailing in and around the Philippines, then to Thailand,, into the Medd for 3+years, and now on our 4th season in the Caribb., I can NOT fault it. Better performance and way less mx of a Maxprop (compared to cruising friends in a HR42 and an Oyster, amoungst others), improved fuel consumption, and significantly reduced-but not eliminated-propwalk.......it absolutely deserves consideration. And NO...I do NOT work for Bruntons!!!

Posted by: sailcrazy2 | January 21, 2018 7:00 PM    Report this comment

After extensive web surfing and research I went with a Gori folding prop for my Bavaria Vision 42 (Volvo 40 hp saildrive) and have been very happy with it. It has the lowest drag of any prop when folded, excellent reverse thrust, little to no prop walk, and an "overdrive" feature that can keep the reverse prop pitch while moving forward, for use when motorsailing or in flat water, resulting in increased fuel efficiency. We sail with the shift in reverse to ensure no windmilling. I found the following articles helpful in my decision-making, in addition to the Yachting Monthly article referenced by Goetzylla (all of these should be find-able with a Google search): "Folding Prop Gear Test", Australian Yachting May/June 2011; "P(r)op Art", Segeln Magazin October 2008 (english translation available); "12 Helices a la Torture" Voile Magazine April 2009 (you'll need to brush up on your French). I hope this helps!

Posted by: augustefortin | January 21, 2018 6:33 PM    Report this comment

I have a Max-Prop classic on my Tayana Vancouver 42 with a full skeg and have never experienced line fouling on the prop or rudder. I strongly considered the Flexofold but went with the Max-Prop because of reported good customer service and location. After four years, including a round trip to Hawaii, my overall experience has been improved sailing speed, a slight reduction in motoring speed, an improved reverse, and no change in fuel consumption. My installation required that the prop shaft be trimmed about 1/2 inch meaning that it could never again be used with a fixed prop; a critical concern to determine for your boat before undertaking...it is a marriage and divorce is always expensive. Consider this factor on any feathering/folding prop selection.

With regard to line fouling on the HR 46, I Googled the boat and noticed that the HR 46 has a 50% skeg rudder which still leaves it susceptible to rudder line fouling albeit better than boats without any skeg at all.

Matching a prop (any prop) to your boat's drive train can be a tiresome undertaking and having to haul the boat to make any adjustment simply adds to the expense. Consult with the prop manufacturer and other owners with your drive train specifications before ordering as a different engine or transmission ratio could mean a different prop selection or installation. After installation test the boat to insure the engine can attain maximum RPM without excess vibration or prop cavitation, both indicating some change is necessary.

~ ~ _/) ~ ~ MJH

Posted by: MJH | January 21, 2018 2:01 PM    Report this comment

I own a 1998 Beneteau Oceanis 321. The previous owner replaced the factory three-bladed fixed prop with a folding geared Austral/Seahawk 10X15R stainless steel propeller with three blades. The boat has a Yanmar 30GM30F (27HP) with a Kanzaki KM29 transmission. After 500 engine hours and 19 years use on Lake Erie and minor wear and tear, the prop was reconditioned in 2017. Martec did the work for around $450.

On Lake Erie, we have commercial fishing nets, plastic bags, and seaweed in the shallow areas. On my prior boat, a Catalina 30 with a two-bladed fixed prop, I was frequently picking something up on the prop. Zero problems with the folding prop.

I get a bit over 7.1 knots at 3400 rpm on flat water with no wind. The prop exhibits good reverse and fuel economy. I race the boat. In a reach under sail, I get 6.8 to 7.2 knots in 10 to 20 knots of wind. I am 100% satisfied with this prop.

Posted by: mark2 | January 21, 2018 1:38 PM    Report this comment

I had both a MaxProp and a Flexofold on my Pearson 365 with a new yanamar and sail drive. There was no question that the Flexofold was easier to maintain and performed beautifully for the 1400 hours I put on it living aboard and cruising from Maine to the out Islands and back.
The MaxProp was an older model that was a pain to dis-assemble and re-assemble each year. Truth be told the reason I switched props was the MaxProp spun off the shaft in reverse (yard mounted and no way to tell it was done incorrectly). Spun off in reverse after about 200 hours after it was mounted. I would go to a Flexofold without hesitation - about 1/2 the cost of the MaxProp and every bit as efficient.

Posted by: aschoenberg | January 21, 2018 1:24 PM    Report this comment

I am not as concerned about stopping power, prop walk, rather catching lines from fish and lobster traps. I am very happy with the service and quality of my 2 bladed Flex-o-fold.

Flex-o-fold had very good results in this test survey.

Google Flex-o-fold reviews by Yachting Monthly.

Posted by: George DuBose | January 21, 2018 1:20 PM    Report this comment

I am not as concerned about stopping power, prop walk, rather catching lines from fish and lobster traps. I am very happy with the service and quality of my 2 bladed Flex-o-fold.

Flex-o-fold had very good results in this test survey.

Google Flex-o-fold reviews by Yachting Monthly.

Posted by: George DuBose | January 21, 2018 1:20 PM    Report this comment

I am not as concerned about stopping power, prop walk, rather catching lines from fish and lobster traps. I am very happy with the service and quality of my 2 bladed Flex-o-fold.

Flex-o-fold had very good results in this test survey.

Google Flex-o-fold reviews by Yachting Monthly.

Posted by: George DuBose | January 21, 2018 1:20 PM    Report this comment

I am not as concerned about stopping power, prop walk, rather catching lines from fish and lobster traps. I am very happy with the service and quality of my 2 bladed Flex-o-fold.

Flex-o-fold had very good results in this test survey.

Google Flex-o-fold reviews by Yachting Monthly.

Posted by: George DuBose | January 21, 2018 1:20 PM    Report this comment

I am not as concerned about stopping power, prop walk, rather catching lines from fish and lobster traps. I am very happy with the service and quality of my 2 bladed Flex-o-fold.

Flex-o-fold had very good results in this test survey.

Google Flex-o-fold reviews by Yachting Monthly.

Posted by: George DuBose | January 21, 2018 1:20 PM    Report this comment

Our Malo 41 came equipped with a Flexofold 3-blade in 2002. We now have just over 2,000 engine hours, nearly all of it in gear. The prop has needed no maintenance yet, other than occasional hard growth removal and polishing. Reverse has been completely reliable and strong, though probably needs more revs than a comparable Maxprop. After shutdown, while underway, we engage reverse to stop freewheeling. The official Flexofold anodes came with a thread lock on the supplied bolt. After switching to less expensive 3rd party anodes which come with untreated bolts, a couple times an anode was missing at replacement time. Now we apply Vibra-tite VC-3 thread locker to the bolts before handoff to the diver. Unlike Locktite, it dries in an hour and is not washed away under water. We also make sure the diver torques the bolt enough. Since that change, no more lost anodes. We like this prop better than maxprop on previous boat.

Posted by: C Stillman | January 21, 2018 12:45 PM    Report this comment

Can someone explain the theory behind the prop walk for the two types of props?

I personally like to be able to bring the boat to a quick stop! I have used a MaxProp on two boats for better than twenty five years.

Ted Spree Outbound 46

Posted by: Outbound4r6 | January 21, 2018 12:12 PM    Report this comment

I bought myself a Sabre 30 a while back and wanted to improve sailing performance. So that meant a new genoa and a folding prop. I replaced a two bladed Volvo Penta with a used Gori folding two blade in fine condition. Given that my boat is under-powered at 14 HP, I was concerned about getting enough oomph to fight the Maine currents and stop the boat in a hurry. Not to worry, the Gori was every bit as powerful as the fixed prop, and at $300 was money well spent. The other advantage is avoidance of lobster buoys. Up here it is inevitable you will hit them no matter how close you keep watch. Feathering props sound fine but my goal is to keep it simple and keep the bank account in the black. For cheapskates like myself, trading up to a folding prop makes sense as those with folding props succumb to feathering marketing.

Posted by: phogan | January 21, 2018 12:00 PM    Report this comment

I have to suspect that the new engine, rather than the prop change, was responsible for the lower fuel consumption. It is also possible that the 95 hp engine at the end of its life was not putting out anywhere near its rated power and thus the rise in top speed might also be related to the repower. Calder's graph shows only about a 0.5 kt difference between the best and the worst.

The Max-Prop needed its rebuild somewhat over every five years or 2500 hours. Come back to us after the equivalent hours have been put on the Flexofold. Anything that moves, wears.

Have to admit that the non-snag folder has a certain advantage here in New England where lobster traps litter channels.

Posted by: Boston Barry | January 21, 2018 11:56 AM    Report this comment

Great article - I just sent an inquiry to Flexofold about a 3 blade prop for my Flicka 20'. I don't think it will fit in the available slot between the keel and the rudder, we'll see.

Do you keep the old fixed prop onboard just in case there is a problem you can't resolve while in some remote port?

Posted by: CaptainJohn49 | January 21, 2018 11:25 AM    Report this comment

Here is a more systematic test of various prop that is quite extensive:
.... just found out that links do not work in the comments..... just google "folding prop test" . It is on "yachting monthly.com "

Posted by: Goetyzlla | January 21, 2018 11:20 AM    Report this comment

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