Flexofold Propeller

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??????????????????????????? This is the standard folding propeller for the Jeanneau 57, a 3-bladed Flexofold.
(2011-07-04 10:04:08 NIKON D7000 with a "18.0-200.0 mm f/3.5-5.6" lens. [f/4.0, 1/60s] ISO 200 Focus 1.33m)
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The propeller is a deceptively small and simple looking device that plays a very important role in transferring the horsepower generated by the engine into moving the boat forwards (and backwards). Propeller design has come a long way over the years and modern blades are very different and much more efficient than those of yesteryear. Generally speaking, the larger the propeller and the more surface area it has, the better the propulsion. Unfortunately the downside of having as big a propeller as possible on a sailboat is that this size creates a lot of drag when turned off and when the sailboat is using her real means of propulsion - the sails. Thus it is necessary to find a compromise between efficiency and drag and there are several approaches.
Propellers have 2 or more blades. Other factors being equal, more blades equate to more power and smoother operation; just as with propellers on aeroplanes. Two blades create the least drag and thus are often seen on race boats, where speed under sail is more important than being able to motor into wind and seas. Older full-keel boats often have two-bladed propellers as well, since the propeller blades create little drag when they are aligned vertically as they are hidden behind the keel when in that position. Most common on sailboats these days are 3-bladed propellers and some very large yachts have 4-bladed ones. The standard 3-bladed propeller is usually manufactured from bronze or sometimes stainless steel and is sturdy device with lots of CAD/CAM engineering time spent in optimising the blade angles, curves and edges for maximum thrust and minimal drag but despite the engineers best efforts a prop will create a lot of drag regardless of whether it is freewheeling or locked in position when sailing. Many gearbox manufacturers recommend that the propeller shaft be locked in place while sailing as a freewheeling shaft (with the engine off) is often not lubricated sufficiently. Generally it is acknowledged that a fixed propeller can slow a boat down by about 1 knot due to all that drag; the effect is more noticeable at slower speeds. One knot on a boat with a maximum speed of 8 knots is quite a bit; it can make the difference between arriving in a harbour in daylight as opposed to in the dark, or making an ocean passage several days shorter.
Enter the two main variants to fixed-blade propellers - the feathering propeller and the folding propeller. Both of these propeller types can reduce drag to 10% or less of the original value and often have the advantage that the power in reverse gets much better and that the phenomenon of “prop-walk” is minimized. Propellers are optimised for forward propulsion as that is what is used most often and for the longest periods, so the blade pitch (or angle) is set at an optimal angle for that propeller diameter and size, for the gear ratio of the engine, and the weight and speed range of the boat. When going in reverse the angles are reversed, but since a blade is not a flat but a complex curved object the forward efficiencies are lost.
Feathering propellers have blades that rotate, allowing them to “feather” or lie parallel to the water flow when not in use. When the shaft is rotating they rotate to form a given angle of pitch and often they rotate 180° so that they are just as efficient going in reverse as forward. Folding propellers have fixed pitch angles, but fold backwards into a streamlined shape when at rest. Both of these systems are much more expensive than a simple fixed-blade propeller and, since they have moving parts, need regular maintenance and lubrication and are more prone to failure or problems. Feathering propellers have a bit more drag than folding ones, and also have a better chance at snagging lines and other debris than their folding brethren. While the folding propeller might seem the best choice, many boats (particularly those with skeg-hung rudders or long keels) don't have the extra space needed for a folding propeller.
At first I opted to get a folding propeller for Zanshin and was left with the choice of which brand and model to get. On Zanshin I had a Gori 3-blade propeller (see Zanshin I Gori modification) and was quite happy with it, so I opted to stick with what I know and put the same type of propeller on Zanshin, a 3-bladed 22“ x 13” model.

After making that decision and communicating with Gori we realized that the 2:1 gear ratio on the VW 140-5 diesel engine would result in a very high propeller speed for this boat weight and the resultant potential problems of cavitation. I then started thinking about my options when I revisited the pictures that I took during the Jeanneau factory tour and started wondering why they had put a temporary folding propeller on the boat - that seems like a waste of resources considering the not insignificant cost of a folding propeller. I checked with the dealer and he confirmed that the boat does indeed come standard with the Flexofold 3-bladed folding model order 22x13-3RH (22 inches diameter, 13 inches pitch, 3-blade, right-handed) one rather than the customary and much cheaper fixed-blade one! Thus a difficult decision was made easy.

Zanshin folding propellor I manually extended the blades of the prop for this picture but need to clean the prop and it is also time to replace the Zinc anode on the shaft.
(2012-02-20 15:47:00 NIKON D7000 with a "35.0 mm f/1.8" lens. [f/6.3, 1/25s] ISO 100 Focus 0.30m)
Zanshin folding propellor
Clean propeller The prop is now nice and shiny after using some elbow grease and 150 grit sandpaper.
[18°24'1.52"N 64°38'1.22"W ]
Clean propeller
Shiny propeller Shiny and smooth blades on the Zanshin's folding propeller after cleaning and sandpapering.
[18°24'1.52"N 64°38'1.22"W ]
Shiny propeller
Flexofold prop with Lanolin The rudder has been covered with a thick coating of Lanocote, a lanolin based compound.
[18°23'53.26"N 64°38'9.43"W ]
Flexofold prop with Lanolin
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