Props: Folding Propellers vs Feathering Propellers - Sail Magazine

Props: Folding Propellers vs Feathering Propellers

Would you buy an automobile whose rear brakes locked up and dragged along the road when going downhill? Not likely. So why do so many sailboat owners do much the same thing by dragging the blades of a propeller through the water behind them when the engine’s off?
Author:
Publish date:
HR1-1310-BWprops

Would you buy an automobile whose rear brakes locked up and dragged along the road when going downhill? Not likely. So why do so many sailboat owners do much the same thing by dragging the blades of a propeller through the water behind them when the engine’s off?

A fixed propeller can create a considerable amount of drag when sailing. A three-blade fixed prop could reduce your boat’s speed by as much as 20 percent. For a medium-sized cruising boat, that might be a loss of between 0.6 and 1.2 knots under sail. Put another way, it could add an hour to a 20-mile daysail. 

There’s also the interminable debate as to whether you should let the prop rotate while sailing, with all the associated noise and stern gland and transmission wear that incurs; or whether you should stop it from spinning by putting it in gear, which might harm the bearings in some types of transmission. Tests have proven that a spinning prop usually creates less drag than a static one, but if you’re planning any long passages it still might be advisable to fit a shaft brake.

Faced with these kinds of choices, why not cut right through the Gordian knot they present and install a propeller that either folds or feathers?

Folding propellers

This tight aperture likely does not have enough room for a folding prop to collapse completely

This tight aperture likely does not have enough room for a folding prop to collapse completely

Test after test has shown that folding props create the least amount of drag when sailing, especially the two-bladed versions. Some early two-blade designs also provided the least amount of thrust—especially when going astern. However, clever blade design has improved thrust considerably; for instance, Flexofold now has a two-blade wheel that performs nearly as well as many three-bladers.

On a few simple folding designs, such as the standard Martec prop that has been around for nearly 50 years, the two blades open independently of each other. However, in my experience this can lead to imbalances and vibrations after just a few weeks of fouling, with one blade opening/closing more than the other.

Martec also offers what it calls its Slipstream model, which has a pair of interlinked blades. As with Flexofold, Gori, Volvo and most other folding props, the blades are interconnected via a toothed gear at the pivot of the blades, which synchronizes their movements and eliminates the risk of one blade being open and the other closed—unless, of course, an evil little barnacle sets up house between the teeth.

Gori’s three- and four-bladed folding props have an interesting “overdrive” feature. The blades reverse automatically when going astern, and if you put the engine into forward gear they will remain in this position to provide a coarser pitch for economic motoring. This feature is useful when motorsailing and helps save fuel when cruising under power in calm conditions.

There are a number of other folding props on the market, including some by the engine manufacturer Volvo. If you have one of its engines, the advantages of choosing the latter are that the company will have already worked out the optimum pitch and diameter to perfectly match your boat.

Depending on the blade shapes and pitch, folding props typically provide better thrust moving forward than feathering props. If they have a weak point it is going astern, where they depend on centrifugal force to hold open the blades against the thrust trying to close them. Once open, in most cases, the shape of the blades is not as efficient as it is going forward.

Feathering props

The self-pitching Autoprop offers a different approach to feathering

The self-pitching Autoprop offers a different approach to feathering

Feathering propellers typically produce nearly as much forward thrust as folding propellers and more forward thrust than those that are fixed. They are also generally more efficient than folding propellers when going astern.

On the downside, feathering props are nearly always the most expensive option, with a two-bladed feathering propeller sometimes costing more than twice as much as a two-bladed folding wheel. Their feathering mechanisms are complex, so they typically also require more regular maintenance. Another disadvantage is that feathering props pick up lines more easily, which is worth considering if you cruise in pot-strewn areas like New England.

Two of the most well-known brands, Max-Prop and Autoprop, also illustrate the two different approaches to feathering-propeller technology. On the Max-Prop the blades flip through 180 degrees when going astern so that pitch and thrust are the same in either direction—in fact, in reverse a Max-Prop provides better performance than a fixed-bladed prop. This could be important if you dock in a congested marina with little turning space.

The Autoprop, on the other hand, is a variable-pitch prop that automatically adjusts itself to the optimum setting depending on engine rpm. At lower revs, the blades, which are shaped rather than flat, as in most feathering props, are pitched more coarsely to provide extra thrust; this can be a great help when motor-sailing and provides better fuel economy at the same time. Because the Autoprop’s blade shapes are identical in forward and reverse, no thrust is lost when backing up, making it another excellent choice for those looking for good performance in reverse.

On virtually all feathering propellers it is possible to adjust the pitch, though this must usually be done with the prop out of the water.

Possible problems

One problem with all low-drag propellers is that they have moving parts and are therefore more prone to suffer wear and corrosion. This, in turn, can impair performance, and instances of blades coming adrift are not unknown. Improvements have been made over the years by using different metals such as Monel (nickel/copper alloy) for fixings and pivot pins, but wear and blade imbalance can still occur.

It’s worth bearing in mind that the center of gravity of many of these props is often further aft than with a fixed prop, which can create additional stress on the shaft and bearings—particularly on boats with long shafts and P-brackets. Shaft whip, where a minor imbalance in the prop and/or an undersized shaft causes the prop shaft to flex and vibrate badly, is a common problem and can result in excessive wear to the stern gland and cutless bearing.

A final factor to be aware of when choosing a low-drag prop is the proximity of the propeller to the rudder. The blades of folding props extend farther aft when collapsed in the sailing position and could foul your rudder when closed—especially on shaft-driven boats. 

Nonetheless, if you spend more time sailing than motoring, or you enjoy the odd race, go ahead and fit a folding prop. If control at close-quarters under power is more important than speed under sail, then go for a feathering prop. Either way, you’re better off than with your standard fixed-bladed propeller, which has pretty much the same effect on your boat speed as a bucket hung over the stern!

Prop Talk

When discussing propellers, the significant terms are pitch, diameter, rotation, and the number of blades they have.

The pitch is the measure of the theoretical distance a propeller would advance in one complete revolution—if the pitch is 16 inches, that is how far the prop will advance in each turn—in theory. (In fact, slippage will decrease the actual advance.) Pitch influences how fast a propeller will push your boat through the water for a given engine speed. The greater the angle at which the blades and any twist are set to the propeller hub, the greater the pitch. If the pitch is too great, the prop will demand more power than the engine can supply; if it is too small, then the engine will have more power than the prop can use.

Diameter is the maximum radius of one blade times two, or the diameter of the circle made by the tips of the rotating blades. Broadly speaking, the more powerful the engine, the greater the diameter of the propeller.

Rotation or “hand” is the direction in which the blades spin. Looking forward along the boat from astern, a right-handed propeller will rotate clockwise when going ahead; a left-handed propeller will rotate counter-clockwise. Catamarans typically have a left-handed propeller on the port engine and a right-handed one on the starboard engine.

Blade numbers vary from two to as many as five. For a given diameter, a three-bladed propeller will accept more power than a two-blader, and so on. Two-bladed folding and feathering propellers can be significantly cheaper than their three-bladed equivalents, but typically will not provide thrust as efficiently. 

Checklist

FOLDING PROPELLERS

Pros:

Cons:

Lower purchase price

Weaker reverse thrust

Lowest drag under sail

Can wear faster

Simpler design

Fixed pitch

Lower maintenance

Exposed gears

Excellent forward thrust

Prop walk (varies by brand)

FEATHERING PROPELLERS

Pros:

Cons:

Superior maneuverability under power

Expensive

Better reverse thrust

Higher drag than folders

Less drag than fixed prop

Need servicing

Reduced prop walk

Adjustable pitch

Read Peter Nielsen's related article "Prop Comparison: Martec Geartec vs Kiwiprop"

Related

daviscards

Davis Instruments: Quick Reference Cards

CHECK THESEIf you’re sailing with new crew this summer or your kids have suddenly and inexplicably started to look up from their phones and take an interest in the finer points of cruising, these Quick Reference Cards from Davis are a great way to further their boating education. ...read more

01-rbir18-596

Another Epic Round Britain Race

There are basically two kinds of offshore sailboat races out there: those that take place annually, like the Fastnet and Chicago-to-Mackinac races; and those that take place every other year, like the Transpac and Newport-Bermuda race, in part so the competitors have sufficient ...read more

01b_WALKING-KEDGE-OUT-cmykpromo

Getting More Use From Kedge Anchors

If you are cruising, you need at least two anchors on board for the simple reason that you must have a backup. Imagine having to slip your anchor on a stormy night with other boats dragging down on yours, or having your rope rode severed by some unseen underwater obstacle, ...read more

SailAwayCharter

How-to: Navigating on a Bareboat Charter

So you graduated from navigation class where you practiced dead reckoning, doubling the angle on the bow and maybe even celestial nav, and you now feel well prepared for your first charter trip. Well, you won’t be doing any of that on vacation—not past the first day, anyway.Most ...read more

04-Turtle-rescue

Turtle Rescue in the Vic-Maui

Strange and often wonderful things can happen in the course of an offshore sailboat race, and one of the strangest and most wonderful things we’ve heard of recently took place during the 2,300-mile 2018 Vic-Maui race, from Victoria, British Columbia, to Lahaina, Hawaii.It ...read more

dorcap-open-blue

ATN Inc: Dorcap

COOL SLEEPYou’re fast asleep in a snug anchorage, forehatch open to catch the breeze, when you’re rudely awakened by a sneaky rain squall. Now you’re not only awake and wet, you’re sweltering with the hatch closed. Sucks, right? That’s why ATN came up with the Dorcap, an ...read more

HIGH-RES-29312-Tahiti-GSP

Ask Sail: Who has the right-of-way

WHO HAS RIGHT-OF-WAY?Q: I sail in Narragansett Bay, which is a relatively narrow body of water that has upwind boats generally going south and downwind boats generally going north. When sailboats are racing, the starboard tack boat has the right-of-way over the port tack boat, so ...read more