Know-How

Why Low-Drag Propellers

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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

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

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 down side, 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.

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 "A Tale of Two Props"

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