Considerations On Choosing A Sailboat Propeller
By Cliff Friesen
There are many old adages or beliefs about sailboat propellers that may have been true in their day. However, today most sailors sail differently from their predecessors. There are still the Lin & Larry Pardy types, engineless diehards that sail between anchorages, but they are now few and far between. The majority of present day sailors are a lot like most Yacht and Sailing Club members. We have lives outside of sailing. This means having to power to be somewhere at a specific time, usually happy hour. Anchoring after dark or late in the day, in the not so great spot because the best spots are all taken also adds to the need to power or power sail to reach the anchorage or dock. Ruth and I intend to sail offshore soon and expect to sail most times, however I expect that the engine will come on to escape a storm or possibly get out of a long calm situation.
So we have engines and to transfer the power to the water efficiently, we have propellers. There is the old adage “a sailor only needs a small engine to get out of the marina into the wind and sail.” Yah sure! And another is “the choice is the two-blade propeller that can be hidden behind the dead wood to reduce drag while sailing.” Another yah sure!
2 Blade Fixed Blade | 3 Blade Fixed Blade | Feathering |
Unique Feathering | 2 Blade Folding | 3 Blade Folding |
Autoprop | Controllable Pitch
Two Blade Fixed Blade Propellers
This old belief is the main reason not to go with a two-blade propeller. Two-blade propellers do not belong on a sailboat. (Very personal opinion) Disturbed water is the main reason. Every keel has disturbed water coming off of it, be it a streamlined fin keel or a full keel. Water meeting on the trailing edge takes time for the flow to smooth out. Two-bladed propellers turning, go through the disturbed water at the same time. This causes a slippage, in turn inefficiencies because it is on/off/on/off etc.
How many skippers with two blade propellers notice the stovetop, lifelines or something else humming, at different engine rpm? How many blamed the engine? This is usually caused by the harmonics set up by the two-blade propeller. Sometimes in combination with the harmonics of the engine, it can cause even more uncomfortable zones in engine rpm.
You also get turbulence created by the over sized propeller. It is oversized to compensate for its lack of efficiency, usually in pitch. The pitch directs water to the other side of the keel or propeller. This happens at the top and bottom blade when vertical behind the keel or strut. This causes turbulence and thus drag with the exception of the very small inefficient propellers.
Three Blade Fixed Propellers
A three blade propeller is the better way to go. You only have one blade moving through the disturbed water at any time. Two blades are still pushing the boat. We have more efficiency, more thrust, more control and less vibration. Depending on the propeller mind you it may cause slightly more drag. For some propellers it is the equivalent of dragging a hand through the water. Most sailboat builders use, as the standard fare, heavens forbid, a powerboat propeller and the drag can be much worse, like dragging a bucket. At best, in order to cut drag, some propeller manufacturers cut down a powerboat propeller to make it have less blade area, loosing the majority of its thrust and ability to control the boat. To compensate, they increase its size either in diameter or pitch. It is quite common to have more drag with a standard two-blade powerboat style propeller than a properly designed sailboat three-blade propeller. A properly designed sail boat propeller will give the vessel good thrust in forward and reverse from minimal horsepower engines and offer low drag when sailing. There are patented sailboat propellers with low blade area (read less drag) that have, through design, maintained and exceeded thrust figures compared to standard style propellers. An added benefit or disadvantage is that they can often reduce the prop walk in reverse. Most skippers like to be able to back in a straight line. Many skippers like the simplicity of a fixed propeller along with its lower price.
If drag were a concern to the ardent sailor, we would recommend a three-blade feathering propeller. A feathering propeller is a broad blade propeller that allows you to have the full use for maximum thrust. It is unique in that each blade rotates on an axis geared to a matching gear on the shaft. In forward the initial half turn of the shaft rotates the blades to a predetermined stop (forward pitch) and gives you full thrust in forward. In reverse, the propeller shaft turning the opposite direction turns the shaft and the blades rotate in the opposite direction to another predetermined stop (reverse pitch) and gives you full reverse thrust. The amount of rotating is limited and is in the initial turns of the propeller shaft. The shaft is turning the propeller after that and operates just like any propeller but with more thrust in each direction.
Another advantage comes when the vessel is sailing and the shaft is locked, usually by putting the transmission in reverse. The force of the water on the broad blades turns the propeller and as it turns it feathers the blades to lie directly ahead, inline with the water flow to decrease dramatically the drag from the propeller. On a coastal cruising boat this can result in a longer happy hour because they will arrive earlier. On an offshore vessel sailing twenty four hours a day it will mean faster daily averages and earlier arrival as well.
Yes, quite a bit more expensive, but competition is bringing the prices down and the advantages can outweigh the disadvantages. Some skippers are concerned about the number of moving parts to make the propeller work along with the higher price.
New products and innovation for feathering propellers in the market allow: vessels with apertures to have feathering propellers; adjustment for pitch in forward; separate pitch for reverse; on the shaft adjustments; all stainless steel construction to reduce galvanic actions and now manganese bronze models from 10″ up to 40″ diameter.
In the past boats with an aperture have been eliminated because most feathering propellers are constructed in such a manner that the axis that the blades actually spin on is extended aft by anywhere from 2 to 6″ from the shaft taper and this interferes with a rudder or hull. There is a feathering propeller available that will fit into some of these applications and the axis is right where a fixed bladed propeller’s blades would be.
The significance of having separate pitch adjustment in forward and reverse allows a skipper to have equal power in reverse as forward. Some transmissions have different ratio in forward compared to reverse. The Hurth transmission are an example, Yanmar another. This allows the engine to run up to the same or lower rpm in reverse to match the same thrust in forward for identical control, instead of having to over rev the engine in reverse to match the control or thrust in forward.
Smaller propellers have now been included by new construction that reduces the size of the hub compared to the diameter of the propeller, with propellers now down to 10″ diameter.
Some of the feathering propellers require hauling the boat and disassembly to change the pitch while newer models allow on the shaft adjustments. A skipper could dive to the prop and make an adjustment to compensate for towing another boat back to dock or to match to a dirty bottom or tired engine.
Unique Feathering Propeller
A most recent addition to the self-feathering propeller line is one with vane style feathering blades. This propeller is unique in that it has a small Bronze hub with three bronze struts protruding where the blades ride. The blades are mounted on these struts and are allowed to vane with in limits of stops that form the pitch for both forward and reverse. This propeller is very unique in that the blades are not geared together to work in unison and when sailing they are free to float in the water stream causing less drag because they vane to the water flow and not the gear position. The blades are a black composite called Zytel by Dupont, the hub is made from Bronze and Dupont Delrin that is very strong and very durable. At present this propeller is available in 15 to 18″ sizes and up to 50 hp. They have both shaft drive and saildrive models. The blades are replaceable and easily changed. It is possible to do this while the boat is in the water.
Two Blade Folding Propellers
It goes against my grain to supply a two blade folding propeller on a sailboat as stated above but we do have them available, like the two blade fixed and feathering. The blades of a folding propeller are hinged, with and without interlocking gears. In use the blades open up by centrifugal force, usually, when the shaft is rotated. The blades that are geared together open in unison, most of the time. The blades that are free and not geared can open at different times. Some skippers may know the feeling of the boat and engine when only one blade opens. The shaking and rattling scares the flaking or rolls out of your sails.
In either case, they open until they reach a stop or the centrifugal force and water flow limits them. From this point on they act like a normal propeller with the exception of reverse. Reverse thrust is reduced by the desire of the propeller to close when pulling in that direction. Skippers with folding propellers usually compensate for this problem by approaching docks very slowly. When sailing the flow of the water closes the blades and reduces drag.
The exception to this might occur in the non-geared folding propeller that may let one blade hang down while sailing. Skippers compensate for this by marking the shaft when the blade edges are vertical and weight won’t open them. They lock the shaft in that position. Geared blades don’t do this because the lower blade can’t lift the upper blade when the water is going by. Folding propellers offer less drag than fixed and offer less chance of catching lines and other debris when sailing than the feathering propeller. When powering in forward all are about the same as any two blade propeller.
Three Blade Folding Propellers
A recent development is the three bladed folding propellers and there is one with an overdrive that allows the engine to push at a reduced rpm and still maintain boat speed. These are a specialty item, even more so than the feathering propellers and usually extremist racers are the few who pay the extra costs to have one.
Autoprop Self-pitching Propellers
Another specialty propeller is one from England called the Autoprop. This propeller features self-pitching as it turns and somewhat reduced drag while sailing. It is an odd looking propeller that has blades that are L shaped with the small leg swiveling on an axis not unlike the feathering propellers blades without the gearing. The off center position of the axis and each blade’s individually freewheeling effect allow it to find its own pitch. The centrifugal force on the blades and the rush of the water into and around it allows it to set its own pitch.
It is extremely efficient and adjusts with any increase in rpm or load such as towing a powerboat, headwinds or seas. The more power you throttle up the more it works keeping the boat going until it reaches maximum hull speed. From that point on it only burns more fuel and doesn’t push any faster because it can’t.
In sailing position it isn’t as drag free as the folding or feathering propellers but it has less than most fixed blades. Some people, especially those in waters with lots of bottom life, take exception to the possibility of the little ball bearings binding up with barnacles and growth. They feel that this in turn will affect its ability to work but it is most likely the cost that keeps them from choosing one.
Controllable Pitch Propellers
Controllable pitch propellers have been in use on the west coast for some time. They are available in very small sizes to large ship sizes including some of our BC Ferry Corp vessels. As the name implies, the pitch of the propeller is adjustable or controlled from within the vessel. The controller can be electrical, mechanical, pneumatic or hydraulic and basically they all work on the same principle of having a piston, cylinder or second internal shaft that adjusts the geared propeller blades to the pitch or position required by the skipper. One benefit of having a controllable pitch propeller on a sailboat is the ability to feather it for sailing. The other main benefit for a sailor is its ability to load up a Diesel engine at its most economical fuel range. Maintaining an engine rpm and bringing up the propeller load to the engine’s maximum capability, at that rpm, will give you maximum propeller thrust. Using rpm where the specific fuel consumption is the best will give you longer range and better engine life. The Diesel is most efficient when it runs hotter and loaded. The disadvantage is that most have mechanical shifting and can take a longer time to go from forward to reverse in control situations. They are also not well represented in our market place, costly to repair and not many people understand their benefits or know how to operate them properly. All Diesel engines with controllable pitch propellers should have pyrometers on them to advise the skipper when the engine is producing efficiently. These engine exhaust temperature gauges would be compared to manufacturer’s figures to determine load at any given rpm to know when to stop the pitch increase. Either that or adjust pitch up until the Diesel smokes black and then back off a couple of rpm.