Getting the most from your yacht propellers
Last updated: 06/06/2016
Hidden under the waterline, out of sight and out of mind, many take the performance and efficiency of their yacht propellers (or ‘props’) for granted.
Yacht propellers can take a beating, even in the best sailing conditions. Mud, floating debris and rocks can bend, chip and crack blades and even minor nicks or scratches can rapidly have a negative affect on the yacht’s performance, which is why good design, regular maintenance and ongoing repair are paramount.
Yachting Pages looks into the design and upkeep of superyacht propellers, giving tips for maintaining props, and pointers on when to replace.
How a yacht propeller works: The science
Carolina Mendoza Pons, managing director of Barcelona Propellers said, “A propeller is possibly one of the most important parts of a yacht, because it ultimately determines the speed that will get and the efficiency it will obtain, so the propeller design must be taken very seriously.
She explained that, as a yacht propeller rotates, the blades push water down and backwards - much like the action performed by your hand when swimming. Water rushes in behind the blade to fill the space that has been left and results in a pressure difference between the two sides of the blade: A positive pressure, or pushing effect, on the underside and a negative pressure, or pulling effect, on the top side.
As Newton said, 'For every action, there is an equal and opposite reaction'. That is, when the propeller pushes the water backward, it propels the yacht forward.
"No two propellers are the same and propulsion calculations must be carefully considered to obtain the best efficiency from the characteristics of each case and yacht; engine power, gearbox ratio, the type of yacht, waterline length, stern post dimensions and cruising rate you wish to achieve etc. must all be taken into account. This is a business for specialists that are able to study, design and manufacture custom propellers for every customer."
What are marine propellers made of?
When commissioning new propellers for a superyacht, it is important to work with a propulsion professional who understands the design, materials and calculations required to get you moving.
There is an ongoing debate among key players within the sector about which is the optimum material from which to manufacture marine propellers. Sergio Moroni of EWOL Propellers, sailing yacht propulsion specialists, described how, historically, bronze alloy is used to manufacture marine propellers. He explained that, in 80-90% of cases, this traditional technology is still applied within the industry today.
Bronze alloy propellers
By its nature, bronze alloy is tough, yet it is not very resistant to corrosion, plus it’s extremely heavy when compared to the more modern-day counterparts of stainless steel, aluminium and titanium.
Sergio explained, “Bronze alloy is very brittle, and so, in the case of hitting a rock or otherwise, there is the strong possibility that some parts of a bronze alloy propeller could detach with consequential strong vibrations and the need for a considerable welding repair date, or even complete replacement of the propeller(s).”
Stainless steel vs. aluminium propellers
When considering other construction materials, there are many arguments for selecting stainless steel or alumininium over bronze alloy.
The main advantage of choosing a stainless steel propeller over aluminum is durability; stainless propellers can withstand more of the damage caused by small rocks, sand and other loose objects in the water. However, there is ‘minimal give’ to the blades, meaning if you hit an object hard enough, there is an increased possibility of causing major damage to the lower unit.
On the other hand, if damage is done to an aluminium propeller, the blades will likely sacrifice themselves before causing any damage to the lower unit.
Sergio explained that, at EWOL Propellers, the team prefers using stainless steel for construction, especially Super-Duplex stainless steel, which has double the resistance of bronze alloy propellers and is much stronger than the standard industry-used AISI 316L stainless steel.
He said, “Super-Duplex parts are normally used in the fabrication of deeply submerged parts of oil platforms, where there is need for an extreme reliability of the part and the cost of replacing the piece would be way higher than using a more expensive material.”
Titanium provides a further advanced solution for propeller blade construction. Sergio said, “Titanium is a fantastic material, extremely light and resistant, it also has a good flexibility which can be useful to absorb some (not big) hits from external objects, coming back to the normal shape without deformation of the blade. Titanium alloy is also self-protecting from barnacles and other incrustations.”
Because of these desirable features, titanium alloys are often traditionally used in the building of military submarine hulls despite the high cost of manufacture, as their lightweight and functional application allows military submarines to increase their depth of operation from around 350m to over 1,000m while reducing energy consumption.
It's all relative
Regardless of which type of alloy is used in the construction of marine props, the actual material of the construction only accounts for approximately 10% of the performance of the propeller. The other 90% of performance is in the blade design.
Simply put, a well-designed aluminum propeller will outperform an average stainless steel propeller, and vice versa.
Maintaining, repairing and replacing superyacht propellers
Once installed, regular inspection and on-going maintenance should be carried out on the yacht’s propellers and adjoining parts to maintain the performance and efficiency of the yacht when underway.
Having a propeller that isn’t in tip-top condition can cause several problems to a yacht’s performance and efficiency, with even a small ding out of one of the blades affecting the performance by up to 10%.
A damaged propeller will not perform well as it will have a negative effect on the balance of the propeller. An out-of-balance prop can cause severe vibration, premature wear on strut bearings, v-drive or transmission bearings and shaft alignment issues. Continuing to use a damaged propeller can result in damage to the gear case and ultimately the power head of your motor.
If increased fuel consumption and/or decreased performance is noticed, it’s impetrative to arrange a propeller survey or inspection quickly to avoid further degradation. Routine yacht propeller maintenance should include the:
- Regular inspection of the boat’s propeller for bent blades, nicks and rolled tips
- Replacement of worn propeller blades to prevent the engine from accelerating beyond the normal range
- The cleaning of hull and boat props to keep them free of marine growth, such as moss, grime, lime, aquatic weeds and barnacles - buildup on a yacht can decrease the performance of an engine by up to 50%
- The greasing of the propeller shafts with a marine-quality waterproof product, both before fitting the propeller and as part of its routine maintenance procedure. Aluminum propellers will need grease on their torsion rods as well
- Most importantly, the scheduling of professional servicing and repairs for propellers and propulsion systems if and when damaged
With proper care and maintenance, you can get the most from your engine, and save money in the process.
When to replace a superyacht propeller
In most cases, a professional can repair propeller damage caused by everyday wear-and-tear, without the need for replacement. However, in cases of severe damage, or where more than one third of the blade is missing, prop replacement will be the only option.
For help with your superyacht propellers, search for propeller and propulsion specialists on Yachtingpages.com