Full scale noise trials onboard a sailboat
Bruntons Propellers make two of the market leading types of sailboat propellers, the Varifold folding propeller and the Autoprop self pitching propeller. But what makes them so special? Well one of the differentiating factors about Bruntons Propellers, and not just their sailboat propeller lines, is the juggernaut of a company behind them. Machining propellers to order based on standard designs is the fare of most of the sailboat market manufacturers, but as for true engineering, research and development… well there the field thins. Bruntons Propellers have been making propellers for commercial craft for almost 100 years and this pedigree gives them a unique insight into propeller design, propeller manufacture and any in service issues that may arise. They actively research, develop and expand their design portfolio with new innovations and products and draw from knowledge gained in the mega-yacht sector and apply it to other sectors, such as the sailboat market. This in particular has allowed them to introduce industry-leading innovations such as helical pitch distributions from the mega yacht industry and apply them to Varifold folding propellers to improve efficiency and noise performance.
One area of research actively pursued by Bruntons Propellers is the reduction of onboard noise and hence the increase in comfort from better propeller design. It is important to note that the propeller is the main source of underwater radiated noise for most vessels, typically from cavitation related phenomena. All propellers will cavitate at some point in their working life and it is the reduction of this cavitation signature, while keeping power levels high, that is the goal of all Bruntons sailboat propeller designs.
This then leads on to the suitability of a particular design for a vessel. Good hard technical advice is being supplanted these days by “Google University” with armchair experts telling the masses what they should be doing without any formal training and qualifications. Experience is great but there is nothing like a little science to prove a theory.
So picking a suitable propeller for a sailboat, now there is a fun topic! It is important to know that all propellers by and large will propel a vessel, but not all propellers are created equal! The ideal condition for any propeller to operate is to spin a large propeller as slow as possible to give the highest efficiency (known as the axial momentum theory), but with engineering limitations such as hull clearances and speed required this is the aim and not the reality. If you think of the beautiful swirling water in your wake as wasted energy you get the point, spinning a larger slower propeller produces less swirl, less swirl means more efficient. Indeed the ideal condition would be some magic device that can accelerate the water under your boat without rotation i.e. axially (hence the name) and push the vessel. For those old enough the movie “Hunt for Red October” springs to mind.
Anyway back to the propellers, the above as I have said is not always practical and indeed powerboats have small propellers highly loaded that operate in a safe (for the most part) cavitation regime. But then we move to sailboats where their primary propulsion is sails, the propeller is really secondary, and only there (for most people) to get in and out of harbor or help in adverse weather condition. So this auxiliary propulsion system does not need the speed or loading of other vessels. If you can meet hull speed of the vessel (a speed where the wave length generated by the boat is the same as the boat length) then this is great. Beyond hull speed you get into semi-displacement regimes and this is not really where a sailboat wants to be! Against this backdrop of ‘power’ and ‘propellers’, it is expected that sailboats are supposed to be quiet, whisper like even, and this is often forgotten in an effort to save half a knot under sail by adopting less than ideal propulsion methods because a dealer recommended them.
The 3 types of Sailboat propeller
So let’s have a look at your options and give a study we were involved in comparing 2 types of propellers for a very large sailboat. So sailboat propellers basically come in 3 flavors, fixed feathering and folding. Depending on your type of sailing you will favor one style rather than the others. Fixed Propellers is really the ‘norm’ and it is what I see most often visiting yards and marinas. They give good forward and good reverse. The later is not as good as forward as the propeller is going backwards but the vessel is maneuverable. Under sail the propeller is locked off or free to rotate and creates drag from the blades through the water. Most sailors will have at some time had to manage the propeller in a scenario like this.
The feathering propeller was borrowed from the aircraft industry and is a flat paddle style propeller. It is aimed at reducing drag under sail by aligning the blades with the flow to make the drag disappear. It gives great thrust at low rpm both in forward and reverse but suffers from performance, efficiency and noise due to the paddle-like blades. Feathering propellers are therefore great for the day sailor who needs marina maneuverability but as you increased the boat size, the loading on the propeller goes up and the performance drops which can be a nuisance for the onboard comfort and damage to the propeller. Performance encompasses the loss of thrust from the flat blades, pressure pulses on the hull from not unloading the tips (thump, thump, thump as the blades pass top dead center), and at wide open throttle cavitation on the blades which if you have never heard before sounds like a couple of quarters bouncing around in an empty tumble dryer!
The final option for a sailboat propeller is to keep the performance of the fixed pitch propeller and fold the blades out of the flow and behind the propeller hub to reduce the drag. In this manner all the benefits of a fixed pitch propeller can be incorporated giving good efficiency, performance and noise characteristics. It is for this reason that all of the big sailboats use folding propellers due to power requirements (cavitation, performance, noise drag) and that racer / cruisers use them for their low drag performance, comparable fixed pitch maneuvering and power performance.
So with the suitability of a particular type of propeller for a sailboat this post gives a summary of a study conducted by Bruntons Propellers and J&A Enterprises for a comparative noise study conducted on a Baltic 147 ft sailboat fitted first with a 3 blade MaxProp propeller and then with a 4 blade Varifold folding propeller over the space of two consecutive sea trials. The study really emphasizes what happens when you load up a flat paddle like propeller on a big boat where the low drag characteristics of the propeller work against it under power, causing noise, vibration and loss of efficiency. In these applications a flat paddle really falls down and you need to optimize the blade for the flow by introducing pitch distributions, blade camber and skew the same techniques you would use for a fixed pitch equivalent propeller.
Noise trials on a 147 ft Baltic Sailboat
The Baltic 147 sailboat used in the trial was a carbon composite hull, fitted with a Caterpilar 3406-E 700 HP engine. The trials were conducted by J&A Enterprises of Marblehead MA and the results were really a validation of the Bruntons approach to sailboat propeller design.
The purpose of the test was to compare the noise and vibration in the aft part of the sailboat using the two types of propellers. The tests were performed by recording the noise for each propeller at various positions of the vessel at various rpm’s. During both sea trials 1/3 octave and narrow band spectra of noise and vibration were recorded. The measurements were conducted at various locations at a cruising speed of about 1515 rpm as well as in the aft crew cabin and the aft cockpit at various boat speeds.
The results were analyzed using industry standard analysis ISO Standard #2923, “Acoustics Measurements of Noise On Board vessels. The results showed that when cruising under power, the MaxProp propeller created low frequency noise and vibration throughout the boat starting at as low as 1500 rpm. The MaxProp was classified as ‘very noisy’ and at cruising speeds between 1800 rpm and 2000 rpm and the MaxProp propeller-induced noise was the biggest contributor to the noise and vibration.
The change to a Varifold propeller dramatically reduced the propeller noise and vibration to levels significantly below the levels of non-propeller related sources.
The above plot shows the noise spectrum of the two propellers with the Varifold folding performing better over the entire measured range recorded for this plot at 1,515 rpm.
Vibration in the frequency range 0 to 100 Hz is the normal range for assessment of tactile comfort. With the MaxProp propeller the only significant vibration (greater than 1 mm/sec) was noted to occur at the propeller blade passing frequency, 31.5 Hz. High amplitudes of 8 mm/sec were measured in the Aft Cockpit and in the Aft Port Crew Cabin. Vibration levels greater than 2 mm/sec are an annoyance, for a sailboat, and vibration levels at 8 mm/sec are normally unacceptable for a sailboat or for a work boat. The excess vibration levels are typical of the flat blade Maxiprop type of propeller.
Vibration generated by the Varifold propeller at the same locations did not exceed 1 mm/sec in the Aft Cabins and 1.5 mm/sec in the Aft Cockpit at a cruising speed of 1515 engine rpm (it was 14 and 8 mm/sec with the Maxprop in the same location).
The MaxProp propeller was noisy due to it’s flat paddle blade design. The propeller started to cavitate at relatively low shaft speeds. Despite the decent propeller tip clearance on the boat (reportedly more than 25% of the propeller diameter), the propeller-induced noise was high. Ironically the MaxProp propeller was chosen on the original outfit for its low drag and feathering characteristics. The same characteristics, which give it low drag when sailing, give it poor motoring efficiency and make it noisy.
The 4 blade Varifold propeller however had low drag when folded, and generated considerably lower levels of vibration and noise due to the shape of the blades which resembled those of an efficient fixed pitch propeller.
“Everybody involved strongly indicates that your Varifold propeller more than lives up to the expectation we had for it. Vibration and noise reduced dramatically plus the power and speed going forward increased substantially” Mr P.G. Johansson of Baltic Yachts
The noise and vibration tests performed aboard the 147 ft Baltic sailboat equipped with a 3-blade MaxProp and with Bruntons Propellers 4-blade Varifold propeller, showed that Varifold propeller generated considerably less vibration and noise while operating at reduced rpm and hence greater efficiency. Baltic Yachts in fact commissioned Bruntons Propeller’s to develop the 4 blade propeller, prior to this test the range was only 2 and 3 blade folding propellers. The resultant 4 blade Varifold folding propeller delivered much improved top speed and noticeably better comfort under power at cruising speeds. Sea trials indicated a startling improvement in performance. The 4-blade Varifold propeller used a conventional propeller blade shape with proper helical pitch distribution. There was a 15% reduction in pitch towards blade tips, to help reduce noise and vibration. This is something Bruntons Propellers incorporated with their fixed propeller designs for luxury Motor Yachts.
In summary then, it is clear that every propeller has its application, but other factors cause them to be applied out of their ideal conditions. Whether it is for budget reasons, no interest in reducing propeller drag, bad advice or the thing I am guilty of – having to have one, propellers end up in all sorts of applications. It is important that your propeller should get your sailboat up to hull speed within the working range of your engine with a little to spare, not reaching it or getting there too early means the propeller is not matched to the engine / gearbox you have. Not getting the best sailing speeds or the propeller is noisy, vibrates or cavities then there is a good chance the design is not right for your boat.
At King Propulsion we offer a range of sailboats from fixed, folding and feathering propellers and we will happy to discuss which one we feel would be beat for your sailboat allowing you to make an informed decision on this relatively expensive purchase and get it right the first time.
Call us today at our Virginia Beach location on (757) 962-9219 and speak to our experts about your boat.