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Propellers - Vibration and a Solution?

Recently, we discussed the vibration issues discovered during initial sea trials on our explorer Yacht, Vanguard. In that Blog, we also briefly mentioned Brunton's propellers and their help mitigating these issues. Now, it's time to dive deeper into how Brunton's knowledge contributed to a resolution.

Why did we select Bruntons Propellers?

No matter how much attention to the theory, reality wants to bite at the most inopportune moments. We sought very efficient propulsion across a wide speed range. Efficient at 3 or 4 knots with an electric drive and at 12 to 14 knots for both diesels at full load, cruising at 10. That's a tall ask for a traditional fixed-pitch propeller optimized for a load range. We expect to operate most voyages on a single engine, extending the time between engine oil changes to 500 hours (250+250) and keeping 100% propulsion redundancy. A traditional second propeller would act as an inefficient fixed vane. On a sailboat, that is often a circa 0.5 knot penalty. In contrast, a sail drive with folding blades under idling conditions seems more appropriate as it reduces propeller drag by around 85% (source -Bruntons). We also wanted low blade surface loading even in a semi-enclosed shaft tunnel where loads are typically higher and slow rotational speed/large diameter to minimize propulsive losses. 

We added those requirements together and initially chose a Brunton Autoprop (Eco*Star). Due to their unique features, these designs target hybrid-drive yachts. For instance, they are efficient as the pitch alters to suit varying rotational speeds, which is helpful for hybrid drive systems. Furthermore, the blades will fold on the non-running propeller, minimizing drag when underway.

Scope of supply

Bruntons supplied propellers and a complete drive line, including the Sigmadrive self-aligning coupling, Autolock shaft brake, shaft line, mechanical seawater seal, cutlass bearings. Within this, we relied on their design expertise to address various challenges, from minimizing shaft whip between bearings to selecting the right propeller diameter/pitch combinations for our application. Their support was invaluable, especially considering our limited knowledge of propellers.

A promise kept

Did they follow through on their promise? Well, I would not be writing this Blog if they had not!

It went further than that; as we reported in our last Blog, we experienced severe vibration issues at shaft speeds over 1800 RPM. Our rope cutters were found to be part of the problem (see installation photo showing incorrect placement of the cutter relative to the bracket). They were removed before another sea trial.

This time, we ran the engines when tied to the dock and again experienced vibration above approximately 1800 RPM. In the crystal clear waters of Antalya Harbor, we took a video and played it in slow motion. Several features were noted:

  • the blade orientation changed from near perpendicular, result of a pitch change between top and bottom of the swing. Being at the dock, the flow under the keel was not representative of underway but did show the blades being forced to alter pitch markedly with each swing.

  • We saw a tip vortex forming at mid speed that became both detached and more annular at higher speeds. Given the very different flow conditions, relatively low frequency of the vibration and light blade loading, this was probably not our core issue but more a facet of the static water flow conditions.

  • The net effect was to reveal the blades fluttering like a bird's wing as they passed through the flow field under the hull. Those deep skegs and partial propeller tunnels have disadvantages even if they protect from grounding and ice. 

Seeking solutions

Bruntons attended a site visit and sea trial followed by long conversation with their helpful staff. It was impractical to suggest alterations to reduce water velocity variations, but instead, we changed the Autoprops for a folding fixed pitch design, a Bruntons Varifold sail drive propeller. The reasoning was:

  • locking the pitch to better cope with the varying flow field experienced each rotation.

  • increasing the blade count from 3 to 4 to increase the disc area in contact with the flow and reduce the resultant pressure variations from blade to blade.

  • reduced the OD from 29.5" to 28", thereby increasing the free water between the hull and blade tips and reducing the maximum tip velocity.

  • A facet of the design placed the Varifold blade disk about 250mm further aft (photo on RHS), in cleaner water flow, less disturbed by the skeg (the auto props did not vibrate in reverse). Folded blades still cleared the leading edge of each rudder.

  • These Varifold propellers also come with a 15% pitch relief on the tips. This reduces vibration where there is interference between the propeller tip vortex and the adjacent hull and unloads the forward low pressure face reducing any tendency for cavitation at the blade tip.

Many positive detail changes will have a significant cumulative effect. Bruntons also indicated they would take the Autoprops back into stock and expedited production of the new Varifold propellers from a typical 8 to a stellar four weeks! We will be back in business soon, and the last major snag preventing departure will be in our wake. A later Blog will report sea trial results, but only a little later!

These are our words but Bruntons own the technical support and suggestions, throughout this process they have been a first-class technical partner and a pleasure to work with. 

Sea Trial Results?

So, how have they performed in practice - see Blog in 5 weeks time!

Useful further reading:  

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