Designing Our Steering System

Updated: Sep 6

I’ve spent my working life working with engineers frequently more knowledgeable on a subject than myself or speaking to educated audiences about my specific topic of the day. There is always the fear of ridicule though invariably, I discover its self-imposed. Now is one of those moments. I want to discuss steering or rather our journey to specifying how we will steer the Explorer Yacht, XPM-002, Vanguard. We found ourselves less than qualified yet required to make a decision.

What already works?

An excellent place to start is often to look at what works already. Mobius maneuvers well and has considerable redundancy in her steering actuator system. It has a duplicate electro-hydraulic actuator and an independent manual hydraulic emergency system. It also has a cutout for removing the propeller shaft if necessary without dropping the rudder and Jefra water-lubricated self-aligning rudder bearings. We looked at the FPB78 and Arksen 85 designs to see what rudder area they were using for a similar-sized, twin rudder setup as ours. I also checked Lloyd’s and DNV rules for rudder stock dimensioning to discover that Mobius has a stock >2x minimum recommended diameter, so all was good in that department.  

The Rudder

Now our own Explorer Yacht. The Yard had designed a supported rudder with a lower bearing part off of the skeg. We have requested twin skegs to allow Vanguard to dry out on a flat or grid for maintenance. That support makes the rudder alignment vulnerable if the hard is uneven. Instead, we have a simple extension to reduce the gap between prop blades and skeg and a cantilevered blade rudder. We also put a large 45-degree chamfer on the leading edge of the rudder blade to reduce the likelihood of fouling ropes or other detritus. We stopped the stock ⅔ the length of the rudder blade and reduced the metal thickness for the lowest ⅓. It is more likely to crumple but remain serviceable in the event of an impact. The area in front of the rudder stock axis was set at 24%. This followed a recommendation from Kebelt for a semi-balanced design. The stock remained at 2x Lloyd’s schedule, and we retained the self-aligning bearings to keep the actuation torque to a minimum. 

Wills Ridley Twin Steering pump package with a common but bifurcated hydraulic tank.

Rudder Actuators

We have two potential actuator suppliers, Kebelt and Wills Ridley. Both have longevity supplying small commercial vessels, so I’d trust the products. Both are electro-hydraulic with 100% redundancy and can run in parallel or singular. I did look at only electrical systems for a while but could not see the advantages compared to a small dedicated hydraulic power pack. I’d instead go for a known technology on this particular system.

Praxis Dynamic Positioning System controlling rudders, Motors and Bow thruster via command and GPS position.

Now for the more exciting part. We have been looking at positioning systems. Principally for the short-handed operation of a rather large (long) yacht. Typically they fit a bow and stern thruster to assist (unless it’s really clever such as the Volvo or Cummins podded drives). I thought that inelegant, so I investigated potential alternatives. Our automation system supplier (Praxis Automation) stepped in at this point and suggested we consider splitting the twin rudder actuation. That way, one engine can position fore/aft, and the other be used for side thrust with the pertaining rudder hard over Port/Sbd.

Vanguard will have a bow thruster in the design from Mobius but needed to re-evaluate it in light of a likely extended run time, proportional controlled thruster at the bow. In all other respects, it’s the same as Mobius. 

Final Design

  1. We now have a rugged semi-balanced rudder design. 

  2. We achieve full redundancy throughout with duplicated infrastructure.

  3. If anything untoward should happen (and it does), we can hydraulically lock either rudder in the center position. Alternatively, we can link them mechanically with a tie bar and drive both from a single actuator.   

  4. We can set tow-in/out of the rudder geometry electronically in non-volatile memory. We can also set the positioning response gain similarly. I’m looking forward to playing with that facility.

  5. Emergency steering can be performed locally in the Engine Room via directly linked controls. 

  6. Finally, we have a redundant ethernet link to Helm, Flybridge, and Transom steering stations.   

Call me “paranoid,” but I think we are good to go.

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