Rudders and their function can fall victim to vibration trouble, too

Feb 8, 2016 by Rich Merhige

Even though a rudder is part of a vessel’s steering system, which is obviously an important mechanism, it usually doesn’t get that much attention. What action items should be addressed for rudders during yard periods?

The rudder turns the stern by way of unequal water pressures. When the rudder is turned, one side is more exposed to the force of the water than the other. This pushes the stern away from the side the rudder is on, causing the vessel to turn as desired.

Most rudders consist of smooth, flat surfaces, are made of metal and pivot off of hinges or around the rudder post or rudder stock, which are vertical shafts. The most efficient rudders are long and narrow, versus ones that are shorter and wider. They’re attached at their forward end to the stern post.

Types of rudders include balanced and semi-balanced. Balanced rudders have some of their area forward of its turning point. An unbalanced rudder has no area projecting forward. The ones most commonly seen on yachts are semi-balanced, and spade rudders. Spade rudders have no support or bearing along the bottom of the blade.

Rudders are moved by torque provided by tiller arms on each rudder. The tiller arms are controlled by hydraulics, which make up the steering system. Tie rods connect the rudders via the tiller arms.

Hydraulic rams produce the force used to turn the rudders. This is activated when the steering wheel is turned. As the wheel turns, it activates the hydraulic ram, causing it to push on the tiller arm, which, in turn, moves the rudder.

Since the rudders are connected by the tie rods, they are, in essence, synced together in this movement. Sometimes, there will be a hydraulic ram on both rudders. This is usually the case if the rudders are heavy, or the vessel is on the larger side.

Just like the shaft line, rudders can also fall victim to misalignment. When rudders are aligned, they should be positioned slightly tow in, or tow out. With the rudders positioned this way, there is less vibration from the water hitting them at these angles. If the rudders are positioned straight, there tends to be a fluttering vibration because the force of the water hitting them is much harsher.

Vibration can also come off the rudders if there is looseness in the tie rods or bushings.

Vertical alignment of the rudder stock is equally important. This is best achieved through the use of a laser alignment system, or shooting an optical scope through the top and bottom bearings.

To keep rudders up to par, there are basic maintenance tasks that should be done on a regular basis.

During haul outs, several items related to a vessel’s rudders should always be covered. Among them:

  1. Measure the space/clearance between the sleeve and bushing. If the clearance is more than .75 mm, then it is worn and should be changed. Misalignment, and deformations caused by damage or corrosion, can cause excessive clearance.
  2. Have the sleeve examined closely and have the fit documented. Is it loose? If it is, corrosion from water could be the culprit.
  3. Check the rudder shaft or stock for pitting by the sleeve. If it’s present, this can be repaired the same way as propeller shafts – with clad welding. When having clad welding done, it’s always best to select a vendor that is ABS recognized.  
  4. If the rudder stock or pintle is fitted into a stainless steel bush, it should be checked for crevice corrosion.
  5. A bump test should be performed to check the wear on the rudder bearings. Bump tests are required during class surveys but, more and more frequently, surveyors are starting to request them even if the boat is not classed.
  6. The plating should be closely examined for fractures. Special attention should be paid to the pintles, shaft brackets, slot welds, and stern frame. The keyway should also be checked by using nondestructive testing. Any deficiencies can be repaired with welding or machining. Again, in a situation like this, it’s always best to look for an ABS-recognized specialist.

As with all components of a vessel’s propulsion system, routine and consistent maintenance of rudders go a long way in preventing costly repairs and major stress.

Rich Merhige is owner of Advanced Mechanical Enterprises and Advanced Maintenance Engineering in Ft. Lauderdale. Contact him through