SHIPS MACHINERY INTERNATIONAL, INC. offers both the Van der Velden hydrodynamic Fishtail rudder and the Barkemeyer Flap rudder. We are able to help guide you in your selection of the best rudder for your application.  Contact us today for the ultimate in steering!

COMPARISON BETWEEN HIGH-LIFT RUDDER TYPES

To compare the efficiency between a Flap rudder and a Fishtail rudder it is necessary to look at two different operational modes;  Course Keeping and Maneuvering.

Course Keeping:

• When a ship is sailing from A to B it needs to maintain a straight line course.  A rudder plays an important factor in this course keeping condition.  No matter how well a ship is designed small rudder corrections will be needed to keep the ship traveling in a straight line, as a result of wind, current, etc.  If the rudder area is well selected these corrections will be always smaller than 10º and usually even smaller than 5º.

For the 0-10º  rudder angle region the following features are important for a good rudder: 

• A good lift-drag ratio.  This helps to cut fuel consumption.
• At small angles the build up of lift should be gentle, in other words, the slope of the lift curve vs. the rudder angle should not be too steep, otherwise, this could easily result in over reaction of the autopilot requiring additional correction.  This would be detrimental to fuel economy.

Comparing a Fishtail rudder to a Flap rudder:

• The lift-drag ratio at rudder angles between 0-10º is comparable for both types of rudders. As the Flap rudder flap moves quite aggressively, the lift-drag ratio will be slightly greater than that of the Fishtail rudder for angles >5º.
• As the Flap rudder flap moves aggressively, this means that there is a flap angle at smaller overall rudder angles, therefore, the lift-force builds up rather more quickly than with a fishtail rudder.  This increased flap angle could more easily result in an over reaction of the autopilot.
• The logical conclusion is that in course keeping a Fishtail rudder has a slight advantage compared to a Flap rudder.

Maneuvering:

• In a port or when using a DP-system the ship is required to make small accurate turns at low ship speed.
• Important factors to consider for maneuvering are:
• The maximum lift force co-efficient and hence the maximum lift force that can be created.  (In this case drag is not important as ships generally will operate at low speed.)
• The ability to redirect the propeller slip stream.

Comparing a Fishtail rudder to a Flap rudder as relates to maximum lift:

• The Flap rudder provides about 7-9% higher maximum lift than the fishtail rudder.
• Flap rudders have a maximum rudder angle of 45º leaving the flap at 90º (perpendicular) to the propeller slipstream.  Fishtail rudders have a maximum angle of 65º in which case the special shape of the fishtail can redirect the water partially backward.  The propeller coverage of a fishtail rudder at 65º compared to a Flap rudder is better for a fishtail rudder, (assuming both rudders have a similar length).

The logical conclusion, taking into account the above factors, is that a Flap rudder will have the advantage in maneuvering. 

The difference between rudder types is not as great as might be concluded on first sight by just comparing the maximum lift co-efficient of both rudders.  When comparing efficiency a Flap rudder is somewhat superior in maneuvering and in course keeping a Fishtail rudder has the advantage depending on the ship speed (at slow speeds rudder over-reaction is not a problem).  The fishtail rudder has an additional benefit of lacking any moving parts and is thus less vulnerable to wear and tear and requires less maintenance overall. 

The selection of either a Fishtail or Flap rudder will represent a significant improvement over an ordinary rudder.  While the perceived difference in rudder performance between these two rudder types will be minimal, main vessel operating characteristics should be carefully considered when selecting which rudder to apply.