Technical Information

 

Firebird Rudder, by Toby Richardson 

Why is the Firebird rudder placed were it is and why is it that shape? The quick answer is to minimize drag and weight. From a builder's point of view the easiest and cheapest thing to do would be to have a transom stern and a transom hung kick-up rudder, just like on most beach cats. The hull is easier the build that way and the rudder is a simple assembly that can be brought in from a fabricator.

HYDRODYNAMIC THEORY/ RUDDER PLACEMENT

On the Firebird the rudder is underneath which means that it can be much smaller because the hull acts as an endplate to prevent air being drawn down from the surface when it is being asked to work hard. On any foil, whether airfoil or hydrofoil, lift is generated when the foil is turned at an angle to the direction of flow (the angle of incidence). This lift is generated on both sides of the foil, as positive or high pressure on the underside (when looked at like an airplane wing) of the foil (the side that is away from the flow). It is easy to see how the positive pressure is generated, the fluid hits the surface and forces it upwards (or across in the case of a rudder). The negative pressure is harder to envisage. This happens because as the fluid flows around the surface of the foil, it is deflected from its original direction toward the direction of the surface of the foil. In doing this it tries to break away and leave a void which causes this low pressure, but as long as it can keep flowing it will keep sucking. Generally only about one third of the lift force is generated on the high pressure side and about two thirds from the negative pressure or suction on the other side.

The force generated goes up as the foil is turned from facing directly into the flow. There is a limit however. The limit happens at very different angles according to speed, the exact section of the foil and the angle that the foil is turned from the direction of flow. The limit comes when the flow around the low pressure side no longer wants to be bent in a smooth flow and becomes turbulent, the suction is too much and the fluid is pulled away from its smooth path (stalling). As soon as this happens then two thirds of the lift just stops. The high pressure side will keep on going to bigger angles but the total force has been reduced drastically.

With a surface piercing foil (in this case a transom hung rudder) working between the dense water and much less dense air we have the problem that as the angle of incidence increases and the important low pressure is building nicely on one side it will suddenly take the easy way out and suck air from the surface down the length of the foil and a large part of the lift just disappears. This will happen at a certain level of suction or force generated by the foil. It will be at different angles according to speed- the higher the speed, the lower the angle. If the same foil is separated from the air, in the case of the rudder mounted underneath the hull, then there is no air for it to suck down and the flow will remain attached to the foil to a bigger angle of incidence so the foil can generate a much greater force.

What that means is that a transom hung rudder has to be significantly larger to generate enough force to steer the boat without sucking air and suddenly losing much of its steering force. That is OK except that pulling a bigger rudder through the water all the time is more drag. At lower speeds most of the resistance to forward motion of the hull is from the friction of water against the hull. The rudder is double sided (friction on both faces) so a bigger rudder can significantly increase the wetted surface area and represent the extra drag that might otherwise be caused by a quite large increase in displacement.

So on the Firebird the rudder is situated under the hull and can be small, in fact it is smaller than on many beach cats yet the boat steers very well. From this we get other benefits. The small rudder means that the center of pressure is closer to the hull, reducing the bending moment on the rudder stock. That means that the stock can be smaller an lighter. The fact that the stock is smaller means that the blade doesn't have to be thickened too much to accommodate its diameter so the rudder section is near optimum. The fact that the rudder is placed forward of the stern and that the method of construction is lighter than a typical rudder stock means that the weight of the steering gear has less pitch inertia. Also because we no longer need a transom to hang the rudder from, the line of the deck can sweep down to the stern reducing the surface area and thus the weight of the hull, further reducing the pitch inertia.

SHAPE

On the shape of the rudder, first of all the Firebird uses a NACA (National Advisory Committee for Aeronautics) foil section to minimize drag whilst getting maximum steering force. This section has to be thickened right at the top of the blade to accommodate the diameter of the rudder stock but by machining away the stock within the blade and transferring the load from the stock into a tapered unidirectional composite reinforcement the blade can be reduced in thickness so that much of it has the optimum thickness to chord ratio. Normally on a spade rudder like this the stock would extend well into the blade, this means that all of the blade would have to be substantially thicker than optimum, greatly increasing the drag at speed and the blade would be heavier. The skin laminate is engineered to provide torsional rigidity with minimum weight but the edges and especially the tim are thickened locally to provide robustness and impact resistance with minimum weight. The rudder has a semi elliptical form. This is to make best use of the area. Any foil that has an exposed end (like the tip of an airplane wing) will have flow spill over the end from the high pressure on the one side to the low pressure on the other. Its a bit like the problem with the surface piercing foil but because it's the same medium, not a considerably less dense one, it doesn't result in catastrophic loss of lift, just loss of efficiency. This flow around the end is distributed according to the pressure distribution along the chord of the section so it will peak about a third of the way back. If the foil tip is extended into the flow around the tip in proportion to the the amount of flow then it can intercept much of that flow and put it to use as lift but without incurring the drag that would normally be associated with that area. This extra area takes the form of the semi ellipse seen at the tip of the Firebird rudder.

RUDDER STOCK POSITION

The position of the rudder stock in the blade is important also. At the top of the blade the stock enters just in front of the section's maximum thickness. This is so that the blade doesn't have to be any thicker than necessary. However the point of maximum thickness will be near the center of pressure so if the whole blade were pivoted about that point it would be close to taking charge, of turning on its own once the rudder was turned a little. Although it is desirable to avoid excessive helm effort, some effort is needed to have "feel". With the Firebird rudder the blade sweeps aft from the point of entry so that if the line of the stock is extended it would pass through the leading edge near the tip. That way the steering is always light even when the rudders are working really hard but the line of pivot is far enough forward to give pleasant feel to the helm.

 

 

Contact: Still Water Design, 1 Winnisimmet Street, Chelsea, MA, 02150, by e-mail: dick@stillwaterdesign.com, tel.: 781.608.3079
Credits: Photos by Lorin Alusic and Claus-Christian Plass