ISSUES & FACTS
There are a great many poorly performing rudders on boats. Unfortunately many of those rudders are used as the pattern for new inadequate rudders. Let's explore some common rudder flaws and how to fix them.
The most common problem is certainly a TOO SMALL RUDDER. The task of maneuvering at lower speeds requires more force than the rudder can provide. The inadequate rudder stalls, losing its grip entirely, and the boat wallows, unable to hold its course in a sea way. Both reactions make the boat less attractive and useful. One problem accentuates another, tending to a cascade of problems, and so it is when your rudder is of inadequate size.
POOR OR MISSING BALANCE is the feature sticks out next. This seriously limits the utility of such rudders. (balance is the portion of the rudder blade forward of the axis of the rudder shaft.)
The balance area aggressively reaches out across the other side of the keel as it turns across the centerline, grabbing water that would otherwise go straight aft. The leading edge of the rudder has the highest pressure with pressure diminishing as it slides aft. The faster you go the more pronounced this phenomenon becomes. Marginal steering systems do not handle these forces well. For this reason many displacement boats are stuck with rudders suitable only for a faster 20+ knot craft. These rudders perform poorly at maneuvering speeds, not only because they may be small, but because they lack enough balance.
To properly understand power boat rudders, they should be considered to function in two modes. One is the conventionally understood " flying foil". At typical displacement speeds, these foils (rudders) stall as they approach attack angles of 20 degrees. There is another phenomenon I call apparent angle of attack. As the hull rotates it quarters into the approaching stream, the water is felt at a diminished angle. For that another 10 degrees is thrown into the tiller. This is how the common 30-35 degree tiller angles are arrived at, but... Even though many designers stop their consideration here, there is that other mode... The propellers' task is to produce a relatively fast jet of water. Out drives and other common systems vector thrust to steer. Rudders do this too (like a chute or reversing clamshell on a marine jet). Properly engineered rudders can efficiently and powerfully do this well.
With some understanding this vectoring thrust, designers often crowd the rudder tightly behind the propeller. A much better practice is to move back, and put on more balance blade, and a flap. The best single panel rudder will have approaching 25 % of balance blade portion. When the rudder is moved back, the propeller vortices hammer it much less fiercely and flow is more defined.
Relatively larger rudders tend to hold their water better, slip, or stall less; therefore they can be used at higher angles of attack. This allows many commercial tugs and fish boats to work well with a full tiller arc of 90 degrees (45 to each side). It is not uncommon that their tasks demand powerful maneuvering and better tracking.
To diminish interference for boats that exceed 30 knots, the better section becomes a wedge to better maintain laminar flow on the sides, but... The problem now is that most of these vessels have little low speed rudder control. As the rudders are small and lack good balance panels, they do not vector the prop wash well. The typical solution is to employ counter-rotating twins. But Again, only 8 knots can be damaging to the boat & the people in a nasty sea way. Though it adds a lot of maneuverability, Counter rotation is no solution here. The rudders were simply designed for faster water and are not versatile enough. Efficient Flap-foil rudders accomplish this task extremely well and give the vessel a broad range of control.
The science of rudders is complex enough and we could devote more time examining common mistakes, but... lets skip to the good news that is the virtues of high lift deflector marine rudders. Why is a flap rudder such an appropriate solution? Because it is a more versatile foil. The flap gives it a variable cross section.
While we're talking foils, some concern is given to the best foil shape. The problem is with every change such as rudder angle, velocity, load, etc., the theoretical optimum shape changes. To stay perfect, a plastic shape would be needed. The purest is flummoxed by practicality. Ever sit at the window on a modern jetliner during descent and landing? You were watching big money compromises on the wing serving beautifully. Your rudder should not be too intricate; after all you are going to stick it in the salt water and in violent flow behind your prop. Instead, we opt for the most effective rudder with the broadest operation range possible, within the constraints of ruggedness and costs.
The term high-lift means that these rudders generate more force. How much? Such factoring is relative. When your rudder has stalled, one that hasn’t is definitely better. To give a sense of proportion, I often say twice as powerful for similar size. This is the sharp knife compared to the dull. Move a dull blade fast enough and it cuts too, but for slower finer work it fails. Without making the rudder bigger, we double its power.
The size of the flap has a large impact on how powerful the rudder is. Naturally, a large flap generates a lot of force, and a strong rudder is needed to transmit that force. Since the trailing edge of the rudder is now acting powerfully, more balance is a big benefit. As much as 40%. As discussed earlier, this captures a lot more flow.
Now we can design a rudder that scoops that prop wash wholesale sends it out at a right angle to shove the stern where we want it. So it is doing good brings good, complementary benefits accrue. With this engineering we could probably make your hull spin in place, at worst cut your turn radius to less than half. We have by now, the benefit of many applications to sharpen our design ability.
I put deflector in the name deflector marine rudder in order to point out the superior ability of these rudders to deflect the propeller wash to more extreme angles. ( we swing tillers 90 deg. 45/cl )
A deflector marine rudder will:
* Enhance your maneuverability drastically ( double + new more acute vectors )
* give you 'at the ready' stern thrusting
* hold you on track, making you more efficient
* reduce your roll in a sea way ( do not underestimate this comfort gain )
* hold you on autopilot ( faster, more powerful, sharp/dull )
The benefits described are available in a multiple and not a small fraction over those of most equipment.