Rudder Theory

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Here is how the Deflector Marine Rudder works.

        A conventional single panel rudder is not functional or effective when swung beyond 45 degrees either side of center. It simply stalls, the flow rolls around both ends, the vessel's headway is impeded and little or no control is established. in one sense the purpose of our device is to allow a greater degree of rudder angle without stalling the rudder. Our rudder can incrementally direct the water to deflect at an angle in excess of 80 degrees from the forward axis. The thrust produced is then better aligned with the desired swing arc for a tighter turning circle.

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      As seen in the diagram of high speed rudders, the leading edge of a conventional single panel rudder has the highest pressure. That pressure diminishes as the flow progresses to the trailing edge where it is much lower. The Deflector Marine Rudder (DMR) establishes a second high-pressure zone on the aft end of the rudder by use of the flap. This generates substantially more turning force. Further, it functions to deflect the propeller wash to a much greater angle as previously mentioned making it capable of static maneuvers that pivot the vessel without making headway. When the main rudder is swung at 45 degrees from center on either side the FLAP is swung an additional 37 + degrees. This makes an effective rudder angle of 81 degrees +/-. When the tiller is centered both blades are also centered.

 

    CONSTRUCTION CONSIDERATIONS:

        The hinge that separates the two rudder panels is crucial to the proper operation of this rudder. In the Deflector Marine Rudder a UHMW pin is used to reduce wear and corrosion. The hinge unit is designed to be full length formed sheet or plate and welded to a box tube for resistance of torsional forces and general rigidity. The hinge knuckles could be constructed in several ways. However the described system has proved superior when all factors are considered.

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Application Versatility

        Deflector Marine Rudders are also custom engineered for each individual boat from 9 ft (3 meters) to 450 ft (150 meters) and can be sized for kayaks, sailboats, power yachts, tugs, trawlers, ferries, dredges, and amphibious landing craft. Our rudders are used on Kort nozzles, and for performance machines of all sizes and types. They can be installed cost effectively on new builds or as retrofits. We build them to be long lasting, durable and stronger than a conventional rudder

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    RUDDER PERFORMANCE AND SHAPES

        "STALLS” Boat rudders stall just as aircraft wings stall for indeed wings in air and rudders in water share many performance characteristics. The main difference is that water is more than 800 times denser than air so there is a great difference in the velocity at which these events occur. Wings are foils. Rudders are as well though often more crudely shaped. Our work is making simple more scientifically effective rudders.

        Stall occurs when the water on the backside and trailing edge fails to flow smoothly. When the rudders angle of attack is too high or the velocity is to low for the loading on the rudder. (Loading = Resistance to turning) the flow roils turbulently and the lift working force of the rudder is interrupted. The hull fails to continue turning. To reestablish the lift force. One must increase the velocity (often not possible) or reduce the rudder's angle of attack (usually necessary). This however dramatically increases the turning circle. Hopefully nothing is in your way.

         Now when considering zincs for your rudder(s) the common practice is to attach zincs to the surface of the rudder’s side panels. This is A NO no; it breaks up laminar flow contact of the rudder’s side panels reducing its efficiency. (Example) It would be the same as attaching protruding blocks on an airplane wing, the cause and effect is NOT GOOD. Notice that DMR rudder(s) DO NOT have zincs on side panels. They are often cylindrical zincs shielded top and bottom on the rudder’s end sections in the path of water already disturbed by the shafts.

        For longer than most of our lifetime's aircraft wings have featured trailing edge flaps. Aircraft do not feature many frivolous features. Part of our work in making more effective rudders is designing simple effective rudder flaps. This creates a type of rudder known as HIGH LIFT. As the name implies it is more versatile and powerful.

        The trailing edge flap feature increases the cross section making a shape more suitable for lower speeds. Slow is of course where most tight maneuvers take place. Note the flap makes the trailing edge work much harder changing the distribution of loading on the rudder blade shifting it aft. With this new state of the loads it is appropriate to rebalance the rudder. The BALANCE BLADE is the portion forward of the main axis shaft. The new flap load allows a good increase in the size of the Balance Blade area. This new extended leading edge balance now reaches further across the centerline and grabs more flow. This can dramatically increase rudder force.

        This new fore and aft dimension is particularly better suited to take the propeller wash and REVECTOR it. This is the only water in motion when you start away from a static position at the dock!.

 

    SHAPES CONSIDERED

            All of us are familiar with slab or PLATE RUDDERS and know they work as they are common. There shape is however less strong and versatile. A flap is difficult to put on a plate rudder as the plate does not have the structural rigidity to handle the additional load and would likely flex and fail. Further the flap hinge section would bulge out as an obstruction reducing smooth laminar flow. Smooth flow does not occur on a dead flat plate. A curving surface maintains flow contact more smoothly. The relatively sharp front entry would seem to have advantages and it does, but it brings some less attractive traits as well.

        An elongated teardrop or properly FOIL SECTION is of course stronger as it is thicker. This gives it the ability to resist stress from more directions, and it gives the bulk to contain a strong flap hinge, without an intrusive bulge. What the bulkier FOIL section does HYDRODYNAMICALY is shift the highest lift zone aft on the rudders panel. This foil rudder is smoother and delays the onset of stall.

        The front edge of the rudder can be either sharp or blunt. Each is chosen for a different purpose. A very high speed boat ( 30+ kph ) should have a needle sharp entry and a wedge section with a flat section across the back. This is intended to hold the water on the sides by presenting the sides. If a hull needs a more accurate or precise feel at the helm then a sharp entry may help. Remember though this shifts the greatest lift zone forward which diminishes stall resistance so it has a price. Our flap feature reacts faster so we get the a more precise feel over center that way. For that reason one is usually better served with a blunter entry. This reduces the whop from the propeller vortices. This place directly in the propeller vortices where the rudder operates is extremely fierce. Roiling irregular blasts of water strike the propellers front surface from opposite directions at the top and bottom. For many a bulbed front helps diminish these impacts by separating the flow laterally.

          Perhaps you noticed that the vertical edges of many of our designs are curved when viewed in profile. There are several reasons for this. Consider that the propeller is a disc shape as is the section of the flow vortices. A curved edge semi disc shape matches this better than does a rectangle. Further the petal shape has less drag, and the missing corners will not protrude into, or snag anything. Another benefit is that the loaded curve of the surface skin is stronger and the varying shape defies the potential for a single harmonic. All of these are important for long trouble free service, which is our aim. This is the characteristic D profile.

          The ASPECT RATIO is the comparison of length to width. The least resistance is obtained by a high aspect ratio panel, meaning that a long slender paddle blade shape creates less drag than one with more width. Rudder drag is only one consideration though. The rudders task is the most important. That the rudder has the strength and, force to do its prime task is the first priority. A hull that is slewing along through the water tracking poorly because the rudder it has is inadequate will have far more drag to say nothing of inefficiency and discomfort. One should not consider rudder drag alone. For this reason many of our designs have aspect ratios closer to one to one.

 

      Using Autopilots

          As previously mentioned our DEFLECTOR MARINE RUDDERS are better suited for autopilot applications than conventional rudders. They provide the crisp fast grip to hold your hull on track. They react quickly and powerfully before your hull is slewed. The timely correction is possible because the flap cups like a turbine blade faster than the main blade and bites even at smaller angles off of dead ahead. Holding course well is being timely.

          Some of the material that follows is from a major marine autopilot manufacturer about how to properly set your autopilot.

          DEFLECTOR MARINE RUDDERS will always require less rudder and other function values than other rudders.

          Theory of autopilots - ‘Rudder’ control is essentially a gain adjust with enough range that allows the autopilot to precisely find a rudder setting that turns any given boat. Counter rudder messages gain allowing the pilot to use full gain to initialize a turn and ramps gain down as the commanded heading is approached counter rudder even puts the helm opposite if the commanded course heading is being approached too quickly. Turn rate allows the pilot to be set so that all turns are done at a rate that is responsive yet safe for any given boat. Yaw is adjusted to prevent the pilot from fighting compass motion introduced by rolling and acceleration, allowing any boat to work more comfortably with the seas the way the hull was designed to work. Fast/slow settings allow for the primary parameters into two memories for different speeds that are automatically or manually selected.

          Rudder set - Every boat is different and many boats very differently when moving slowly compared to normal cruising speeds. Autopilots that consider this feature independent settings for when the boat is moving slow or fast. In any case, upon initial sea trails, the first thing that must be established for any given boat is the amount of rudder required to make that boat turn. Because counter rudder messages rudder, set counter to 0 or 1 while establishing rudder. Counter rudder will be adjusted next. Because we need the boat to turn during this exercise. Turn rate should be set mid scale (4or5) to be fine tuned later. Yaw should be set low(1) to be fine tuned later. At normal cruising speed, with autopilot in fast mode, begin by commanding 30or 40 degree course change in either direction. Continue to increase rudder until the boat comes to the commanded course heading, over shoots, correct back and then correct again. Two or three overshoots across the course before settling indicate that the amount of rudder gain has been established to turn the boat. More than 2 or 3 overshoots indicate excessive rudder. Reduce rudder accordingly.

          Counter rudder set - Next, continue to command 30 or 40 degrees course changes. Increase counter rudder one point at a time until the autopilot brings the boat to the commanded course and goes straight without over shoot. If the boat stalls before coming to course. Reduce counter rudder.

          Turn rate set - Turn rate controls how many degrees per second the autopilot will allow the boat to turn. Continue to apply large course changes. Adjusting turn rate until the boat turns responsively yet safely. Note if turn rate is left in at a lower set point because very tame turns are desired in normal conditions, know the upper (responsive/safe) setting. This setting will work best in heavier seas.

          Yaw set - Yaw is the control most often adjusted when sea conditions change. The above are now for the boat and typically do not need much adjustment across changing conditions. To properly set yaw. Simply watch the left/right correction indicators on the display. If the autopilot is over active. Increase yaw. Let the boat work with the seas the way she was designed to do. If the autopilot is under active, decrease yaw. In the normal conditions the autopilot should show correction activity about 15% to 20% of the time. If the boat tracks straight on her won lines, yaw can be reduced to minimize pilot activity. This saves much wear and tear on the mechanical steering rear and saves greatly on power consumption aboard sailboat. Autopilots are equipped with automatic rudder trim. If corrections are consistently to one side, the autopilot will eventually set rudder position to compensate. This clock based feature and is always readjusting.

          Slow/fast modes - Now that you have set up your autopilot in the fast mode, set the autopilot in slow mode and repeat the above at a slow boat speed. Once both modes are set, they are retained in memory but do write your settings down in your manual. If you want your autopilot to switch automatically between settings based on the speed input from you 'GPS'. Enter a speed toggle point.

    Ultra-High Molecular Material

         tech04Cavello-S UHMW possesses a unique combination of physical and mechanical properties, which enable it to perform well under the most rigorous conditions of wear and environment. It has the highest known impact strength of any thermoplastic presently made, plus high resistance to abrasion against a wide variety of metals. These properties make UHMW an exceptional material for marine applications. For polyethylene to be certified as UHMW the molecular weight of the material must be at least 4 million (4 PPM). UHMW is usually certified at 4-6 million molecular weight. Any material with less than a 4 million molecular weight is either high density or low density (HD or LD) polyethylene. These products are tech05UHMWINSTALL-Smuch less expensive to manufacture than UHMW, but the physical properties are correspondingly less. We are at present, in the design stages of producing high lift flap rudders made of Ultra-High Molecular Weight Material (see technical section). UHMW is lightweight, absolutely non-corrosive, repels marine growth, and is high impact resistant. We are very excited about our new direction in UHMW rudder technology .

    Update UHMW Flap Rudder


    Matthew Cavello, This UHMW Rudder Was Put On A 36ft Crusing Yacht! In The Summer Of 2006, Deflector Marine Rudder Fitted A Prototype UHMW Flap Rudder To A Converted Gillnetter Used For Cruising On The Columbia River. The Rudder Performed Very Well And Provided All The Advantages Of The Stainless Steel Rudders. UHMW (Ultra High Molecular Weight Polyethylene) Resists Abrasion, Impact And Marine Organisms As Well And Offers The Possibility Of Compression Molding Standard Rudders To Save Production Time.

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