• Drive Train

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    Except for R/C sail boats, all R/C model boats need a system that transmits power from the engine to the propeller. On outboards, this transmission is inside the engine and is not visible. What you see is a cylinder, much like that of an airplane motor, normally wrapped by a water jacket - although some of the more modern K&B engines are using air cooling - and a lower end with a propeller on tip . Inside, a flex cable links the motor crankshaft to the prop. For all other engines, internally mounted in the hull, some kind of transmission must be made between the engine and propeller. Mostly used types are the rigid shaft (picture below), which demands 1 or more u-joints to allow for the inclined angle from the engine to the propeller shaft, parallel to the bottom of the hull. This kind of transmission is normally limited to electric boats or glow engines with low HP. and comes with kits for assembling this kind of boats. For engines with higher HP and for gas engines, the transmission is via a flexible cable that looks like the speedometer cable of your car, but much more larger and stronger. This "flex shaft" runs through, and is supported by a brass tube with or without a teflon liner. This tube can be bent to create the correct angle from the engine and become parallel to the bottom of the boat, linking to the prop axle. One of the tips of the flex cable links to the engine crankshaft by means of a special coupler screwed onto the engine crankshaft on one side and with a square hole on the other side in which fits the flex cable. On the other end of the flex cable we solder the ferrule, screwed to the prop shaft. In USA there are some types of drives available called "out drives" (pics below) that, besides being extremely efficient, ease the assembly of the boat. For very special applications, other types of drives known as "jet drives" are available in the USA, like the one we show in the below picture, they work like a jet ski, getting water from the bottom and directing it at fast speed out the transom.

    Rigid shaft transmission

    Flex cable transmission

    M&D designs Racing transmission (USA)

    Precision in Perfection drive. Can be adjusted by means of a third radio channel.

    Water jet transmission

    "Made in Germany" drive trains.

    Yet just a curiosity - the performance is less than proportional to the price and complication evolved - but really very interesting. A jet propulsion on a David Cadman boat:

    Some models of drive

    A- Shark Racing

    B- Int'l R/C Cobra Drive

    C- Enforcer

    D- MDd-Drive (M&D designs Racing)


    When choosing a propeller, those variables must be considered:

    • diameter, that is its size;
    • pitch: the inclination of props blades, which determines the theoretical forward movement the propeller makes at each rotation;
    • number of blades: normally 2 or 3 on RC boats, although there are special props for submarines with 5 or more blades;
    • shaft diameter: The most common sizes are 3/16 (.187") and 1/4 (.250").

    Besides that, we must take in considerations whether the prop will be used in a surface drive configuration wherever the prop will be used at the surface - a situation where half prop is under water and half out of water - or subsurface, in which the whole prop is under water. The first type - surface drive - gives better performance and for that reason is used on boats where performance is the prime requirement.. Because the of the vast variety of types, sizes, dimensions and materials of props commercially available in the USA your options are only limited by your pocket.

    Improving the propeller:

    (This one is a post from Jim Nissen himself on his board. As he says, was not tested, but makes a lot of sense)

    A tip I read about and have yet to try!

    Spray machinist die on the props. Run the boat several laps and bring it in. Look at the color of the die on the props. The areas that have the die removed are doing the most work. The dark areas are areas that improvement can be made and/or material can be removed. The ideal prop would have a nice even area of die removed. This would indicate that the entire blade surface is working together to move the water.
    Also will show if one blade is working harder than the other.

    I hear the big boys are using this technique with great success.

    Compensation for prop walk

    If sharpening is not enough to eliminate prop walk, you should adjust the side to side angle of your strut. Seen from back, the prop shaft must be angled 2-4' to the left. That way, the prop will tend to push the stern to the right, diverting the bow to the left, offsetting the natural tendency to the right. Use washers to do this so you can return easily to the original setup, if necessary. Use the least angle that corrects the problem. As a alternative, the rudder trim could be adjusted a little to the left (1 or 2 clicks on transmitter trim). More than this and excessive drag will occur, with a resulting performance loss. A wider or deeper rudder can also be tried.

    Two and three blades props

    The props normally used in R/C boating have 2 or 3 blades. More blades = more area = more pushing force but also more drag and more power demands on the engine. Three bladed props are more often used on cats and hydros, because they produce more lift. A two blade prop is normally faster. A three blade prop is equivalent to a 2 blade prop with 1 or 2 mm larger diameter - with the same pitch.

    Wedge type rudder:

    As stated on Assembling your R/C boat segment, the wedge type rudder is the best one. However, commercially available wedge rudders, probably because of production facility, are not of a true wedge design. Don Betz showed a very interesting drawing on how they are and how they should be. It's requires some work to fix them, but certainly it worth.

    Rudder to servo rod

    Stainless steel wheels spokes can be used as neat rudder to servo rods. If you have a 4-40 die, fix the non-threaded tip to the servo arm and cut the other (threaded) tip to the right length and use the die to make another thread. If happens that you don't have the tool, save the threaded tip, cut the other to the length and make a Z bend to fix it to the servo arm.

    Stuffing Tube

    Sometimes, it is necessary to elongate the stuffing tube, because you change engines or because you want to change the C.G. of the boat. If the stuffing tube is fiber glassed to the hull, taking it off, replace it and fixing the new one is a hard task. An easier and quicker alternative: use a tube with a diameter just a bit bigger than the s.t. fixed to the hull, solder inside it a piece of tube with the same diameter as the s.t. and long enough to achieve the desired length and solder the support tube to the s.t. Look at the picture bellow.

    Releasing a set screw from the ferrule

    If you already tried Corrosion X, heat, all the bad words you know and even a few others that you didn't remember you knew, that is the ultimate solution: a Dremel, emery-stone and there goes the set screw AND the ferrule.

    And now, a much better tip: don't ever, never, forget to disassemble the flex cable at the end of the day, mainly if you are running in salt water. No need to ask how I know, right?