• RC Boat Definitions

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    New comers to the R/C Model Boat hobby are often confused by the terms used. This page gives definitions for common R/C terms and are grouped in a logical way.

    General:

    ARR: Almost Ready to Run boat received from the vendor almost ready to go. Very little assembly is required.

    ARF: Almost Ready to Float synonymous of ARR.

    Cavitation: A situation in which a propeller slips in the water, much like a slippery clutch on a car. In this condition the propeller runs at high RPM but almost no forward movement is created. Normally caused by too small a propeller or/and too little pitch.

    Rooster Tail: The water spray from the propeller behind the boat. Too much could be a indication of poor alignment of your strut - but it's beautiful thing to see.

    The picture bellow belongs to a Dom Mauro's ACLaser45



    Engine:


    Unlike a 4 stroke engine, almost all 2 stroke engines that are used on RC boats have no valves. During each revolution the oil/gas mixture is admitted under the piston, compressed in the crankcase and pushed to the combustion chamber trough the transfer ports. Another fundamental difference is the fact that each complete turn of the crank produces power. Besides this, the same physical principals apply to both motors.

    Top Dead Center - TDC: The place where the piston is at it's highest position. A degree wheel fixed to the crank should read, at this point, 0.

    Bottom Dead Center BDC: place where the piston is at it's lowest position. A degree wheel fixed to the crank should read, at this point, 180.

    Admission timing (also known as intake timing): Indication, in degrees of rotation of the crank, of how long the intake port stays open, allowing the gas/air mixture to enter the engine.

    Exhaust timing: Indication, in degrees of rotation of the crank, of how long the exhaust port stays open, allowing the burned charge to exit the engine.

    Ignition timing: Indication, in degrees of rotation of the crankshaft, where the spark plug ignites, burning the fresh charge.

    Blowdow: Difference, in degrees, between the start of the exhaust port and the start of the transfer port.

    CC - cubic centimeters: Metric unit of measuring the engine's displacement. Gas engines normally found on RC boats are from 18 cc to 45 cc. Even in the USA the unit is CC, not cubic inches, presumably because the engines are mostly from Europe or Japan. (1 cubic inch=16,38 cc).
    Hull:



    Bow: The front of the boat, as opposed to the stern.

    Stern: The rear of the boat.

    Transom: The vertical portion of the stern where the rudder an strut are secured.



    Beam: The width of the hull, at it's widest point.

    Starboard: Right side of the boat, as seen from behind.

    Port side: Left side of the boat, as seen from behind.

    Freeboard: the lateral panel of the boat, over the water line

    Chine: the lower edge of the freeboard, the seam with the bottom of the boat

    Sheer: the upper edge of the freeboard, the seam with the deck

    Keel: the centerline of the boat's bottom

    Lift: lifting of the stern caused by the turning propeller, by design and/or placement of the rudder or design of the hull.

    Porpoising: bow oscillations, up and down, when running in a straight line.

    Torque roll: tendency of the hull to lean to the right as a reaction of the engine torque.


    Chine walk:
    the boat rocks from side to side (chine to chine). Usually happens on hulls with a flat "ride pad". As speeds increase the boat looks like it is walking across the water. It can usually, but not always, be cured with the trim tabs.

    Hook: A concave area on the bottom of the boat near the transom, acting like a big trim tab.

    Rocker: The opposite of hook. It is a convex area near the transom that actually acts to lift the bow. More accurately it creates suction at the transom which in turn lifts the bow like a lever.

    Deadrise: The angle of the bottom or Vee.



    Variable deadrise: Means the Vee angle changes down the length of the hull. Sharper angle toward the bow and less angle toward the transom. Ok for slower boats, but it puts a limit on the hull speed.

    C/G Center of gravity: The boat's balance point from bow to stern. Must be established with the boat totally assembled, ready to go to water, except fuel. Normally is situated between 27 - 30 % of the total length of the boat, measured from the stern.

    Engine rails (also incorrectly known as stringers): 2 pieces of wood or other material, bonded to the interior of the hull, parallel to the boat's keel. To the rails fastened the engine, the radio box and the gas tank. On gas boat they are, normally, 5 inches apart.

    Pads: parts of the hull which touch the water when the boat is moving. Of course, the area varies according the velocity and CG of the boat. Ideally, we must look for the ideal point between the least wet area possible and the desired stability. Ride pads are flat portions designed into the hull to aid acceleration and/or top speed.

    Skeg: a small fin, normally found on crackerboxes, attached to the bottom of the hull, that helps the boat on turns.

    Spray rail: A step on the side of the hull over the water, along the length of the boat, designed to avoid the water climbing the hull. Helps to keep the interior of the hull dry and may also help performance.

    Steps: Located at the bottom of the hull, the steps allow air under the hull, making bubbles and lessening the hull's drag. Steps run perpendicular or angled to the movement direction and are normally found on catamarans and deep-vees, boats with wide wet surfaces.



    Strakes: Raised portions on the hull that run parallel to the keel from the stern forward. Strakes help the hull to plane and also help keep the hull from side slipping when turning. Normally there are 2 external strakes (also know as chines), for lower speeds, and 2 inner strakes, for higher speeds. On fun boats those strakes can run for the entire length of the boat, possibly causing excessive lift. On faster boats they are short, because excess lift causes lateral movement and loss of stability.


    Strakes on a fun boat


    Strakes on a boat trimmed for speed

    Hull's types:

    Mono hull (mono):
    The hull has a continuous wet surface. It's the most common design for real boats. Monos can be:

    Deep V: Viewed from the front a deep Vee shows a very pronounced V shape (more than 16deg). Once more, that's the design commonly found on real boats.

    Cracker box: The hull has an almost flat bottom, in opposition to a deep V.

    Hydroplane: in generic terms, any hull with more than one wet surface. They can be:

    • * Unlimited hydroplane: A scale copy of the real hydroplanes. They have 2 sponsons which go from the bow to 1/3 of the hull length. At speed, just the sponsons and the back portion of the hull touch the water. They demand calm water to operate.
    • Outrigger: the fastest boats in the R/C field, there is no counterpart real boat. Basically, they have a rectangular box containing the radio box and engine and 2 sponsons linked to the central box by booms. At speed, the boat is supported by the back portion of the sponsons and by the propeller. They need calm water to operate.
    • 4 points hydro: similar to the outrigger, with 2 additional small sponsons on the back of the hull.
    • Tunnel hull: the hull has 2 sponsons the entire length of the hull, creating a air cushion, allowing the boat to float on the cushion of air, reducing drag . Because of it's reduced space, tunnel hulls normally run outboard engines.
    • Catamaran: a mix between a V and a Tunnel. Very good stability, can be operated on less calm waters and it's inner space allows the utilization of in or outboard engines.


    Drive train: Those are the parts that transmit power from the engine to the propeller.

    Square drive: Drive assembly using an adapter screwed on the engine shaft, which has a square hole on the other end, where the square ended flex cable is inserted. Transmits the rotation of engine to the propeller shaft. Another type is the collet drive, where the square drive adapter is replaced with a device that squeezes to hold the round flex cable, much like the way a electric drill holds the bit.


    Collet drive for flex cable with round tip

    Flex cable (also flex shaft): A cable that connects the engine crankshaft to the propeller shaft, allowing the prop rotation. Being flexible, it can be curved for going from the hull interior to the exterior, behind or under the transom. It's normally linked to the prop shaft by means of the ferrule.

    Stub Shaft Cable: A flex cable that has the prop shaft already welded to the flex cable.

    Ferrule: A brass part soldered to the end of the flex cable and secured by set screws to the prop shaft. Transmits the rotation from the flex cable to the prop shaft.



    Propeller shaft: A stainless steel shaft, linked to the flex cable, that drives the prop.

    Negative angle shaft: a shaft that points downwards and has a tendency of lifting the transom.

    Positive angle shaft: a shaft that points upwards and has a tendency of pushing the transom down.

    Drive dog: A collar, secured to the prop shaft, that has notches matching the propeller to transmit the prop shaft rotation to the prop.



    Propeller/prop: The device which during it's rotation, pushes the water rearward, moving the boat forward.

    Prop walk:
    Tendency of the prop to turn the boat to the right even with the rudder in a straight line. This is due to the prop encountering resistance when the blade pierces the surface of the water. The prop tries to raise the hull and "walk" sideways across the water.

    Tail nut: A nut screwed onto the prop axle, to hold the prop on the shaft

    Drive train position:
    The prop position in relation to the water surface:

    Subsurface: The stuffing tube exits through the bottom of the hull and goes under the transom. The prop is always completely under water.



    Surface: The stuffing tube exits the hull at the transom. At speed, just lower half of the prop is under water.



    Types:

    Direct drive: the prop is linked to the engine by a solid shaft or a flex cable, so for each rotation of the engine, corresponds a rotation of the prop.

    Geared:
    the prop is linked to the engine by gears. Although this can also result in a 1/1 relation between engine and prop, commonly the gears are used to increase (gas engines) or decrease (electric engines) the prop speed. We also see this kind of utilization when 2 engines are coupled to one prop or 2 props are linked to just one engine.

    Transmission:

    Straight shaft: A solid shaft linked to the engine by means of a coupler and to the prop by screw and nut. Not normally used on gas boats.

    Straight shaft with U joints:
    same as above, with universal joints at the tips, that allow adjustment in the shaft angle relative to the prop and correction of minor misalignment between engine and prop. Not normally used on gas boats

    Flex cable: Universally used on boats with gas engines, it uses the same type of flex cable that comes with the string trimmers or weed eaters from which the engines are derived. Thanks to their flexibility they conform easily to the stuffing tube's curvature, allowing for the adjustment between the engine angle, tilted in relation to the hull, and the prop, parallel to it.

    Rudder:

    A metal fin fixed to the transom via a pivot which is turned by a servo commanded by the transmitter, that allows changing the direction of the boat.

    Skeg:
    A small fin fixed to the bottom of the hull to avoid excess side slippage in turns. It's mainly used on flat bottom and crackerbox hulls

    Strut: Support for the prop shaft and sometimes to the rudder(s), fixed to the transom

    Stuffing tube: A tube, normally brass, that, from the engine, goes through the hull and ends near the prop shaft and is used to contain the flex cable. The stuffing tube may or may not be teflon lined.

    Trim tabs:
    Metal fins at a 90 angle, affixed to the transom to adjust the boat's attitude on the water. The bigger the angle, the more the transon lift. The more sophisticated tabs allow adjustment while underway by means of a servo commanded by the transmitter.



    Turn fin: A metal fin fixed to the transom of a mono hull, the right sponson of a hydroplane, or the sponson of a tunnel hull in such a way that, on turns, helps to maintain the boat's stability and prevent side slipping.



    Radio:


    Battery pack: A set of rechargeable cells that provides electricity to the transmitter and, on the boat, to the receiver and servos.

    Cell: Each one of rechargeable cells with 1.2 volts that make a battery pack.

    Radio box:
    Box, presumable water proof, containing the servos, receiver and battery pack.

    Servo:
    A small case containing an electric motor, gear assembly and electronics. The servo is controlled by the receiver upon receiving commands from the transmitter. On gas boats 2 servos are used: one for rudder, the other for carburetor.

    Carlos Andrade