• RC Boat Hull Attitude


    What we call attitude is the way a R/C boat behaves on water. This page explains the theory to help get the best performance out of your R/C Model Boat. Theoretically, when moving a perfect boat should:

    • have a reduced wet area: just a minimum part of the hull touching the water
    • not have prop walk: tendency to turn right even with the rudder on a neutral position
    • have no tendency to chine walk: side to side movements when traveling in a straight line
    • have no tendency to porposing: bow oscillation, up and down when traveling in a straight line
    • have no tendency to torque roll: rolling to the starboard as a reaction to the engine torque
    • be stable: not slip too much and not roll on turns.


    This is the theory. In practice, things happen in a slightly different way. There is not such thing as an ideal boat and so, we must adjust it to make it behave as close to this ideal as possible. Let's exam each on of those items and discuss how to correct "bad habits".

    Before any adjustment, it's important to have in mind that a boat on the water is in equilibrium over two lines. One of them cuts it from bow to transom and other transverses from port to starboard, at the boat's Center of Gravity - CG. This understanding is important to make it clear that all adjustments are complexes and seldom produce a single effect. For instance, lifting the transom makes the bow go down into water as a consequence. Lifting starboard makes the port side deep into water. To extend the prop far from the transom to reduce prop walk may result in porpoising. In other words, whatever you do in one side induces a counter-reaction on the opposite side; sometimes correcting a problem causes another.

    Center of Balance - CB: line that cuts the boat in two halves from bow to transom. Movements from starboard to port or v.v. occur alongside this line.



    Center of Gravity - CG: point of equilibrium of the boat, between bow and transom. Bow to transom movements occur over a shaft that runs through this point.



    The higher over the CB the Center of Gravity, the higher the tendency for the boat to lean on turns. Higher/heavier engines tends to raise the CG - the farther from the keel the CG, more unstable the boat will be. Compare the race cars with their street cousins: lower=more stable.

    Wet area:
    the smaller the portion of the boat in contact with the water, the higher the speed. This is pretty obvious because the drag of the hull is proportional to the area in contact with the water. The limit is attained when the wet area is so small that we lost the control of the boat.

    How to correct the attitude of the boat

    Center of Gravity: The chapter How to Assemble a R/C Boat, in the Technical session, shows how to calculate and adjust the CG. It's easy to understand that a CG towards the front tends to maintain the bow down into water, increasing the wet area and hampering the planning of the boat when at speed. On the other side, a CG too close to the transom tends to raise the bow too much, making the boat unstable at speed. As a rule of thumb, CG must be between 27% and 30% of the total length of the boat, measured from transom, for a completely assembled boat - ready to run except for the fuel. As very powerful engines tend to raise the boat's bow, under those circumstances a CG over the upper limit (30%) may be advisable. On the other hand, mild engines will be happy with the 27% rule.

    Strut angle: A strut has a positive angle when the prop shaft points upwards, far from the water surface. In this situation, the transom is pressed down. A negative angle occurs when the prop shaft points down, towards the water surface, raising the transom. If you imagine the boat supported on a transverse shaft that goes exactly through the CG, it's easy understand that, when the transom is lowered the bow goes up, reducing the wet area. The contrary occurs with a negative angle for the strut. Although apparently we could use the strut angle to correct any incorrect attitude, the truth is that the neutral angle is the ideal, just a few degrees off are allowed to adjust the wet area. If more than 2 degrees are necessary, something else should be wrong.


    (from a original design by Ron Frank)

    Trim tabs: Normally 2 are used on each side of the boat. The outer ones take care of the behavior of the boat on turns, the inner ones of the attitude when traveling in a straight line. We will take care of those from now. Increasing the trim tabs angles - orienting down the adjustable portion - raises the transom, dropping the bow - the boat increases the wet area. Inversely, reducing the trim tabs angle - directing up the adjustable component - lowers the transom and raises the bow, reducing the wet area.

    Outer trim tabs act on turns. Remember, adjusting them to rise the transom makes the nose dig on turns, making the turns eventually sharper than desired.

    Once again, the degree of possible actuation is limited - if correcting the attitude demands more than few degrees of actuation, we must look in another place the solution of the problem.



    Hooks and Rockers: Hook is a concave surface on the bottom of the hull, near the stern, that acts like a big trim tab and raises the transom - lowering the bow and increasing the wet area. Rocker is exactly the opposite - a convex area near the stern, that lowers the transom and raises the bow.
    Some manufactures claim that those surfaces are made on purpose on their hulls, to determinate the attitude of the boat on water. I don't like the solution, I prefer to believe that both are defects that must be corrected. The attitude of the boat must be determined by devices that allow adjustments and not by this way.

    The correction is very complicate. The rocker must sanded until be level with the bottom of the hull, the hook must be filled with and sanded.
    Both corrections are complex and time consuming, reason why I suggest you to avoid a hull with any of those characteristics.

    Propellers: Some models of props have as a characteristic to raise the transom, they are the high lift props. Among Octura props they are the ones with no letter on the identification number; the low lift props have a letter on their codes: X or V series belong to this kind. Check if you are using the proper prop and change if necessary.

    Drives: Some drives may have a tendency of raising the transom. Possible corrections are:
    • adjusting the drive angle, making it a little positive;
    • if the drive has a straight and level surface contacting the water, consider rounding this surface on a lathe.


    To move the prop far from the transom: extending the prop far from the transom creates an automatic correction for the excessive raising of the bow because every time the bow raises the prop goes deep into water and raises the transom, counter reacting the raising of the bow - again an excessive correction raises another problem - in this case porpoising.

    Other elements that have an influence on the boat attitude on water are the strakes and steps, which are part of the hull design and are not susceptible to correction. If those characteristics are not suitable, choose another hull.

    Prop walk: The counter-clockwise rotation of the prop acts like a paddle wheel and drives the stern to the left. Consequently, the bow is deviated to the right; the boat doesn't run in a straight line even with the rudder on a neutral position. Possible corrections are:

    • Sharpen the prop:The sharper, the less the tendency to prop walk, there is no counter effect to this solution;
    • Deviate the strut to the left, to counter react this effect. Again, the angle should be very mild because the deviation to left tends to raise the port side of the boat. Added to the raise caused by torque roll, it can result in an excessive raising and a boat with tendency to chine walk.
    • Adjust the transmitter trim, so the rudder is a few degrees to the left. Again, little correction is advisable because the rudder on this position cause drag and reduces performance.


    Chine Walk: It's a lateral movement, from side to side - the boat looks like walking over water - if the other adjustments were made into the narrow limits allowed, you may try to correct chine walk adjusting the trim tabs. Check which side raises and reduce the trim tab angle at this side and/or increase the angle of the opposite trim tab. Remember that; raising the stern makes the bow down at the same amount - the inverse is also true. So, if the correction implies dropping the port side this may result in an excessive raising of the bow and make the boat unstable. A better solution could be raising the starboard, adjusting the right trim tab. Same is true if correction implies raising the transom of a boat that already has a big wet area: lowering the other side of the transom would be a better solution.

    Porposing: Up and down movement, when running in a straight line. Could result from a prop too far from the transom, by a too positive strut angle or a too shallow prop. Possible corrections are:

    • Bring the prop near to the transom, 1/2 inch each time, testing after each adjustment;
    • Adjust the strut angle;
    • Lower the strut.


    Torque roll: As a reaction to the engine torque, that turns counter-clockwise, the hull leans to the right - port side raises and starboard goes deep into water. Possible correction are:

    • Mount the strut to the right of the keel (not more than 1/8 inch). So, the lifting force of the prop is applied more at the right, raising the starboard and compensating for the torque roll;
    • Lower the starboard trim tab;
    • Move to the left everything that is not fixed to the hull (battery, fuel tank, etc.)


    Stability on turns:
    Every boat sideslips and leans on turns. If side slipping/leaning are excessive, impairing the turn negotiation, those are possible corrections:

    • Adjust the external trim tabs, raising the transom and lowering the bow at turns; as a consequence, the hull will tend to make sharp turns and minimize the side slip.
    • Use a bigger rudder and lower it: the bigger and lower rudder tends to raise the transom and minimize the side slip.
    • Lower the CG. Possible correction is lower the engine/tuned pipe.


    I hope I could make it clear that there not a magic solution. Correction of a boat attitude demands time and a lot of tests. And remember: the boat is over a seesaw, whenever you do something to a side, a reaction occurs on the opposite side. Some corrections - maybe all - are not a consensus. For instance, using trim tabs for correcting the boat attitude is condemned by Jim Nissen. He say that trim tabs are crutches. And a final word: adjusts for correcting a problem can cause others. Complicate? Sure. But this makes the delights of the hobby.

    Finally:

    We may put together everything detailed above in a few drawings that concentrate the multitude of available options for adjusting your boat. Naturally, the drawings are exaggerated on purpose, to make the idea clear. If you need so radical adjustments, something is strongly wrong with your boat.

    Center of gravity





    Center of rolling





    Strut angle




    Strut deviation





    Rudder deep





    Rudder angle



    Rudder size





    Propeller deep





    Prop distance from transom





    Trim tabs





    Addendum: Ron Frank posted this at Jim's site. His secrets to adjusting his boats for racing. Read and compare.

    SETTING UP A MONO HULL FOR OVAL RACING

    BASICS TO SETUP A MONO HULL FOR OVAL RACING

    While there will certainly be conflicting opinions, this is what works for me. In my opinion a well setup boat should run straight & turn predictably under full throttle. The only reason to let off the throttle is to avoid an obstacle or correct from hitting a wake or wave.

    NOTE: Use of a GPS, Radar gun or speedometer is mandatory. A well setup, stable boat will look slower than a poorly setup boat & it's difficult to see a 2-3 mph speed difference.

    1. STRUT:
    With No tabs or turn fin, rudder set at neutral on radio & parallel to the keel. Use a known good prop.

    Run straight, shim strut mount as necessary to run straight. When correct the boat will run straight �hands off�
    Adjust strut angle for ride attitude.
    Adjust strut depth for max. speed.
    Move tank, battery etc. to compensate for any torque roll.
    If you have a good prop you should have little if any torque roll.

    2. RUDDER:
    Set D/R to low setting.
    Turn both ways & trim rudder depth till turning degrades.
    Adjust D/R till you can hold the turn well.
    NOTE: A mono will slide in the turn at this point, the turn fin (STEP 3) will correct this on a right turn.
    Another note: If the rudder is to short you won't turn left. This is because as the hull leans left it pulls the (right side mounted) rudder up, maybe completely out of the water!

    Rudder angle: To lower the bow in the turn, angle the rudder forward.
    To raise the bow in the turn kick the rudder back.

    3. TURN FIN:
    Install the turn fin, make sure it is parallel to the keel. If the boat now turns either way, the turn fin is not parallel to the keel. Adjust T.F. mount as necessary to run straight.
    Try different fins till the boat will hold a full throttle right turn without sliding, skipping or hooking.
    NOTE: If the turn fin is in the water while running straight it may just be the drag causing a slight right turn, adjust with rudder trim or re-shim the strut mount.

    4. TRIM TABS:
    John Finch's excellent article says it all. www.intlwaters.com/fvrcb/monomani.htm
    It's a great reference for setting tabs as well as other areas of mono hull setup.

    Basically the tabs are used to correct deficiencies in the hull and adjust the boat in differing conditions. The tabs will affect ride attitude, turning and directional stability. With tab angle, a little goes a long way. Make small adjustments & TEST TEST TEST.

    5. PROPS:
    Props will affect every aspect of the setup, straight running, turning, ride height etc. The only way to find the best prop for your boat is by trial & error. Again, TEST TEST TEST.

    Speed in oval racing is not the primary objective. Any good handling mono in the 45-50 MPH range is capable of winning oval races.
    The fastest boat usually doesn't win.

    Comments & criticism graciously accepted.

    Good luck
    Ron



    Carlos Andrade