ESTABLISHING SUBMERGED AND SURFACE TRIM
If your r/c submarine is of the 'wet-hull' type, the hull must flood and drain quickly and completely when in the water.
Improperly placed or an inadequate number of vent holes in the top of a free-flooding type hull is why so many r/c submarines experience angle and depth control problems while running submerged. Entrapped bubbles under the hulls deck - moving for and aft with changes in model dive angle, ever changing the submarines center of buoyancy - makes for an impossible or, at the very least, a very difficult to operate model submarine. In fact, getting the correct number and placement of vent holes in the top of your model is so important that we have done it for you already.
SUBMARINE STATIC STABILITY
Static stability, the submarines ability to automatically right itself when upset in the roll or pitch plane, is dependent on the vertical alignment and distance apart of two opposing forces. One force representing the accumulated gravity - pushing down. The other force line represents the accumulated buoyancy - pushing up.
Static stability exists when the focal point of the two forces are vertically aligned with the center of buoyancy (c.b.) above the center of gravity (c.g.).
The greater the vertical distance between these two force focal points, the greater the roll and pitch righting moment and the more stable the model becomes.
(insert sketch of side-view of submarine with c.g and c.b)
You may have observed that much more fixed lead weight is provided with the model kit than is required to achieve surfaced and dived trim. The additional weight lowers the center of gravity (c.g.), enhancing the models static stability.
Since there is no control surface to provide righting forces in the roll plane, we rely entirely on static stability to right the boat in roll when an outside force acts on the sail to roll the model over. However, static stability forces are no match against stern plane or hull induced pitching forces at any but the lowest underwater speed.
DYNAMIC FORCES ACTING ON THE SUBMERGED SUBMARINE
During 'sea trials' you will observe that a submerged model will evidence a pronounced tendency to roll inboard of a sharp turn. This is do to dynamic forces acting on the sail. Sail induced roll is countered only by the models static stability in the roll plane. It follows that models with exceptionally large sail areas (the model class has the most massive sail structure for size of hull I know of) need exceptionally massive amounts of fixed ballast and floatation to overcome the sail induced roll.
(insert sketch showing yaw, roll, pitch axis)
The leverage the stern planes and rudder(s) exert to control angle (pitch) and course (yaw) is dependant in part on their distance from the models center of rotation - a point between the c.g. and c.b. The other factors relating to the force exerted by the submarines control surfaces include: their area; amount of deflection (between 0 and 35 degrees); velocity of water over them; and blanketing effects presented by hull, stabilizers, and other turbulence inducing structures.
With your model model, the area of the stern planes is a value fixed by design, as is the maximum amount of deflection (35 degrees is typical). Using the recommended WTC-3, the models center of rotation is far enough forward to assure quick yaw and pitch rates in response to rudder and stern plane deflections.
CORRECT PLACEMENT OF THE MODELS CENTER OF GRAVITY
Check that your model balances with the c.g. near the center of the WTC-3 ballast tank. Put the model in the water to work out the amount of fixed buoyant material needed. Placed within the free-flooding spaces, the foam counters the weight of the WTC-3 and the fixed ballast weight you just installed. The majority of fixed buoyant material goes in the bow.
First thing, before putting the model in the test tank, stick as much foam in the bow possible. Button up the model (turn on the transmitter and WTC) and stick it in the water. Command a vent. Holding the model underwater with one hand, shake it a bit to dislodge any bubbles that may be hanging up in the hull or ballast tank. Does it sink like a rock or float? is it nearly neutral; does it stay at any depth with little vertical travel?
ESTABLISHING SUBMERGED TRIM
Correct submerged trim is achieved when the submerged model (with completely flooded ballast tank) is near a 'neutrally' buoyant condition in the water.
Charge the on-board bottle fully with Propel, this adds about 1.5 ounces of weight to the model. Place the model in the water and flood the ballast tank. If the model is heavy (sinks when released), add closed cell foam pieces high within the hull, but below the designed surface trim water line, until near neutral buoyancy is achieved.
Determining placement and amount of buoyant foam is conveniently done by holding foam pieces against the hull with rubber bands. Later, after determining the amount and location of the foam pieces, they are placed within the hull and secured with RTV adhesive. Remember, no foam should be placed higher than the models designed water line.
The opposed gravity and buoyancy force lines act to right the model to an even keel when released. For you beginners, good submerged trim is achieved with the sail sticking about 1/2 inch out of the water with a full charge of Propel on board. After the propel charge is expended, the models submerged trim will place the water line about 2 inches down from the top of the sail. Now you can appreciate why the center of rotation is at the point on the hull where the on-board bottle sits: as Propel is consumed, the model becomes lighter, but longitudinal balance is unaffected.
ESTABLISHING SURFACE TRIM
The fixed volume of the WTC-3 ballast tank produces enough buoyant force to lift your model to the correct water lines. Blow the ballast tank to surface the model to normal surface trim. Providing your have placed all of the buoyant foam beneath the submarines designed water line, the boat should assume correct surface trim when the ballast tank is dry.