Hull CG Position (Center of Gravity)

What is CG: center of gravity, the point makes the boat on balance, if you place an empty hull on a brass tube and make the hull stay still on the PVC tube not fall to either side, The CG is the point on the bottom of the hull between keel and the PVC tube which make the hull balances fore to aft. Correct CG will determine the attitude of your boat on water and is very important for a well performance boat, too forward of CG will make the boat runs with the bow deep into water, will be hard to plan and get speed up; and too far back of CG will cause a high lifted bow and makes the boat running unstably, rolling on the sides and sliding on the turns. However, the CG will be varying for different type of hulls: 

Deep-Vee Hull: should balance between about 27% and 30% of their length from the transom - so a 45 inch boat should balance between about 12 and 13.5 inches from its transom (the back of the hull). If there's too much weight toward the front of the boat, it will run "wet" or bow heavy with too much hull in the water. It could be hard to steer and will not perform to its speed potential.
If there's too much weight at the back, the boat will be slower to lift on plane, will probably have excessive bow rise as it accelerates from rest, and it will have a tendency to "porpoise" with the bow rising and falling whilst running along.

Catamaran: These style hulls are more sensitive to balance than Deep-Vee hulls and usually balance more toward the front at between 30% and 35% of their length from the transom - so a 45 inch cat should balance between about 13.5 and 15.75 inches from its transom.
The level of power in a catamaran can affect the best balance point too. A "stock" engine will require a balance point around the 30% mark, whereas a more powerful engine in the same hull will need the balance point between 33% and 35%.
If a cat is too heavy at the front, it will run "wet", lose speed and not turn very well. A cat that has too much weight toward the back will be "flighty" and can lift right off the water and flip over backwards - especially when running at speed into a strong breeze.

Hydroplane hulls: These hulls are even more critical than cats for balance and the best balance will vary depending on the style of hydro - sport hydro, race hydro, front sponsons, aft sponsons or outrigger-style hydro. With these boats, it is important to obtain the best balance point from the manufacturer as just a slight imbalance can make a big difference to top speed and handling. Balance points for hydros are even further forward than for cats, and can be around the 51% mark (of length from the transom) or around the leading edge of the turn fin.
Whilst also true for monohulls, the positioning and angle of the strut and rudder can particularly affect how cats and hydros handle, as does the choice of prop. All these factors inter-act with each other, so changing any one could well require changing the others if ultimate performance is to be obtained.

Tunnel Hulls: It is quite difficult to set up the correct CG for tunnel hull, it is somewhere around 33%-36% measure from transom of the hull, in most cases, the gas tunnel hull will use outboard drive with the gas engine on the top of the drive, this way, it will add a great amount of weight on the back of the back, therefore,  you will need to add weights to the bow or adding heavy battery pack into front servo box to obtain the correct CG, also install your servo box as forward as possible to make up the heavy weight in the back of the hull, the weights needs to add into the hull depend on the size of the hull and the according to the power engine you use for your hull. The CG could be more forward if you use more powerful engine.

How to obtain correct CG for your boat

First you will need to custom made a CG measuring instrument similar to the one we made at above picture, install all the transom hardware and engine into your hull, Arrow Shark offer smart billet rail which allows you to install all hardware on the rail outside of the hull then move the assembly rails into the hull, this way you can adjust the rail with hardware installed forward or backward to reach the correct CG spot.

Please keep in mind that as more powerful engine will bring the CG more forward, therefore, if you install a stock engine to a Deep-Vee hull, the CG would be around 27-30% measure from the transom, however, if you install a 6HP+ engine to a same boat, then, the CG would become around 30-33% measure from the transom.

Make a mark on the hull for the CG spot according to your hull type, then place the complete RTR boat on the CG measuring instrument, and match mark on the hull to the arrow mark on the CG measuring instrument, as now, the hull might lean to either front or back; adjust your smart rail assembly by pull it forward or backward according the hull leaning status till it gets complete balance on the CG measuring instrument.
The mild imbalance will not affect performance too much, it can always be trimmed by adjusting the trim tab or angle of the drive, but the closer to an exact lateral balance you can get, the better the boat will run on you and less adjustment you will need to do later.

However, if the imbalance is seriously, and there is no more space you can move the smart rail assembly further, then, you might consider to add some weight to either front or back to make it close to balance.