Hydro System Tips for RC Boats

I will explain the most important setup aspects for sport hydro RC boats. The points covered can also be applied to some outrigger hydros. I will not cover things like center of gravity and engine placement as these should be supplied by the boat manufacturer. In most hydro’s the engine will end up being considerably forward of the leading edge of the turn fin. The CG will end up closer to the leading edge of the turn fin. This is generalizing and varies from one boat to another. The turn fin is what the boat "pivots" on when the rudder is turned. Moving the CG away from the point of pivot changes the turning characteristics of a hydro considerably.

There are three (3) pieces of hardware to adjust and fine tune on most hydro’s. These are:

The strut

The rudder

The turn fin

Adjusting the Strut: 
To do this, it's usually easier to remove the rudder blade. If the rudder extends past the back of the strut you can leave it on, otherwise remove it so you can move the boat forward on the table and have the entire strut bottom over the table surface. This is important when adjusting the strut angle.

In most cases the manufacturer will recommend 0 angles on the strut. They should also recommend a starting depth for the strut, usually measured from the bottom from the boat at the transom to the center or bottom of the strut. Lightly tighten the adjustment bolt(s) and double check your height. Make sure the strut is still flat on the table. This is checked by ensuring you do not see light under the strut when looking at it from the side, parallel to the table. Fasten the adjustment bolts and triple check the strut.

Setting the rudder: 
Setting the rudder is pretty straight forward. Unless the manufacturer specifies otherwise, you will usually want the rudder perpendicular with the riding surface of the boat. In this case, this means perpendicular to the table surface you just used to set the strut. For this reason it is VERY IMPORTANT to set the strut first!
With the strut adjusted, install the rudder blade and check that it is 90degrees with the surface of the table. The boat must be sitting flat with the strut and both sponsors on the table. If the manufacturer recommends some inner or outer tilt, you can check and adjust with a protractor again using the table as reference.
Setting the turn fin: 
There are two different types of turn fins more commonly seen on hydros. One has a straight bend and the other is a curved type. In most cases the manufacturer will specify where to mount the turn fin and how to adjust it. Again this varies from boat to boat.
Tilting the fin forward will have a tendency to make the boat track to the left and exert some lifting force on the right (starboard) sponson.
Tilting the fin back will have a tendency to make the boat track to the right and pull down on the right (starboard) sponson.

I hope this will help some of you fine tune and get the best possible performance out of your hydros!

How to correct the scheme of the boat

Center of  Gravity: The chapter How to Assemble 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 Angel: 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 to 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.

Trims 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 raise 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 scheme 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. 

These are some tips to adjust the scheme of the boat for those who want to build RC boats on their own.