Hydrodynamics vs.Aerodynamics

"Hydrodynamics vs Aerodynamics"

Propeller, Rudder & Turn Fin:

In optimizing the competition RC boat, careful attention must be paid to any item which comes in contact with the water. This is the "hydrodynamic area" we must all be VERY concerned with. Why? Because anything which touches the water is subject to hydrodynamic drag which is 10 times, yes 10 times more significant than aerodynamic drag.
You can see that any small change in the shape, sharpness, etc. of your propeller will have huge effects on the performance of the boat. You may have experienced a boat which starts to pull to the right when you change to a specific prop. If you will check, the prop is probably not very sharp along the edges. What happens, in an exaggerated example, is the edges hitting the water are like a flat board which don't want to penetrate the water. If they are VERY sharp, they will cut into the water easily. You can see that the prop with the "Square Edges" will try to walk across the water and not penetrate the water. You MUST spend a lot of time sharpening you prop to a very sharp edge.
The rudder is another very important item which is running submerged all the time. It has a significant sectional area which can be substantial drag. It is very important that the leading edge of the rudder be very sharp. The rear of the rudder blade would be much better if it came to a tear drop, however after trying this I found that if you round off the trailing edge of the blade, you will get a continual oscillation of the blade at high speed. I found that by having the rear of the blade square, it worked as well as any way I tried. The bottom of the rudder blade should be rounded so as not to lift the boat on the flat rudder bottom. You should also make sure that the sides of the rudder blade are not concaved. Some of the commercially available blades have this concave and it makes the blade drag a great deal. If you find a concave, file or sand it out so the blade sides are straight.
You should also pay very careful attention to the turn fin. This is a great contributor to total drag. I sharpen my turn fins ONLY on the outside (outside of the turn) because I believe that any sharpening in the inside tends to make the fin try to lift out of the water. Again, make the turn fin very sharp and make the trailing edge square.

Sponsons & Strut:

Some Say.....the sponsons should have very sharp edges in order to keep water from adhering to the edges in a "boundary layer". I don't spend much time keeping my edges sharp, since I find that VERY sharp edges tend to not hold paint very well. The edges seem to chip on the sharp edges and make the sharp edge rough. Paint will also tend to round the edges on its own. I like to have the edges of my sponsons slightly rounded to keep the paint on well. The strut should have very well defined edges if you use a square bottom strut, and very smooth if you use a round strut.

Additional Considerations:

After you have ALL your aerodynamic boat components optimized, then it is time to look at aerodynamic items. As you can see, the hydrodynamics are 10 times more important than the aerodynamic.