by hmvh

Propellers are one of the key elements of a fast radio controlled boat, but few boaters could tell you much about how they work, or how to choose the best one and optimize its power. Once you have this knowledge, you’ll be able to maximize your boat’s speed without damaging your motor.

Pitch vs. Slippage

Propellers are sometimes compared to a screw, but there is one significant difference. The “pitch” of a screw is measured by how many rotations it takes to go in one inch. This is the same as the number of threads per inch. So, a screw with a pitch of 12 threads per inch will require 12 full turns to go in one inch.

Propellers aren’t quite as efficient because they “slip” a bit pushing through the water. A prop with a 2 inch pitch would theoretically move the boat forward 2 inches for every revolution, but because of slippage moves somewhat less than that. The amount of slippage depends on the design of the prop, the motor that drives it and the speed of the boat. A light hydro boat moving at top speed might have a slippage of only 10% or so, but most boats will fall into the range of 20-30%.

Diameter and Pitch

The diameter of a prop is measured from tip to tip of the blades. The larger the diameter, the more water it can push. A larger prop also needs a larger motor to power it, or else a gear drive that will apply sufficient thrust without overheating the smaller motor.

The diameter and pitch of the prop are the two numbers you’ll need to know in order to choose the best prop for your boat.

Choosing the Best Prop

If you buy a kit or ready-to-run boat, it will most likely come with an inexpensive, low performing motor and a prop that will give a long run time with low amp draw.

The reason the manufacturers do this is so beginners can have fun right away with a minimum amount of frustration. More advanced boaters can upgrade the same model with more powerful motors and propellers.

Once you’re ready to upgrade your fast electric boat, first try a prop with a slightly larger pitch, then one with a larger diameter. Run the boat at top speed for one minute and then feel the motor. If it’s too hot to touch, the prop is too big, so try a smaller diameter or pitch.

If you’ve tried to upgrade your boat’s speed with a hot new motor, you should also change the prop, which was matched to the stock motor. The new motor will draw more amps at lower RPMs than the stock motor, so you’ll probably need to put on a smaller prop to keep the new engine from overheating. The smaller prop will allow the new motor to obtain higher RPMs and thus higher speeds for your radio controlled boat.

Another design factor that affects your choice of prop is its lifting ability. Two props with identical diameter and pitch may have their blades shaped differently. A lifting prop could be useful on a hydro to get its transom out of the water, thereby reducing drag and increasing speed.

Prop Materials

The three most common materials for fast electric boat props are plastic, aluminum, or beryllium copper.

The best plastic props usually have graphite fibers embedded as a stiffener. These props usually come well-balanced and are inexpensive.

Aluminum props are light with little flex. The rarely require balancing and are easy to polish. However, they can crack if bent too far and cost the same as props made of harder materials. Aluminum is too soft to hold a sharp edge, which is necessary for top performance.

Props made of beryllium copper and similar hard materials are the choice of most serious boaters and most records are held by boats using this material. They can be sharpened to a razor’s edge and polished until they gleam, but they will require balancing.

Balancing Propellers

Props made from light materials like plastic and aluminum rarely cause vibrations at high RPMs, so they’re rarely out of balance. However, heavier blades made from beryllium or bronze turning at 20,000 RPMs or more can produce enough vibration to interfere with radio transmissions and can even crack the hull. To avoid these problems, high performance props need to be properly balanced.

You can make a balancer with scrap wood and razor blades, but if you do a lot, you be better off buying one designed for boat props. Some balancers designed for airplane props aren’t sensitive enough for the smaller props used in radio controlled boats.

To balance a prop, place the axle on the razors and see which propeller blade drops to the bottom. This is the heavier end and needs to have some material removed to lighten it. Using a metal file, start at the leading edge and remove material from the side of the prop that faces the bow. Check the balance often and stop when the prop rotates very little when placed on the balancer.

Be sure to use goggles and a good respirator when filing a beryllium prop to keep the poisonous metal particles out of your eyes and lungs. Use 100-grit wet-or-dry emery paper to remove the file scratches. Then finish with a 400-grit paper.

Once you’ve put this much work into modifying your props for maximum performance, you’ll want to take good care of them. A plastic box for storing fishing flies works well for this.

Upgrading your boat’s performance by choosing and balancing the propeller is a matter of some trial and error, but is definitely worth it. Have fun!