Gas Prop Essentials

The prop

The prop is responsible for transforming the movement of the motor’s power. It accomplishes this task by displacing a mass of water when turning. One of the first laws of physic states that with each action there is an opposite and equal reaction of the same intensity, and of contrary direction. As a result, the water movement is displaced backwards, and the prop is pushed forward. There is a rigid link to the hull, that results in the boat moving forward in a speed proportional to the volume of water being displaced. The prop rests in the water and moves forward, by the amount of it's pitch – in theory. As water is not a solid mass, the prop slips and the forward movement is less then the theoretical forward motion. 

So, the larger the pitch, the higher the speed, as a result of increased forward movement at the same time unit. This is true to a certain point. Sometimes, the pitch is so great, the engine can't reach it's maximum RPM. And less RPM, means less forward movement within the same time unit, = less speed. Compared to your car, a higher gear reaches higher speed – in normal circumstances. On a steep hill for instance, the load on the engine causes a reduction in RPM and a decrease in speed. Under these conditions, the 3rd gear may be faster. Like the car gear box, smaller pitch has more acceleration and less top speed, while greater pitch has more top speed with less acceleration. (not always true)

Conversely, too little pitch makes the engine over-rev, with possible costly engine damage.   

As your boat's engine doesn't have a gear box, the prop has to be carefully chosen to overcome these restrictions. 

Diameter is also of prime importance when choosing the right prop: larger diameter = more thrust, and greater capacity for moving a heavy hull. Increasing the pitch generally demands a reduction in diameter, lessening drag and increasing speed. Again, this has a limitation: a minimal diameter is often required to guarantee the necessary thrust.

Moreover, the diameter helps control slippage, larger diameter is less prone to prop slippage, until the larger diameter causes too much drag and reduces engine speed. In a way, diameter directly effects speed, but it's major influence is on water displacement.

The number of blades are also important. The ideal prop should only have one blade, eliminating drag of the other blades that are not producing useful work. Two bladed props are standard with most hulls, on other type hulls, 3 or even 4 blades may be required for additional acceleration and lift. Outriggers and Hydros are the best examples of boats that demand lift at the transom, to ride properly on a 3 point stance.

Blade Thickness  (thinner is best). A thin and sharp blade cuts the water and reduces prop-walk tendency (walking to the right as the transom goes to the left, because of the props turning effect).

Preliminary Definitions:

Blade: is the prop part that cuts the water and thrusts it back. In R/C speed boats we use 2 or 3 bladed props, with very few 4 blade props. 

Hub: central portion of the prop, where the blades and prop shaft are attached, The Hub provides most of the strength of the propeller.

Diameter: prop size, measured between the tips of the blades. Or 2 times the measurement between the tip of the blade and the center of the hub. Larger diameter props are normally used on large and heavy hulls. With R/C gas boats, 65 to 80 mm is the standard.

Pitch: theoretical distance the prop travels on each rotation. This is theoretical because water is not a solid medium and the prop slips. 10 to 30% is normal, the lower numbers are found only on surface drive hi-performance props, specially prepared.

Constant Pitch: the pitch is the same across the entire propeller’s working surface, or blade face.

Progressive pitch: the pitch is lower at the leading edge and increases progressively along the trailing edge. 

Consider a prop that runs in a liquid media with a constant pitch, the tip of the prop rotates faster than the hub. Conversely, if the angle at the tip is lower, the water speed on all prop surfaces will be the same. In this case, the performance is far better. Progressive pitch props, offer better planning performance.

High lift: a prop which tends to lift the transom when the boat is running.

Low lift: a prop which does not have this tendency.

Aspect ratio (AR): relationship between pitch and diameter, = (pitch divided by diameter). Normally a higher AR means a faster prop. Generally, a faster prop has a higher (AR), more Pitch, with less Dia. However this holds true to a certain extent, as adequate Dia. is necessary to sustained push!

Cavitation: is water vaporizing due to the extreme reduction of pressure on the back of the propeller blade. Many props partially cavitate during normal operation, but excessive cavitation can result in damage to the prop's blade surface. Note: Cavitation is often confused with Ventilation.

Ventilation: often confused with cavitation, it's normally induced by an external source, not by the prop itself. It occurs when air is directed on the prop surface. Eventually, it may help by reducing cavitation. On real boats, this is sometimes accomplished by directing the exhaust over the prop. On R/C boats this is not a common practice.

Cupping: Is a curvature added at the props trailing edge, normally very slight, towards the blade center, shaped like a spoon. It reduces cavitation and maximize thrust, increasing the effective prop pitch. Additionally, it lifts the bow and narrows the thrust cone. It looks like a magic medicine, the solution for all our problems but, like any medicine, it has to be taken carefully. Besides, cupping has to be equal in all blades, or you will have an imbalanced prop, that may damage your engine.

Camber: Arching curve from leading edge to trailing edge, like a spread out cup. It makes a progressive pitch at the prop. 

Rake: Is the angle of the blade attachment to the hub, and the degree that the blades slant forward or backwards. It's normally use to correct ventilation or cavitating situations. Furthermore, rake lifts the bow and so the speed. Typically, low rake props are used on sub-surface drives with heavy hulls. Higher Rake blades with surface drive application, tend to re-direct and condense the thrust cone further aft.

Prop Polishing

         Generally speaking, a prop that is high polished on both the (anterior & posterior) face & back of the blades, including the hub; will achieve the highest top speed. However there’s some advantage to a rough or less smooth surface, on the "back" of the blade. It gives more bite for short course acceleration, however this can cause drag at higher speeds.

 I believe there's little to no difference between a satin finish on the back of the blade, vs. high polish on both sides, until very high speeds are reached.After reading this, what lessons have you learned? The most important: there is no single prop that is ideal for all possible hull and engine configurations. The basic rules are listed, but consulting the experts and lots of experimentation, documentation, and patience is fundamental.

Generally speaking, a prop that is high polished on both the face & back, including the hub; will  achieve the highest top speed. However there is some advantage to a rough or less smooth surface, on the (back) of the blade area. It gives more bite for short course acceleration, however can cause drag at higher speeds. I believe there's little to no difference between a satin finish on the back of the blade vs high polish on both sides, until (very high speeds) are reached. I've tested the above polished vs satin props on the same boat, back to back. Satin = 72mph and polished = 72.4mph. You can your own conclusions weather it's worth polishing your props, however I polish mine for max speed potential.

Increased Pitch / Reduced Lift & Reduced Slip

" I believe this is very important to understand "

          With Surface Drive, you can gain additional speed with most props including the H50, by running it deeper............reducing slip. This is the single best trick for higher speed. In general surface drive props suffer from anywhere between 20 > 35+% Slip. The Speed resulting from High RPM is greatly diminished by excess slippage. The (Spoon Shape) blade narrows the thrust cone as water exits the blades. It is re-directed further aft for increased forward thrust and also reduces lift, as water exiting the submerged blade is also re-directed back. Not straight off the blade in a downward direction. This tends to raise the transom, causing unwanted "Lift".

         The trick is to modify the blade shape, to accomplish the above without loading the engine, and lowering RPM! With the (Spoon Shape) Mod you win 3 fold, (Increased Pitch), (Reduced Lift) & (Reduced Slip)!

Prop Wash Diagram

This diagram shows how the rooster tail is effected, when the strut or drive angle is raised or lowered. It also displays the condensed prop wash, that is a result of a modified (Spooned) prop. A narrow thrust cone causes increased forward push, that results in faster acceleration and top speed. A prop with higher blade rake : (trailing edge of blades are attached to hub at a higher angle, the blades extend further back) This also condenses the thrust cone, as the wash exits the angled back blades. Prop "Lift" is also effected by the above drive angle, rake, and spooned blades.