Model Boat Propeller Direction and finding a prop for my Springer Tug

Rant about Model Boat propellers

I want to rant about boat propellers. I've recently finished a Springer Tug (video above, my young son on the sticks), and buying the prop has been a frustrating experience. I'll blog later about the actual build itself, but this post is about my prop-frustration and what I've learned. The basic issue is that some manufacturers label the orientation of their props unconventionally, or not at all. Due to this, I have had difficulty selecting a prop, and even had one spin off in the water. Thankfully, my boat was safely rescued, but the prop remains resting at the bottom of the pond. This article will help builders understand prop direction, why it is important and how to obtain a prop with the correct orientation. 

The main factors in a prop are:

• diameter
• number of blades
• material (e.g. nylon, brass)
• manufacturing process (e.g. moulded, soldered)
• hub type/mounting method (e.g. threaded hub, plain, dog drive)
• shape, which crucially includes both pitch (how far it travels forward every turn) and rotation direction. 

Prop Direction

Conventionally, in marine engineering, prop direction is as follows, quoting directly from Tupper 1996 and Techet 2005 (see References below): 

Handing of propellers: If, when viewed from aft, a propeller turns clockwise to produce ahead thrust it is said to be right handed. If it turns anti-clockwise for ahead thrust it is said to be left handed. 

Right Handed vs. Left Handed: A propeller’s “handedness” affects its shape. A right handed propeller rotates clockwise when propelling a vessel forward, as viewed from the stern of the ship. A left handed propeller rotates counter-clockwise, as viewed from the stern, when in a forward propulsion mode. 

When viewing a propeller from astern, the leading edges of the blades will always be farther away from you than the trailing edges. The propeller rotates clockwise, and is right handed, if the leading edges are on the right. 

A propeller’s handedness is fixed. A right handed propeller can never be exchanged with a left handed propeller, and vice versa. 

Selecting a prop for your model

This all sounds great and selecting your prop should be easy, right? Well, Raboesch model boat propellers follow the convention. Everything looks lovely, consistent. These side-on shots from the Raboesch site demonstrate the description above about leading edges and rotation direction:

Left Handed:

Right Handed:

That's all lovely. And this prop from Logic RC is labelled conventionally: 

Pack is marked Left Handed:

View from stern:

Side view: 

This Logic RC one is definitely a left handed prop as per the convention. 

Here are some props that do not follow the convention: 

Packet, showing "links" for Left: 

View from stern:

Side view. Although marked Left handed, it's a Right handed prop:

And here are two Caldercraft props (RO Marine brand), which are both marked opposite to convention: 

Then I've seen some advertised with no direction information, like this one from Kavan: 

On inspection, it is clearly Left Handed:

What does all this mean?

Orientation is important because props are designed to work best in one rotation direction. They will work if you spin them the other way, but less efficiently. 

In my case, I needed a left hand rotating prop because my prop shaft is threaded. So when moving forward, the prop will tend to tighten up on the shaft because the shaft turns anti-clockwise when viewed from the stern. The resistance from the water will tend to tighten the prop on the shaft as the boat moves forwards. That said, if I run in reverse, then that will tend to loosen the prop, and I ensure that whoever sails my boat knows to be gentle in reverse. 

Originally, I had fitted a RH prop, which meant that the shaft was rotating clockwise when viewed from the stern, tending to loosen the prop in forward propulsion. When given full forward throttle, the prop spun off and now lies on the murky pond bed. 

There are other reasons to know prop direction. Tupper 1996 states: "In twin screw ships the starboard propeller is usually right handed and the port propeller left handed. In that case the propellers are said to be outward turning. Should the reverse apply they are said to be inward turning." So in twin prop set ups, you need oppositely rotating shafts, and therefore need to know what orientation each prop is. 

When buying a prop, it's not easy because some manufacturers show the conventional direction, others label it opposite to convention, yet still other makers show no direction information. Ideally, inspect the prop before you buy it, or see an actual photo of it. Don’t rely on the manufacturer’s handedness, unless they are one of the makers that follow naval engineering convention in their labelling. I have finally found props that I like for this boat and it was enjoyable to swap them out and see what works best. 

Have fun experimenting and try not to lose any in the water! 

References

Techet, A. 2005, Marine Propellers, Hydrodynamics 2.016 Reading #10,  MIT. Available at: https://web.mit.edu/2.016/www/handouts/2005Reading10.pdf 

Tupper, E.C. 1996 Introduction to Naval Architecture, 3rd ed. Elsevier Science: Oxford, at page 217. Available at: https://home.hvl.no/ansatte/gste/ftp/MarinLab_files/Litteratur/IntroductionToNavalArchitecture_Tupper1996.pdf