Home        Store        Learn        Blog

Ducted Propellers - Today’s Rabbit Hole

Today on YouTube I was looking through video from RCModelReviews and found one titled “How ducting a propeller increases efficiency and thrust".

What really caught my attention is when the discussion of what I am calling a "coanda ducted fan”. This starts around 11:15 into the video.

It seems to me that the BlueRobotics thrusters might be using this principle to improve efficiency. Is my assumption correct?

Why this is important to me is that I am using “weed guards” on my thrusters and it seems to me that the weed guards might have a negative effect on a “Coanda ducted fan” design.

Also part of me thinks BlueRobotics thrusters might NOT be using a “Coanda ducted fan” design. The reason is that the propeller blades are not all that close to the sides of the duct. In the above video, the distance between the propeller tips and the duct must be very close to avoid "prop tip vortices” which from my understanding, have a negative effect on efficiency. The "prop tip vortices” issue may be addressed through BlueRobotics design of their props. Also in this video, it is mentioned that any ducting should be very light because any efficiency gained by the ducting can be reduced or eliminated if the ducting is too heavy.

Assuming that air and water are both fluids, I am guessing that even though all the information described above were for propellers and ducting in air, that the same principles apply in water.

I am planning on building a simple load-cell test setup to measure the thrust of a T200 in water, with and without the weed guard, but if BlueRobotics can tells me my weed guards should not make a difference in performance. I will bag my tests.


Hi Phil, what’s your application? I haven’t noticed much of a difference when using the same weed guards. I’m going assume there is some loss, but running with guards has always been a higher priority for me than running without and getting fouled.

Hi Kevin,

My application is kayak propulsion (trolling motor(s)). See the Flickr link in my original post.
I have always assumed there was some loss but when I stumbled on this “ducted fan” topic, I was curious. I am with you as to the weed guard(s) being a priority. I am going to go ahead and do some thrust measurements with and without the weed guards and if BlueRobotics can answer the question related to the design of the ducting, it should tell me if it a big or small contributing factor as to any loss.

In my case, running one T200 or two T200 thrusters with tiller steering on my 10’ kayak works great. Same goes with my outrigger skid-steer setup.


1 Like

Ah got it, missed the kayak the first time I looked at your first picture. Best of luck with your tests! By the way, we’ve found 2x T200 thrusters with the guards move a surface vehicle weighing 300-350lbs at around 3kts at full power. We never had an issue with torque or power either.

Hi Phil,

You are correct that a ducted propeller significantly increases the efficiency of a thruster, and we are using this feature to great effect in our thrusters. In fact, the Kort nozzles on our thrusters roughly double their efficiency versus a non-ducted design! Prop tip vortices are reduced, and static thrust (most important for underwater applications) increases. Maximum speed is reduced and drag increased, but that’s where the M100 and M200 can step in if it is important for an application.

We haven’t done any comparative testing of this with and without a weed guard, but I imagine that efficiency and thrust would be reduced. Perhaps most critically, drag would also increase, but in surface applications where fouling is a hazard I think the trade off is clearly worth it.

I’m curious to hear how your testing goes!


Hi Adam,
Many Thanks!

Hi !

I was reading the post regarding the weed gaurds and was curious to know how much loss was there in the efficiency and thrust. Some inputs on this would be really helpful for my project.

Further does BR have a ready made weed gaurd? or any cadd model for the same so we can take a 3D print out?

With regards,

think I answered all your questions in your previous question found here

I don’t this BlueRobotics has weed guards for their thrusters at this time.

Here’s a post with some stl files. Are these the ones you are using @pakoregon?

did you click on links… the designer disable these a long time ago.

I didn’t :(. There’s some others here.

I have some of weed guards from the guy in your last link but never used them. The other weed guards are very nice and work great. I was able to get the files by messaging the designer.

Thanks for the links, I have downloaded the defelctor design and also messaged Sally for his design. Look forward to get that also.