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Surface Vessel: Dual Thrusters or Single Thruster/Rudder


Hi All -

Long time fan of BlueRobotics, but haven’t gotten to employ any products yet.

I’m dreaming up a surface AUV vessel. Not too big, maybe four to six feet in length and two to three feet wide. On the propulsion side I’m thinking through a couple options and would love to hear of your thoughts and experience.

The options are

  1. Dual Thrusters - static mounting, steering via differential thrusting.
  2. Single Thruster - static mounting, steering via some mechanical rudder added.
  3. Single Thruster - mounted on pole that can rotate. That is aim thruster in desired direction.
There are pros and cons to each. I like the idea of reducing the mechanical complexity by going with option 1, however I'm not sure how well it would work.

My use case right now is on a lake, relatively calm.

Thank you.

(Richard) #2

Option 1:

Simple, easy to implement, can turn without forward motion, easily coupled to gyro and compass for automatic steering (autopilot).

(Paul) #3

It rather depends on whether your vessel will be moving along a track or whether you will need it to station keep and stay in one location.

If it will be surveying and primarily be sailing forward, following a track, option 1 will be the easiest to implement. There is already code out there to help you with this.

If you want it to station keep, then I might suggest considering 3 or 4 vectored thrusters, similar to what BlueROV2 has. This will be the most efficient in moving the vessel in all directions without turning.


Thank you Paul and Richard for your feedback.

This vessel will primary be in motion, so it sounds like option 1 is the best for me. Perhaps a bit more expensive, but the ease of controls and electronics and lack of mechanical sure make up for that.

Now, to decide if it is T100 or T200 I’d go with. I bet I could go either way. I’m thinking T200 since I may upgrade the vessel in the future and can just move parts over.

(Kevin) #5

Hey Tim,

I’ve been working on something exactly like this for the past few months. I took a Bali 6 child kayak (I needed something short and inexpensive) and mounted 2x T-200s to the underside. The beauty is that I only had two moving parts on the whole vehicle, which made it pretty rugged. It worked fine in MANUAL mode, but I was having some tracking issues with AUTO, most likely due to the Rev1 design on the BlueESCs, which Rusty told me about.



After some life drama, I’m starting it over, and installing stock T-200s with Basic ESCs mounted in the Nav Box and upgrading to an RFD900+ for my telemetry. I may also try out using a 10,000 mAh Li-po instead of the marine battery I had, simply because that was pretty heavy.

I’ll keep my Ardupilot thread going as I build the new one.


Thanks you Kevin for sharing your experience. I didn’t realize there were child kayaks and it looks like they would be a good size to work with.

I was thinking more of a catamaran style boat using 4" PVC pipe. About four feet long, and a two foot by two foot or two foot by three foot platform.

Tell me more about your tracking issue and the BlueESC? I see a new ESC is coming out, I’m curious what the problem was.

(Paul) #7

I think you’ll find that Kevin’s kayak is the most cost effective solution. You can pick one up at Walmart for around $100USD.

A catamaran would give you greater stability, but 4" PVC pipe won’t give you much flotation and schedule 40 pipe is pretty heavy. Instead, I would recommend you either build your own hulls (thin plywood and fiberglass would work) or shop for a pair of “kayak outrigger floats”. That’s one of the options I’m considering.


True, very true Paul on price. I like the idea of stability, that is why I was going with catamaran. I hadn’t used PVC yet in this application, so I’m glad to hear your comments about weight and flotation.

A couple years ago when I was first exploring this idea I came across Expandacraft (http://expandacraft.com). I like their product because I could easily assemble and dismantle for transport in my car. It also has a nice “nose cone” instead of a blunt PVC pipe. The price though is a bit more, so if this was a serious project (commercially funded) I’d totally consider this method. At the moment is it only hobby funded, though I’m open to building on on a commission if one wanted to pay me ;-).

Here is a pic illustrating the use of the Expandacraft. I think I got this from their Facebook feed.

I’ll also look at the kayak outrigger option too.


(Paul) #9

I like the Expandacraft floats, very modern “wave piercing” hull design. But not cheap and also doesn’t provide that much buoyancy for length. Here’s what I am considering using:


Shorter than what you probably want, but at $100 per hull I think it’s considerably cheaper than Expandacraft.

(Kevin) #10

Further clarification on my issue: The Rev1 thrusters I had (originals from the Kickstarter) worked fine when I was doing my bench testing and were very responsive. When I placed them in saltwater, however, the port thruster seemed to have a “stuttering” problem. It would work, but in a diminished capacity and the vehichle wouldn’t turn left properly. Turning right was fine. I have a verbal that this issue will be fixed in the Rev2 design, so you should be fine to use them once they are available in the store again. Until then, the stock T-200s work just fine and it is easier to replace an ESC board.


Thank you Kevin for clarifying. I’m eager to learn about the new BlueESC when that becomes available.

I’ll start another topic on hull design ideas.

(Spally) #12

Tim, I thought about this with my boat. I went down the the Azimuth Mount thruster path.

Two fixed thrusters (Skid Steer)

  • Good- Easy to implement, easy to install. Nothing really to go wrong.
  • Bad - You have to back off the power to one thruster to turn. Also if you are travelling across wind, you would spend the whole time with one thruster running slower than the other just to keep straight.
  • Good - Easy to implement, install and are great at holding line in a cross wind/current situation. No loss of power during turning.
  • Bad - Need water flow to be effective. Not the best for tight maneuvering. Another thing that can be fouled by debris
Azimuth Mount Thrusters
  • Good - Great for holding line in cross wind/currents. excellent maneuverability. No loss of power during turning.
  • Bad - Need mounting system. (See azimuth mount thread in the build forums). I have gone through many revisions to get the final design I have. I get them 3D printed and they work extremely well (over 500hrs of use now).
I am sure others can add to this list or disagree but they were some of the reasons I went the way I did.