counter rotating necessary?

[derelicte]

Is there any advantage to using counter rotating thrusters?

I'm trying to figure out if it is worth tracking down the counter rotating prop.

Thanks!

update (8-12-11):

after playing with my rov I have found that yes, counter rotating props help.

[AHarris]

The only advantage of a Counter Rotating prop is a more accurate course (however I do not believe this is a major problem when it is such a small ROV that is being used and with such a low power motor) and it has the ability to turn on it's pivot (more easily.)
As you can see it has plenty of advantages however from the way you asked the question it sounds like you are having issues with finding one.
I don't think it shall be a problem using two props that spin in the same way however I have not been able to test it (as my ROV is still in the construction stage.... again).

[derelicte]

Thanks for the reply!

I'm trying to find counter rotating props at my local hobby store. I have found a couple that look like they will work, however they were temporarily out of stock on one side. It sounds like it would be worth it to wait a few days to get another in stock.

few more questions though:

1. If you look at the rov from the back, is the normal rotating (not the counter-rotating one) motor spinning clockwise?

2. Is the normal rotating motor on the right side of the rov?

3. Is the normal rotating prop a right hand prop or left hand?

Thanks again!

[AHarris]

I unfortunately don't know, however I do know that the prop on boats spins clockwise when going forward and anti-clockwise when reversing.
Sorry for the lack of information.

[derelicte]

that seems to jive with other info I've found online.

thanks!

[PaulC]

Let's ask a couple of questions here first. One, how big is the ROV? Two, how much torque is the motor actually putting out? Three, is it really necessary to have counter rotating props? With those questions in mind now then in my opinion since this a "hobby" and in my experience in the three years I have been building ROV's (and not using counter rotating props) and since 99% of all my ROV's lie within a 12" square (for the most part) no I don't think that it is an issue that you have counter rotating props. If I knew the answers to at least the first two questions then my answer might change, but I still think that since it is just a hobby, no. Typically when we see it becoming necessary to have counter rotating props we are talking about building an ROV that is usually in the range of a ROV that is designed with high torque, and is big (like the size of a minivan.) ROV's of this type are usually out of the league that we hobbyist are looking at building. Remember that this a hobby, and that we are just out there to have for the most part fun. Build on my friend build on. In answer to your three question though... The counter rotating prop should be on the left and the other on the right, but in all actuality it doesn't really matter. How many motors are you planning on using? Meaning, if you are using two verts. and two for front/ back then the main thing to do is to match both sides. I always think of my ROV as (left and right,) (front and back), and not as a whole. That way I always have the pivot at the absolute middle. Think of the absolute middle of your ROV as the middle point of a "X,Y,Z" coordinate system. 3d space. This aids in greater ability of turning and balance. That is why if you look at my work in progress all my motors are right along the center point of the whole ROV. Easier in my opinion to achieve neutral buoyancy if everything is in balance. Also, with having a ROV set up this way it gives the builder greater options.

[derelicte]

Thanks Paul,

You are probably right about our thrusters lacking enough torque to need counter rotating props.

I'm building a small rov for our pool. It is 10" wide x 10" tall x 12" long. I'm using three johnson/mayfield 500 gph bilge pump cartridges for thrusters. I've found some props locally from a traxxas rc boat that has counter rotating props that look like they might work ok:



http://www3.towerhobbies.com/cgi-bin/wt ... LXGBH1&P=0

they are 47mm in diameter (pitch unknown) and have m4 threaded inserts with a locking set screw. They fit on the pump shaft without an adapter with the set screw locking it in place (hopefully!).

Since I first posted, the store got the missing prop in stock so I now have a set of counter rotating props for the horizontal thrusters and a regular prop for the vertical thruster.

I'm hoping I can get them mounted this weekend and try them out in the pool!

Thanks again everyone!

[derelicte]

I have finally got my rov in the pool!

One of the first things I noticed is the rov will slowly spin around the vertical axis when the vertical thruster is running.

I guess counter rotating props help no mater how small the craft is.

The rov moves forward and backwards perfectly level, though!

[SoakedinVancouver]

Coil your umbilical in the opposite direction of your vertical rotation, so as to somewhat counteract it with the mechanical resistance of said umbilical. Just an idea.

[derelicte]

that might work about half of the time. the rov spins in the other direction when the vertical thruster is reversed.