Adding a Tail!


#1


Cannon made an interesting modification to his ROV - he added a tail! Sounds like testing has gone well. I'm eager to see the video!


#2

Cool! Once I finish my ROV, I am planning on adding an arm.


#3

Cannon, very interested in the water trials! You probably already know this but the reason the ROV pitches and rolls so much is because the center of mass is very close to the center of buoyancy, with little righting moment (torque via the distance and linear displacement between CG and CB). It's the curse of small ROVs the world over. You are going to change the center of mass with the longer fairing, so I would think you'd need weight compensation on the bow end, thus displacing (lowering) the center of mass, which is good. However, I'm not sure what the buoyancy properties of acrylic are but you'll also affect the center of buoyancy, most likely shifting it stern. But you may lengthen the moment arm and add stability to the craft with the additional weight (the question is where). It's a great experiment in the proportional nature of buoyancy locations, mass location, and righting moments. Post vids as soon as you have them. Curious as to the over handling. There's a good story about a Naval sub that during sea trails, every time they made a left or right turn at speed, the entire boat would rotate due the angular momentum rotating the ship about the center of mass...which happened to be the center of buoyancy. It would have been a fun ride on that sub. (I think I have that right. Please correct me if I'm off mark)

Sam, I've 3D printed a few hands for larger ROVs but not one small enough for the OpenROV that is practical. Do you have a design(s) in mind? There's a few good posts in the forums about it.


#4

However, a zeroBG (distance between CB and CG) you have a very maneuverable craft capable of all kins of aqua-batics.


#5

Hey Jim. I do not have any design right now, but after I finish my ROV. I might have one.


#6

I have also built an adjustable rear fin for my OpenROV ser # 315. It is vary simple and can be adjusted to correct speed dive and rotation. I have installed counter rotating props on my OpenROV so rotion is not a problem now. Before the new props it was.

The fin is made from .030" plastic sheet. It could be a little thicker. You might be able to use a plastic binder cover though that might be to thin and not stiff enough.

Two #8-32 screws with washers and locking nuts attach the fin to the ROV. You adjust the trim by moving the rear of the fin up or down and you can put a twist in it to counter rotation when you are use the old props. Make sure the screws are tight after adjusting the fin.

Frank




#7

When I first put my ROV in the water it was rear heavy and was very positively buoyant. I acrylic is very dense and therefor is not buoyant. So by adding the tail on the back I made the weight distribution problem worse, however,I still needed to add weight to make it neutrally buoyant overall. I managed to do this by adding a zinc alloy ring that I machined down to the right weight (at least close enough…) and put that on the forward cross bolt. This brought the nose of the ROV down to level and made the craft neutrally buoyant.

The drag on the front and rear (vertically) is about the same. So when you go full power to surface or dive you get an almost level flight pattern.

I have a small 300 gallon tank that I use for testing of some other stuff, and in that tank I’ve noticed a lot more stability. Hopefully this weekend I’ll be able to get up to somewhere that isn’t frozen over to catch some outer video.


#8

Hi Jim:

You're right. At the first stages of fully underwater hydrodynamic type subs development(Class U-21 to Albacora designs) , the hull rotated around the longitudinal axis when making a fast turn heeling a lot.

The "easy" solution was to fit governing planes in an X arrangement, instead of the classical plane-like T shape.

About PMMA buoyancy properties. Its density is1190 Kg/m³.

Hence, buoyancy force in fresh water results 190 gr/cm³ and 164 gr/cm³ in standard sea water(1026 Kg/m³).

Cannon:

Quite a nice design and simple solution. That PMMA plate looks thick enough for trying to sharpen the trailing edges while rounding the leading ones.

As that long tail, increases the skin friction, fairing the fins section (I think -no numbers done- that not a total fairing is required) would help to compensate for the increased dragg, reducing battery comsuption.

Yes. may be I'm too purist, but a big ammount of little details can finally arrive to a remarkable difference.

Regards


#9

Hey Guys here is a little (boring) video of the ROV with the tail on it. https://www.youtube.com/watch?v=aVI4RHLzS68

-Cannon