Marine Rotary Shaft Seal


I am looking at installing a rudder onto a pressurized cylindrical chamber - PVC (the main body for my underwater drone) the setup is a servo motor feeding from the inside through the chamber where it is connected to a double split collar and connected to a fin. Obviously i need to water seal the shaft feeding through. from my research i found the best option was to use some form of rotary shaft seal,

However I am not sure about details of the practical process to install a seal like such into the side of a cylindrical body, as well as the specs and form of the seal components.

I am looking for a cost effect method, and the best place to buy such components for the sealing and its framework, (I already have the pipe/motor/collar and fins)

the application of the drone is going down to 100m deep, with constant micro movements of the rudder, maximum 45 degrees.

I would love to get some advice on this topic as I am flying blind here.

cheers andy


I recommend to use things like that.

This kind of seal are used on big oil filled thruster in the offshore industry.
there is a fixed part, the second is the rotary one. the rotary seal is made by ceramic plan sucked in each other (the white part).

Attention to not grease the ceramic, or it will be not be sealed, these needs to be dry mounted !

hope I helped here.


Hi @andybabbage,

I don’t have experience with marine shaft seals, but oil-based pressure compensation can simplify the sealing aspect.

Alternatively you might want to look into getting an underwater servo that you mount on the outside of the enclosure, and just feed the wires in through a normal penetration in an end-cap.

It depends on how deep you want to go. Talk to someone in the hydraulic pump business. They use O-rings and lip seals to thousands of psi.
I used a simple servo and a 1/4" shaft seal from an old military radio. The seal is on the left in the picture and has an O-ring to seal the fitting and an internal O-ring to seal the shaft. It has never leaked but I never took it very deep either. Counterbore the hole in the housing to make a flat surface for the O-ring to seal. The shaft seal bushing clamps the metal plate to the curved surface of my ROV. Keep it simple and improve when it gives trouble is my philosophy.
The problem with lip seals is they need more force to turn as the pressure increases so minimize the shaft diameter. One picture shows my side thruster shaft lip seal.
My two side thrusters rotate 180 degrees and their mounting shafts are sealed with O-rings. A photo shows the shaft; on the left is a brass bushing with pipe thread that screws into my ROV housing. It was drilled and reamed to 1/2" for the O-ring seal. The circlip goes on the inside and the brass spacer acts as a thrust bearing against water pressure on the outside. The thruster housing mounts on the pipe thread on the right. I built this before BlueRobotics was available to provide T200 thrusters, and before ESCs and brushless thrusters were available.
I still have a need for rotating shafts for my Ping Altimeter that I want to rotate. I have a posting elsewhere on this forum but so far there are no suggestions.

This picture of the inside of my ROV shows the main thruster motor in the center, the rudder and elevator servos above and to the right of it, the side thruster rotate assembly on the left, and the brass pipe thread bushing for the O-ring of the other side rotate assembly just visible on the right.
The flat plates are drawer slider on which my electronics mounts.
Yes, brass, aluminum, galvanic corrosion, etc. but it works fine in river and lake water.
Eliot; can you point me to underwater servos? I know someone was looking at using a flooded brushless motor to rotate objects.

Hi Peter -
Blue Trails Engineering has a servo product line