Rudder stops with hydraulic cylinders

On all my monohull motorsailer wheelhouse Vetus hydrolic steering systems i placed my stops at 30deg. and have never had a problem. As a matter of fact i think they recommend the 30 deg. stops---geo.

 

What I have difficulty with is when your full on and you can feel your at the end who in their right mind continues trying to turn and rips the rudder stock out.

I have a cat with hydraulics and I never use steering in a marina using both engines and why would I need full lock at sea and then why would I keep trying to turn. And yes the rams stop in themselves, there is no rudder stops

I would call a little bit on operator error here too. Unless he left the auto pilot on and slowed to a drift--again operator error.

 

Frosty is spot on with the "pressure relief valve" suggestion. When I design hydraulics I prefer the cylinders not to go full stroke.

Depending on how the system is operating I normally use external limit switches to stop travel or magnetic reed switches operated by a steel piston.

This is for material handling automation and go through a PLC.

 

Rudder stops

Paragraph 6.1.4 of ISO 12215 Small craft — Hull construction and scantlings — Part 8: Rudders, states that rudder stocks that are actuated by remote control shall be fitted with stops, limiting the angle between 30° and 45° on each side.

However it does not specify whether hydraulic rams can go to full extension, preventing the arms from hitting the stops, so I assume that it is up to the cylinder manufacturer.

 

I realised this morning while having my oats and chopped grapefruit and apple that I was assuming that a hydraulic cylinder for steering would have the same configeration as the systems I put in materials handling.

But, I realise that you can't have a cylinder driven by a directional valve as I would normally do.

This is because you need to lock the rudder in one position. For example if you wanted to move the rudder five degrees, it needs to move five degrees and then lock into position.

This can be acheived by having equal pressure both sides of the piston. So the piston is moved by pressure diferential.

By pushing a button or a lever it operates a switch that energises an electirically operated pressure relief valve that reduces the pressure in the direction you want the piston to move.

The stop would be a limit switch activated by the rudder arm that interupts the signal to the pressure relief valve and returns both sides of the cylinder to the same pressure, locking the piston.

That is the type of design I would be looking at if I had to design a rudder control.

Although the regulations require a "stop" an electrically operated stop or a hydraulic shut of valve type stop wouls still be classified as a stop.

Using the structure of a boat doesn't seem to me to be logical.

 

As an interested amateur, I am puzzled by the schematics of the hydraulics drawn with the pressure arm having 'fixed' ends (four bolts). ie, cant move left or right on the horizontal axis

Surely the fastened(inner) ends of the hydraulic cylinder should be on a 'swivel' to make sure that the push/pull effort doesn't force alignment problems on the arm.

In the attached rejig of the schema, it appears that at full stretch, the hydraulic arm is being forced out of alignment, and the pressure of the mis-alignment may weaken the point where it is attached to the rudder. In the illustration, the blue arrow end extending from the square blue box is where it appears the fully extended rudder attachment point would want to be.

How was it actually installed ?

 

Hey a couple actually 3 simple solutions.
1) easy math problem if you keep the same stroke length of your hydraulic cylinder then legthen your fulcrum and mount your cylinder further away from the pivot point. this will allow cylider to max out it's stroke with the same degrees and same stops.

2) If you want to you could buy a shorter stroke length of cylinder and do # 1 to keep the mounting position of the cylider the same

3) See a hydraulic supplier, if it is a name brand cylinder such as a Royal, get them to install tube stops inside the cylinder itself.

I hope this helps. You really don't need a proportional direction control or reliefs or dumps to limit stoke length. Get the math/ geometry right.As a journeyman Millwright an Mechanical Eng in industrial plants for 25 years my advice is keep it a simple, clean design. The pressure should only be the force required to do the job and simple system relief for safety.

 

I have immediately assumed there is a swivel point because without one something would snap or bend even before an end stop is reached.
But it was only a crude sketch that doesn't show where the cylinder is bolted to.

Because of your comment and the fact that the end stops were on the rudder shaft housing where they shouldn't be, I start to have doubts.

 

A short general comment from someone who, for the last 25 years, designs and delivers servo-hydraulic rudder, stabilizer fin, trim tab and interceptor control systems, and has them routinely approved by ABS, DnV, etc:

1. Any system that uses a linear hydraulic cylinder should never, ever use any structure external to the cylinder to limit cylinder travel. Cylinder travel will always be limited to the required range by internal stops only. External motion stops for the device being moved, if they are deemed necessary, must be placed beyond the limits of the normal travel range between cylinder mechanical limits. If it is expected or intended that the cylinder will routinely be used to stop motion, cushioned non-linear internal piston stops are routinely specified and included.

2. The selection of the mechanical cylinder stroke limits must be based on known characteristics of the servo control. i.e if the active servo control range is plus/minus 30 degrees of travel, the physical stroke limits of the cylinder must exceed that range by sufficient margin to prevent 'end bumping' due to servo overshoot. (The desired control range and the cylinder physical limit range can be nearly identical in systems that are not high-response servo controlled but are instead low slew-rate systems such as a simple conventional rudder control package; see then note 1 above about end cushions).

 

Rudder stops

Indeed there is a swivel point with the cylinder !

I have looked at the Installation and Owner's manual for the Hydrive (Australian) cylinders, their guidance is the same as Lecomble & Schmitt:

"RUDDER STOPS should always be fitted to limit the rudder movement, and should ensure the stops engage before the cylinder reaches the end of the stroke.
This is to prevent cylinder damage in the event of underwater collision."

Thanks, all contributors, for your input.

 

Very odd. So using that approach, then one has to design all structure between the mounting of the cylinder and the external stop to easily withstand actuator stall force transmitted to and through it. In theory, that is usually done anyway since one can never guarantee that something will not jam and the actuator stall force reached. But to make that a permanent condition....well its something we've always carefully avoided.

 

Espirit Marine - With all due respect, I feel that you are misinterpreting this information.

You are assuming that the stop is a physical stop that the rudder bangs against to stall the cylinder. This in my opinion is wrong.

A stop can be a stop anywhere as long as it stops.

I believe they are talking about a stop that cuts the hydraulic pressure so it protects the hydraulic cylinder from damage when the rudder reaches its extremities.

Isn't there a schematic of the hydraulic system, or did someone just make it up as they were going along?

 

Poida, you might be right, I have already asked clarification to Hydrive. However an hydraulic cut would not prevent cylinder damage in case of underwater collision... The hydraulic schematic is in post #9.

The answer from Lecomble & Schmitt was (in French) "Nos appareils à gouverner sont dimensionnés à partir de la norme ISO 10592. De ce fait, les butées mécaniques doivent être atteintes avant les butées vérin." which is translated to"Our steering products are dimensioned from ISO 10592 standard. Hence the mechanical stops must be reached before the cylinder stops."; so they refer to a physical stop.

BMcF, yes it is a requirement of mandated standard ISO 12215-8 par. 6.1.5 that the stops shall be able to transmit the shaft torque.

 

I will check to be sure but as per my post #16 I think Vetus recommends external mechanical stops at 30deg. settings either that or I read it somewhere regarding hydrolic steering installations. After some 35 to 40 yrs. of installs using this practise, and reading the above posts I am now questioning the proper install. Possibly different manufactures require different proceedures ???? Maybe it's a throwback from the old chain rudder stops of iron men and wooden boats --Geo.

 

how about a nice pic of where the damage occured, even if

of a different, undamaged boat.


Seems simple enough to me:

1)system must have enough power to easily turn rudders

2)stops must be stronger that system's relief valve setting


AND....

the part of the rudder that might hit something underwater need to be able to breakaway and be replaceable without disabling the boat.



My guess is relief valve setting was WAY TOO HIGH and completely wrong for a light boat with small twin rudders.