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  #1  
Old 04-06-2011
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Hydraulic Engineers

Do we have any hydraulic engineers on this forum? I have a one off (20 year old) Navtec system that was designed as a shock absorber for the keel (4,000 lbs of lead) on a swing keel boat and I am trying to determine how much pressure should go into a reserve tank. I think I understand how the system is supposed to work and I have all the part numbers, but I am not sure how to determine the required back pressure in this reserve tank.

I have contacted Navtec sales and service and I am hopeful that they may eventually help me, but I thought I might check here for the appropriate expertise.

thanks,
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Old 04-07-2011
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I do a bunch of tilting and dont follow as i can see it lifting it up and down but am not understanding the shock absorber function ?

unless there are some flow controls the Pressure tank would let it rise at some point but also let it slam down
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Old 04-08-2011
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Keel shock absorber system

Tommay,
There is a Navtec -17 hydraulic cylinder


attached to a 3 to 1 reverse purchase system

that runs to the aft point of the keel (in line with the keel pivot pin).

As the keel is raised the cylinder fills with fluid and retracts, if the keel should suddenly drop the fluid must pass thru a flow controller back into the reserve tank and thus slow the descent of the keel. The N2 head on the reserve tanks is what fills the cylinder with fluid as the keel is raised. Under normal operations everything moves slowly so this happens fine. But if you run over something (like a shallow reef) and the keel comes up quickly you want the reserve tank to refill the cylinder before you pass over the reef (or log or whatever) so that the keel does not just drop after it has passed the object. The flow controller between the -17 hydraulic cylinder and the reserve tank consists of a SUN Hydraulics GAA Manifold with a model NCCB adjustable needle valve with reverse flow check. the specs on the flow controller are here:
Sun Hydraulics - Cartridge - NCCB-LAN

and the Manifold specs are here:
Sun Hydraulics - Manifold - GAA

the questions is what head pressure do I want in the reserve tank? The reserve tank is rated at 2K psi, so you don't want to exceed that under any conditions.

I know 'clear as mud' but its the best I can do without diagrams and such.
It is an interesting question (so I have been told by several sales reps) but one that know one has been able to answer and I don't know who designed this system as it was only done for this single boat.
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Old 04-08-2011
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I would think your talking about the bladder pressure on the tank ?

Gotta think about this as when the keel is down the pressure is low but there needs to be enough reserve presure in the tank to overcome the friction in the system and rasie the cylinder taking up the slack
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Old 04-08-2011
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Pressure low or high?

Tommy,

Yes I am talking about bladder pressure, but there is no bladder in the reserve tank, they simply mounted a pony dive tank upside down, so when there is both hydraulic fluid and N2 in the tank the N2 is up above the fluid pushing it down (and out to the cylinder).

I am not sure about high and low pressure. When the keel is down, the hydraulic cylinder is full extended. Which means, I think, that the fluid has been pushed back into the reserve tank and that would raise the pressure of the N2 head. When the keel is retracted the cylinder is compressed (shortest) and is full of hydraulic fluid. So that if the keel should drop, from the raised position, it would have to force the fluid thru the flow control valve back in to the reserve tank. That restriction is what would slow the keel from just dropping freely. It does not stop the keel or help raise it, because it will always bleed away, it just slows it as it drops.

the original owner, 20 years ago, had this installed because he did just as I suggested, ran over a reef that brought the keel up and it dropped on the other side damaging the keel swing pin area and it had to be rebuilt. That all seems fine now (no leaks) and I can avoid the issue by not running over any shallow reef's. But the system does provide another safety feature in that if the raising line should run free and the keel drops fast, someone could get tangled in that line and hurt, much like a heavy anchor line. We certainly take all precautions to avoid this, but this system is installed and I would like it to work. The original owner only recalls that the system had low pressure reading on a gauge (there is a pressure gauge measured in line with the reserve tank).

thanks for your thoughts.

Ron
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Old 04-09-2011
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I have never seen a system like this so excuse my apparent ignorance.

Is there a tank that pressurises when the keel drops and in that process damps the descent of the keel? That would require the tank to remain under pressure while the keel is down. Sounds onerous to me.

To me the easiest way to manage that (if I had to build my own)would be to use the tank as simply an accumulator and have a variable non-return valve in the line. In other words while the keel is being lifted the valve allows fluid to pass unimpeded from the accumulator to the cylinder but when the keel is lowered the valve allows an adjustable restricted (controlled) flow of fluid back to the accumulator.

Is that the part that is in question? Because that is relatively simple to engineer.
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Old 04-09-2011
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That's it !

Omatako,
I believe, what you describe is what is intended with this system. The hydraulic fluid moves back and forth between the hydraulic cylinder and the reserve tank (accumulator). When the keel is up the fluid is mostly in the cylinder. As the keel drops the fluid is forced thru a flow control valve back to the accumulator and slows the rate of descent. When the keel is down most of the fluid is in this reserve tank (accumulator). (Note: there has never been any hydraulic fluid leak anywhere, so I assume there is already enough in the system.) The head pressure in the accumulator has to be enough to force the fluid back to the cylinder if the keel should lift quickly (say within 10 seconds instead of the minutes it take to raise it manually).

I believe the SUN hydraulics check valve and manifold that I listed are what provide the flow control. They do not appear to be expensive so I could replace them (as I cannot guarantee they are working properly now as old as they are).

The question is what head pressure do I want in that accumulator tank (I currently have 60 psi)? There is about 20 feet of high pressure hose line ( Jo8M2 - 15 Kpsi burst pressure) between the accumulator and the cylinder as it snakes it's way under the flooring and up to the cylinder.

It does seem simple enough (and all hefty components), but I am reluctant to stress test the system to see if it is actually working. The best I can do is judge the resistance as I lower the keel quickly.

thanks for thinking!

Ron
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Old 04-09-2011
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I am not a hydraulic engineer but have a pretty good idea how this system works from your description. I doubt Navtec can advise you on this system unless they designed it. It sounds like this is a one-off design from your description. Are all of the components from Navtec? Do you know what the N2 pressures are now when the keel is down and up?

The following is based on the assumption that the keel lifting device is not this cylinder. If the cylinder lifts the keel and also provides shock absorption functions too then I would need a more thorough description of the system.

My guess is that the required pressure in the accumulator is determined by maximum cylinder speed when the keel is lifted suddenly. There should be an oil flow regulator that limits how fast the keel can fall no matter what pressure is in the accumulator. So the pressure in the accumulator combined with another flow regulator determines how fast the cylinder moves as the keel rises.

One way to figure this out is to find out all you can about the flow regulator device that limits the cylinder speed when the keel rises suddenly. Flow regulators usually have a minimum and maximum differential pressure specification. Between these two pressures the flow regulator will create a constant flow and thus a constant cylinder speed. The correct pressure can be found from this data as about 3/4 way between rated minimum and maximum when the keel is down. The accumulator pressure will fall as the keel rises and hopefully it will still be high enough to be in the range of the flow regulator when the keel is all the way up. Note that the cylinder itself does not factor much into the pressure specifications of flow regulators.

But if the system is working as it should now then recording the keel up and down pressure would be the best reference points unless you can find the original designer.

Dan
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Old 04-09-2011
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I'm assuming the rope seen in your picture is the CB pennant? and that when the keel is down the hydraulic cylinder is (fully?) extended as shown?

If that's correct all I can see is the pennant slacking if the keel lifts, I'm having a hard time seeing the cylinder being able to adjust for that....

..but I'm probably missing something.

EDIT:.... wait.. maybe that's not the lifting cylinder, is it?
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Old 04-09-2011
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you only need enough pressure to move the cylinder to take up the slack in the line during a grounding so the cylinder will be ready to dampen the fall of the keel in the event the keel drops suddenly. This pressure is only overcoming the friction in the cylinder and there should not be any flow control in that direction. the only way to test the system is to have a shut off valve it the line which you shut after you lower the keel, then raise the keel manualy. then open the valve to see the speed the cylinder is retracted and how fast it takes up the slack. 60 psi should be plenty to move the cylinder to full travel in about a second, alot faster then a grounding would raise a 4000 lb. keel. the flow control is set so the keel drops only slightly faster then it drops when you lower it manualy. seem to me if the cylinder is following the keels movement during normal raising and lowreing that the system is working as it is entended.
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