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    Piston Ballast Systems

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    John.redearth
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    Piston Ballast Systems - Page 6 Empty piston uses

    Post  John.redearth on Mon Nov 10, 2014 3:09 am

    Hi

    FYI my last two boats had scratch built pistons which fitted inside the central ballast tank that was operated by a pump in pump out system. A smaller piston does not give the pressure or imbalance problems the larger ones do but has all the advantages. As soon as the surface is breached, turn on the pump and empty the ballast. I haven't seen many boats with this config (none actually) but it is a very good option. The piston give fantasic fine control to get neutral buoyancy, and is triggered by the failsafe if the signal dies.

    I can't actually think of a downside.

    Cheers

    John
    david f
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    Post  david f on Mon Nov 10, 2014 9:52 am

    Hi John,

    Funnily enough this tank combination came up in a conversation I had with Nigel E recently and we both thought it sounded really good but we couldn't think of anyone using it! So it is good to hear from you!

    It does seem to combine the best of both worlds with just a bit of extra complexity. Probably very good for WW2 boats where you need a low down waterline? No high pressure or overpressure problems etc.

    My Resurgam model suffers a little from a too small piston tank (only about 80ml) and your idea could sort that out.

    So basically you just inset the piston tank into a tank. Then use a low pressure pump for surface pump out?

    Any chance of a photo?

    David



    nigele(ADMIN)2
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    Post  nigele(ADMIN)2 on Mon Nov 10, 2014 10:16 am

    Yes,I used this system about twenty years ago hydraulic pistons on a central tank,it worked very well but the pressure produced would cause leaks on the pump and failure of the pump seals mainly when the boat had been unused for a few weeks and the piston seals had grabbed the cylinders,after a couple of years I changed to central tank with two electric piston control tanks,and I have used the same system on larger boats ever since,U12 s191 has had that system for seventeen years now,and I have never had to do any work on it at all.
    nigele
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    John.redearth
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    Piston Ballast Systems - Page 6 Empty piston in tank

    Post  John.redearth on Tue Nov 11, 2014 10:40 am

    The way to go about that is to have the tank intake and outlet managed using a valve that i initially saw described by subtech. Lets water in ad out with just a little pressure.

    John
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    Post  david f on Thu Nov 13, 2014 10:52 am

    So you need some form of valve to prevent water coming in to the tank at the surface?

    (I happened to mention this thread on the phone to Nigel and I think his previous post refers to a different system (a piston moved in and out by a water pump?)

    David
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    Post  John.redearth on Mon Dec 01, 2014 7:04 am

    Hi

    The valve is a doddle. Have a look at this..

    https://youtu.be/2YsIfgtrXhM

    Best John
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    Post  david f on Thu Dec 04, 2014 4:41 pm

    That is very ingenious, John!

    So the ball bearing is functioning as a non return valve and a (low) pressure relief valve? By low pressure I mean a few inches of water. It makes pump selection a doddle.

    I wonder if a single one could also be used as a vent (to prevent over pressure) on an air filled bag system?
    I confess I have never used a low pressure water pumped system or an airbag (I've only used pressure tank water systems and piston tanks.)but they are often talked about as solutions. But the necessary detail is often glossed over.

    Low pressure water tanks + smaller piston tanks do offer benefits (as discussed earlier.)


    David

    PS I see that Arduinos can be programmed in C. I will give an update on my learning C shortly!


    Last edited by david f on Fri Dec 05, 2014 11:09 am; edited 1 time in total
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    John.redearth
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    Post  John.redearth on Thu Dec 04, 2014 8:43 pm

    Yes to all of that.. keep in tough with your programming. We should exchange scripts as we go. I am not that good at it but it works so far..
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    Post  david f on Sat Dec 19, 2015 3:29 pm

    Piston Ballast Systems - Page 6 Img_5417

    Just an update on the piston tank saga. These are home made ones I did over the course of 2015. The top one is the one used for my Holland and the lower one is used in Resurgam and the Nordenfelts. Both are now pretty much standard designs using the proportional controller but I adjust the dimensions to suit the model.

    They use a standard (Nitrile) O ring. The fit is all important and does involve a bit of trial and error. Spin the piston and take small amounts out of the groove with a needle file. My rule of thumb is that you have to be able to move the piston up and down the cylinder BY HAND. (If you can't it is too tight and could stall the motor.)

    The basic construction is as made by German modellers. The two photos below show the idea (This tank was originally for Resurgam but was too undersized - err on the large size! This tank uses a silicone O ring - a bit softer and with less friction. Friction seems to be a problem with smaller diameter tanks, for some reason.)

    Piston Ballast Systems - Page 6 Img_5418

    Piston Ballast Systems - Page 6 Img_5419
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    Post  John.redearth on Sat Dec 19, 2015 9:59 pm

    An added thought. Very intersting about orings and pistons. I am in the process of building another one right now. I have found that if you cut the groove too deeply and there is not enough pressure, self sealing butyl tape can be used in the base of the groove under the o ring and keeps the seal nicely.
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    Crossie
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    Post  Crossie on Sun Dec 20, 2015 9:00 pm


    To get the best level of sealing with the level of friction appropriate for the surface finish of the cylinder, it's quite important to make sure that you cut the groove to the right dimensions as per a proper o-ring chart for dynamic conditions and not static fitment which has a higher % of ''squeeze'' and hence friction. A few minutes google-time will yield plenty of charts to get the right fit.


    Trevor.
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    Post  david f on Tue Dec 22, 2015 12:22 pm

    You are right Trevor. The surprising thing is how wide the groove must be to allow the O ring to "roll."

    With my limited machining skills I have great difficulty in turning grooves to accurate dimensions. Any suggestions on how this is best achieved? Can you perhaps make a special tool to do it in one? (I use a narrow parting off tool - but it is not the best.)

    David
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    Post  tsenecal on Tue Dec 22, 2015 8:46 pm

    a quick google search found this....


    http://www.applerubber.com/oring-gland-calculator/
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    Post  John.redearth on Tue Dec 22, 2015 8:51 pm

    Hi

    I am doing it now and putting together a video on it.  I think the best way to go is to start by machining a close sliding fit with the cylinder with practically no play.  This takes time and small cuts but is definitely worth it.   Then cut the O ring groove depth using the cross slide to 85% the width of the o ring, and the groove width to 120% width.  Then take a nice cut off the whole diameter to get a looser fit for the end cap..  maybe 5- 10 thousands of an inch.  Then youv'e got it.  The hardest thing to do using this method is to turn the initial sliding fit to work from.  The hardest thing to do is to measure the internal bore accurately as there are 'variables,' and this elimiates this problem.

    Cheers
    david f
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    Post  david f on Mon Jan 04, 2016 10:32 am

    Hi John and a Happy New Year!

    Very interesting what you say about starting out with the cylinder a good sliding fit.

    I have perhaps been less than scrupulous about this in the past therefore with some rather "random" O ring fits.

    The good sliding fit is the starting point for the other dimensions.

    David
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    Post  John.redearth on Mon Jan 04, 2016 12:40 pm

    Here is a start. It worked pretty well

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    Post  david f on Tue Mar 19, 2019 10:46 am

    Just showing my growing collection of DIY piston tanks. The latest one is for the Pioneer. Several have been in use over a number of years. DIY does mean that they can be tailored to fit different hulls. Diameter is the key variable but length is also often a problem.

    Piston Ballast Systems - Page 6 20190313
    Piston Ballast Systems - Page 6 20190311
    Piston Ballast Systems - Page 6 20190312

    Sorry about the well-painted bins and the tie-wraps but they work!

    David


    Last edited by david f on Thu Mar 21, 2019 8:45 am; edited 1 time in total
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    Post  merriman on Wed Mar 20, 2019 12:16 am

    david f wrote:Just showing my growing collection of DIY piston tanks. The latest one is for the Pioneer. Several have been in use over a number of years. DIY does mean that they can be tailored to fit different hulls. Diameter is the key variable but length is also often a problem.

    Piston Ballast Systems - Page 6 20190313
    Piston Ballast Systems - Page 6 20190311
    Piston Ballast Systems - Page 6 20190312

    Sorry about the bins!

    David

    Most instructive, sir.

    David

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