Water ballast tank placement

Discussion in 'Boat Design' started by Matthew Dunk, Feb 27, 2023.

  1. Matthew Dunk
    Joined: Apr 2019
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    Likes: 10, Points: 8
    Location: Adelaide, Australia

    Matthew Dunk GILow

    So, I have completely stripped out my Kelly Peterson 44 and have commenced the refit.
    She is around 2000 kg lighter from removing the old timber interior, all of the iron and stainless steel tanks and the bronze portlights. I will soon be fitting a significantly lighter engine and lithium batteries. The interior is being built almost entirely from PVC honeycomb core panels with FRP skins. The cabin sole will form the fresh water storage, around 1500 litres below the floor, split into three 500 litre tanks, midship port and starboard plus a forward tank.

    Now, I want to add salt water ballast tanks. The plan is to build them against the sides of the hull, making use of the curved hull by fitting a flat panel from the hull at the cabin sole level, up to the underside of the deck. This has a number of advantages, simple to construct, uses space that’s pretty hard to access anyway and it makes the interior refit easier by presenting straight sides right where you want them, behind the galley on the port side and behind the furniture on the starboard side.

    Rough estimates suggest 1000 litres per side should be easy to fit in, and this sort of volume is consistent with what has been fitted to somewhat similar boats in the past. At this size I should get a useful reduction in heel.

    However, I have a concern about fore and aft trim with these tanks. I had planned to centre them midships, in a way that would not affect fore and aft trim. But looking at the few boats I can find with water ballast, the designers seem to have gone to some trouble to position the tanks further aft. (I am ignoring extreme race boats because I understand their behaviour under sail is vastly different to the Kelly Peterson 44.). I see boats like the Hunter HC50 and some designs by Chuck Payne are a little more like my old KP and they have their ballast tanks well aft.

    Is there a reason for this that I should consider? Would I be wise to move my ballast tanks aft a bit?
     
  2. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    It would be convenient to know the general arrangement of the boat, as it remains after the reform, as well as the body lines plan, and recalculate the total weight and its center of gravity, to clearly define the amount and position of the ballast. But, on the other hand, if that saltwater ballast is going to be fixed, it would be preferable to consider a solid ballast, located somewhere on the keel.
    They are simple opinions, because it would be necessary, as I say, to know more about the boat.
     
  3. Matthew Dunk
    Joined: Apr 2019
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    Location: Adelaide, Australia

    Matthew Dunk GILow

    No, my question is about placement of the tanks and whether they should, in fact, alter the trim fore and aft.

    The fore and aft static trim is already sorted out.
     
  4. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    For other readers here is some background info about the Kelly Peterson 44 on Sailboat Data :
    SailboatData.com - KELLY-PETERSON 44 Sailboat https://sailboatdata.com/sailboat/kelly-peterson-44

    kelly-peterson_44_drawing.jpg

    Why on earth are you getting rid of those wonderful bronze portlights?
    Their weight is not going to make one iota of difference to your future performance. What do you intend to replace them with?
    Re the fore and aft location of your water ballast tanks, you ideally want to have them port and starboard of your LCG (Longitudinal Centre of Gravity) when the boat is in full cruising displacement mode.
    You cannot just 'move your tanks aft a bit' simply because other boats seem to be doing this.

    Edit - we posted at the same time just now.
    Re your question, you do not want the salt water ballast tanks to adversely alter your fore and aft trim.
    @TANSL, I think that Matthew wants to have ballast that can be shifted from side to side, like a race boat.

    Edit 2 - @Matthew Dunk did you build the tanks in your keel on your previous Swanson 42, as per this old thread of yours?
    Vinyl ester keel tanks https://www.boatdesign.net/threads/vinyl-ester-keel-tanks.62195/page-2
     
    Last edited: Feb 27, 2023
  5. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Yes, surely you are right and I have not been able to realize it. But that system is so complicated that I find it strange that he would want to adopt it. Maybe it's good for a racing boat, I don't know, but the Matthew's boat doesn't seem to be a racing boat.
     
    bajansailor likes this.
  6. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    If he is using the tanks for shifting ballast, they need to be 2000 liter per side. If they are used only to keep the boat on her lines, the expense of lightweight materials was an exercise in futility.
     
  7. TANSL
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    TANSL Senior Member

    @gonzo I don't understand how you could calculate that it needs 2000 liters per side.
    From the second part of your post, the truth is that I do not understand anything. Could you please explain yourself in less "technical" terms? Thank you.
     
  8. gonzo
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    gonzo Senior Member

    It has to do with the density of water. Fresh water weighs 1 kg per liter. 2 tons are 2000 kilos. Therefore 2000 kilos divided by 1 liter/kilo equals 2000 liters. Is this too "technical" for you?
     
  9. skaraborgcraft
    Joined: Dec 2020
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    skaraborgcraft Senior Member

    I think his point is, how did you come up with the 2 ton as being "required".
     
  10. TANSL
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    TANSL Senior Member

    From experience I know how laborious, sometimes complicated, it is to calculate the amount of ballast necessary for a boat. For this reason, I will forget the error of considering a weight of 1 kg/liter for seawater, which only represents an error of 2.5%, and I will insist on asking about the procedure that @gonzo has followed to, without knowing what the boat is like, calculate 2000 kg per side are necessary.
    That's what's amazing about the first part of his post. Too technical for me? no, just amazing. (and, of course, thanks for the revelation: 2 tons is 2000 kilos !!!!!!!. Awesome too)
    The second part of his post is the one that I don't understand, perhaps because it is expressed in more technical language than I usually use.
    Thank you @skaraborgcraft , you have understood me perfectly (which, between you and me, was not difficult to understand given my poor technical language)
     
  11. gonzo
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    gonzo Senior Member

    He probably calculated the difference in material weights.
     
  12. Matthew Dunk
    Joined: Apr 2019
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    Location: Adelaide, Australia

    Matthew Dunk GILow

    The tanks, as I said, would be against the hull sides. 1000 litres of salt water at the hull side is the equivalent of ten or more grown men sitting on the rail. That is a significant righting force.

    Gonzo seems distracted by the weight reduction I have achieved by stripping the boat of the original fitout. The aim of the ballast tanks is not to replace that weight but to provide righting force, as it is with any typical water ballast tank. The weight reduction figures were included to help people understand that this ballast would not overload the boat beyond its original designed displacement.

    What I am trying to work out is whether the tanks should also provide some kind of fore and aft trim adjustment as well as righting force. I ask because designers of similar boats with water ballast appear to have done just this, placing the tanks aft of the centre of buoyancy. And I assume the designers of these boats knew what they were doing.
     
  13. Matthew Dunk
    Joined: Apr 2019
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    Matthew Dunk GILow

    It is a cruising boat.
    Heel angle is actually more of an issue on a cruising boat than a racing boat, from my perspective. On my last trip I spent two and a half days on one tack. I was heartily sick of the heel angle by the end of those two and a half days. Any reduction in heel angle will improve comfort on long passages.

    Plus, it is well understood that any reduction of heel angle increases sail efficiency and reduces weather helm. This is not one of those IOR race boats that were designed to sail better on her ear, she was designed as a cruiser and the hull shape suggests she will be best sailed as upright as possible.
     
  14. Matthew Dunk
    Joined: Apr 2019
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    Matthew Dunk GILow

    No, I’ve used the formula for immersion and recorded the change in waterline. Easy enough to do and very accurate. (But I have kept a rough running total of weights as I went to the tip and the scrap metal merchant.)
     

  15. Matthew Dunk
    Joined: Apr 2019
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    Likes: 10, Points: 8
    Location: Adelaide, Australia

    Matthew Dunk GILow


    As I noted, I think Gonzo got distracted by the number given for the reduced weight.
    As for how much ballast I should add, I simply put out a call on another forum for all of the boats people could think of that had water ballast systems and checked to see how much was carried by each. I took the logic that, unlike me, the designers of those boats knew what they were doing.

    I found a number of boats of somewhat similar design to mine, older, conservative displacement hulls. They ranged from 30 feet up to over 50 feet. I looked at the ballast carried by each and determined that 1000 kg (plus a bit for salt water) was going to be way more than enough.

    Incidentally, most of the people I have spoken to online or in person who have sailed boats with water ballast felt it was a worthwhile addition to the boat. One sailor even observed that his friend’s identical boat with water ballast out sailed his own boat without the additional ballast tanks.
     
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