Water ballast tank placement

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

  1. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    I don't know the boat and therefore cannot give advice on what you should do on it. I can only tell my opinion about ballast in general and its various variants.
    If you have changed, removed or added some elements, you will have changed the CoG of the set so you should calculate not only the amount of ballast, depending on the total displacement you want, but also the position. Ballast, if fixed, will improve trim in certain conditions and worsen trim in all others. If it is variable ballast, you get yourself into complications that you are not at all interested in addressing.
    Increasing the keel ballast will lower the position of the CoG and therefore increase the initial stability and also prevent the ship from heeling more than is comfortable. On the other hand, the liquid ballast on the sides, which is logically higher, will have a much smaller effect on stability and heeling.
    Side ballast tanks decrease living space while solid keel ballast does not.
    There may be further advantages of solid keel ballast over liquid side ballast which could be discussed later if you like.
    It would be very convenient if, instead of talking about opinions or what friends have done, you could check, through some specialized study, what is best for your boat.
     
  2. Matthew Dunk
    Joined: Apr 2019
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    Location: Adelaide, Australia

    Matthew Dunk GILow

    Regarding the portlights…. Yes, they were lovely when they were new. But now they are very tired and corroded. I can restore them, but to what purpose? They face the wrong way for a boat at anchor (this boat will not be a dock queen but will spend ten months of each year at anchor). They slope inwards on the cabin trunk sides so pool water on the sills meaning you get a face full of water from them when you open them. But most importantly, they let in very little light. The improvement in interior light levels from removing them is extraordinary. The bronze frames are very bulky resulting in an actual transparent component less than half the area of the hole they create in the hull. (Incidentally, their total weight is close to 70 kg, the weight of a small adult.)

    I did build the keel tank on the Swanson and it works well. Still have that boat, I’m on it now.

    Thank you for answering my question about trim, but can you think of any reason why designers of other water ballasted boats appear to have placed the tanks aft of the centre of buoyancy?
     
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  3. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Well, this is certainly an entertaining thread.

    I've avoided stepping into it, mosty because I'm not sailor.

    But my boat has a CoM/CoB problem which forces the bow down. And while this has little to do with your boat; it does help answer a question or at least gives us a clue as to why aft is preferred.

    If you add 1000 pounds to your boat and you pick one extreme end to add it to, which would be preferred? Certainly and without question the stern. The bow dropping down would increase pitching and decrease rudder.

    Also, if you err aft with your ballast; the effect on the boat is less because the aft sections of the hull have more available buoyancy. And if the CoB moves aft; it would be less a move for the same amount of error forward. So, an error is assumed and by placing it aft; it is mitigated.

    Perhaps someone more educated can articulate my point better.
     
  4. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell . . . _ _ _ . . . _ _ _

    MD,

    This seems absurd to me.
    You've already tripled the fresh water stores from 500 litres to 1500 litres in port, strd and fwd tanks.
    Now you want to add two 1000 litre tanks to move 1000 litres of sea water from side to side as ballast.
    And you're looking to a public forum for direction...
    No drawings or sketches just a bunch of hearsay...
    Li batteries, a lighter engine, sounds like a "more money than brains" scenario.
    Hire a Naval Architect.

    Best of luck with your refit.

    Edit: I've sailed a Peterson 44, you don't need ballast tanks.
     
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  5. Matthew Dunk
    Joined: Apr 2019
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    Location: Adelaide, Australia

    Matthew Dunk GILow

    Pretty well articulated. Yes, I suspect something like this might be in the minds of the designers. That being said, older designs like the KP44 have pretty narrow sterns and, I have heard, are a sensitive to too much weight aft.
     
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  6. fallguy
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    fallguy Senior Member

    His question was relatively simple, but I definitely agree that this is a job for a pro.
     
  7. Matthew Dunk
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    Matthew Dunk GILow

    Thank you for your opinion.
     
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  8. fallguy
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    fallguy Senior Member

    You have to have the waterplane area to make any reasonable guesses even.. My guess is that the stern is wider (than for'd sections) for the same distance from the CoG. I'm only trying to offer a chance for someone wiser to coin in. Your drawings don't have the wp..
     
  9. Matthew Dunk
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    Matthew Dunk GILow

    Yes, about 10% wider. She was originally drawn as a canoe stern, which can be seen when she’s out of the water.
     

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  10. fallguy
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    fallguy Senior Member

    I would also wonder the effect on the vessel if the ballast is in the wrong tank further forward than aft...and in pitching..even with baffles, etc
     
  11. Matthew Dunk
    Joined: Apr 2019
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    Matthew Dunk GILow

    I don’t quite understand the question? There are no plans for fore and aft ballast tanks but I can move the fresh water around a bit. The problem with fresh water ballast is that, as a cruiser, you are always reluctant to jettison the stuff. (Even with the two water makers I have on board.)

    I read of a few boats with fresh water ballast and the owners admitted they never used the system as it was designed because they tried to just keep the whole system at full capacity.
     
    Last edited: Feb 27, 2023
  12. Matthew Dunk
    Joined: Apr 2019
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    Location: Adelaide, Australia

    Matthew Dunk GILow

    I suspect these sorts of questions are a little outside people’s comfort zone. That’s ok, but it might help for those that are scared of such systems to think of the old “rail meat” analogy. I am sure every one here has crewed on a boat where the skipper had all spare crew they could rustle up sitting on the rail with their legs over the side.

    Well, on a cruising boat we rarely have ten muscled up sailors sitting around with nothing to do (well, maybe they do on SOME cruising boats, but this isn’t THAT sort of cruising in boat.)

    Think of the ballast tanks in this situation as somewhere between one and ten muscled up sailors, ready to sit on the rail, without the associated problems of feeding them or keeping up the beer supply.
     
  13. Rumars
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    Rumars Senior Member

    You see tanks placed further aft because for two reasons, either that's where the COG is in those designs, or they are ment to compensate for a bow down attitude when powered up. From a trim perspective you have three major cases, in light wind you want to be bow down to keep the stern from dragging, in heavy weather beating you want the weight midships to minimize pitching, and downwind you want the ballast aft to compensate the pressure on the bow. That's why racing boats have multiple tanks and a chart with wind conditions and fill distribution supplied by the designer.
    There are two positions for watertanks, along the waterline and completely over the waterline. Tanks completely over the waterline are only good for a limited range of heel, after a certain angle they begin to work in the other direction (towards capsizing). Cruising boats usually avoid this type of tank, and even racers don't usually have the tanks up to the deck.
     
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  14. philSweet
    Joined: May 2008
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    philSweet Senior Member

    Don't worry about trying to push the tanks aft. That is for racers, and they may be taking on ballast at speeds as low as 6 or 7 knots TWS. Their tanks are as much for light air surface area reduction and down-wind bow prying as for RM. A racing boat would draft 2 feet more and have a bulb keel and a mast 15 feet higher. They have much greater moment arms. Just center the tanks.

    If you plan to have a crossover pipe and valve, I recommend you go big - like 2.5" or 3". RV sewer knife valves with remote cable actuation are cheap and durable. The tanks should gravity switch 100 l at 11 or 12 degrees of heel. Tapering the tanks helps a lot with this. Design them for 115-120 l in order to switch 100 l. You'll want a vent line connecting the tanks as well.

    Good luck. And don't ever tell us you are getting rid of good bronze, or even bad bronze ;)
     
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  15. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    I appreciate what Rumars says above, but in your case I think that ideally, you want to position your port and starboard ballast tanks such that when full they will not create a trimming moment either way regarding your new designed cruising displacement - but if you want to err on the side of caution, it is always better to have a bit of trim by the stern, rather than by the bow.
    Just thinking laterally, have you considered keeping them as fresh water ballast tanks rather than salt water?
    If you now have 1,500 litres in the bilge, you could pump a thousand litres into the windward tank if you have to sail a long passage to windward on one tack.
    And if you want to (eg) cross the Pacific, going downhill the whole way, and you want to have daily showers etc, while not being totally reliant on your watermaker, then you could do the sums to see where she might float with all tanks full of fresh water?

    Matthew, I suppose the deed is done now, and you have taken these portlights to the scrap merchant - but I would be a bit dubious re your claim that they were 'corroded'.
    Bronze will turn green if you do not polish it (were yours green?), but one wonderful aspect about it is that it does not 'corrode' as such, unlike aluminium or steel.
     
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