Water ballast tank placement

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

  1. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    @Rumars - you beat me to the post button.
     
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  2. Matthew Dunk
    Joined: Apr 2019
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    Location: Adelaide, Australia

    Matthew Dunk GILow

    Ok, now we get to the good stuff.

    Some well reasoned responses to the fore and aft trim question, thank you all.

    I suppose get them close as possible to the fore and aft centre of buoyancy, but with, maybe, just a hint aft, just a hint. ( I did consider splitting them vertically to allow them to be filled in a way that changes the fore and aft trim, but the plumbing complexity just doesn’t seem worth it.)

    Regarding the crossover pipe idea, I’ve decided against it for a few reasons. Simplicity being the main one. I can install simple centrifugal pumps in each tank to empty them. There’s no shortage of salt water so dumping it has no downside. Also, the nature of the cruising life seems to be days of sailing on one tack, followed by pulling into an anchorage. Different to the racing boats who tack up wind and hence need to transfer ballast fast. A big hose routed across the boat would be difficult and very, very messy if it split. (Look what happened to one of the VOR racers a few years ago when their ballast transfer hose blew.). I don’t mind if my tanks take an hour or more to fill, but it seems I can afford pumps that will do it in twenty minutes.

    Regarding fresh water… I agree, can’t have too much. But 1500 litres is pretty generous, and the construction of the under floor fresh water tank system as been a HUGE task. To build the tanks in such a way that they won’t contaminate the drinking water is very complicated. To make the ballast tanks suitable for holding drinking water was going to be a massive task. And keeping them clean was even harder. So, on balance, I figured to keep them just for salt water which just doesn’t matter.

    The bronze portlights were not corroded, no, that was the wrong word. They were tarnished. Or, if you like, they have a lovely aged patina. They will be sold online, FB marketplace , the local chandlery, wherever. Somebody will want them. There are ten of them, so a few to sell. Worth a few bob at a guess.
     
  3. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Can one of you fine gents explain to me the mechanics of how side ballast tanks work?

    I envision a boat leaving harbor with tanks full of saltwater. Sails are raised and the boat heels. With a fixed amount of mass on each side, the offsetting mass keeps the boat at a lower angle.

    But isn't there a disadvantage to the not windy side ballast tank? i,e, Is it increasing heel? So would it be preferrable to allow it to dump? Of course, all this dynamic business sounds awful. So, I am trying to grasp the difference between valved tanks, etc. vs the simpler case.
     
  4. Matthew Dunk
    Joined: Apr 2019
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    Location: Adelaide, Australia

    Matthew Dunk GILow

    You only fill the ballast tank on the upwind side.

    High speed race boats do this by lowering a carefully shaped, forward facing pickup tube into the water below the hull and use the velocity of the boat to force water up the pickup into the tanks. It is spectacularly fast, but then so are those boats.

    The same boats, just before they tack, open a transfer hose and allow the water in the upwind tank to drain into the downwind tank by gravity, hence references to crossover hoses. They then tack. It is one of the odd properties of these setups that having water in the downwind tank has less of a destabilising effect than anticipated. A good drawing showing the various centres of buoyancy etc explain why this is, but the main thing is that it means it is not dangerous to have the downwind tank full should you be forced to tack in a hurry.

    Because my boat is not even remotely fast enough to use the pickup tube approach, I will have to use centrifugal pumps to fill and empty the tanks. Such pumps produce very low pressure but great rates of flow. Thankfully the pressure needed is not all that great, the top of the tanks will be less than one meter above the waterline. I anticipate it will take at least fifteen minutes, maybe half an hour to fill and empty the tanks. That’s ok, because when cruising you usually know hours in advance of when you will want to tack.

    Finally, I came at this idea from a different issue to righting moments, I actually wanted to settle the boat down when at anchor. Since her two ton weight reduction she has been a little jittery. This makes sense when you see the hull shape. Like most boats of her era and style, the hull turns in sharply at the waterline so her beam at the waterline is greatly reduced as she floats higher. Filling BOTH tanks while at anchor should improve her comfort a little.
     
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  5. skaraborgcraft
    Joined: Dec 2020
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    Location: sweden

    skaraborgcraft Senior Member

    I will just say the boat was not designed to have them. By shedding some weight in the hull, you are already increasing the righting moment of the existing keel, by adding more fresh water capacity down below, is also doing the same. You need to come up with a new figure for the righting moment with your suggested tanks, as you might find you need to replace the entire rig.
     
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  6. Matthew Dunk
    Joined: Apr 2019
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    Location: Adelaide, Australia

    Matthew Dunk GILow

    The rig will be plenty strong enough, I’m going dyneema.
     
  7. skaraborgcraft
    Joined: Dec 2020
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    Location: sweden

    skaraborgcraft Senior Member

    Its not just standing rigging, its the mast section, it will be under more compression.
     
  8. Matthew Dunk
    Joined: Apr 2019
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    Matthew Dunk GILow

    Yes, but her mast is already massively over spec, plus I’m converting to external chain plates. And she’s a cruising boat, I’m not going to be sailing her under full canvas in strong winds.

    Those sorts of concerns relate to race boats with noodle masts.
     
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  9. ChrisVJ
    Joined: Nov 2021
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    Location: Kelowna

    ChrisVJ Junior Member

    Two things,
    Weight at the longtitunal ends of the boat will decrease pitching compared to weight added at the centre. Longer moments mean it requires more effort to disrupt the longitudinal stability.

    Weight added at the topsides is an order of magnitude more effective at decreasing heel than weight added on the keel, (for small angles of heel.) When the boat heels, say 10 deg. the exisitn ballast and the centre of pressure diverge only a few inched for a thirty odd foot boat of this kind of design but a thousand lbs added at the topsides exert a force of, say, 1,000 x 6 foot lbs.

    Personally, rather than the enormous effort of building watertight (and forever) tanks I would buy or build a couple of cylinder tanks and put them against the bottom of the topsides at the back under the benches. They would need to be well baffled but it seems an easier option. Might allow easier transfer too.
     
  10. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    pretty sure adding weight to the ends of the boat increases polar moments and this increases pitching

    Contributors like @jehardiman are much better at these subtleties.

    I think the issue is not whether balanced weights at the ends are more or less pitching, but an errant weight forward, so please read my remark carefully. But I look forward to an NA commenting on this issue.

    Things like pitching and damping are also not the same.
     
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  11. trip the light fandango
    Joined: Apr 2018
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    This thread caught my messabout backyarder eye, having baffles could make building the tanks a bit easier, and mb safer when filling[?], if that was the case then it doesn't seem like much of a leap to be able to have some choice as to how much weight is moved for and aft and allow for tailoring to the conditions and point of sail, cheers.
     
  12. Matthew Dunk
    Joined: Apr 2019
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    Matthew Dunk GILow

    Depends on the state of the interior. In my case, easier to install full height tanks along the hull sides.
     

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  13. Matthew Dunk
    Joined: Apr 2019
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    Location: Adelaide, Australia

    Matthew Dunk GILow

    It certainly is an option. However, baffles usually have limber holes at the bottom to distribute water evenly between tanks. And the forces on the baffles are relatively minor as you don’t have a huge imbalance of water on one side and air on the other.
    To build these tanks with a fire and aft split would increase the build complexity hugely, and add plumbing complexities that I think would become a maintenance headache over time.
    I did give the idea some thought, but in the end simplicity wins. (This may sound strange given I am adding complexity by adding the ballast tanks, but actually, building them the way I plan is very simple and makes the rest of the interior refit much easier again.)
     
  14. skaraborgcraft
    Joined: Dec 2020
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    Location: sweden

    skaraborgcraft Senior Member

    Sounds to me like someone has never slept in a pilot berth next to or on top of an unbaffled tank.
    Wish you luck with your project.
     

  15. Matthew Dunk
    Joined: Apr 2019
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    Location: Adelaide, Australia

    Matthew Dunk GILow

    I can confidently say I’ve done a lot more sea miles than you have. What a patronising and foolish remark.

    And I didn’t say there would be no baffles, I said they’d have limber holes. So the water would be distributed evenly around the baffles, thus reducing the unbalanced forces on the baffles.

    If you are going to be patronising I suggest you work on your reading comprehension beforehand.
     
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