Seaworthiness: The Forgotten Factor- C.A Marchaj

Discussion in 'Boat Design' started by Velsia, Sep 13, 2020.

  1. Will Gilmore
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    Will Gilmore Senior Member

    Compressed air can be heavier than seawater and can also be released or refilled at will. A 3000psi aluminum dive tank sinks when full and floats when empty.

    Maybe a system of a single compressed air tank that physically moves to windward or is discharged, using the discharge to drive a generator or something, when turning downwind.

    If we are exploring the development of modern, computer controlled tech, why rule anything out? There are high speed compressors that do a pretty good job of moving large volumes of air into a tank.

    -Will (Dragonfly)
     
  2. BlueBell
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    BlueBell Ahhhhh...

    Compressed air can not be heavier than sea water, but I see your point.
    CO2 is the way to go for utilizing a compressed gas efficiently.

    I'd be interested to hear Russell Brown's comments on water ballasting.
    He has a lot of experience with it on his single-handed cat.
    He's a member here. You could try Personal Messaging him.
     
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  3. gonzo
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    gonzo Senior Member

    Not at all. Dragging the stern makes the boat understeer. I have yet to read any of your claims in this forum show any data to back them up. The water ballast on ski boats is to control the wake. Filling them makes a steeper wake.
     
  4. Velsia
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    Velsia Floater

    Filling the ballast tanks in this instance probably pulls the whole boat down and not just the stern. The bow might have a better angle of attack/narrower entry angle in this trim.
     
  5. DogCavalry
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    DogCavalry Senior Member

    Well, this got hijacked almost immediately. I was hoping someone would mention my personal favorite: Marchaj's Aerohydrodynamics of Sail. Bedtime reading for me until it disintegrated.

    As an aside, no way in hell there could ever be an active ballast system like the one described. Not even theoretically possible. Sudden response to dangerous heeling moment has the momentum transfer in the wrong/lethal direction. If such a thing were actually necessary, the sole option is stability through gyroscopic precession. Note! This does not mean sensing for feedback loops to detect a divergence between altitude commanded, and attitude detected. The gyroscope is the physical mechanism by which righting moments can be applied. Look up single rail trains, if yiu have too much equanimity.
     
  6. Will Gilmore
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    Will Gilmore Senior Member

    Taking a little deeper look into this, you are right, air at 3000 psi isn't heavier than sea water.
    "common Aluminum 80 (Al80) cylinder is an aluminum cylinder which has a nominal 'free gas' capacity of 80 cubic feet (2,300 L) when pressurized to 3,000 pounds per square inch (210 bar). It has an internal volume of approximately 11 litres (0.39 cu ft)."
    Seawater is about 2.25 pounds per liter, times 11 litres equals almost 55 pounds, where the air in a full 80 cf tank only weighs 6.5 pounds.

    There are water tank systems that move counter weight water from port to starboard ballast tanks. I don't think any of them can respond to sudden emergency wind shifts fast enough to be considered a countermeasure to such a situation. Mechanical shifting of weight from rail to rail, just as crew would move, would be the only current technological option. Maybe gyroscope stabilizers could do it.

    Since dinghies don't usually tip over when a 180 pounds helmsman shifts from one tack to another, I wouldn't expect dead weight movement to do it either.

    -Will (Dragonfly)
     
  7. DogCavalry
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    DogCavalry Senior Member

    Will, your intuition fails you. The situation is not that of a dingy sailor deciding to tack, and moving across in a coordinated fashion. It is that of a sailor, heeled hard to port, when a sudden gust heels his dinghy violently to starboard and he's already hiked out. Shoving off hard against the starboard side to get to port, shoves his dinghy even more to starboard when rolling starboard was already his emergency. Newton's laws of motion make it a literal physical impossibility to move to port without pushing something starboard with the identical momentum.
     
  8. DogCavalry
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    DogCavalry Senior Member

    This is still a hijack. It's interesting, but... New thread?
     
  9. Velsia
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    Velsia Floater

    My grandfather left me the predecessor of this book "Sailing Theory and Practice" and it was a fave. Fortunately its still just about intact! Though I am sure I heard somewhere that some theory's in the book have since been refuted? Maybe I more learned member of this forum can comment on this statement as I very well could be mistaken.
     
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  10. Will Gilmore
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    Will Gilmore Senior Member

    That could be. And a very nice way of saying I'm wrong. It happens, np.

    However, I understood what you were saying about that. I was merely talking about the fact that those dynamic, non- emergent ballast systems are already in use. The dangers you mention are, apparently, ignored. Perhaps they need to be used with caution or with an understanding of the dangers.

    On the other hand,
    If the shifting ballast is kept below the cg, there would tend to be righting force until the stop at the end. The lateral plane of resistance below the waterline would have some affect, as well. Acceleration and deceleration are concerns when forces get high enough.

    Since this is suppose to be a thread about seakindliness, discussions about ballast systems, even far fetched ones, seem appropriate. Tangential subjects do often seem to generate a sort of self-perpetuation beyond the main thread.

    I haven't read a lot about seakindliness, but I've experienced and thought about the extremes. I lived aboard a skinny 56 foot schooner that was very sea kindly and my father had a fishing trawler that rolled like the dickens. My personal philosophy is about stability over comfort and comfort over performance. However, there are ideas around each of those factors that I feel are worth considering and trade-offs or sacrifice might not be made that are inconsistent with such a standard.

    For instance, I'm a fan of the planing hull for performance. In a sea-going sailboat, that flat bottom, low rockered hull form wouldn't be the most comfortable. I would design for form stability supplemented by ballast to both keep the boat shallow and flat bottomed. This means wide and wide means rolly. Because hull form is used for stability in addition to ballast, when the water tilts, the boat tilts, where a skinny boat with mostly ballast stability would remain upright on the side of a wave.

    The wide flat shallow hull would also have sudden shifts in attitude at the crest of a wave because it's ends would be temporarily suspended above the water before over-balancing to the other side. In the trough, it would be a different story. Less violent transitions for the wide boat because the bilge flotation would keep the boat from sinking completely to the bottom of the trough and thus tend to smooth it out.

    For sailing preforms and comfort, the wide boat has better stiffness, but when the wind is strong enough to really lay her over, there is a lot of lifting and falling. The up and down motion can become exaggerated on the windward side.

    I don't have any conclusions, just interesting ideas I'd love the chance to experiment with.

    -Will (Dragonfly)
     
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  11. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Not at all. Dragging the stern makes the boat understeer. I have yet to read any of your claims in this forum show any data to back them up. The water ballast on ski boats is to control the wake. Filling them makes a steeper wake.
    Filling the ballast tanks in this instance probably pulls the whole boat down and not just the stern. The bow might have a better angle of attack/narrower entry angle in this trim.
    Correct Velsia. "any data"? here is some data, it a pic of Phat Sac designed to put as much mass as far FORWARD as possible as part of evenly spread displacement increase. "steeper"? Not sure, but desired result is BIGGER wakes at a given speed. Casual observation of boats with sacs operated by people that seem to know what they are doing don't show big change in boat's angle of attack, just increase in wake size and decrease in freeboard, and I hear increase in "ride quality" for non-hotdogging passengers. Its having an easily variable increase in ride quality and also sea worthiness (with only temporary decrease in MPG and top speed) that I'm after with my proposal of systems of bladders, tubes and pumps.


    Chief Engineer wanted to take on water ballast but Captain didn't want to run seawater in fuel lines. No idea if that is accurate and such ships used same pipes such operations. No doubt lots of flushing, spilling and wasting fuel to return lines to fuel-mode.

    I just think semi-built in system to add selected amounts and locations of water ballast could be a cheap and easy way to fine tune seaworthiness.
     

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  12. jehardiman
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    jehardiman Senior Member

    Sometimes it won't matter, it will be a matter of fate. See Typhoons VIPER and COBRA 1944.
    H-039-2: Typhoon Cobra Overview https://www.history.navy.mil/about-us/leadership/director/directors-corner/h-grams/h-gram-039/h-039-2.html
    Admiral Nimitz's Pacific Fleet Confidential Letter on Lessons of Damage in Typhoon https://www.history.navy.mil/research/library/online-reading-room/title-list-alphabetically/p/pacific-typhoon-18-december-1944/admiral-nimitzs-pacific-fleet-confidential-letter-on-lessons-of-damage-in-typhoon.html

    I always used this paragraph when conducting mission briefings.
     
  13. leaky
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    leaky Senior Member

    Active ballast systems in consumer boats & not ships I don't know but I think there is a lot of room for ballast systems.

    For instance it's hard to rig a boat that is well balanced with relatively full and empty fuel tanks. Usually the engine(s) are given priority then everything else is fit around. Water can be used to re-balance for lost fuel or offset the way full tanks leave a boat heavy fore or aft.

    Also trim angle can be adjusted with weight, even listing. Trim tabs are not all that ideal on say an inboard or pod drive, a nice clean transom is interrupted by gadgets sticking out to catch docks, fishing lines, or even swimmers. A ballast system can accomplish the same from within the boat.
     
  14. gonzo
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    gonzo Senior Member

    Ballast and trim tabs do not accomplish the same. The first is weight and generates a downwards force. The second is dynamic lift and generates an upwards force.
     

  15. leaky
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    leaky Senior Member

    I don't disagree but if the boat planes with the bow way up and you pump some weight up there you could change the angle just the same, some boats may even realize a speed increase. Actually if overweight at the transom, adding weight in the bow helps even lift the stern at rest, like a seesaw, can get low scuppers up out of the water.

    If you are listing to port underway or at rest, similarly pumping weight to starboard could even everything out.

    I used to fill my livewell that was located on the transom prior to running through the inlet when it was dark for instance, at certain speeds it allowed me to run the boat more "proud", afforded some safety when I wasn't quite sure how big the breaking waves were.

    Trim tabs have downsides both to speed & efficiency, so does adding weight, seems like they can accomplish most of the same though as far as solving various problems - no?
     
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