Seaworthiness: The Forgotten Factor- C.A Marchaj

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

  1. Velsia
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    Velsia Floater

    This was a formative text for me during my design education and it covers an aspect of yacht design (seakindleness) which is sadly not considered by some as much as I would like.

    Does anyone have any good recommendations for further reading on the subject?
     
  2. jehardiman
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    jehardiman Senior Member

    The concept or the science?
     
  3. Velsia
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    Velsia Floater

    Both really
     
  4. jehardiman
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    jehardiman Senior Member

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  5. Will Gilmore
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    Will Gilmore Senior Member

  6. Velsia
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    Velsia Floater

    These are good links. Thank you both for your help. Some fun bedtime reading!
     
  7. Squidly-Diddly
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    Squidly-Diddly Senior Member

    IMO in this day and age it should be SOP even for private yachts to have integrated, designed in, computer control enhanced, redundant, variable, dynamic system of Water Ballast and Reserve Buoyancy.

    This would mostly be water and air bladders in various nooks and crannies controlled by tubes and hoses. POSSIBLY including fuel bladders but more likely air, sea-water and fresh water bladders. Computer control would optimize everything after being set by human for desired combo of speed, safety, sea-keeping etc.

    These "phat sacs" seem like good starting point and my experts say the pumps handle silt laden water for "years" but maybe upgrade if more of PIA to change out in big finished yacht. Amazon.com : fat sac ballast https://www.amazon.com/fat-sac-ballast/s?k=fat+sac+ballast
     
  8. jehardiman
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    jehardiman Senior Member

    For what exactly? What do you think you could control/mitigate?
     
  9. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I heard side to side (water) ballast is a few times more efficient on a healing monohull than fixed center ballast.
    Maybe in heavy or choppy seas you'd want to ballast down, but in calm water go faster with lighter boat.
    Maybe Flume Tanks that could be empty to lighten boat when not needed and double as emergency flotation.
    You could load heavy supplies and not worry about spreading them out to keep boat balanced. Like a self leveling car suspension.
     
  10. clmanges
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    clmanges Senior Member

    That's easy to imagine, and you're probably the umpty-thousanth person to have had the idea. Are these systems in use anywhere? All the technology is available off-the-shelf right now.

    One thing such a system would have to do is to predict the future so it can start the pumps early and have the ballast-water where it's needed in time to counter the effect of, say, a beam-on wave, which will certainly move displacement a lot faster than that plumbing can ... unless (or even if) you've used up an outrageous amount of hull volume with enormous pipes and pumps and tanks and extra engines to power it all. I think it would need to be on a scale with maneuvering thrusters to work at all.

    Maybe this is why I've been reading that some vessels are equipped with gyroscopes.

    I'll wait for someone to tell me I'm wrong.
     
  11. jehardiman
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    jehardiman Senior Member

    I'm not trying to be negative or critical here, but water ballast is fraught with problems when used as a major part of stability, especially dynamic control of stability. When professional mariners using computer controlled systems manage to catastrophically roll and/or loll a ship real time in benign water, I don't put too much hope in the casual sailor even with an expert system in being able to manage water and free surface in a significant seaway, and I know because I've had to try and fix the design of someones bright idea.

    clmanges pretty much states the reason you don't see systems like this. It just takes to long to have any significant change unless you put it up waaaay high in the vessel or have multiple insane pumps. The other thing is the randomness of the sea way, even with near real time 3D lidar/radar topographic scans (yes, they exist off the shelf for ride level control systems), you may have to get the mass back in less than half the time it took to put it there, even when said mass is already moving in the opposite direction. Think about hiking on a dingy or being rail meat on a ULDB; think about how random and fast you have to move to keep the boat balanced. Go have a look at every measurement you would need to make a viable PID controller and on the fly tuning for this. I'm not saying that a computer couldn't do what you do, I'm saying that a pumping system to move mass that fast is not practical (and yes, I know what the system has to look like...it isn't pretty and very, very, power hungry).

    Yes; water ballast make sense on Vendee Globe ULDB's that are going to be on one tack for 5,000 NM. Yes; computer controlled ride leveling works on high speed vessel with rapid control surfaces or slower vessels with weighty gyros. Real time control of water ballast to improve seakeeping?....not so much.
     
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  12. Velsia
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    Velsia Floater

    Any system which is computer controlled inherently has the problem of failing and the human charged with fixing also failing. The boat is no longer seakindly or seaworthy if the system stops functioning. If vessels move in this direction, seafarers will have to add computer science to their repertoire and good seamanship will take on another facet. This probably requires a thread of its own.

    Anecdotally I have sailed a boat with water ballast in some largish beam seas and when the water was pumped to windward, the radius of the roll was minimized which helped with her seakindliness. (2000kg worth of water on a 22,000kg boat so about 10% of disp). In smaller beam seas, the same volume of water ballast pumped to windward righted the boat so quickly that the fast rolling motion would make her less seakindly.

    I think it is worth clarifying that although in the most part the Vendee boats are very seaworthy (forgetting the foils), the speed and stiffness of these boats make them terribly un-seakindly. Any boat where you have to crawl about to stay safe does not signify comfort!
     
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  13. Squidly-Diddly
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    Squidly-Diddly Senior Member

    The MacGregor requires water ballast when carrying much weight (people besides the driver) and motoring fast. No required water ballast and "operator error" was fingered in that capsize and death of several (5 or 6 IIRC) on some lake. I hear about both cargo and warships taking on water ballast in storms, and that matches my exp. with smaller boats. Just using side-to-side water ballast on a long reach to keep a monohull less heeled, or to compensate for uneven loading is about as dynamic as I'm thinking, except for Flume Tanks. Nothing quicker than 2 minutes "dynamic". But it might be nice to have a computer sense the motion of the boat and after a few minutes make adjustments more precise and nuanced than a human. I'm thinking less dynamic and more trying to give a boat a wider range of personality, sort of like a MacGregor but also able to do side to side, front to back and even moment arm factors (whether you want extra mass toward the center or out on the periphery).
     
  14. jehardiman
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    jehardiman Senior Member

    Realistically, the time frame you envision for changing "ballast", it only changes the mass to WP and CG to CB relationships a very small amount. Sometimes this is sufficient to de-tune response, other times not; but in either case it is better to design the hullform to give the response you want in most conditions. As I have pointed out in numerous threads, water "ballast" only works when it is above the waterline, otherwise your just inefficiently changing the hull shape, so in the case of the MacGregor they added weight (people) high and added buoyancy low (i.e. didn't remove volume low) which drove a BG couple too large for BM (Iwp/Volume) to insure a positive GM and righting moment through dynamic changes to the WP caused by the seaway. Yes, ships ballast down, but what they are doing is changing the relationship of center of mass to the variations in magnitude and location of buoyant force. Note that in the case of static stability, water in tanks below the waterline does not effect the BG couple, but water entrained in tanks does effect the center of mass in the case of dynamic response to a seaway...whether this is helpful or hurtful to overall dynamic stability depends on the individual case (c.f. EL FARO ballasting down to control wind list).

    Automatic control systems (analog or digital) have their own issues (both dither and lag). They are not, as Velsia points out, prescient; and for the most part require human intervention, usually by setting a "gain" of some form.
     
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  15. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Example: when a ski boat/wake board boat isn't pulling boarders but just cruising some middleaged people around outside the ski area in what is mildly choppy water, filling the Fat Sacs greatly smooths out the ride, at least at certain levels of chop. Guys that fished offshore in SF Bay with modified ex-US Navy Viet Nam river boats would use water ballast to make boat better in rougher seas. IIRC they'd take on ballast during actual fishing operations with men on deck.
     
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