The same but different? Design trade off

Discussion in 'Boat Design' started by comfisherman, Dec 10, 2023.

  1. comfisherman
    Joined: Apr 2009
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    comfisherman Senior Member

    Lively coffee debate this morning with the old guy crew, courtesy to much cold wind and snow. Figure boat design might have some fun weighing in.


    We have a max boat size that more than half the new boats the last 10 years have been built to. Length restricted to 58 feet, most are 27'6" wide to fit in the common haul out width and all roughly hit curb at a very similar weight. Two design philosophies have come out of it, one is far more simplified and optimized for production speed. Essentially the differences boil down to hull thickness and shape. The production boat being thinner steel with large ballast and a less complex shape, but cost 300-500K less than the competitors.

    Both are now proven capable boat designs, albeit the more expensive are perceptibly faster loaded with equivalent power plants.


    The big question is ride quality, the big intangible hard to quantify question and the source of much grumbling over coffee. Does the location of the weight i.e. heavier plate spread throughout with minimal keel ballast equate to a better ride than a thinner hull with a larger ballast in the keel?

    Muddied water was that the weight spread had more input on sea handling than more complex shape (complex roll bent shapes vs mostly flat plate design).


    What would you prefer, and how large a number difference I weight would you feel is needed for human perception?
     
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  2. BlueBell
    Joined: May 2017
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    BlueBell . . . _ _ _ . . . _ _ _

    So many variables!
    Generally, the lightest boat with additional keel weight is going to feel / ride better.
    However, that may only be in 51% of the cases.
    Too many variables to make anything more than a general statement.
    And, that's just my humble opinion.
     
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  3. gonzo
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    gonzo Senior Member

    A boat with a distributed weight, rather than it more concentrated in the middle, will have an easier and slower motion.
     
  4. comfisherman
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    comfisherman Senior Member

    That was the consensus of the old man table, the debate was how much was real vs perceived.
     
  5. Barry
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    Barry Senior Member

  6. gonzo
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    gonzo Senior Member

    It's basic physics. The increased moment of inertia makes the accelerations smaller. However, the oscillations will be larger. For example, the boat will roll to a larger angle, but will do it slower. High accelerations make motions uncomfortable.
     
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  7. comfisherman
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    comfisherman Senior Member

    No argument on the physics, debate mainly centered around if the difference on 400k pounds was perceptible enough to actually be perceived or was the 20k ish spread enough to take it out of significant figures.

    That little device could be fun. Have been in some storms that made me wish I could numerically quantify what was felt.... better metric than the fear in a greenhorns eyes.
     
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  8. Rumars
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    Rumars Senior Member

    The accelerometer that's already in your phone is easily as capable as a stand alone one, all you need is to run one of the dozens of apps available. The results are enough for discussions about how the crew perceives the difference, for example at the helmsman seat, on the workdeck or in the crew quarters, as long as the boats are running in the same conditions (easiest when the fleet goes out or if you fish with a buddy boat).
    If you want to get scientific you would have to place a dozen sensors in the exact same places referenced by a common point, for example the calculated center of buoyancy.

    What muddies the waters in the first place is your 2.1 L/B ratio, form stability will often override the ballast. A second factor is load condition, in lightship mode the light boats should be stiffer, but the question is if you can perceive it against the form stability. Load distribution isn't as simple as the ballast, the lighter boats could have heavier engines and/or carry more fuel, and of course all this is against the background of buoyancy distribution.
    At full load the differences should become smaller because the majority of the weight is now concentrated in both types of boat, but again the buoyancy distribution is working against you, the more rounded boats will have more buoyancy, so even if they have heavier ends their motion could be dampened better.

    The question I'm interested in is an economic one. The more expensive boats are clearly able to compete, be it by speed, fuel economy or range, so what are the advantages of the cheaper ones besides needing less initial capital? Over their lifetimes wich makes more money? Are there enough boats of both types so that the captains luck can be averaged out? I ask because owners and even crew aren't ultimately going to choose the "better" boat, they are going to choose the one that makes more money, comfort be dammed as long as we make it back.
     
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  9. Will Gilmore
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    Will Gilmore Senior Member

    A complete amateur weighing in. The central location of the keel weight isn't as big an issue, versus the distributed weight, as the distance that weight is from the enter of moment (I believe is the term for the point around quick the vessel rotates.

    For a pendulum, as well as a rotating mass, the farther form the center of rotation the mass is, the slower the motion will be. That is easy enough to test by sitting in a child's swing and wrapping the chair around and around the chains, then letting yourself go to spin. As you spin, extend your legs and lay back into a horizontal position, then pull your self up and your legs in, and compare the motion of the two postures. You spin much faster when pulled in towards the center of rotation.

    You can also test it with a string and bob pendulum. Just hold the pendulum up with a couple of feet of string and make it swing. Observe its period. While it is swinging from one hand, grab the string at the top with the other hand and slide your fingers slowly down the length of the string. As the string effectively gets shorter, so does the period.

    For a boat, the same dynamics are in play, plus the added effects of seawater on the hull shape. A skinny hull with weighted keel will be less affected by the wave motion. In broadside wave conditions, the skinnier hull will have less floating away from her midship's weighted center. She will be slower to react to the lifting forces of the waves.

    In a head-on sea, the finer bow and stern present a smaller area of floatation to the lifting seas. The same will be true of a following sea.

    As far as weight distribution in these conditions, the heavier boat will be slower to react to the various, but equal forces, upon the two boats, light versus heavy. That is simply the application of inertia from the laws of motion. A long keel, will add to the virtual hull weight in head-on or following seas.

    I would say, in conclusion, that a long, skinny hull with a full keel will be more comfortable than a wide boat and dropping the weight farther from the center of motion, will also allow the period of rotation. Although, I believe the ocean medium will affect that dynamic to a point that the difference will be much smaller than a few motion swing.

    -Will
     
  10. comfisherman
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    comfisherman Senior Member

    External dimensions and tankage are almost the same, as they are dictated by regulatory factors. Sadly for the last 30-40 years it's been and ever increasing race to make the "least worst " platform. Issue isn't isolated to alaksa, it's common in the Canadian maritime, great Brittain and Norway. Anywhere there is incentive for running up to the e line of legislation.

    Think the reason the slower more lively boats boiled down to the intangibles of the company. Better bridge funding, shorter lead times and more hands off input won out. Let's say 25 boats in this category have been built. Half came from a single company and most were sold to owners or corporations with hired skippers. Basically you picked from one of two mains and the color.

    Conversly the others came from the shells of former companies that were either dormant or focused on other things. Usually the operator was part owner or owner operator, construction was overseen almost g.c. like. Took a very hands on approach to a years construction.

    Going to be interesting to see long term how they all survive. We're going through what ranchers did in 80s and farmers in the 70s with massive consolidation of resources. These boats are only increasing in width, with no sign of that reversing course. I'm positioned decent compared to peers for staying in the game longer, but will either need to get out or pony up to the larger platform.
     
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  11. sharpii2
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    sharpii2 Senior Member

    I would think that a lot of the choice would be based on how the boat was being paid for.

    I imagine that if I owned a fleet and could afford to pay cash, the more expensive boat may be the better deal. But on credit? Imagine what that extra 300 to 500 k does to the payments. And then consider the added interest. These factors may tip the scale in favor of the less expensive boat. That is unless the more expensive boat performs so much better that it makes up for these extra costs.

    I read, in a book about ocean liners, that the European built ones were seen as more comfortable. They had an easier motion. The US built ones were considered less comfortable but safer. They had to meet USCG standards even in that day. I conclude that there can be a big difference between "sea kindliness" and "seaworthines." I think the two overlap a bit, but can end up being on opposite ends of a spectrum.

    When having to load fish onto a boat in all but the worst weather, I would certainly look for the overlap, rather than look for something on either end of that spectrum.
     
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  12. comfisherman
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    comfisherman Senior Member

    The general rule of thumb between the schools of thought were owner operators bought the faster slightly better riding boats and the others were predomiatly corporate bought or owned by larger groups where the chance the majority owner was not likely to actually do the fishing.

    Obviously it's not a 100% observation, there is definitely some cross over between those groups.

    Speed of construction definitely played a roll, it was a several month savings. No trivial difference depending on what 3 months and seasons were represented. Missing July and August could cost me half my yearly income.


    Me personally, If the time comes for me to put the money on the table I'll opt for the thicker hull, with better lines. If I was over 50 as opposed to under 40, probably make the opposite decision.
     
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  13. DogCavalry
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    DogCavalry Senior Member

    If it was a choice I had tomake, I'd do the same, @comfisherman . I take more care with my tools, toolbelt, workboots etc, than the other side of my life. I'm working 70 hour weeks, 20-30 days at a time, so most of my waking life is work, not rest. How much of your life are you on that boat?
     
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  14. comfisherman
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    comfisherman Senior Member

    Not much these days, probably a cumulative of 4 to 5 months. If I got back in the 58 game that would double. In 15 it was near 11 months aboard or in the yard, was only off Thanksgiving to Christmas. Well that's partially true, still do gear work like we are now.

    Hard part of both categories of vessels, the quickly begin to own the owner....
     

  15. Milehog
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    Milehog Clever Quip

    Being a gyppo logger is a lot like being an independent fisherman.
    I looked at buying a logging truck in late '88/early '89 and saw the same truck owning the owner-operator phenomenon.
    Luckily for me another opportunity came along in early '89 and I took it... 6 months before the bottom fell out of the logging industry.
    Decades later our small towns on the coast that were based on fishing and logging have not recovered.
     
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