Effect of Chine at Bow

Discussion in 'Powerboats' started by barrelback, Feb 27, 2018.

  1. barrelback
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    barrelback Junior Member

    Mankind has been sailing for millennia compared to flying for just over 100yrs, yet I seem to be able to find at least twice the information on aircraft engineering than I can find for boat engineering, and when I do find something for boat engineering it seems to be displacement hulls more often than not. Searches containing CHINE bring a bunch of trash hits on CHINA.

    Ok, rant over, now on with my questions.

    ONE) In a sheer view, planing hulls typically wet the chine from abt 40% and aft, and free it from 40% fore. I'm guessing this curve is to soften the ride as the bow rises and falls, but this is only a guess.
    What is the effect of leaving the chine wet the full length, or bringing it 6" free as opposed to 12" or all the way up to the sheer clamp/breasthook? Is the placement of the chine at the bow a ratio of draft? Does it affect Cp?

    TWO) Looking at wikipedia I get hung up on guessing whether the equations are standard/metric/interchangeable, or what the Greek letters refer to, or circular arguments where Fd = (equation with Cd), and Cd= (equation with Fd)
    Can anyone recommend an engineering resource with all the equations in one place and the symbols explained?
     
    Last edited: Feb 28, 2018
  2. DCockey
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    DCockey Senior Member

  3. barrelback
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    barrelback Junior Member

    Thank you, but *whistle* that price is maybe a little TOO comprehensive for what I'm looking for. Not saying it's overpriced, just that I can't justify 20 lbs of platinum when 2-bits worth of copper will suffice. In other words, I don't need the theory and equations explained (I can generally find that in various sources without asking for help), just the units and definitions of the letters and Greek symbols. As it is, I FINALLY managed to find the Savitsky papers (and a few others including Blount-Fox 1976) and they do a pretty good job with the units and symbols. I also found the Dingo spreadsheet so I'm well on my way (I think).

    Could still use a rule of thumb on the chine question. For some reason, search engines LITERALLY think I'm searching for information on the country of China, or the material used to make dinnerware.
     
    Last edited: Feb 28, 2018
  4. gonzo
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    gonzo Senior Member

    ONE)All the engineering resources would take a library. Blount's book is written for engineering students, so it assumes you already know statics, dynamics, strength of materials, etc. I think that probably the problem is that the question does not really have a straightforward answer. Chine, on the profile view, has to be considered along with the rest of the design. A hull is a 3D object and any dimension will affect all the others. For example a narrow boat will have a different curve on the chine than a beamy one.
    TWO) Depends on the formula. Some are unitless and others are unit based. For example, beam to length ratio is dimensionless (it is a ratio not dependent on units). Hull speed has a correction factor based on the units used.
     
  5. DCockey
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    DCockey Senior Member

    Yes, written with one of its intended audiences being engineering students. But the statics knowledge assumed is very basic, dynamics not much beyond definition of acceleration, and I don't recall any discussion of materials.

    Have you actually read the book?
     
  6. TANSL
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    TANSL Senior Member

    May be this could help you.
     

    Attached Files:

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  7. PAR
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    PAR Yacht Designer/Builder

    This is a difficult set of questions to answer, because full plane chine hulls have as various a set of requirements, as do most anything else, so the decisions made can vary wildly, in regard to where and how the chine line falls. Way to many variables to consider, plus the black magic also associated with them (chines, shapes and locations, etc.) is a bit like pulling teeth. The Blount book is pricey, but well worth it for understanding and given it's value on the market, can be sold after you've gotten what you need from it. I got lucky and received one a few Christmases ago, by someone who likes me . . .

    Maybe it would be easier to tell us what you're trying to accomplish, rather than a dissertation on yacht design variables.
     
  8. barrelback
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    barrelback Junior Member

    My aim is a basic runabout for cruising a reservoir, Lwl/Bwl abt 5*17.5, with beam near amidship or slightly afore. The design should be a comfortable compromise between screaming performance on 10HP, and 300HP to crawl at 5kn. I understand there are MANY variables, change something here and affect three things elsewhere, etc. A few specific questions...
    Would a fully wetted chine make the hull draggier i.e. need more power to come to plane? increase or decrease porpoising? make the planing hull slam harder as it rides along plane? affect the tendency to pitch, roll, yaw, heave, or sway?
    Does the profile of free chine affect bow wave as much as the Lwl afore the chine?
     
    Last edited: Feb 28, 2018
  9. barrelback
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    barrelback Junior Member

    BEAUTIFUL! Thank you. Sometimes the best answers are the simple ones.
     
  10. gonzo
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    gonzo Senior Member

    I assume by "free chine" you mean the section that is not wet. If it is , it does not affect the bow wave since it doesn't touch the water.
     
  11. TANSL
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    TANSL Senior Member

    It affects, and much, in that, with the same longitudinal profile, depending on the height of the chine, the "V" of the bow is more open or more closed and, therefore, the way in which the boat attacks the surface of the water is very different.
     
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  12. barrelback
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    barrelback Junior Member

    Marine VHF transcript.
    Party 1: "Unidentified vessel, suggest mutual course adjustments 8deg starboard to avoid collision."
    Party 2: "Suggest you adjust your course 12deg port to avoid collision."
    Party 1: "This is the Captain of the Navy Destroyer on official maneuvers. I see you on collision course to our portside. That's our RED navigation light you see. By the authority of the mighty government I'm ordering you to give way to starboard or we shall open fire."
    Party 2: "This is the coast guard lighthouse. Open water is to your portside."

    Hull speed is related to Lwl is related to wavelength generated by hull. So it only makes sense that the amount of bow at wl afore the chine will affect bow wave. I suspected something along those lines but had a fixation/mental block as to how to predict the performance (as well as having made a mistake in my lines drawing). It seems I need an equation that takes into account the angle of chine and the angle of stem and the distance at wl between stem and chine. It's one of those questions where if I knew how to ask it I would have seen the answer without asking. All good. Thank you both.
    (Aren't ya'll glad I ain't asking about a bulbous bow? ROFL!)
     
  13. gonzo
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    gonzo Senior Member

    Hull speed is the speed at which the amount of power necessary to accelerate a displacement hull increases hugely. It is not a definite number. Also, it depends on other factors besides length. For example, the length/beam and length/depth ratios.
    Is you question about the ratio of submerged to dry chine while planing? In that case, "hull speed" does not apply since the vessel is outside the "displacement speed".
     
  14. TANSL
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    TANSL Senior Member

    Olé ¡¡¡ for the scientific and detailed definitions. Let's see what other "scientists" think :
    Snap12.jpg Snap13.jpg

    Power not mentioned at all.

    Explanatory note: "Olé" is the exclamation that Spaniards, full of emotion, pronounce when a bullfighter performs a very artistic and very courageous action.
     
    Last edited: Mar 1, 2018
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  15. PAR
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    PAR Yacht Designer/Builder

    The typical math to gain the S/L ratio isn't written in stone and is usually inaccurate, of course depending on several variables. For most reasonably shaped displacement craft this formula works to a degree, though most displacement craft, unless powered rarely can sustain the max S/L ratio.

    To address you questions, any changes to the chine will affect the attributes of the boat, from handling, flow and wave penetration, bow rise, time and ability to get up on plane, efficiency, etc. I'd still recommend you accept that you're not really ready to self design and instead purchase a set of plans, making aesthetic changes only. The math is done, it'll likely float with the decks facing up on launch day, it'll likely not force you to swim to home, when farther from shore than you can swim back to, etc. If you insist on self designing, you'll have to perform the studies necessary. Hunt and pecking for ratios and formulas will require a lot of time, when simply opening a few books can get you through much quicker.

    Simply put, if you have to ask if a "fully wetted chine" will make the hull more draggy, well you're just not ready to self design yet. No personal offence intended, just an observation, based on what appears your understanding.
     
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