Yacht design the hard way. learning from Kav800 (most beautiful boat EVER...etc LOL!)

Discussion in 'Sailboats' started by Schoonner, Jan 12, 2012.

  1. Schoonner
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    Schoonner Senior Member

    Sorry for making a new thread, but I think I might get more answers by separating questions from my build log.

    The reason for this thread is not just about the K800, but I am trying to learn Naval Architecture at home online for the cost of model building supplies since I'm mentally disabled with fixed (really low) income. I can't seem to find a school where I can get financial aid because none of the schools seem to actually reward a degree of any kind.:mad:

    My first question is about hull speed of the Kav 800.

    First off, I am curious whether this is a displacement hull, or a planing hull. Since I see high potential for it to pick the bow up and plane I'm guessing that it is a planing hull. On the other hand, I could see it moving past it's hull speed determined by the formula for a displacement hull by punching past it's bow wake, or rising up on top of it a little bit so that the water line shrinks front to back.

    If this is the correct formula, multiply the square root of the length at the water line by 1.34, then I think the hull speed will be slightly more than 7 knots. Under sail I would think you could actually exceed that, but probably not with an OBM without being extremely inefficient and probably over weight and exceeding transom stresses.

    Is hull speed different for salt and fresh water? It seems to me that because the displacement of a boat varies with fresh and salt water, then the actual hull speed would change accordingly.

    Please understand that I have tried very diligently to come up with my own conclusions and am not looking for an 'easy way out' by asking questions like this.
     
  2. Alik
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    Alik Senior Member

    'Hull speed' is a term used exclusively for heavy displacement hulls which shapes are not designed to reach higher speeds. Resistance curve of such boat is such that hump at Fn=0.4...0.5 is practical limit of speed.
    The hull of K800 is relatively light boat of semi-planing type (with wide stern and smooth rise of buttocks aft); there is no sense to apply 'hull speed' concept in its case.
     
  3. quequen
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    quequen Senior Member

    Schooner, I think on Hull speed as a "step" on the water, usualy heavy-deep boats can't pass over that step, k800 and many light sailboats having this kind of linesplan, overpass the step easily.
    To analyze Performance of a boat, hull speed means nothing. You should use a VPP program instead, and print the polars of the boats you want to compare (using the same VPP, not other previously printed polars).
    About K800, take account on other issues, not just linesplan: this boat is very thin, has no corridors on deck, and deck is thinner than maximum Beam, all this pulls down Crew Righting Moment. Also, it has a relatively light keel without bulb, so hull RM could be less than optimum. All this can give a low stability, wich can make this boat hard to control on hard winds.
    But can be a very funny boat for sheltered waters and light-medium winds, and it's also easy to transport! ;)
    Salt water and Low Temperatures of water makes Hull Resistance (both Frictional and Residuary) a little bigger.
     
  4. Alik
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    Alik Senior Member

    This boat is designed for sheltered waters (cat 'C' nowdays) and not intended for offshore passages, at least in centreboard version. And the boat is trailerable so beam is limited.

    There are TWO options of foils: a) centreboard plus shallow fin b) fixed bulbkeel
     
  5. Schoonner
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    Schoonner Senior Member

    COOL! I didn't know there was another option. I was kinda right then :D It will pick up and over the bow wave and go pretty fast.

    Where are drawings for the bulb keel? I want to make my RC use a bulb keel because light linds will be like huge winds for her. (Lightweight foam core RC model)

    Is there a list of formula abbreviations? Fn=0.4...0.5 looks a little scary since I have no idea what Fn means, but I could imagine it has to do with a friction co efficient because of context.
     
  6. JRD
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    JRD Senior Member

    Nope - you will need to do some real reading if you want to start predicting hull drag. There is no single co-efficient that explains it, that would be far too simple (and all boats would look the same). Suggest you read Principles of Yacht Design by Larrson and Elliason.

    For a better understanding on the effects of yacht proportions also search out Mr Eric Sponbergs "Design Ratios" which was presented on this site a couple of years ago and summarised in pdf format (also posted on this site). Also check out website www.sponbergyachtdesign.com where Eric has kindly shared quite a number of helpfull resources for those with an interest in yacht design.

    Fn is short for Froude Number which explains the relationship of wavelength and speed. This relates to the speed to length relationship for displacement hulls.
     
  7. Schoonner
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    Schoonner Senior Member

    THANKS! I like your site. First thing that caught my eye was the free standing mast. :D I have been working on a braid for carbon fiber yarns that is actually a fractal made from triangles that make up hexagons. Very strong in 60cm by 4cm lengths without any resins even. I don't know how to test them other than support the end and reel in 50lb line on a fishing reel nailed to the wall and measure the deflection before breaking. That idea was a bust because the carbon rods (6 of them) won't break in the same place on the mast. While the material is obviously compromised it is still in one general piece, just with cracks and breaks in it. I also found that the mast still retains most of it's deflection strength even after a couple of the rods fracture. The carbon rods I have fracture lengthwise for about 4cm on either side of the beginning crack and actually tighten the yarn because the diameter increases. I didn't have any actual carbon yarn, so I used monofilament fishing line instead because it doesn't stretch much and I doubt that carbon yarns stretch much before breaking.

    EDIT:: One of the problems with this is the time it takes to braid and that it will require 6 smaller mount points for a mast rather than one big one.
     
  8. JRD
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    JRD Senior Member

    You need to consider the principles of proportionality when converting a real life design to a model. If you do an exact scale down there will be problems if you plan to actually sail it.

    Form stability is proportional to the 3rd power of the beam, meaning that your RC model will be much less stable relative to the real boat even after scale factor is considered.

    Have a look at some websites for RC racing yachts. The first thing you will notice is the really long keels. Racing RC yachts have developed shapes and ratios which are fastest based on their actual size. Many of them now only have passing resemblance with real keel yacht designs. However methods to achieve stability are the biggest departure from full size boats.
     
  9. Doug Lord
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    Doug Lord Flight Ready

    American Model Yachting Association(rc sailboats,suppliers, and racing classes): http://www.theamya.org/


    Design Ratio's paper by Eric Sponberg:
     

    Attached Files:

  10. Schoonner
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    Schoonner Senior Member

    When calculating the CwP why are the hull appendages not considered? The rudder for example in fig 2 on the PDF which is linked in the previous post. Doesn't the rudder displace water and extend the actual Lwl in the pictured design? Perhaps it is included, but is not shaded for some reason?

    EDIT:: That's actually figure 3 not fig 2

    Nevermind the answer is located on later pages of the PDF.
     
  11. Alik
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    Alik Senior Member

    Appendages are not included in coefficients because their drag is taken separately from canoe body. Coefficients of canoe body are used to calculate only canoe body drag.
     
  12. JRD
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    JRD Senior Member

    Schooner, You need to read the whole thing first, then read it again, then start asking questions.
     
  13. Schoonner
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    Schoonner Senior Member

    Sorry, I got a different answer from Alik, and I like his answer better. :D

    So, when calculating the Fn it is kind of like calculating flow past the hull:?: I mean like looking at it as though the hull were stationary, and the water is what is actually moving. :confused:

    I was taught that Fn was not used much since it was only applicable to projects like calculating flow through a spout on a watering can.

    So, instead it is a dimensionless number like Reynolds number for airfoils.


    UGH! I think I might just stick to designing unbreakable masts. :rolleyes:

    I need better math skills too.
     
  14. Alik
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    Alik Senior Member

    Fn is Froude number that is a measure of wavemaking; it is also a relative speed same as 'speed-length ratio' but in more scientific form.

    There are Froude numbers based on length and on displacement. Displacement-based is used mostly for high-speed craft.

    Fn=FnL=v/(g*L)^0.5
    FnV=v/(g*(V^0.333))^0.5

    And now You have to go read something; Gerr's 'Esoterica' and then Larsson's 'Principles' would be excellent start. There is no way to study naval architecture on forum.
     

  15. Schoonner
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    Schoonner Senior Member

    Okay. Thanks. I already have more books than storage space. :D I guess a couple more of the right ones won't hurt. They can go on my coffee table.:p Who else do you know who lives in a 9 meter RV and has a full sized coffee table, a long range wireless network, (36 meters away through three aluminum trailers and a window ('parabola')) and more cookware than your grandmother!?!:p

    Oh, but I already have. LOL! Not understand everything of what is being discussed, but you know...:rolleyes:

    Some people say fat thick quarter keels are better, some say thin fin keels are better, Some say beam to foil thickness is the formula to follow, others say on a fin it doesn't matter as long as it is a parabola and has tapered sides, while others say that the lift/drag ratio matters.

    All I care about is coefficient of frictional resistance with varied Reynolds numbers and foils and then damping because it seems like the rest should kinda take care of itself on a hull like the K800 has. Well, with the exception of roll and heel characteristics which I have not really gotten into yet. :D
     
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