6.5 Meter Racing Sailboat

Discussion in 'Sailboats' started by kvsgkvng, Jun 21, 2013.

  1. kvsgkvng
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    kvsgkvng Senior Member

    In my everlasting pursuit of glimmering mirages on horizon, I made up a model for what I think might approximate 6.5-meter racing sailboat. At least in my mind I hope I followed common conscience, some information I read in the books and of course, plagiarism of existing net pictures of similar boats. Not to fool myself and anyone, I am modeling this boat after 6.5 meter transat sailboats. My task was to recognize what was behind those triangular shapes, rounded transoms and blunt bows; and come up with what I think is a plausible model. I tried to find similar info on the net but could not find anything really revealing.

    Most likely there are mistakes, that is why I am trying to ask for someone experienced to kindly point me those blunders or not so obvious to me problems. I just want to mention that this is not a complete sailboat, it is only a hull. Sails-Ruder-Keel is not here but I will add them later.

    So, having said all this, I am asking if anyone would point me in the right direction, hopefully without foul remarks.
    Thank you very much.
     

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    Last edited: Jun 21, 2013
  2. Petros
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    Petros Senior Member

    the bow looks to my eye needs to be more fine, smother transistion to aft part of the hull. Seems that design is also going to be real tender on its keel, it will want to heel over real easy.

    Also, I do not understand the hard chines up in the bow, they appear to curve upward too far forward. I do not see how these can be helpful, it would reduce directional stablity and could contribute to broaching. It should be more rounded up in the bow, it seems those hard chines that far forward are a mistake. Most designers would have a smoother bow and have it transition into a hard chine at the max beam and aft to assist in coming up on plane.

    I think I would make the cock pit larger as well, more room for friends to come along. Unless this is designed for solo only, more seating area will make the design more useful.
     
  3. kvsgkvng
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    kvsgkvng Senior Member

    First arguments

    Thank you, Petros for your comments.

    First of all I would like to admit that I am not a professional and my arguments may prove to be fruitless.

    However, here are my reasons for hard chine bow: I tried both smooth and hard chines in the bow. Smooth bow LWL is about one foot shorter than hard chine bow. The chine in the aft created broken water lines (I don't know how to name this phenomenon). Hard aft chine also has larger wetted surface area. If I understood correctly longer LWL has higher hull speed if not planning.

    The upward curvature of crease edge is beyond my easy control... I would have to play with finer definition by splitting the end of crease into several smaller radial creases. At this point I would think it is a waste of time. This model is very coarse -- just for testing and exercise.

    I wanted to have fuller bow because it would not dip in the water that much while running down the wind. It also limits dipping while heeled. Another benefit is that it keeps waterplane parallel to the keel. This also may reduce drag.

    You are correct in presuming this is a solo boat. At most it would be two men crew.

    Thank you once more for quick and informative opinion.
     

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  4. messabout
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    messabout Senior Member

    The profile view shows a rather abrupt upward curve of the keel line as it approaches the transom. That is typical of a tug boat not a race boat.

    Think of the 1/4 beam buttock. The angle that it should make, with respect to waterline, as it approaches the transom should be as small as displacement considerations will allow. That buttock can be almost a straight line. The deepest part of the bottom on fast boats are often forward of the mid point in order to cause the run line to conform to your small angle wishes. Maximum width is aft of the mid point in many cases so that the curve of areas can be arranged to your liking. These are characteristics of hot rod planeing sail boats.

    Truth to tell, you won't be planeing all of the time so consideration must be given to pure displacement mode. Part of the mystery and fun of boat design is to fiddle with the requirements of the two modes and manage to compromise them appropriately.
     
  5. kvsgkvng
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    kvsgkvng Senior Member

    6.5 Meter Racing Sailboat - Rev.1 - unusual resistance curve question

    Thank you, messabout

    Your very straightforward suggestion about buttock lines helped me a lot. Comparing comments in the past made by PAR about "rounded lines... sucking water... " and your comment, I think that I started to understand the concept of water flow and its viscous separation from the hull at curvature areas.

    So, I incorporated your and Petros comments in Rev.1 of the model. I wonder if these changes go along with your suggestions. I also would like to thank PAR for earlier suggestions, slightly cryptic but to the point, to be to fair.

    I have a question about resistance analysis. With the Rev.1 hull heeled, the resistance curve is a strange looking snake. Is it normal, or does it indicate that the model or whatever is bad? I attached PDF files with charts and numbers.

    Thank you, very much.
     

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  6. messabout
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    messabout Senior Member

    The revision is better than the first draft. It is probably not there yet. Move the deepest part of the buttocks even farther forward and flatten the run a little. The sharp corner of the transom may dig when the boat is heeled. Transition from the section just forward of the transom is severe.

    Heel the boat over to 10 degrees or so and examine the waterlines at that attitude. Pay some attention to the section area distributions when heeled. And if that is not enough, then take a look at the heeled section centroid alignments. If those are twisted or too curvy then helm will be affected, sometimes seriously. Inland lakes scows are good examples of attention to centroid alignment. It is easier to do with the scow planforn than it is with the pointy ended ones.

    Best scheme is to look at line drawings of some proven boats......Flying Dutchman, 49er, 505, Int. 14, Aussie 18s, int. 10 meter Canoe, etc. Find some of Uffa Foxes planeing designs from the past. All that stuff is somewhere on the net but may require some searching.

    The computer program will deliver whatever you tell it to do but it does not know how to design a good boat. You have to give it input based on things that you have researched. .

    Have fun.
     
  7. messabout
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    messabout Senior Member

    There is a current thread with the word "Velocity". Go to that thread and scroll down to post number 36. A link is there that will be valuable to you. The link is; Savitsky report. Click on that and read the text of a currently acknowledged expert on design principles. He tells of the reasons for being careful with aft buttock lines among many other things.
     
  8. kvsgkvng
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    kvsgkvng Senior Member

    6.5 Meter Racing Sailboat - Rev.2 "Drastic"

    Thank you Petros and Messabout!

    As both of you suggested I changed the hull shape as you suggested. Now the transom is flatter and more spread and flattened, buttocks deepest point moved forward. I did some number crunching and as you said “had fun” in playing with heeling angles, feathering of curves and documenting it in a simple combined picture. I also uploaded PDF files for those three cases.

    What I got is completely different from what I was initially laying out. What a change! Now the hull shell resembles ultramodern dinghy. I wonder if this is predictable result for this optimization. I am really surprised how much difference it made in hydrostatic and resistance calculations…
    I will definitely read the reference you suggested and will do my best to comprehend it.

    Meanwhile, would you be so kind as to give me your advice regarding this hull. Rev.2? Again, this is only the hull shell, nothing else.

    Thank you very much!
     
  9. messabout
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    messabout Senior Member

    You are getting there.

    It appears that the deck is pretty wide at the transom. If you anticipate that will be a trapeze platform then alright but it does add more weight than is desirable. I expect that as a trapeze platform it is too far aft for anything but a wild broad reach in a half gale. That would be a different deal on a catamaran whose evil tendency is to bury its lee bow.

    While you are doing these exercises, look for places or ways to conserve weight in your design. Light weight is an important matter for a hot rod boat. In racing you will encounter puffs. The lighter the total weight of boat and crew, the better it will respond to puffs. Newton gave us his magic little equation.....F= Ma. For a given force, less mass will enable better acceleration. Those little accelerations make a difference in close racing situations. (truth to tell, the skill with which the skipper responds to the puff or micro burst is more influential than a Kg or two of weight.)

    Smaller accelerations are not the worst of the penalty. A boat with heavy ends will pitch more deeply than one with light ends. Pitching motions slow the boat. That is because there are short term immersions that are unfavorable. But that is not all. Just think of what the mast is doing when the bow pitches up. The top of the mast is going the wrong way and the air streaming across the sail is disturbed badly. Keep the ends light.

    Your forefoot is a little bit deeper than I might draw it. There are two reasons for keeping it up nearer the surface. One reason is that a deep forefoot will make the boat behave erratically when quartering waves or wakes. The boat will become "twitchy". The other reason is that the boat will not tack as willingly if there is too much lateral area in the water at the stem. Keep in mind that the boat will pivot, approximately, about the center board. If the bow is too deeply immersed it will resist the pivoting motion.

    There, I have over done it again. I hope these comments will give you some food for thought.

    Cheers.
     
  10. capt vimes
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    capt vimes Senior Member

    those lines resembles remarkably the 12ft skiff lines... ;)

    edit:
    good article messabout... but mainly for motorcrafts - sailboats follow slightly different principles to my understanding...
    just one point which i've seen flicking through the paper: motorboats put much emphasis on the block coefficient while for sailcrafts this is of rather little importance...
    prismatic coefficient on the other hand is a good indicator for sailing performance at different froude numbers where it is almost neglected for motor yacht designs...

    here is a good thread on the design ratios for sailing yachts:
    http://www.boatdesign.net/forums/boat-design/center-flotation-calculation-implications-30857.html
    here is the summary of all that thread in a pdf download:
    http://www.sponbergyachtdesign.com/THE DESIGN RATIOS.pdf
     

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  11. kvsgkvng
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    kvsgkvng Senior Member

    6.5M Racing Sailboat Rev.3 "Decisive"

    Thank you Messabout and Capt. Vimes!

    I diligently read your comment and adjusted my model a little. I called it Rev. 3 “Decisive.” Surprisingly I arrived at a similar design done by Brendan Eagan and Jim Walsh earlier without knowing about is statistically inevitable. It just proves that same physical laws would generate same forms :) I attached graphical comparison of these two forms. Not that I didn’t see those high-tech boats flashing all over the net, but my concept was guided by a simple shape made out of plain sheet of paper. I am almost sure that the hull would be better if stern has bulbous formation where the hole is. Maybe I would include this feature in the future model(s).

    I must say that I would never be able to come up with something like this without helpful suggestions of the forum members. And again I ask for a kind help and a few pointers in the right direction by anyone who would want to help me. Thank you once more.
     
  12. hambamble
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    hambamble Junior Member

    My thoughts, and by no means are they criticisms:

    Bmax of the hull (maximum beam) is too far aft (said before)

    Modern sportsboats tend to have a wide stern for planing, yours is quite narrow. Thats ok but if you want to get up on the plane, it may not be quite what you expected.
    I would make the hull more U shaped forward, than your V shape. the the boxy-er your design, the more righting moment it will tend to have (for the same displacement). You can reduce your wetted surface area by sailing slightly heeled, its how lots of big, beamy racing yachts do it upwind, then downwind they flatten out and use their width to plane. The minis do it too.

    I personally would put a bit more rocker in the bow.

    The transom/stern I would make vertical. It looks nice, but on a small boat, if you want to get crew weight far aft, you can't, it takes up space that you could use for an extra person, and its more complicated to build round than flat.

    If you love mini's (like I do) i would check out the speedfeet 18, its a french sportsboat and you can see the influence of the french mini designs in her lines. (http://www.speedfeet.fr/ in french or http://sailboatdata.com/viewrecord.asp?class_id=5647 is an english overview)


    What are you going to build her from? Some materials suit shapes better than others, particularly for a one off. The i550 is probably the best looking ply hard chine boat i have seen... just looks fast. If your going for glass or timber, you can get that nice compound curvature hull shape. If you know what material it might be worth getting a preliminary weights and moment tables so you
     
  13. kvsgkvng
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    kvsgkvng Senior Member

    It is just an exercise

    The reason behind wide transom was to account for extreme heeling. With narrow aft sections it would sink deeper and alter smooth waterlines. With 30-degree heel, there is still some freeboard left.

    Perhaps it would be more effective sailing vessel by raising freeboard and making hull narrower – that I would not know. Here where come my pleas for help from more knowledgeable people. Otherwise, one person may say something, which would sound very scientific and true like, but without statistical backing or mathematical modeling, it would boil down into “hear-say” category.

    For example, I agree that for a small dinghy straighter stern sections would benefit in terms that a person may lean back more when going downwind, but for a larger vessel it becomes immaterial and weight saving and cross wind area would prevail design reasoning. In the end, the differences may tumble to nothing with inexperienced crew.

    Having said so, my reasoning for a wide transom is to maintain heeled geometry resembling lines of a leveled hull. If aft is artificially narrowed, waterlines suffer and buttocks would not be as straight as many people suggested. It does make sense.

    The shell geometry and balancing are most important for this exercise and other considerations are secondary. For example, in this particular case I care less if water would splash inside cockpit in following seas as long as it would flash right out through open transom.

    Any kind of questions force me to re-think initial reasoning and are only beneficial to my efforts. Please keep them coming. Thank you more.
     
  14. messabout
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    messabout Senior Member

    It is not necessarily true that heeling a narrow transomed boat tends to shorten the water lines. In fact there are many cases when the WLs are lengthened when heeled.

    Since you are into analyses, you can prove this to yourself. The easiest hull form to explore is the Sharpie or Flattie. Lay out a simple flat bottomed skiff. Experiment with different heel angles and then change the rocker dimensions a little at a time. Observe not only that the WLs get longer when heeled but they all become narrower and wet surface will diminish. Not only that but the section centroids can be made to fall into a perfectly straight line. This is not to suggest that you make your boat flat bottomed or Sharpie like. It is to get a quick and easy graphic understanding of how the waterlines behave under heeled conditions.

    As for heeling, you have used as much as 30 degrees for examination of the several items of interest. That severe degree of heel is interesting only to the extent that you want to determine righting moment. There is very little likelihood that you would ever deliberately sail the boat at that angle. To do so would diminish drive from the sails more than necessary or desired. It is highly likely that a severe helm will accompany too much heel. For a small single or double handed racer, near upright is usually the best attitude for performance.

    There are boats that like to be heeled, such as scows and many of the flat bottomed skiff like boats. I can think of none that would want much more than 15 degrees. Well done sharpies usually hit the sweet spot at about 10 to 12 degrees in which case sail drive losses and underwater shapes and drag figures can be compared.

    Did you do the spoon experiment that Savitsky mentioned in his paper? The spoon reaction is counter intuitive but does give visible evidence of an important principle of physics. The spoon will work that way in a concentrated air stream too. Physics experiments are fun.
     

  15. Petros
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    Petros Senior Member

    much improved, a few thoughts: a flatter bottom will allow it to come up on plan at lower speeds, and it will be less tender or tippy when just using it recreationally. the rounded bottom however will have less drag in the lower displacement speeds and likely less drag when up on plane, but it will take higher speeds to get it up on plane.

    Less rocker will also bring it up on plane faster, but make turning somewhat more draggy and slow you down more at the lower displacement speeds. If you are not going to have a weighed keel than the crew weight is everything you have to counter heeling moment, which means the wider the gunwale the better, even if the water line width is more narrow, to meet other design objectives. So a large amount of hull flair could be what you want if you expect to be hiked out a lot.

    The shape will depend on where you want to optimize performance, or to compromise it between one end or the other of its expected performance envelope.

    so first you have to consider do you want to, or expect to, always be on a high seed reach, or do you want it to point better in more modest wind (which usually means better displacement performance). Once you decided that than optimizing the hull shape is less of a guess.
     
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