15 foot sports recreational dinghy

Discussion in 'Sailboats' started by kvsgkvng, Jul 14, 2012.

  1. kvsgkvng
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    kvsgkvng Senior Member

    I am learning naval architecture and as an exercise, I am trying to design a recreational two adults sailing dinghy for lakes, beach and protected waters. My goal is to have the correct feasible plans and not to build one. I attached a composite picture with the information. I wonder if real naval architects would be kind and give me a few pointers.
    Thank you for the help.
     

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  2. PAR
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    PAR Yacht Designer/Builder

    The best thing you can do is study yacht design. Being given a few "pointers' isn't going to help you much, without a solid understanding of the physics involved. Try the book store here, searching "yacht design" for some reference and research text offerings.
     
  3. CT 249
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    CT 249 Senior Member

    Kv, in dinghy design the "real naval architects" have shown little superiority over those who lack qualifications as NAs. I'm not a designer but I have interviewed most of the top designers in the world and almost all of them say that what counts is sailing experience and the ability to learn from existing designs, which means respecting them and understanding what the designer was trying to achieve.

    There are very, very, very good reasons that the combined skills, experience, deep thought and knowledge of generations of dinghy designers have lead them to create the typical shape of modern dinghies. Your boat is dramatically different in just about every way.

    You could perhaps go to Ebay and find an old copy of Ian Proctor's book on racing dinghy design. Other excellent information is on the websites for development classes such as Merlin Rockets, National 12s, International 14s, International Canoes, and Cherubs, although since the information is not presented as a textbook it takes a lot of work to understand the lessons of time. These are racing boats but the same rules apply to cruising dinghies.

    A lot of the best information can only be obtained by watching boats on the water, talking to sailors about the way various shapes behave, and going into museums and spending hours reading about the way that designs have developed over the decades. But just googling can teach you a lot. For example, one could learn a huge amount from the Bieker PT 15, which is designed by one of the world's greatest small boat designers. At

    http://www.biekerboats.com/Bieker_Boats/PT_Dinghy_Particulars.html

    you will find a significant amount of information about a boat similar to yours in role, but very different in shape. And for an insight into the sort of thinking Bieker uses, Google will lead you to pieces like

    http://www.stevencallahan.net/images/proboat/bieker-dec2002.pdf

    where you can get an understanding of the trade-offs he thinks about.

    Another excellent piece on dinghy design is Jim Champ's interview with leading designers at

    http://www.devboats.co.uk/99interview.html

    Again, it is about boats that are very different from yours, but the same concepts of dynamics, dimensions and trade-offs apply.
     
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  4. messabout
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    messabout Senior Member

    KV the other respondents have given you sage advice. Neither of them was specific about the particular elements of your design that may be questionable.

    OK here is only one comment. The underbody sections are probably too rounded unless your boat is to be extremely heavy and will draw a lot of water.

    Second comment; Ditch the computer program for a while. Practice doing your drawings the old fashioned way with a pencil and a sheet of drawing paper.. Practice until you get the feel for just how a spline wants to bend.

    Do the calculations the hard way. You'll get a feel for the numbers and ratios, and all that, by doing them yourself. After you have done those things for a while, then you can streamline the process with the computer program. Build a firm foundation first, only then use the magic box.
     
  5. kvsgkvng
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    kvsgkvng Senior Member

    I did go throuth the book by Lars Larsson and Rolf Eliasson "Yacht Design" pretty thoroghly and believe I do understand the physics, statics and some basic fluid dynamics. I also studied lots and lots of existing sail plans for medium and small sailboats. I think I understood what different boat "designs" thrive to achieve. That is why I am asking for what I did not so-right because I tried to account for what was important for me. The only good thing I was able to get is a remark about "probable too round bottom" and "your boat is dramatically different in just about every way." Suggestion to lay off the computer is good too.

    I wonder what is wrong with it specifiaclly, not why it is dramatically different in just about every way. Whatever the book said about ratios, coefficients, displacement, length, "centers," heeled waterlines, trim, etc. etc. appears to fall in the "good" category and those calculated printouts suggest it. I did have a bit high freeboard because I wanted this dinghy to be able to cope with choppy waves in coastal waters and to have a small bow compartment to keep personal items and food dry. What is wrong with the round bottom? As far as I can compare it is pretty close to my Laser I sail weekly. It is also very close to JY15 which I sail as well in my club. Agreeably, their freeboard sucks and that is why they have a wet ride. That is why I have a bit hight freeboard.

    Anyways, I thank everyone who bothered to answer me for some free good advice.
     
    Last edited: Jul 15, 2012
  6. gggGuest
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    gggGuest ...

    The trouble is that because its so different in every way from the mainstream its hard to pick up on individual points. Your described aims are very vague too so its difficult to figure out where your targets are. Its also difficult to pull in much of a critique without knowing much about what your aims are: recreational/cruising is such a vague description. How many people do you want to carry? How fast do you want to go (this affects cruising range) how much do you expect to carry? Is a capsize business as usual/not unexpected/undesirable/a disaster? Will you sail in tide/current? How big will the waves be? Will you sail in full wetsuits/full waterproofs when windy/maybe a spraytop but bitterly resent getting splashed even when its seriously breezy? Will it sit on a mooring? Will you want to bring the boat shore without a trolley? Will you want toretrieve it with trolley up a soft sand or shingle beach, or just on hard standing? How will you sail: do you intend your crew to just sit in the boat on thwarts, sit on the side/lean out on toestraps/use trapeze?

    I guess when I look at that my first thought is "slow". Now speed is not necessarilly a virtue, nor slowness a vice, they are just design features, but it really depends what your aims are. If I have to get home for supper against an adverse tide/current for instance then I am going to want as much speed as I can get for instance. But if I intend to have my sandwiches and thermos flask out and have my lunch under way and a capsize means I lose it all then a very fast boat with all the extra precision in handling and attention needed is a problem too: and its unwise to drink hot coffee whislt being launched off the top of a wave at 15 knots.

    You mention low freeboard on Lasers and things, but low freeboard brings with it virtues too - reduced structure weight for one. All else being equal double the freeboard and you double the weight. That's a big issue with off the water handling if you are planning to pull the boat above the high tide mark and camp overnight.

    For instance you seem to have put effort into the lines when sailing heeled, but every dinghy is sailed bolt upright if possible these days, so maybe that's an over consideration.

    The bow looks very full to me: I think it will kick up an enormous bow wave. Modern dinghies tend to the opposite.

    Another concern of mind would be dynamic behaviour of the hull and how it rolls when under way. As a boat rolls it will tend to change direction: the hull steers the boat one way and then the other. This can accentuate the roll. I think I see a lot of that in your boat with the round bilge and the way the stern is treated. I remember sailing a dinghy back in the 70s which had vaguely similar sten lines which became uncontrollable at speed because of this behaviour. A hard chine boat with less stern tuck in may be less prone to that.

    Then the masthead rig with a big genoa is not one I would like to handle. Its only virtue, as far as I can understand, is that you get the maximum sailcloth up with the minimum size of spar. I would have a none overlapping jib on a small recreational boat because a big genoa will tip the crew in the water every chance it gets: its really a quite horrible sail on a small boat - maybe any boat.
    I'd also have no more than 75% forestay height, because that way the rig yields and loses power when hit by a gust: with the masthead rig the mast will bend the wrong way and tend to power up the sail, which causes the boat to be very hard work in anything but the steadiest of wind.

    The rudder seems way undersize to me. Modern dinghies use both the rudder and the centreboard for leeway resistance, so increased rudder size, as well as increasing control, also trades off in reduced centreboard size. Biut the centreboard also seems small. Having the rudder tucked under the hull is an unnecessary complication. Stern hung is better in almost every way.

    I think you'd do well to read both volumes of Bethwaite: especially the first. Bethwaite is very focussed on racing craft, but he put in more serious empirical numerical research into dinghy design and behaviour than any other three designers you care to mention.
     
  7. CT 249
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    CT 249 Senior Member

    NAs who also design dinghies have remarked how different they are from yachts. For example, high performance yachts tend to have similar DLRs, whereas high performance dinghies range from as low as 32 (International Canoe) to as high as 120 (12 Foot Skiff). This creates major changes in design factors; an International 14 must have quite high volume so it has U-shaped sections, whereas an 18 can have low volume and moves so fast that wave impact is a major factor so it has a V shaped bow.

    While these are different types to yours, it's an example of the reasons why some NAs say that dinghy design is much more complex than big boat design. Therefore a design that falls into the "good" category of L & A may not work in a dinghy - for example your PC is very different from the one that Bieker (an NA, multiple world champ and America's Cup designer) says you should have in a medium-speed boat - see the linked article.

    Specifically;

    Your entry waterlines appear to be extremely wide, and get even wider higher above the water. Entry angles vary dramatically according to many factors including LOA, speed, purpose etc so perhaps the best thing is to measure a similar boat like a JY15 or the the PT15. Without a lines plan I can't judge, but yours appear to be about 10 degrees wider than a modern boat which is similar in length. Even in the 1930s, Uffa Fox's Int 14s had bow angles of 19 degrees. They have been getting finer ever since.

    Your bow becomes much fuller above the waterline, whereas one of the big trends over a long term has been to reduce fullness above the waterline, partly because of changes in rig weight and self bailing. For example, one NA ensures that his bow angle is around 14 degrees around half-way up the topsides, whereas his entry angle is 12 degrees (but this is in a 16 Foot Skiff, which has a fine entry). The point is that modern boats show that you should not have topsides that are much fuller than the waterline.

    U-shaped sections carry more volume/buoyancy down low and create a "planing flat" along the centre which increases planing lift and allows you to have narrower waterlines. These are almost universal among modern boats apart from BEthwaite designs, but because they are so long and high-powered he places a higher emphasis on wave penetration.

    You mentioned the Laser - Kirby's design for it was strongly influenced by the fact that it had to be a cartopper, therefore it has less freeboard and therefore it needs more flare above the waterline. In his other designs he has gone for less flare. See his Int 14 designs in the Int 14 class history, which is also extremely useful as it provides an excellent history in dinghy design. The Laser can handle its flared bow because it is very narrow, but to get it going well upwind in a chop demands a lot of hard work and technique, which shows the problems created by extra flare. Your bow appears much fuller and IMHO it would be extremely hard work upwind in chop.

    I have Laser lines and they are much finer in the bow and fuller in the stern than your boat. Your stern appear more like a Europe to me; that transom shape is nice but how it will work with the very full and flat mid section is another matter.

    I have been lucky enough to speak with the designer of the Laser and JY15 and both of them are generally in favour of U shaped sections with less flare, as well as more conventional rocker lines. Both of them draw boats with elliptical mid-sections and stern sections as they provide more lift and less wetted surface area.

    Your rocker is very flat in the centre but curved aft. That tends to be slow in a breeze and as Gguuest says, in combination with the round in the stern it creates handling problems.

    Gguuest is not "just" an amateur designer, but also someone who has done an enormous amount of research in the field and his remarks are definitely worth listening to. I can only second his remarks.

    In the interview with designers I linked to, you may note that even Bieker - arguably the world's #1 skiff designer and IMHO the world's #2 dinghy designer behind Morrison - says that when he started designing he learned a lot from looking at the plans of older Int 14s. Every other designer I have spoken to emphasises the same thing about learning from earlier designs. Perhaps you could take some measurements of a Tasar, JY15, Wayfarer, Vanguard 15 etc and study them? As someone who is an amateur like you, I learned a lot from just getting ball park measurements in that way.
     
  8. kvsgkvng
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    kvsgkvng Senior Member

    Thanks, the two replies above are very nice and informative. Moreover, it shows an expertise and knowledge. What is interesting, that both gggGuest and CT249 picked up those features which I had in mind without knowing it! Indeed, the emphasis was more on recreation and cruising then racing, the hull weight was not a concern during this exercise. Both experts picked up undeveloped ruder and centerboard and effort to have smooth heeled lines. It is amazing to see how this hull got picked up and dissected. Thanks this is exactly what I was looking for! I much appreciate this input and will continue sink my teeth into this fun hobby. Thanks again, kind regards.
     
  9. Stumble
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    Stumble Senior Member

    One of the things that struck me (I am not a NA just a sailor), is the almost complete lack of form stability. Particularly on small boats having them this unstable at rest is going to make loading and launching difficult and uncomfortable since weight must always be kept low and centered.

    This is also going to result in a boat that will always sail at some significant degree of heel, which is usually considered undesirable in a dinghy.

    Comparing this to a VX OD, which is one of the easiest boats I know to sail I think my be instructive. http://vxonedesignracing.com/vxodr/pages/664 notice the VX has a very wide transom, hard chines, and a pretty fine entry (she is 19' not 15' btw). This keeps her flat even at the dock, though it does limit carrying capacity.
     
  10. sean9c
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    sean9c Senior Member

    Awfully full forward and slack bilged aft. Why rake the stem so much? You're just giving away length. Why not try making the stem more vertical and use that to stretch out the fullness forward, though it will still be awfully full.
    Blades are odd, too much taper.
    Rig is odd and won't work. Masthead on a dinghy? Spreaders are too high and long. No way you'll be able to make that genoa sheet around those spreaders. Spreaders are so long and chainplates so far aft you'll not be able to let the mainsail out.
    As mentioned before look at other reputable designers work, and go sailing, get practical experience
     
  11. Doug Lord
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    Doug Lord Flight Ready

    ======================
    You might take a look at the design of the 15' Albacore by Uffa Fox in 1948. It has an entry angle at the waterline of about 30 degrees, a displacement with two crew of 620 lb., a boat weight of 290 lb.. It is a "cruising/racing dinghy". Steve Killing says: "This boat is forgiving-it has enough stability provided by the hull shape that an error in judgement or a slow transition of the crew from port to starboard won't result in a capsize. This is in sharp contrast to the International 14 which has a narrow waterline and little hull form stability. The Albacore is raced with two, but is just as happy with one or six." This is from Steve Killings book "Yacht Designed Explained" which is an interesting, if basic, book on the subject. I'm posting this because the Albacore may be close to what you are trying to achieve.
    I agree with GGuest but I'd be very, very leery of trying to achieve entry angles of 12-14 degrees as suggested by "249"-especially on a daysailer. The best thing
    thing you can do is research other designs, measure boats that you have access to and try to look at as many plans for different types of boats as possible. Sail as many small boats as you possibly can-this along with your design research will help you get a "feel" for designing and help give meaning to theoretical concepts. Another good book besides Bethwaites and Killings is "Design of Sailing Yachts" by Pierre Gutelle which is one of the only design books out there with a dinghy specific section. Good Luck! Research, Sail, Research, Sail!

    http://albacore.org/

    See Dinghy Statistics pdf below-

    click on the image for better detail:
     

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  12. CT249
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    CT249 Senior Member

    I said nothing of the sort. I referred to the difference between waterline entry angle and the entry angle higher up the topsides and gave as an example a designer who has two degrees difference between these two areas. I said that the point I was making was that modern designs do not have much difference between the two angles. I quite clearly said the figures were from a 16 Foot Skiff, which (as I noted) has fine entry angles and I did NOT suggest that such fine angles would be suitable for this boat. In fact I referred to the fact that this proposed's boats use as a daysailer was relevant to bow angles, when I said "Entry angles vary dramatically according to many factors including LOA, speed, purpose etc so perhaps the best thing is to measure a similar boat like a JY15 or the the PT15".
     
  13. kvsgkvng
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    kvsgkvng Senior Member

    Thanks for very good suggestions. This “design” is certainly fun and I may bother you again sometime in the future. Perhaps in my mind I had more of a cruising small boat, and not lightweight tender dinghy. Maybe I used the wrong terms.

    Regarding the transom – its angle coincides with the backstay. This way all strain in the transom stays in-plane of a thin shell.

    I wonder if anyone would be interested to look at the second iteration of this boat? Thanks again for help and suggestions.
     
  14. CutOnce

    CutOnce Previous Member

    You have received a lot of good feedback in the thread.

    I don't think a second look would be beneficial or show respect to folks like PAR until you actually understand why they've tried to steer you towards better solutions.

    Everything you've posted clearly indicates a keelboat mentality (hullform, fixed backstays, genoas etc.) that are not present in modern dinghy designs - even cruising designs. Doug gave you great advice - go sailing in lots of dinghies. Until you really understand the problems and issues, presenting your computer software-generated solutions is a little premature.

    I wish you well in your desire to understand design tools, but real understanding comes from understanding the design requirements and compromises.

    --
    CutOnce
     
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  15. Doug Lord
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    Doug Lord Flight Ready

    ==========================
    I would. I'm interested in every iteration of your design as you learn more and narrow down your requirements to something you're satisfied with.
     
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