Help with small sailboat, sail's area - Beginner

Discussion in 'Boat Design' started by cpodest, Mar 10, 2014.

  1. cpodest
    Joined: Mar 2014
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    cpodest The Beginner

    Dear Friens,

    This is my first post and I would like to ask for some help with "my" first design.
    The design consist in an very small sailboat that I will use in my own sail training.

    I calculated the sail's area (main and jib), using two diferentes sources
    - From the book "how to design a boat" by Jhon Teale, Pag 38, Table 18. For 0,237 tonnes of displacement I calculated a sail of 125 sqft, aprox 11,6 mts2
    - From book "Elements of yatch design" by Norman L Skene. For LwL = 11,12ft, sails area = 200 sqft, aprox 18,58mt2

    I have no experiencie in boat design, neither sailing, but for me, the sail area seems too big compared with other similars small boats.

    Maybe, the formulas and the tables from where I based the calculations are not good for this kind of small boats?

    Desing features:
    Design length : 3.619 m
    Length over all : 3.620 m
    Design beam : 0.865 m
    Beam over all : 0.864 m
    Midship location : 1.810 m
    Displacement : 0.237 tonnes

    I really appreciate any comment or sugestion about this issue

    Thanks in advance

    Carlos
     
  2. philSweet
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    philSweet Senior Member

    That's an extremely narrow boat. It sounds like a Moth. 1.4 m beam would be about average, and your legs wouldn't be hanging off the low side out the boat.

    It's better to list the displacement of the empty, rigged boat, and the weight of the crew separately on these little jobs.

    You can get some comparables here - just search for your LOA and browse the offerings.

    http://sailboatdata.com/
     
  3. Skyak
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    Skyak Senior Member

    In addition to what Phil said,you are not listing ballast weight and position but I think the texts you list only cover keel boats, not dingys, and your dimensions are outside common keel boat design. You are right, those do seem high but 125sf is not too crazy if 60% of the weight was a lead bulb 6ft below the surface. If you don't have a ballast bulb those calculations are not appropriate -follow Phil's advice.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Designing a sailboat and not really having the sailing dynamics understanding, experience and yacht design education, just doesn't sound like you have an especially positive possibility of success.

    What is the wetted area of this boat? Considering the beam/length ratio, obviously you'll need to whittle down the area and/or aspect ratio, unless you're going to be in a trap and don't mind a wet, twitchy ride. Self training in a boat like this, sounds like you'll be capsizing a lot as you learn.
     
  5. SukiSolo
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    SukiSolo Senior Member

    It might be a rather wide 2.4 Meter Class boat?

    To give some idea, a Devoti D1 has 11.5sq m sail (without assymetric) an RS 100 8.4sq m or the larger 10.2 sq m rig. But they weigh peanuts 55-60 odd Kg (+ and -) and go well. Hell, an Int Moth is 30 Kg with 7.4 sq m and goes errr fast.

    237Kg is a light 2.4 relatively btw. The nominal sail area of a 2.4 is about 7.4 Sq m or 80 sq ft depends upon design. If I look at conventional UK classes the N12' has in reality about 10 sq m for 2 people and 2m beam. The Enterprise is 13' and has slightly more sail but narrower, pretty much nothing except modern skiffs or keel boats carry that much sail for the beam you quote. There are a string of single handed classes in that length range 11-12' odd, Blaze, British Moth, Europe, Illusion (keel boat), Solution, Solo, almost the Laser, etc etc.

    With the benefit of the doubt, you might as well design something that fits the 2.4m rule or an Int Moth. I would suggest getting a ride in a few different types of boat, prior to choosing your design direction. The different possibilities are at opposite ends of the spectrum both speed wise and also the physical demands of the sport. A dinghy you will sit 'on' so to speak but a keel boat you sit in and at 12' long it will be like a 2.4 to be stable. BTW those 2.4s' do submerge and fill up too, so make sure you have positive buoyancy that will float the boat and lead.....
     
  6. messabout
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    messabout Senior Member

    Cpodest; The dimensions of your boat are more like a canoe than a sailboat. It is very narrow and will not be well suited for a beginner sailor. In fact it would be challenging for an experienced sailor.

    To build the boat with those dimensions you will need to be a good swimmer if you use more than 35 or 40 square feet of sail. I suggest that you
    widen the boat to at least 1.3 meters. Then it will be more manageable. At that width I would suggest 60 square feet of sail. Mainsail only, no jib. It will be much easier and more pleasing to learn sailing technique if you use modest sail area. Using a Jib will only complicate the learning process.

    All the above comments presume that you are designing a simple dinghy.

    Skenes Elements Of Yacht Design deals almost exclusively with large keel boats. If you are using that book to guide your design then you are building a boat that will need a lot of ballast enclosed within a deep keel. Your displacement figure suggest that you will have some ballast. If that is the case, then you can let the boat be narrow and use more sail than I have mentioned above.

    Please describe the type of boat that you have in mind. You can get better advice when we know what you intend to build.
     
  7. WhiteDwarf
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    WhiteDwarf White Dwarf

    One step at a time?

    Cpodest,

    Respectfully, you want to design a boat in which to learn to sail. Individually, good ideas, but not in combination. Small sailing craft have developed over thousands of years. Traditionally, each design is taylored to the environment in which it will operate, as well as available materials, etc, etc.

    To start from scratch, as you propose, risks creating a bad design of boat, perhaps deficient in just one minor subtlety, but functionally inadequate. Such a craft might be a profound disappointment and even discourage you from sailing. You have also left out the third group of skills - boatbuilding.

    Would you consider giving details of the design you have conceived and the environment in which you will use it? There are many on this site far more knowledgeable than me who might offer valuable comments, you could also compare your design with existing designs. I would recommend Selway Fisher, Mike Storer and Tom Dunderdale's Campion Designs www.campionboats.co.uk.

    I would humbly also suggest that you look at http://firebug.co.nz - I used one as an introduction to boatbuilding. If you are skilled with tools, you will finish the project in two to three weeks - and 8 years later I am still having fun in mine. It was the first boat I built for myself after nearly 50 years of sailing.

    Regardless of your decision - good luck.
     
    Last edited: Mar 10, 2014
  8. Skyak
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    Skyak Senior Member

    The thread has morphed into advice on beginner building and sailing so I can't resist offering my opinion.

    Studying, building and sailing small boats is fun. If you are enjoying the time you spend doing it, don't stop. The books you mention are fine but in many respects useless in designing a dingy -a boat that relies on crew weight shifting for righting moment. Some good resources are cited above and there are more. This blog is a good resource. You can get 10 of the top 3 problems of any design you post in a day or two, many from talented pros. The thing I would like you to keep in mind is that it isn't that hard to design and build a boat that will not kill you or even do you serious harm.

    The one tidbit of advice I will give is to build a sail rig that reefs down fast, easy, and effectively. For a beginning dingy sailor, having the right sail is critical because the force of wind is proportional to velocity squared. Most dingys have no ability to adjust sail area and instead there are simply beginner boats and boats for advanced capabilities. You don't have to build a beginner boat, but if you don't want to risk getting thoroughly trashed you do have to be able to lower the sail power or limit yourself to precise optimal conditions.
     
  9. Doug Lord
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    Doug Lord Flight Ready

    Here are two documents with a lot of dinghy specs, one has both Imperial and metric dimensions. They may give you some idea about the designs of other dinghies:
     

    Attached Files:

  10. PAR
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    PAR Yacht Designer/Builder

    The two ideas are mutually exclusive and a novice doing both, well, not a pretty prospect. A sail trainer is precisely what the above dimensions don't define, suggesting my point is valid.
     
  11. Skyak
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    Skyak Senior Member

    What design do you think would be best for a beginning sailor if they are a talented woodworker? Preferably a design that would still be desirable as they develop some skill.

    The list from Doug is full of racers. The design White pointed out is clearly a beginner boat.

    My first dingy was a used Laser I purchased as an adult. My sailing experience to that point was limited to sailing a square beginner dingy as a child (frustrating inability to make progress to windward) and some crew time on a 47ft keel boat (wheel steer). My first Laser sail was in 15 to 25mph wind from a lee cove, and I realized after my first tack that my right hand had enough skill on the tiller to keep me going and upright, but my left did not! I turtled at least twice and capsized more than I can remember. At one point a pontoon came by an deployed a kid in a life vest to to help -hilarious! I made it back skinned and bruised but it makes the point of race boats in inexperienced hands. To this day, if I could reef the sail down my laser would get a lot more use.
     
  12. PAR
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    PAR Yacht Designer/Builder

    There are many performance oriented dinghies, that will do well as a trainer, while still keeping the skipper interested once they gather up some skills. I don't recommend Laser like boats, simply because it can be too disappointing, too quickly. I would recommend a Bay River Skiff (15) or a Core Sound-15 as a good home built starter, with some performance potential (particularly the CS-15) as the skipper learns. You have to screw up pretty bad to capsize one of these (especially the Bay River Skiff). I'm fairly sure both can be had as plans or kits. I too have some designs in this vain, as do most every designer in the world, so the options are wide open. A quick cruise through Glen-L's site and Bateau.com's offering will turn up some "reasonable" designs, where continuous capsize will not result on the first few outings.

    Carlos hasn't returned to this thread, maybe he's been knocked off his dream already or is over at SA, truly getting a beating.
     
  13. Skyak
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    Skyak Senior Member

    Yes I noticed Carlos has not been back but thanks for the recommendations. The blog will live on and it is a common question. I regret that this blog often discourages beginners so I try to find a positive alternative without avoiding the truth. The CS15 is a good all around boat even with experience and a beginner could take a teacher on his first outing. I am not familiar with the bay river skiff and I am surprised you don't mention any of your designs. What does a guy have to do to get you to plug your designs?

    Clearly the Laser is better suited to humbling than learning. If I wasn't so hard headed my first sail would have been my last.
     
  14. cpodest
    Joined: Mar 2014
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    cpodest The Beginner

    Thanks

    Dear Friends,

    I really appreciate your help on this issue.

    When I posted this issue in this blog I never thought to get so many answers. Some of them are very good and they are very helpful, but others to be honest, are really daunting. (I don´t want to go into depth). For that rason, thanks again to all people that support people like me, I mean, beginners.

    My intention is to build a very small boat, A boat for 2 person, max 3. A dinghy or a flattie, and then try to learn how to sail it. I live since 4 months in front of some beautiful beaches and i would like to take advantage of that.
    The features that I posted, are not the final features, I posted, in order to ilustrate the dimensions of the boat and the huge sails area calculated in base of the books that i´m reading.

    One option to solve my doubt, in order to the sail area is take a look at other similar boats, and use a similar area. Is not a bad idea, but i would like to know from where this calculations came from... or is only an empiric method? (for small boats)

    I will refine my design and will present in the forum to receive your precious feedback.

    Thanks again,
    Carlos
     

  15. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Welcome back Carlos. Don't be discouraged, though we get these requests all the time, some folks actually take up the charge and learn what they need to learn to do a real, safe and effective design. Nothing is meant to be taken personally, but frank answer (I'm guilty as much as anyone) will come, if the concept is "out there" a bit, as yours is.

    In order for comparisons to work, with some level of reason, your basic hull shapes and dimensions also have to be within normal ranges. Currently they are not, which will make any comparisons essentially meaningless. Can you post a drawing or sketch of your proposed boat?

    Skyak, I generally try not to plug my stuff here as it's not the right venue, in my way of looking at things and I also feel it's inappropriate. On occasion, I'll take a design request to email or offer one of mine as a reference example, but no plugs. In this vain, my Murphy (RYD-16.2) design would work well for him, but so will hundreds of other designs.

    Back to Carlos, your question of calculating area for a given model can get really complex really quickly, especially if you're well outside the "norms" for hull form and/or dimensions. This is why I asked you what the wetted surface of the hull was. There are a number of parameters that we take into consideration, when calculating sail area. If you can't provide the basics in terms of hydro figures (CG, CB, CLP, metacenter, etc.), it will be little more than a guess as to how much area you'll need.
     
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