15 foot sports recreational dinghy

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

  1. sawmaster
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    sawmaster Senior Member

    Hey Doug:My apologies in advance since this doesnt relate directly to Kvsgkvng's design (or maybe it does)--Im curious about about marchaj's formula of 500 sq ft of sail area per 2240 lbs. for planing.Maybe Im missing something but isnt there a third variable--that being wind speed?--At what windspeed is 1 sq ft of sailarea/4.48 lbs adequate for planing?(I THINK thats what the formula equates to)--Also, why wouldnt planing with a higher lb/sq ft be possible given sufficient wind strength?--(Albacores are a little heavy for SA but planes anyway)I dont see how to use this formula without comparing different designs in the same windspeed.Given enough wind,couldnt a heavy,undercanvassed boat also plane?--Im thinking of petros' 700 lb gross wt hardware store class and what kind of hurricane would it take to plane with 100 sq ft of sail area.
     
  2. Doug Lord
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    Doug Lord Flight Ready

    ==================
    Marchaj's old rule of thumb is just that-a rule of thumb. The best indicators are numbers from successful planing boats-particularly total weight and sail area integrated into accepted ratios-and most of all your own experience upclose from sailing. Bethwaites SCP/ Total weight theoretically says that if the result is .3 or higher the boat will plane upwind. I would never take just one rule of thumb-I'd compare a design of mine with boats that are close to it-in fact I did and showed the results for the comparative ratios for the Crossbow and Windmill in the Crossbow thread.
    I don't think you can use lb.per sq.ft. wind pressure to predict planing because rigs are very different in how they translate wind pressure to power and hulls react very differently as well. Comparing to a boat you have direct experience with is the best way.
    For instance, the Windmill planes in 8-10 knots off the wind and is very fast in light air-I know that from years of racing. According to Marchaj's rule of thumb it doesn't quite have 500 sq.ft. per ton(493). So you have to take a rule of thumb with a grain of salt and compare boats with accepted ratios like DLR, SA/D,SA/WS then put all the info and your own experience together and come up with a reasonable evaluation.
    I think I added Erics ratios pdf in this thread somewhere -if not and you(or anyone else) would like a copy just pm me.
    Hope this helps.....
     
  3. messabout
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    messabout Senior Member

    I think that somewhere in Skenes there is a chart that purports to equate driving force with Beaufort numbers and sail area. That is fine and dandy but it still does not tell us when a boat might plane.

    Far be it from me to question Marchaj. I do suspect that his statistical data was compiled for a variety of dinghys that were not hot rods to begin with. He did, however, use the IC as a benchmark quite often. The IC (International 10 sq. Meter Canoe) enjoys a sail area to weight ratio of less than three. So yes, that one is definitely a hot rod.


    Along with Doug, I have several years experience with the Windmill class boat. It will indeed plane in a medium breeze if you can keep it flat. Contrast the Mill with a Snipe. Both boats are about the same size. But the Snipe will only plane in a half gale if at all. (Oops, I may have offended some of the excellent Snipe sailors with this. Ok, I will change the comparison to a Comet.)

    My current toy boat is a 16 foot flattie/sharpie with only 60 ft^2 of sprit boomed sail. It falls way short of the 500ft per ton deal. The little boat is light with an all up weight of 335#. That puts it at about 80% of what it needs for sail. Nonetheless, It will plane nicely with about 12 to 14 knots of breeze. The reason it can do that is largely because of the nice flat bottom and a very small quarter beam buttock angle.

    Consider sailboards too. A board with a 170 pound sailor and a 3 sq. meter sail will plane like fury, given some wind. That gives it about 5.7 pounds per sq. foot.

    In conclusion, I reckon that we can use all those various ratios for a start but we can not really bet the farm on them.
     
  4. CutOnce

    CutOnce Previous Member

    There is a lot more in play than just weight, sail area and windspeed. Formulas are just forensically developed equations that allow people to quantify something. Using formulas for predicting performance is only worthwhile if all other conditions (sea state, hull shape, surface area, drag, weight distribution etc.) are the same as the boat the formula was derived from.

    Hull shape is a huge factor, hence PAR's unheeded (and ignored) comments. It is perfectly possible to meet weight and sail area criteria in a fundamentally unsailable, planing-not-possible boat. It is entirely possible to make a boat that planes, but is unusable in all other conditions. Tweaking sail area, weight, hull surface area, righting moment and other variables will have no effect if the hull shape is wrong.

    This is why this discussion is honestly pointless - there is no apparent fundamental understanding of all the issues in play. PAR, Paul B. and others have politely tried to point out that the original poster needs less mouse time and more time studying theory and fundamentals - and why time in small boats is a critical asset to developing an implicit understanding of the fundamentals through experience. The original poster (in my opinion) has dismissed the people who could point him in the right direction, and just continued to post computer-generated "designs" while ignoring the help offered.

    I'm all for the original poster designing dinghies and using good tools - but a tool is only as good as the experience and education between the chair and the keyboard.

    --
    CutOnce
     
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  5. Petros
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    Petros Senior Member

    I think even more important that theory and fundamentals, is getting some time in small boats, and as many different types as possible. I do not really think it is possible to create a good design without having a proper feel for how it behaves when you make changes to the hull design.
     
  6. kvsgkvng
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    kvsgkvng Senior Member

    Thank you all for spending time on this, perhaps, useless for you matter. I am very grateful for any input. I do study theory as time goes by. Again, thank you all for support and lessons. Kind regards.
     
  7. CutOnce

    CutOnce Previous Member

    Speaking for myself only, I appreciate your polite response.

    Trying to help people on this forum is NOT useless to the participants, both those asking the questions and those that take time out of their day to respond.

    The trick I find is to very carefully consider and listen ... to both questions and responses - even those that are not in agreement with my thoughts. In this forum and many others I've found that keeping an open mind and objective perspective has helped me avoid repeating many mistakes others have made before me. Avoiding known mistakes is the huge advantage gained that can be the difference between a failed project and a success. Avoiding known mistakes can save money and that most precious commodity - our limited time.

    My eighty year old father is currently dealing with age-related loss of mobility and stability. His beautiful 24x36 shop, populated with superior tools and materials now lies quiet. Although he hasn't come out and said it audibly, I know the cruel frustration he is seething with now that he has the time, knowledge and experience to execute his dream projects - and his body has let him down to the point he is afraid to turn on his General cabinet saw.

    I know I've sounded a little bit negative about your delight focusing efforts on incredible computer design tools. Go sail dinghies every spare moment if you are interested in small boat design. Spend time in the off-season studying theory and current state of the art designs. If your goal is to develop a comprehensive understanding of naval architecture as it relates to small sail boat design you will get there far faster this way. Then you will be able to do a great job with CAD tools ... or realise you didn't need them in the first place.

    I reached a personal epiphany one day when I found out my Lee Valley hand plane was a far more effective tool than my Bosch router for doing finish work on a project. The technically superior tool enabled me to make more horrible mistakes faster than the manual tool.

    Best wishes,

    --
    CutOnce
     
  8. kvsgkvng
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    kvsgkvng Senior Member

    Hello again. Here is my next step in the process of dinghy design as an exercise. Perhaps in my mind I have something closer to a “pocket cruiser” rather than “dinghy” because I think of a weighted centerboard. I think there is enough displacement capacity for this purpose. Eventually I hope to build it and to travel a little along the coast lines. Again, this is not final; this is only the hull, sails, balances, speed and resistance approximation. I hope that instead of general references to “read books and study” I may receive constructive comments regarding real things which are off the established practice. Thank you all for your time and patience. Kind regards.
     

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  9. Petros
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    Petros Senior Member

    much better concept, but several issues come to mind looking over the drawings.

    1. You will have very large tiller forces if you build it that way, it will not be pleasant to sail at all. Move the dagger board back behind the mast, you do not want that much lateral pressure on the rudder. Also I think it will be directionally unstable the way it is, taking constant tiller input to keep it on course, very tiring for the helmsman. By moving the dagger board aft so the center of lateral resistance is approximately at the center of effort for the sails it will balance much nicer and be much less tiring to sail. there is a reason almost all sloops have the center board/dagger board aft of the mast.

    2. the transom will be a lot of extra work to build as designed, and not give much benefit. I would either go with a flat transom or add another foot to the length and make the hull a double ender (traditional "boat tail"). The below water line shape should not be affected by either change.

    3. Your rudder is too high aspect ratio, it will not be very effective and will easily stall and not allow good control. You always want the aspect ratio of the rudder to be much less than the centerboard/dagger board so you do not loose control of the steering. The high aspect ratio surfaces are more efficient, but they area also much more likely to stall or be ineffective due to surface irregularities. A high aspect ratio surface will stall at only about 15 or 16 degrees, after that it becomes ineffective and the drag goes way up. A low aspect ratio surface can tolerate up to 40 deg angle of attack before it stalls. So unless you are building a racing boat, best stay away from high aspect ratio rudders, especially if you intend this to be an easy sailor. I have been in boats with both types of rudders, high aspect ratio rudders suck for general recreational boating, suck big time. I would never have one except in a racing boat and only with a very skilled helmsman who knows the limitations of rudder and boat in general. You will not harm a thing by having a low aspect ratio rudder that is larger than you think you need, it will be effective and be responsive and a joy to use.
     
  10. DCockey
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    DCockey Senior Member

    Petros' comments are very good. The shape of the stern with the tight radius areas is likely to be higher drag than a simple transom stern. Have a look at the lines of small sailboats.
     
  11. kvsgkvng
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    kvsgkvng Senior Member

    First of all thank you from the bottom of my heart! I deeply appreciate your outreach to help me with your comments. I will work on them diligently and will try to digest them very methodically.

    Right away, I don't understand why the tiller forces should be large. If I understood the theory correctly, then with proper attachment details there shouldn't be any forces because of lateral effort. As shown on the attached sketch, this tiller and rudder arrangement may be a bit sensitive to stir, but if I am correct there should not be any lateral resultant causing any additional torque about vertical axis. That is why ( I think ) old sailboats had slanted rudders, in order to place the theoretical pivot point between rotaion attachments. In this case left and rigth half would equalize each other. All lateral pressure would be taken into the sailboat hull by means of the rudder attachment details. I will also try to address placement of the keel later on, as I did not understand basic mechanics behind your statement as well.

    Please would you let me know if I am wrong? Perhaps, I did not understand basic mechanics of this arrangement. Otherwise my studying is under question.

    For better or worse -- thank you very much for your help.
     

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  12. kvsgkvng
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    kvsgkvng Senior Member

    Thank you, for your input. With this statement all my schooling goes down. When I was working on this, I placed all those tight radii physically above theoretical water lines. Of course in real sailing waves and uneven trim will bring those location in contact with water, but so will it will be for the simple right angle transom. Having hard corner will probably generate more drag than rounded part. Again, I am not a specialist, perhaps my idea is simply wrong.

    On the other hand, having something rounded in the water looks better to me comparing to sharp 90 degree angles. Do my arguments have sound grounds?

    Thanks again for your help.
     
  13. DCockey
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    DCockey Senior Member

    The center of pressure on the rudder will actually be around 1/4 chord aft of the leading edge, not at the center of the area. Ther pressure distribution at an angle of attack is not symmetric fore-aft. Rather the pressue difference between faces of the rudder is large near the leading edge and goes to zero at the trailing edge. The design you show would try to go hard over to one side or the other as soon as you were underway.

    It appears that you are including the rudder area in you lateral area calculation. This generally leads to a centerboard location which is more forward than if you did not include it, which in turn leads to a large lateral force on the rudder.

    How much experience do you have with boats? Do you have any type of technical background?
     
  14. kvsgkvng
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    kvsgkvng Senior Member

    You absolutely right on all counts. But the argument was about "tiller forces." If the rudder would be properly balanced, then the helsman should not be subject to the lateral pressure on the rudder. Of course, with slight changes in rudder angle, the center of effort may change its location, but I believe, this change would be not so prominent.

    Here is my logic behind including the rudder in the calculations. If the rudder is out of the balance equation and the keel is optimally located without rudder, then as soon as the rudder is introduced in the lateral equilibrium, the center of effor of the combined areas of the keel and rudder is off. It moves aft and this creates a force couple about vertical axis. This couple tends to swing the boat a bit. The helsman must adjust tiller to compensate the course and this creates additional drag in the form that the hull is moving at an angle along the theoretical passage line. This may be the case of insignificant magnitude, but I hope it is true.

    Now, if the rudder would be a part of the balance equilibrium, properly balanced, securely attached to the hull and the hull would be reinforced enough to take the load, why is it sounds like it is bad for sailing? Because it would be very sensitive and precise? Or because a helsman would not be able to hold exact course with such sensitive stirring? Or, because when rudder is at various angles, the lateral component moves much and thus creates tiller force?

    Please let me know, because I am humbly looking to absorb any knowledge. Thank you for your time and help.

    I have small sailing boat experinence like sunfish and laser. I also have 14'~16' catamaran experience. I don't have formal theoretical naval education, but I have very good technical background in statics and dynamics, lest fluid dynamics.
     

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  15. DCockey
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    DCockey Senior Member

    The balance of the boat when sailing (as opposed to on paper or in the computer) is determined by the three dimensional balance of the aerodynamic and hydrodyanmic forces. The "center of effort" approximation is only an approximation and one which doesn't align with the acutal physics very well. But it is a useful tool when designing as long as not too much is read into it. Search for other threads discussing it.

    Two possibilities out of an infinite number.

    1) The boat is balanced when the rudder is free to rotate. Assuming the rudder post is forward then the rudder will align with the water flow past it so that no lateral force is generated, and the rudder will not be affecting the balance of the boat (other than a negligable amount due to the drag of the rudder).

    2) The rudder is held so that it is aligned with the centerline of the boat. The amount of torque and therefore force on the tiller needed to hold the rudder in this position will depend on how for forward the rudder post is.

    The usual goal is to be somewhere between these. Case 1 will usually result in a boat which needs to be more actively steered. Case 2 may result in more force required to hold the rudder in position than desirable.

    Again, the CE method only corresponds to the actual 3D physics of a boat going to windward in a very, very approximate manner. It is best used in comparison with similar boats which are known to perform as desired.
     
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