Stability criteria of pond sailboats

Discussion in 'Stability' started by George S, Oct 25, 2008.

  1. messabout
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    messabout Senior Member

    George;
    I doubt very much that you can have a 36" boat that will displace 25 to 30 pounds and still sail well. The boat will need to be either very wide or very deep or both to get that kind of displacement in 36" of length. If you adhere to some degree of authenticity or scale, the plot thickens because you may not be able to use suitable hull section dimensions without violating scale.

    I suggest that you get a copy of the book; The Nature of Boats by author Dave Gerr. The book deals with basic design subjects more than adequately for your purpose. It does so with a minimum of mathematical acrobatics as well.

    If you are a history buff then a boat such as Mayflower is perhaps appropriate. It will, however, carry with it some pesty problems. Not least of which is that it will be too complicated, in terms of sail control, to sail with any degree of satisfaction. I suggest that an an old time boat such as Joshua Slocum's Spray would be much easier to deal with while maintaining a degree of historical signifigance.
     
  2. ancient kayaker
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    ancient kayaker aka Terry Haines

    A half-submerged log 36" long and 10" dia will displace 51 lb. 30 lb is a bit on the light side for a 36" hull, I think. The ships of the Mayflower's day had very wide beams.
     
  3. waikikin
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    waikikin Senior Member

    George s, if you want a sailing model with some real cool history & performance, google up "balmain bugs" & check em out, might suit you! Regards from Jeff.
     
  4. George S
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    George S The old fellow

    Fellows, what you have said is very helpful. I bought plans for Mayflower II but loaned them out and have not gotten them back yet. They are for an 11 1/8th inch lwl static model hull. So currently I am "working" from the littlle 5 inch plans in the book "Mayflower and other colonial vessels."
    I am thinking of a "look alike" model so what I need is for the hull above the waterline to "look like Mayflower II". Underwater I can adjust things to get a ready to sail weight of 25-30 lbs.
    I am thinking of making the underwater sections semicircles so I can just calculate their areas rather than measuring them, and also adjust the waterlines aft so the transom just touches the water.
    Several books say to measure the section areas using the trapazoidal rule then plot the section areas on a baseline and make the length of the vertical lines proportional to the section areas, then measure the plot using the trapazoidal rule to get the displacement.
    Is there an alternative method of calculating the underwater volume? Is there a mathematical way to multiply each section area by the distance to the next section and sum these figures to get he underwater volume? Thanks again.
     
  5. waikikin
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    waikikin Senior Member

    George, you can use Simpsons multiplyers in tabular form for calcs of area under curves & for volumes too, although its about 1/4 century since I've done this, trapiziodal rule is ok for quick & dirty calcs or tanks or barges or even straitish sectioned hulls.Regards from Jeff.
     
  6. messabout
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    messabout Senior Member

    George; There is a quick and dirty method for calculating displacement. As follows: Calculate the areas of the sections in square inches.. Sum them, divide by the number of sections. This will give you the average of the underwater areas. Use that average figure multiplied by the length of the waterline in inches. You now have a reasonable approximation of the immersed volume. Multiply that number, which will be cubic inches, by the constant 0.03611....That will give you the displacement in pounds. Simpsons rule or the trapezoidal rule may be a tad more accurate but the method described is simple and fast and will get you real close.

    If the book tells you the prismatic coefficient, Cp, of the boat you can do the math even quicker . Find the area of the central or largest station...mutiply by the Cp. I would guess that the Cp of a tubby boat like the Mayflower would be 0.60 or more. Note that this is always a decimal number less than one. Take the product of the main section area times the Cp. Now multiply by the waterline length. Multiply by the constant 0.03611. You have the displacement in pounds. This is very easy with nothing more than a hand held calculator.

    The bit about plotting the individual areas on a graph (curve of areas) is useful for determining where the center of bouyancy is likely to occur but also the manner in which the water will be displaced. I see no reason to bother with that as you are going to build a somewhat boxy boat and there is no need to make the design more involved than necessary or useful.
     
  7. George S
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    George S The old fellow

    This is all very helpful. Baker did NOT give a prismatic coefficient. He spends a little time on how he drew the sections using circles and straight lines. Then he gives the tonnage rule as follows:

    keel length x breadth x depth divided by 100 should equal about 180 (for Mayflower)
    Baker's results for Mayflower were length of keel 58 ft, breadth 25 ft. and depth 12.5 ft. gave a burden of 181.
    He selected a rake aft of the stern post of 4 ft, the rake of the two circle stem was 21 feet.
    He located the midship section 21 ft. aft of the forward end of the keel.
    Then he fiddled with the floor rising line etc. All of which is interesting but my objective is only to design and build a model that LOOKS LIKE Mayflower ABOVE THE WATER and design an underbody that will support between 25 and 30 pounds of boat and lead (as much lead as possible in a fin) all with a draft limitation of about 14 inches (due to the limitations of our local pond.)
    And to get away from the Mayflower design I think I should call the boat "Sally of Salem"!!! So I think that I will use Messabout's method of calculations.
    (I still have an itch to move the "date of construction" forward so I can get the transom out of the water, add topgallant masts and sails AND add staysails for really poor sailing conditions.)
    The first model will be free sailed. Later I will try and figure out radio control of rudder and sails. (Steel, Chapman & Hutchinson Ltd uses an invisible stainless steel bentinck boom in some of their courses. This eliminates sheets and tacks and their braces go from the tops to about 3 inches from the CENTER of the controlled yard. The sails and yards above just "follow along."
    A spritsail and sprit topsail WOULD ADD COMPLICATIONS!
     
    Last edited: Nov 10, 2008
  8. messabout
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    messabout Senior Member

    George;
    Ancient Kayaker has some good advice. Make a really simple, unfinished model first. Tinker with the stability by using a basket or whatever you can dream up for finding the right ballast weight. Really racey models have ballast ratios on the order of 3 pounds of ballast to one pound of boat. I dont think that applies here because Mayflower (or Sally) is very wide. Models such as the meter classes (one meter 39.375 inches long) have beams on the order of 8 to 10 inches on deck and less at the waterline. They have 60" tall masts and perhaps 600 sq. inches of sail. Your 58' x 25' x 12.5' boat will scale to 0.62 inches to the foot of full sized boat. Might as well call it five eigths inch to the foot. So your model will be 15.625 in beam. That will give it a lot of initial stability and lessen the need for such a big ballast ratio. You do need enough ballast to right the boat in case it is knocked down. And remember that the sails and rigging will be wet.

    For a quicky underwater platform, or merely an example of something that would hold the 30 pounds or so, you could use a pirogue or canoe shape with square chines and flat bottom. That type would be an easy build. A double ended box with a midsection beam if 15 inches more or less and a draft of 4 inches will give you somewhere around 37 pounds of bouyancy. I'd give the box about 3 inches of rocker. Rocker is needed or the boat will be difficult to turn. That may not be what you are after because ships of the subject type have rather full ends. So a box that loosely resembles the shape of the planform would carry its' width pretty far fore and aft. In such a case you would neeed less draft because there will be more interior volume in the shape. Less draft might be good because you can have a deeper fin and bulb while still keeping maximum depth within the 14 inches you suggest. In any case, a square chine type underbody will give decent results while being simple to construct and perhaps easier to replace when/if the spirit moves you.
     
  9. ancient kayaker
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    ancient kayaker aka Terry Haines

    Speaking of radio control of sails, I am not up on square rigging, but is it practical to control the sails by mast rotation?
     
  10. George S
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    George S The old fellow

    1/22/09 Thanks fellows. I have Baker's plans from Plimoth for $25. They are for a 11 inch model. I have stretched them to 36 inches lwl, and calculated the displacement at about 7-8 lbs. I decided to increase the depth of the hull 1 1/2 inches to get more displacement and will add a false keel so the draft is 12-14 inches. I have cut semicircular frames (bulkheads) to glue under the lower deck and will next draw the upper frames (bulkheads ) etc. shortly.
    George
     
  11. George S
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    George S The old fellow

    Sources of info on pond sailors

    Traditional Model Yacht Design by Thomas Moore's Text and Diagrams.
    A caution about scale models in "Elements of Yacht Design" by N. L. Skene 1904 from Sheridan House ISBN 1 57409 134 4 P. 243 "Scalle models of larger vessels do not perform well. Their stability is insufficient for the wind velocity is not scaled down and stability varies as the fourth power of a lineal dimention, whereas heeling effect varies as the cube. Therefore scale models have relatively much less stability." etc. It is good on pre-computer design. THE STORY OF SAIL Illustrated with 1000 scale drawings" ISBN1 55750 896 8 will give you lots of ideas. "An introduction to Radio Controlled Scale Sailing Models" by P. V. Williams ISBN 1 900471 20 0 is very helpful, but the figures on page 20 are wrong. The center of gravity does not move. THE GALLEON ... ISBN 1 55750 300 1 Gets you into these great ships. MERCHANT SAILING SHIPS ...1775-1815, ISBN 0-87021-418-7 is good. SCALE MODEL SAILING SHIPS ISBN 0 8317 7700 1 is very helpful in counteracting the scale effect. YACHT DESIGN EXPLAINED ... ISBN 0 393 04646 X has great details about the scaling effect. ISBN 1 55750 098 3 Details a model ship build.
     
  12. George S
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    George S The old fellow

    MESSAGE TO MANUFACTURERS OF MODEL SAILBOATS: Please include the weight of the lead bulb at the base of the fin on your boats. With that we can estimate the righting moment at 30 degrees angle of heel and estimate sail area and get an idea of stability.
     
  13. George S
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    George S The old fellow

    Mayflower Iii

    I bought a copy of Baker's Mayflower II plans for a static 11 inch model and expanded them to about 36 inches LWL. I have the model about 5% built and will add 1 1/2 inches to the depth of the hull and add a fin keel so the draft will be between 12-14 inches (the depth of "our" pond) and then I will add lead to the bottom of the fin and see what sort of stability I get.
    I asked the same question in a thread at <rcgroups.com> under sailboats, hoping for some numerical data but, while we had an interesting discussion, I did not get numbers.
    While I am "at it" I plan to add a sprit topmast and staysails so I have changed the name to "Cape Ann" and the date of the design to about 1675.
     
  14. George S
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    George S The old fellow

    Progress report 1 MAYFLOWER III

    Feb 18, 2009 The model is about 5% done. I have taken Baker's Mayflower II lines for an 11 inch static model and expandeed them so the water line is about 36 inches long. I also altered the lines a little so the transom will not "drag" in the water. I also converted the sections to semicircles so I could calculate the areas rather than measure them. I leveled the lower deck so it is level fore and aft. Displacement to this waterline is calculated at about 9 lbs. The half angle of entrance of the LWL at the bow is about 60 degrees. That is all for now

    George
     

  15. ancient kayaker
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    ancient kayaker aka Terry Haines

    I haven't been able to find a picture of the Myflower's lines. However, noting how short and wide she is compared to modern designs, and how high the masts are, it occurs to me that she may lean forward under sail, thereby lifting the transom. Thus it may not be necessary to modify the underwater lines.
     
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