question about pdracer

Discussion in 'Sailboats' started by sail102, Nov 27, 2011.

  1. ancient kayaker
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    ancient kayaker aka Terry Haines

    Stability is roughly proportional to beam cubed; the boat will lose 58% of its stability at 3/4 beam. My own sailboat has a 4' beam and is stable enough to walk around in without problems. My canoe was 3' in beam and was very tippy even sitting down. Other factors have an effect of course but I don't think you're going to like a 3' wide PDR.
     
  2. messabout
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    messabout Senior Member

    Terry; I agree that a 36" PDR is going to be tippy. too tippy for anyone short of ballerina status. Tinkering with the beam cubed ratios, I get conflicting results when compared to actual static RM calculations. I think that this might be a can of worms that challenges my modest abilities. It is fun to contemplate however.

    Just for giggles I compared the PDR with the NZ Firebug. Firebug has the same 8 foot by 4 foot planform as PDR. It is also a scow or punt type. There is a really big difference in initial stability calcs. Firebug has 24 inch ends and beveled chines. It is not as butt ugly as PDR but will be much more demanding of its' skippers agility.
     
  3. ancient kayaker
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    ancient kayaker aka Terry Haines

    I didn't know about the Firebug: nice looking little boat. The beveled bilges and narrower ends are going to make a big difference to the primary stability but it will stiffen up nicely as it heels. I am tempted to say it will go better than a PDR in a light wind or under oars due to lower wetted surface, but my own 10 x 4" flattie is so good in both those departments that is hard to know what to believe anymore.

    It is amazing how good a simple boat can be. Boat design is largely magic and if thought it was safe I'd ask the designer (our own PAR) if he is related to Harry Potter, but I don't want some humongeous snowy owl showing up on my doorstep - the neighbours worry enough about me as it is . . .
     
  4. JRD
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    JRD Senior Member


    A good point. The firebug is a kids learner boat, from what Ive seen they arent having too many problems keeping them upright. When they do capsize the boat is easy to bring upright again and becasue of the wide side decks hardly any water ends up in the cockpit.

    The Firebug was one of the last designs of the late John Spencer, known in NZ as the plywood king due to nearly all of his designs being hard chine, but ussually embarasingly fast. He designed the firebug to encourage parents and kids to build their own boats, which has been reasonably succesful in NZ and Australia. They have been used in a number of school and community building programmes here.
     
  5. Petros
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    Petros Senior Member

    obviously you have to make a larger door from your basement. You have to get your priorities right, boat or house?

    build a trimaran or a catamiran, join the hulls after it is out of the basement. way more fun than an ugly PD racer.
     
  6. WhiteDwarf
    Joined: Jun 2011
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    WhiteDwarf White Dwarf

    JRD The Firebug is no mere "kids' boat" . I've sailed one for the last 5 years. Won our handicap class two years running,etc. I weigh 90kg and have sailed for over 50 years. Alot of fun, is thr Bug.

    As the acknowledged 8 footer guy at our club, I was asked to try our a newly built PDR for a builder. It was the worst boat I have ever sailed by miles. The heavy rig made it unstable, there was no visibility, under the sail and poor manouverability for so small a boat. From that one experience only, I see the PDR as a device to discourage people from sailing.
     
  7. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Sail rig and area are open under PDRacer class rules, so what you didn't like were the choices of the particular builder, not PDRacers in general. Class rules here.

    Would you condem a rowing boat because the owner selected oars which were too short and heavy?
     
  8. WhiteDwarf
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    WhiteDwarf White Dwarf

    David, Sorry if I offended you, regarding the PDR. I did stress that "From that one experience only."

    WhiteDwarf
     
  9. Collin
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    Collin Senior Member

    If you cut out all the parts before assembly (glue chines, and bulkhead pieces together) you can literally nail it together in an afternoon and paint it. Let the paint dry overnight and you're done. The boat can live outside just fine.

    You can even tack it together inside to make sure it all fits together, withdraw screws, and take it outside with screw holes to line everything back up. There's no reason to do any complicated half hulls or modify the design.
     
  10. gggGuest
    Joined: Feb 2005
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    gggGuest ...

    If you're going to make it narrower then you'll get some stability back if you can make it longer, and it will be a better boat in many other ways. 14' * 3'6 might be a nice size for a recreational craft.

    I don't know that the PDracer is necessarilly a good model though.

    The Firebug is an astonishingly clever piece of design. Spence was always an unsung genius, but that way he got such a sweet shape out such ridiculously simple construction features just boggles my mind.
     
  11. sharpii2
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    sharpii2 Senior Member

    In case you might be interested, I have a design that has never been built yet that is 3ft x 12 ft.

    Unlike a pdracer, it has deeper rockers, so it can heel some before the transoms get immersed. By heeling some, it is supposed to present a 'V' to the water, instead of a flat section. I once presented the idea to Phil Bolger and he said he thought it would be fairly fast. I still have the letter.

    It has a max design displacement of 480 lbs.

    Your proposed 3ft duckette would displace about 457 lbs before the bow and stern transom start to go under.

    My design was intended to be built inside a mobile home and to be driven around in my S10 pickup truck.

    I'm thinking of shortening it by about 9 inches to make the stern transom deep enough to hang the rudder on and so less boat will hang out the back of the truck.
     

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  12. sharpii2
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    sharpii2 Senior Member

    Not quite true.

    I did do a math experiment drawing a very crude boat. I started with a L/B ratio of 2.0 and went up from there. I made the boat progressively deeper as I lengthened it, maintaining the same displacement.

    I found whetted area continued to decrease until the L/B ratio was close to 10. The Beam/Draft then was about 2.0.

    Further increases in length and depth caused the whetted area to start increasing.

    At its best point the narrower boat had roughly 16% less whetted surface than the original boat. And that included boards and the rudder.

    This may explain why motor ships have the proportions they do.

    With sailboats its a whole different matter.

    As the Beam increases, the whetted area increases, but so too does the sail carrying ability. In fact, the sail carrying ability increases in direct proportion to the initial stability of the hull. And that increases with the cube of the Beam increase, if the boat is kept the same length, and approximately the square of the Beam increase if the narrower boat is lengthened to make the B/D go down less.

    This is for two reasons:

    1.)The longer narrow boat has a greater B/D than the shorter narrow boat and
    2.) the longer narrow boat can set up more smaller sails (which have low VCA's) than the shorter narrow boat can.

    This all, of course, assumes no deep external ballast.

    So, if a boat is made wide and shallow, it ends up with greater whetted surface, but a much higher S/D and, most likely a much higher sail area/whetted surface area as well.
     
  13. brehm62
    Joined: Mar 2011
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    brehm62 Junior Member

    No, you've actually brought up the central point which is that even today a PDR is often more of a builder's boat than a sailor's boat. I don't know of anyone who has a set of well documented plans, rigging, and operations.

    People build these with sides from 10" - 18"; that's a big range.

    People build them with end tanks, sides tanks, or foam floatation. That's a big range.

    People use lug sails, sprit sails, sloop, leg of mutton, and others. This is a big range.

    People use daggerboards, leeboards, side dagger boards, and swinging side boards, sometimes one and sometimes two.

    People use rudders of varying depths and width and shape.

    People use rub rails or bilge keels or a single keel or none.

    People use sails of varying area from around 40 - 60 sq ft.

    It's no surprise that the results vary so much.
     
  14. Collin
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    Collin Senior Member

    It's more like you're looking for the perfect date and each person's preference is different :D

    I have no idea why you present a dichotomy between a sailor's boat and a builder's boat (whatever that means.) I'd bet money that the average PD builder does more sailing than any other boat owner.
     

  15. brehm62
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    brehm62 Junior Member

    Well, a sailor wants a boat to sail, building is a means to an end. This person wants detailed and clear instructions; they don't want to experiment or find their own solutions. Building should be straightforward. When finished, a poor boat would be a complete failure and this person would probably never try building again without hands-on guidance. If the boat were good this person would probably not work on another one unless there were either a clear need or they were helping someone else.

    A builder wants to build, sailing is an excuse rather than an end. This person only needs general plans and instructions; they want to experiment and find their own solutions. Building can be chaotic. When finished, a poor boat would just be another step to a better boat and this person is likely to build others even if the first boat is good.

    Daysailers in general tend to be used more often than larger boats because they don't really require planning or extra help. If the wind is up and it's a clear day you can quickly get out on the water and sail. Except for the current rough state of the plans a PDR presents few obstacles for someone who wants to sail. They can be built fairly quickly, they don't cost a lot of money, they can be set up without assistance, they don't require a boat trailer, and they carry two adults more easily than something like a Snark or El Toro. I'm sure a Sunfish, Laser, or Snark would also see a lot of use but more likely by someone younger rather than an adult.
     
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