PDR design

Discussion in 'Sailboats' started by brehm62, Jun 24, 2012.

  1. brehm62
    Joined: Mar 2011
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    brehm62 Junior Member

    I finally got enough repairs completed on my 20' sailboat to try it out at the local lake. The last time I went out my brother went sailing with me. He seemed to feel that my boat was too big and complex. So, I've been thinking about building a PDR. I think he could build one but I can't completely completely recommend it until I've built and tried one myself.

    However, I've noticed that the available plans are rather rough with seemingly a lot more suggestions than actual build details. I picked up some 1/4" plywood but I need to get some things figured out before I start cutting.

    I like the room with end floatation tanks but I don't really care for how much water is left when the boat is righted after being capsized. So, I'm thinking about compromising with a diamond shaped cockpit area. Basically use end tanks but the center angles instead of running straight across. In other words the tanks are wider at the sides than in the center. Or conversely the leg area is longest at the boat center-line and less at either side. I'm thinking:

    Aft center-line tank bulkhead vee joint: 12" from transom.
    Aft hull side/tank bulkhead joint: 24" from transom.

    Fore center-line tank bulkhead vee joint: 12" from bow.
    Fore hull side/tank bulkhead joint: 30" from bow.

    The mast would also be at the fore vee joint.

    I've thought about stays for the mast and using a slotted mast with sail slugs. Most likely I would use 1/4" braided rope for the stays. If I have to cut a slot then the mast would likely have to be wood.

    In order to vary the amount of sail more I've thought about using a jib with a clubfoot and single sheet. This is fairly simple and should be easier to control than two sheets.

    The sail that seems the best is the leg of mutton because it doesn't need a boom vang and can get the boom high enough to not smack you in the head.

    I would probably use hardware pulleys. At the moment I'm thinking along the lines of a sewn tarp sail. I do happen to have some white polytarp material. And I have heavy polyester thread. I've done repairs on my old main with my sewing machine so I'm certain it can sew the tarp. Of course, I also have 18 yards of 1.6 ounce, ripstop nylon. I was going to use it to make a drifter but it seems like my boat can sail with the genoa even in very light wind so that doesn't seem necessary. I'm not sure the nylon is stable enough for the main but it might okay for the smaller jib. I think if I did use the nylon on the main I would have to cut it flat and just let the naturally stretch give it some camber. And, perhaps I could run the weave from each corner rather than joining the whole sail horizontally. Certainly the white, blue, and yellow nylon would make a prettier sail but white polytarp isn't bad.

    The wood I got for the rudder and keel boards is 1"x6". I noticed that most people seem to use much larger lumber but I'm thinking this should be adequate if there is some additional keel on the bottom. That would make sense because I think something would be necessary to help reinforce the 1/4" hull bottom.

    I already bought latex exterior primer and latex glossy porch paint. I have epoxy and fabric to reinforce the joints and I have other acrylic colors for decoration. I will probably use Titebond glue since I have a whole bottle of it.

    I was thinking that it would be a good idea to bevel all of the plywood edges so that the end grain is mostly covered. That seems straightforward to me but I haven't heard of anyone else doing it. The only place I could see where it would be a problem would be the hull bottom since due to the curve you can't really predict the exact length. So I would have to cut it slightly long and then trim it to fit. Those 1/4" plywood decks are also not likely to hold up my weight so I'll need some reinforcement there too.

    I'm undecided about access to the tanks. Traditional covers are very flimsy so you can't walk on them. However, a threaded PVC clean-out while sturdy also has that annoying square lug sticking up. I could just leave two open holes on the either side of the center-line but I'm concerned that water left in the cockpit could slosh inside. That would only leave the option of actual hatch covers. Maybe.

    Hopefully I'll be able to get this figured out and start cutting soon. I suppose I could draw something and take a picture; that might easier than trying to use Windows Paint. Or maybe I can find a drafting program.
     
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  2. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    Hi, Brehms.

    The diamond shape cockpit is an interesting idea. I have thought of it myself.

    The only problem is your bulkheads will be intersecting the bottom curve at an angle. This poses two problems:

    1.) the bottom curve of the bulkheads is going to be considerably different than that of the sides, and
    2.) Any chine or clamp board is going to have to be carefully shaped to match the bottom curve. Just springing it to the bulkhead curve is not going to work.

    That is not to say it is not doable. That joint can be a taped seam one with the deck being held on with a clamp board.

    Also, with a surform plane and a long board (long enough to reach from side to side), you can carefully shape a bottom/bulkhead clamp board so the bottom fits smugly.

    As far as the opening inspection ports go, those can be installed in the bulkheads, instead of the decks. Then you don't end up standing or sitting on them. Besides, most of the time they will be removed. They will only be in place while the boat is in use.

    I fail to see the advantage of a diamond shaped cockpit. I do see a 'coolness' factor' but no practical advantage, except, perhaps, sleeping on board (if the tips of the diamond extend all the way to the transoms).
     
  3. messabout
    Joined: Jan 2006
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    messabout Senior Member

    When you say leg O mutton sail I think you are referring to a sprit boomed sail. They work well and are simple. The only adjustment is the snotter, simple is good. A sail of that kind will have a practical limit of area. For the PDR with a 16 foot mast you are looking at no more than 60 square feet of sail area. That might be plenty for the PDR but it seems that a lot of people pile on more sail. Consider a lug sail. It will have only two strings to pull and will give you a lot more area on a shorter mast. .....

    Why would you even consider the complication of a jib, genoa, or drifter? You will not need shrouds or stays if you omit any sort of foresail. WIth no shrouds you can let the sail weather vane when appropriate. That is a real convenience and even a safety consideration.

    This is a little boat that needs to be kept as simple and quick to rig as you can arrange. The easier to rig, the more you will use it. The sprit boom sail can be stowed by simply turning the mast to wind the sail up. You can even use a lanyard to rotate the mast for almost instant dousing and stowage.

    The PDR guys seem to like the side tank layout better than end tanks. If you build the 16 inch side model then a 6 inch wide tank on each side will give you plenty of flotation, nearly 200 pounds per side, and will ship little or no water when righted from a capsize. The verical tank sides also reinforce the bottom and the transoms. You will still have ample room in the cockpit which can be almost eight feet long. The side deck/tank tops give you a place to sit when hiking. You can use a simple board cross piece with a hole for the mast. It can be made adjustable and fine tuned fore or aft as needed to find the right location.



    You will almost surely want more rudder than a 1 x 6 will make. Consider edge joining two or more of them.
     
  4. brehm62
    Joined: Mar 2011
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    brehm62 Junior Member

    Well, I hadn't thought of that. I wonder how far off I would be if I lofted it in a straight line from the center joint (known height) to the hull side joint (known height). Looks like I'm going to have to do that on a piece of cardboard and see how it fits. I could probably then adjust it from that.

    If I can get it close enough I would only need to sand it a bit and that wouldn't be too bad. But I can't sand off a 1/4".

    Yes, but I'm thinking wooden hinged doors to keep water from sloshing in. I don't think they would need to be water tight but maybe a bit of weatherstripping wouldn't hurt.

    I have to say this really puzzled me when I read it. Then I realized that you haven't been sitting around thinking about building a PDR so you might not be looking at some of the things. Here's a picture of a boat with side tanks being righted:
    [​IMG]

    Looks good. It floats high and you don't get water inside when you flip it back up. Unfortunately those side tanks are 7" each. And, I don't really want to reduce the cockpit to 34" wide. Now, here's a boat with end tanks:
    [​IMG]

    Yeah, that looks like it is half full of water. I was thinking that if I extended the tanks at an angle down the sides I could reduce that somewhat. I actually think I can cut that in half. You can also see that without the access hole cover he would probably be getting water in the tanks.
     
  5. brehm62
    Joined: Mar 2011
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    brehm62 Junior Member

    The first boat I actually sailed was a sprit rigged pirogue. It was loose footed so it was a lot like this:
    [​IMG]

    The above is how I think of sprit sail: four sided with the sprit pole going up at an angle. This is a leg of mutton:
    [​IMG]

    Similar but with the sprit boom going out sideways.

    Right, the snotter on a LOM essentially is the outhaul. And the lower part of the sail works as a boom vang.

    Well, 60 sq ft is probably the maximum I would ever think of using but I will probably have a shorter mast than 16'. At the moment I'm thinking about 13' from hull bottom to mast head.

    My 20', 1,300 lb sailboat only has about 150 sq ft. So logically 50 would be enough.

    I'm confused why I would need more. Also, the LOM seems to be a better shape.

    Well, I considered a drifter on my 20' boat until I realized that the genoa could move it in a very light wind. So, I'm not really thinking about it anymore. My 20' boat already has a genoa so I'm not really thinking about that either. However, I am considering a straight jib on the PDR because it would give me more area flexibility. And, a jib is pretty simple if you use a club foot. The tricky part is avoiding this:
    [​IMG]

    A rig like the above doesn't really have enough room for a jib. I'm thinking:
    Mast - 11' 8" above the foredeck. That could increase by 12"
    Sprit boom - 7' 6"
    Bowsprit - 2' forward of the bow
    Nominal total sail area - 50 sq ft

    Well, that seems kind of backwards. Shrouds are a function of mast stiffness; they aren't really related to a foresail. However shrouds do prevent using a lug sail.

    So, if my 20' sailboat can't do this then why is it necessary for a smaller boat? Or do you mean if it has too much sail area?

    Yes, assuming the mast is freestanding, round, and in a mast sleeve.

    This is from the PDR website:

    The most popular configuration of the PDRacer is the "End Airboxes", called that because the flotation airboxes are built into the ends. The nice thing about this configuration is that it has a lot of interior room

    I get 278 lbs. This is calculated by a straight line from station to station so the actual number is slightly higher. Allowing for the curve adds about 3 lbs.

    A little bit but that still leaves an unsupported span of 36" between tank sides across the bottom. So you would still need bottom bracing.

    Again, very little. You still have 36" across the back between tanks. The primary bracing of the tanks is for a side deck and the hull sides as well if there are interior bulkheads. They really aren't that different because you either add bracing exposed as a gunwale or you put it inside as a panel joiner.

    For one person, yes. But for two people you only have 7' between transom and mast so 3 1/2' per person. Oddly enough with my layout the shortest span on the side is 3 1/2'. and 6' at the center. So that averages 4 3/4'. However, in both cases I think a shorter cockpit seat would work the best and give both people plenty of room.

    At this point I have no plans whatsoever to do any hiking.

    Well, yes and no. If you had some kind of rack on both sides you could move the mast brace from position to position. However, you would also have to have matching mast seats on the hull bottom; I guess that would be almost like a fife rail except it would be sized for the mast butt instead of belaying pins. Let's see what the stresses would be. A leg of mutton or sloop rig would have the center of effort at about 1/3 of the height. That would be about 5' above the mast brace. With a 15 mph wind and 60 sq ft sail you have a push of about 47 lbs. So 47 x 5 = 235 foot lbs. That's not too bad. A lug sail would have a higher COE closer to 7.5. So 47 x 7.5 = 352 foot lbs. That's not too bad but obviously it would need to be stronger. Logically you would put a center brace under the fife rail on the other side of the hull bottom and probably through bolt them to sandwich the plywood in between. I suppose you could do the same thing with the side racks.

    Theoretically you need 2% of sail area. If I have 50 sq ft of sail area then:
    50 x .02 = 1 sq ft or 144 sq inches
    144 / 5.5 = 26"
    So, I would need 26" of depth when using a 1x6. Perhaps. But if that does prove to be a problem I do have 1x8 boards. To make certain that this is realistic I'll need to compare it with the waterline beam to lever ratio on my 20' sailboat rudder. On the PDR website he briefly talks about lever arm saying that some people put the side boards too close to the rudder. However, he doesn't mention the beam ratio. There is a difference between having 6' between centerboard and rudder with a 3' waterline beam and having the same 6' lever with a 6' waterline beam. The wider beam degrades the rudder effectiveness. Without measuring it the maximum waterline beam is probably about 6' and the lever arm is probably about 9' for a 1.5 ratio. If that were true for the PDR then with 4' of beam I would need a lever of 6'. The mast is 7' from the transom so I need to set the centerboards forward of 4.5' from the transom but with the jib I need it forward of 5'. So, that seems within the ballpark but it might very well take a 1x8 to avoid having a lot of depth. The lee boards would need 12" - 18" of depth on a 1x6. So, maybe 18" deep for both.
     
  6. lewisboats
    Joined: Oct 2002
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    lewisboats Obsessed Member

    You really seem to want to over complicate things. A crappy PD that still sails at 90+% of a well built one can be done in a week end. A well built one can be done up in about 10 days. You want the boards (DB or LB and rudder) to be big and beefy because these things can sail in 25+ mph winds and can break things. I saw a guy snap his 10" x 36" x 1/2" thick daggerboard in half just sailing...but the boat wouldn't go over. They are very stable and sail like they are on rails so having a (possibly) oversized rudder is much better than having too little...same with the lateral resistance. I built my daughter one that is 10 ft long, has 3mm inner and outer side tank panels and 1/4" deck and bottom panels. It comes in at about 65-70 lbs or so. The rudder is 3/4" thick and around 180 sq in. submerged. It could use more. On this kind of boat...you don't worry about drag because you just throw a couple of square feet more of sail on it. The bottom flexes a little in a chop but I expect the boat will last a long time in spite of it. I weigh over 200 lbs and I can stand on the side decks (8") wide and not dip the gunnel in the water. I have side air boxes and front and aft boxes too. If you could balance the boat on it's side in the water you wouldn't get any in the cockpit... not even with my daughter parked on top of it. The side decks are perfect for sitting on and controlling the boat... much better than trying to sit on a thwart or the bottom. I use a 30" (submerged) x 10" x 3/4" thick solid Meranti daggerboard in an off center case that is attached to one of the side box's inner walls. We used a 60 sf Sprit rig (4 sided with sprit spar and at first a sprit boom then a regular boom) and I probably could have used another 15-20 sf to really get her cooking along.

    [​IMG]

    Here is the building thread if you are interested.
    http://www.boatdesign.net/forums/wooden-boat-building-restoration/eider-duck-build-41237.html
     
  7. brehm62
    Joined: Mar 2011
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    brehm62 Junior Member

    Perhaps. But I think it is good to design completely before starting the build. If there were any decent plans available I would have already started building. I have 3 sheets of 1/4" plywood ready to go. But, since there aren't any really good plans available I'm going to have to document my build.

    The shortest build I've ever heard of is 6 days.

    That's what I was thinking, 10-14 days.

    The maximum stress I would ever think reasonable would be 500 ft lbs of torque. You could get that with a lug sail of 30 sq ft in 25 mph winds. Of course, that would also exceed the weight of one person so you would turn over unless you hiked out.

    That would make sense. His dagger board would have only 22% of the strength of mine.

    Yes, that is why when sharpii2 questioned it I decided I would probably need a 1x8 on the rudder instead of a 1x6. The lateral resistance is different since ideally it would have 3 sources. However, if I didn't feel that I had enough from bottom strakes then I could make the lee boards larger as well.

    Pretty boat.

    That makes sense; you only used the 1/8" ply on the shortest span. That would save about 12 lbs so you would more than break even over the extra length. That would explain why you are lighter than 75 lbs. I assume that weight is without the standing rigging.

    That would theoretically be enough for a 60 sq ft sail (which I see is what you have).

    However, it looks like you only have about 4' from rudder to centerboard which I don't think is enough leverage for the beam. I'm guessing you would need about 50% more rudder to balance.

    Now that's a stable boat.

    Yes, that is a consideration of mine as well: having enough seat area and being able to move your weight so that it stays trimmed. With side tanks obviously you could move forward.

    If you don't have bottom strakes I would figure it as a 1x12 cut 30" long. Yours is 87% of that. That might be enough either with bottom strakes or with the chine effect.

    Well, I don't fancy myself a nautical engineer but it looks to me like your sail COE is too far forward compared to the CLR of your dagger board. I would have placed the daggerboard trunk forward about 12". That would give you more rudder authority and move the COE slightly behind the CLR (giving it a little weather helm) which would make the boat roundup with higher wind. To my eye it looks like you would have a slight lee helm as it is.

    Yes, I looked at both the building and the earlier design thread.
     
  8. brehm62
    Joined: Mar 2011
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    brehm62 Junior Member

    Okay, I took some measurements on my 20' Explorer.

    Rudder: 36" x 11"
    Lever arm: 9' 4"
    Beam: 5' 8"

    Lever advantage: 1.65
    Rudder to sail ratio: 1.83%

    So that gives a number of about 3

    For PDR
    If the lee boards are at 5' from transom
    Lever arm: 5'
    Beam: 4'

    Lever advantage: 1.25

    3 / 1.25 = 2.4%

    50 sq ft x 0.024 = 1.2 sq ft
    1.2 x 144 / 7.5 = 23
    That would mean 23" with a 1x8"
    1.2 x 144 / 9.5 = 18.2"

    With lee boards at 6' 5"
    6.4 / 4 = 1.6 lever advantage
    3 / 1.6 = 1.875%
    50 x 0.01875 = 0.9375
    0.9375 x 144 / 7.5 = 18"
    So, to keep the rudder at 18" I would either need to move up to a 1x10" board or I would need to move the lee boards forward to 6' 5" from the transom.
     
  9. messabout
    Joined: Jan 2006
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    messabout Senior Member

    I will not quarrel about the leg of mutton description. Bolgers book; 100 Small Boat Rigs calls any triangular sail a leg of mutton. He goes so far as to call a short gaff a "shoulder of mutton".

    The distinction between two varieties of sprit sail is that the squarish one is a peak sprit and the trangular one is sprit boomed. I have a sprit boomed sail on my 16 foot sharpie/flattie. Just right for a lazy old guy like me. It has a round mast and the furling lanyard that I mentioned. Elapsed time to unrig the boat; 90 seconds.

    I think that you might be shooting yourself in the foot when using the the larger 20 footer as a definitive reference point. The PDR is a floating box whose characteristics are at technical odds with a more conventional boat.

    You appear to have given this project considerable thought. That is commendable. On the other hand, a PDR may not deserve an excess of cerebral exercise. We will be pleased to learn of the outcome of your project...maybe some pix.

    Fair winds.
     
  10. brehm62
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    brehm62 Junior Member

    I've thought about possible rudder hardware. I don't really like the idea of using regular hinges even though they would theoretically work if you attached them to a bayonet type mount. You would then remove the hinge with the rudder. Fabric or X lacing would also need a bayonet mount since these hinges are semi-permanent.

    The only practical way of making a gudgeon seems to be to use 90 degree aluminum angle. I'm thinking 1/8" x 1" would probably work. Unfortunately my local hardware store doesn't carry it. So, I guess the next time I'm in Terre Haute or Bloomington I'll have to get some at Lowes. I think two pieces each for top and bottom gudgeons would work. Put one angle up and the other down with a spacer in between. That would probably be a wooden spacer so 3/4". However, if I needed something beefier I could also possibly stack two layers of the 1/4" x 1" stock that I have to make a 1/2" spacer. That would give 3/4" of aluminum bearing surface. I can't think of any reason that wouldn't work.

    So, that leaves the pintles.
     
  11. lewisboats
    Joined: Oct 2002
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    lewisboats Obsessed Member

    Actually the helm balances rather nicely if you don't sit all the way aft. Don't forget that in a boat this size...weight shift can steer it alone because it changes the attitude of the boat immensely with only a small shift of the butt. Sitting forward puts the forward portion of the chine into the water and lifts the after portion out which shifts the CLR forward. Don't get too hung up on a static profile CLR/CE ratio... in reality it shifts all the time...and in a smaller boat is always on the move and is more noticeable too. You compensate without noticing really.
     
  12. brehm62
    Joined: Mar 2011
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    brehm62 Junior Member

    I know how much real rudder hardware is. It costs $3 each for gudgeons made by Race Lite. It costs $6 for a short pintle and $7 for a long pintle. Then it costs $9 for shipping. That's $28. It's worth it for stainless steel parts but that does seem a little high on a low end boat.

    So far though, I haven't found much of an alternative except perhaps gate hardware.

    There are strap hinges. A 6" strap hinge like this runs about $3.30.

    [​IMG]

    There are pintles. One like this runs about $5.

    [​IMG]

    These are common hardware so you an buy them or order them locally. That would be $16.60 (plus tax) for zinc plated steel. You also don't have the advantage of one shorter than the other.

    Since this is visible I guess I don't really have to worry about rust. Of course if I were taking it in salt then I would definitely go for the stainless.

    So, it looks like these are my two options. I could either do gate hardware at about half the cost of real rudder hardware. Or I could make a bayonet fitting from wood and make an X laced hinge from a few cents worth of rope. Maybe I should make a prototype and see how solid it feels.
     
  13. brehm62
    Joined: Mar 2011
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    brehm62 Junior Member

    Right, functionally, a peak sprit works similar to a gaff rig whereas a boom sprit works similar to a sloop rig. The obvious disadvantage being the interference when the sail rests against the sprit pole. It doesn't matter to me what we call it as long as we know what we are referring to (that's why I included pictures). Sometimes you hear people call this a "spirit" sail instead of sprit. I don't normally correct them. Top sprit, head sprit, gaff sprit, peak sprit probably all imply that type of sail whereas clew sprit, sloop sprit, foot sprit, or boom sprit would probably imply the other. That makes sense though that any sail tensioned with a pole would be a sprit regardless of where the pole attaches.

    Yes, that compares favorably with the nearly 30 minutes it takes me to set up my Explorer. Being able to just roll up the sail would be nice but I have some other considerations.

    My feet are easy targets: size 10, E width. Well, it has occurred to me that the hard chine on a PDR would give some lateral resistance while the softly rounded hull on my Explorer would not. Which part do you think I am estimating wrong? I can see how the trim would change with shifting weight. I'll have to think about that.

    I'll be pleased if I get started.
     
  14. messabout
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    messabout Senior Member

    You could increase the rudder lever arm and do away with gudgeons, pintles and tiller. Just mount an oarlock or a pair of thole pins on the transom and use an oar for a rudder. You could scull with the oar too. Many a small boat has gotten along quite nicely with this simple ploy. You can also use the oar in a dammit situation if you get caught in irons or wish to bash a rude jetskier.
     

  15. brehm62
    Joined: Mar 2011
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    brehm62 Junior Member

    Well, I do have a canoe paddle I could use as backup. But my preference would be a balanced rudder. On my Explorer the rudder forces are considerably higher with the rudder up versus down.
     
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