25' trailer sailer

Discussion in 'Sailboats' started by LP, Jul 25, 2005.

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

    There aren't many hard and fast rules that apply to your question about an optimum trim angle at 15 degrees on this hull form.

    You have a bit of a ******* there and direct comparisons are difficult, so the form needs dynamic study (shove it through some software) to determine what, where and when things will occur. Deciding if these qualities are desirable is pure speculation, though experience with similar modifications or types can increase your chances of success and it's this, that is the hallmark of good yacht design.

    I like the tucked in or wine glassish stern sections and deadrise there, but don't like the quick rise in the run. I think you should flatten out the midship section a bit more and I can't see enough of the forward sections to comment, but would recommend they be "U" shaped, rather then "V", you can always toss in some flair at the top of the forward frames to provide the "look". The tumblehome aft is a nice styling touch, helping keep the "in the theme of" thing going. A boat with these recommend revisions will produce a craft with the motion you could expect form your base model, though it will be a livelier boat due to the D/L reduction, not one that is quirky.

    Built down sections refer to the reverse curve found in many older designs as the planking approaches the garboard and keel. This reverse provides displacement down low, that is foil shaped, deepens the bilge for stowage, lowers freeboard and cabin top height needs plus can dramatically soften up the ride qualities of a yacht. On the other hand, it adds wetted surface, increases planking difficulty, increases drag and a few other less desirable traits if weight and speed are more important in the design compromises made to complete the brief. Since you've drastically reduced the immersed volume, you'll be better of with a "U" forward section, firm bilged with flattish midship section and "V" shaped stern sections. This will give you some of the qualities found in your base model. The high D/L in the original hull form contributed to it's sailing attributes a great deal, making a soft riding, solid feeling craft that great security could be had in. Your version will be quicker in most every regard, but should retain a lot of the sailing qualities, except in heavy air, where her lower D/L will slam you around a bit more.

    People that make generalizations like ""V" sections not being fast, or not sailing well, frankly haven't been on many boats of different type or hull forms. I was on a L. F. Herreshoff - Bounty copy a few years back, spent the whole afternoon battling a fleet of plastic cruiser/racers. The winds got up into the 20's and they were reefing down, changing headsails and tying things down, we were laughing at not spilling our beers and debating, rather calmly, when we should begin to think about a reef in the main. At the end of the day we all got together at the dock and they were all very wet, we were quite dry. Their sloop and cutter rigs got us to windward, but we kicked they butts once freed up a little. We had mahogany, raised panel seating and dining areas to enjoy, they sat on sail bags, full of wet rags and plastic settees with sopping wet cushions. We had a very easy slosh of it, they battled hard for the few gains they got. Which yacht would you like to cross the puddle in?

    The point is, a lot of the design decisions made are reflected in the type of sailing you want to do. You have a cruiser there and will win few matches against modern production craft, but you'll be closer to what you want, then they likely will be. At this point you need a full hydrodynamic work up and as much comparison as you can with similar types (good luck) to see if you're in the ball park for your desires in this little yacht. Stop worrying so much about the latest arcane developments in theory, engineering, structure and whatever else. You focus should be calculating reliable, practical, solid scantlings for this vessel, after you've developed it's shape for it's intended service, which should match you needs as closely as the necessary compromises will permit.
     
  2. Mark 42
    Joined: Feb 2005
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    Mark 42 Senior Member

    Personally, I like the retractable keels on the modern boats (e.g. Melges 24),
    but they don't lend themselves well to traditional shape hulls.
    I would angle the keel so seaweed doesn't always stay wrapped around it.
    ...And where does it take us?
    A "Full Swing Keel" or should we say "Jack Knife Keel"
     
  3. LP
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    LP Flying Boatman

    Design Criteria

    PAR,

    I am greatful for all of your input towards my project.

    I love the wineglass stern of a shapely yacht. I can't say that my design is shapely, but maybe I can pass along a little of the elegance and beauty of a bygone era. Some of the defining elements of this design are to correct the shortcomings of my current boat. First, in pounds to a stop in a chop (powerboat wakes). Secondly, it drags it's stern through the water. Makes a heck of a racket back there.

    So anyway, I've put in a very fine entry with a fair amount of flam to start the bow back up in a controlled manner. My concern is that it might be too fine and the bow would have a tendency to get buried. I'd like to take your suggestion on a "U" shaped section forward. I've tried to in corporate that idea, but one of the ways to increase lateral area is with the prominent forefoot of the Casco design. Blending the forefoot into the forward sections produces a "V" section by nature. It's kind of a convoluted section. Lower portion flaring from the keel to the waterline, then flam to the sheer.

    I've put some powerboat theory into the mid and aftersections of the craft. From midships aft there is a constant deadrise of 18 deg. I don't expect to plane in this craft. The constant deadrise section of a planing craft produces less drag than the changing deadrise sections of planing craft from the turn of the century (and possibly through the mid part of this century). I'm probably applying some theory erroneosly, but by maintaining a constant deadrise, the accelerations on the water surrounding the hull are minimized by such a section. BUT, does that really matter in a displacement hulled craft? Actually, I had forgotten that I had put that much deadrise into the hull. The sections in the afterbody of the Caasco are closer to 14-15 deg. Maybe I should target that. I like a craft that "talks" to me. So, having a a craft that sails on heel gives me security. Gut feeling tells me that a "deadrise" hull will heel sooner, but will shift it's lateral CB earlier, as the weather bilge will rise above the surface sooner, and will begin to stiften earlier than a hull with flatter sections. Sounds like a contradiction to me. :) I guess that's where a numerical analysis comes into play. If you were to flatten the midsection, would you strive to maintain the current displacement or would you accept a greater displacement and the possible requirement for more ballast. If this weren't a trailer sailer, I'd go with the added displacement and ballast.

    I left the built-down sections out for two reasons. 1) Ease of construction, 2) Reduction in displacment. I think it would be great to have the sections of the Casco type. Actually, I think the wetted surface would be reduced. I'm looking to strip-build this craft. I just couldn't visualize the transition at the garboard. I could start the built-down sections and finish them with an epoxy fillet at the keel. Just a thought.

    I'm open the the hydro-dynamic work up you've mentioned. Any suggestions? My location is land-locked in Kansas. Not too many NA's around here with the right kind of software.

    In the meantime, I'll try to fatten...I mean flatten the midsection, reduce the deadrise a few degrees and shift the heeled CB forward a bit (fatter foreward sections, Argh).

    Thanks again for all of you insight.

    P.S. The water ballst might go by the wayside. Maybe there is still hope for me. :p
     
  4. LP
    Joined: Jul 2005
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    LP Flying Boatman

    Swingin

    Right now, I have no intention of having a swing-keel. I feel like I have sufficient lateral plane area, but I'm not ignoring the idea. I'm looking for simplicity in the build and in sailing. Thanks for the input. All things are always subject to change.
     
  5. asathor
    Joined: Oct 2004
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    asathor Senior Member

    I would agree that simplicity is great but looking at the map of Kansas I see a lot of interesting reservoir lakes most most of which follow river valleys. To sail them you want a boat that goes high on the wind comfortably otherwise you will be tacking until after the cows come home. To me that means a reasonably sophisticated rigging and sails and some kind of lee board, blade, keel - whatever it takes for your hull. My little Freedom (shoal) does not get up to full speed until I tack at 120 degrees (60 each way on the true wind) going closer only yields a better VMG when the water is flat (with my old sails - new would be a little better). Also shallow hull/keels tend to stall and generate far too much leeway without speed.

    If Kansas is anything like the Dakotas you will have plenty of wind so trailering is your primary weight/displacement limitation and you do not have to have the tallest rig in town. A cutter rig with a full roach main with battens may be an idea that could give you a lot of power with a low sail plan. You can size the main to match some of the catamerans and buy cheap used sails.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Asathor brings up a good point, in that lake and river piloting requires a fair amount of close quarters maneuvering. That long, shallow, draggy keel isn't going to help you much in this regard.

    I'm not sure if the deeper forefoot is going to help with your concerns over getting slapped around by chop, as you current boat does. Small craft get beat about, mostly because they lack the mass necessary to carry through. Shallow forward sections can make matters worse, but that isn't typically a problem with the older shapes you seem to have an interest in.

    You sketch looks to carry a reasonable amount of draft to provide sufficient lateral plane without trying to add more in the forefoot. A more centralized appendage, still with the moderate draft and well shaped forward sections can be had with a reduction or increase in the displacement. I'd be inclined to try to maintain the qualities of the boat (built down sections and all) by moving the waterlines around to get the desired displacement.

    Log on to http://www.atkinboatplans.com/ and take a look at some older hull shapes that you may enjoy. The displacements will be higher then the standards currently employed, but this can be changed to suit your needs and you'll retain the sailing qualities of the craft.

    As currently designed your boat will not have the qualities of the model you're working from. Altering these older designs and retaining the abilities of the vessel can be quite difficult, but not imposable.
     
  7. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    Should LP's water ballast take a dump?

    Dear LP:

    A couple of things.

    A 'V' section is only more stable than a flat one because it is possible to get the ballast lower without also much lowering the sectional center of bouyancy. Since ballast is usually more dense than water, the needed amount of it usually fits nicely into the lower part of the 'V', where as the CB stays in the upper part of the 'V'. There is no contradiction. The 'V' hull will heel sooner and to a greater degree than a flat section. But. But, if there is enough ballast, and the 'V' section is deep enough, The 'V' section will obstinately refuse to capsize. It will let the wind knock it all the way over on its side, then pull itself upright at the first let up. (A deep, ballasted, fin keel on a shallow flat bottomed section, by the way, would do the same thing)

    The flat, straight sided section, on the other hand, will be initially stiff, especially in your normal sailing range of heel, but will quickly lose stability as the windward chine rises. There are two reasons for this. One, that the flat section to float rightside up at all without an enormous amount of ballast, needs to be relatively shallow. And two, when it is this shallow, it can stand a very high center of gravity and still be stiff enough to carry sail. This can make it very dangerous. It will feel perfectly secure until it's too late. It's interesting to note, however, that the same section, but ballasted and much deeper, is even more tender and even more resolute about not flipping than the comparable 'V' but is so much heavier that it requires much more sail for its given beam and sails at a much sharper heel. When it does this, The leeward chine presents a sharp 'V' to the sea so it will, paradoxily, pound less. Phil Bolger's earlier sharpie designs had this characteristic.

    The biggest fault I can find with your proposed design is the extreme double wedge shape. When this shape heels, The bow tends to bury because the heeled center of bouyancy tends to be much further aft than the level center of bouyancy. This has a very bad habit of making the boat round up into the wind. My boat had this fault and I once cracked a rudder stock trying to keep it on its course during a strong gust. For this reason, you may find yourself reefing due to controllability reasons long before you do so for stability. This can be lived with. Just know what you're getting into. The long, sharp bow may cleave the sea nicely. But that may be all it wants to do. Just because a little more of something is good, doesn't neccessarily mean a lot more is better. I would move the maximum beam forward by about half a station (5% of the boat's length).

    Other than that, I would make no changes.

    Of course, a full preliminary wieght study needs to be made to determine where the center of ballast weight needs to be. Since this boat is to be strip planked, the the cg of the hull itself is a major concern. Also, whether or not you are going to use an inboard or strap an out board to the transom needs to be taken into account as well as how many people you will usually have in the cockpit. My guess is that the center of ballast may have to be forward somewhat of the max Beam.

    As for water ballast.

    It's interesting to note that the original boats used stones for ballast. And the stones were probably granit. I read somewhere that granit weighs approx. 175lbs/cft. Now water weighs 62lbs/cft which is, of course, much less. However, that is not the whole story. The original boats had only internal ballast. Other than plain soakage, there was no weight on the keel structure at all, making it neutrally bouyant at best. If you took a 3x3in square steel bar of about ten ft in length (300lbs) and made it the very bottom of your keel structure, you could, perhaps, use water for the other 700lbs of 1000lb estimated ballast.

    The hope is that the vertical distance of the cg of the steel would at least somewhat make up for its lack of weight. It would provide the lion's share of the righting moment where as the water would provide the wave splitting heft. Maybe with this combination you can get away with fact that it would have a combined density of slightly less than half that of the granit stones. Making the keel hollow by making it out of two 1/2in plywood sides would further help the cause by eliminating approx. 100lbs of bouyancy down where it would do the most harm.

    This would reduce your water ballast tank to approx. 10cft capacity while trimming your launch ramp weight by approx. 23%.

    When I started my own water ballasted project, I decided that I didn't want to deal with valves of any kind beneath the wl. In exchange for that, I was willing to put up with long filling/draining times of at least 1/2 hour each. This way, I needed just little holes in the bottom as well as a single bleed tube on the top of each tank. My boat was to have four tanks. By capping the air bleed tubes, I hoped to cut the drainage rate by at least 1/2. Now the tanks would take at least an hour to drain out. That and the narrow, high sided proportions of the boat were supposed to insure she self righted long before she lost much of her 1400lbs of ballast.

    I am sure that for your project, this would be unacceptable. My boat was to be trailered very infrequently (which is why I abandoned water ballast to begin with). But, by the looks of it, yours would be trailered all the time. I would suggest for you a single one way valve which would have a pull string that would end at the top of a much larger bleed tube that would have cap on it. The draining system would work much the way an average toilet works ;). You would first get the boat on the trailer, pull the string, then slowly pull the trailer up the ramp. A fill/drain rate of about 40gal/min should be quite sufficient. Within 3 minutes it would be drained dry.

    Well. That's my $0.02 worth.

    Best of luck on your project.

    Bob

    P.S.- Did you build that gourgeous looking boat, you now own, yourself?
     
  8. LP
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    LP Flying Boatman

    More bilge water

    Bob,

    Thanks for the input. I did mean hydrodynamics. I can work my way through the hydrostatics with out too much trouble. I'm re-evaluating the whole water ballast idea. If I can gain even 3-4" of head room in the cabin, I think it will be worth it. I'm also looking at building the garboards down as PAR suggests. I'm hoping to still keep the displacement under control. I initially had trouble visualizing the construction, but have designed the solution. I'm also going to move the CB forward slightly and move the lines around until the heeled CB is better behaved. The Casco type also had a bit of a foil shape to the keel. I might be able to hide a swing-keel/centerboard in there. Maybe some rocks instead. :) Anyway, I've got some drawing and calculating to do.

    Oh, and "yes." I built my current boat. It's a Karl Stambaugh design. A Meadowbird. Originally created by S. S. Rabl as a "Titmouse."
     
  9. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    Dear LP:

    Have you done the displacement calculations yet? I could not read the text on your drawing. It appears that your design is approx.
    8ft wide.

    By my calculations it should displace at least 4500lbs, maybe as much as 5000lbs. You might reduce the Beam a bit and keep the deadrise proportionate. If you did that, your displacement would go down considerably.

    By my estimation, reducing the Beam by 1ft would reduce your deisplacement by at least 1000lbs.

    This way you would also be able to move the Max Beam forward some and still keep the same finess in the bow. Wave penitrating power comes from either sail carrying power, or heft or a combination of the two. Bow finess helps too.
    What I mean by heft is wieght in relationship to Beam not length. By making a narrower boat, it is possible to have plenty of heft while, at the same time, having a relatively low D/L ratio. Multis, with their slim hulls, use this trick all the time.

    Monos with good heft (with proper CG location) are not only easier to self right but also harder to capsize in the first place. Joshua Slocum's spray had quite a bit of heft dispite its enormous Beam. I doubt, however, that she had a CG low enough to recover from a capsize (a knock down of more than 90deg.) But because of her heft, as well as her long keel, and prudent handling by her skipper she ended up being so succesful that, even today (with some modern improvements such as external ballst) she is much admired and copied.

    My own personal project would be 20ft long, 5ft wide, and displace 2800 to 3000lbs. 300 of which would be concrete and 600 of which would be sand bags. The good news is that she should self right from a 140deg capsize. The bad news is that she would have such high sides and deep deadrise that she would only be able to carry around 150sft of sail. This would make her, in performance terms, a dog.

    That being said, she would also be convieniet, well mannered, and have good to excellent course keeping characteristics. And she would split channel chop rather than be stopped by it. Everything I would want for a mini voyager.
    Her vertues on top of her vices make her a perfect custom boat. Not one many people would want to copy.

    So too may be your boat. Just be sure you are aware of your hierarchy of requirements. Boat design, like economics, is a dismal science. For everything you get there is something you must give away.

    Good luck on your project.

    Bob

    P.S- Incase you are curious; here is my 'heft formula'

    20*displ. vol./Length*Beam^2

    With moderate ballast(33%) and moderate draft(1/2 Beam) My ratios are:

    0.75 is low, for good to excellent sail carrying but poor wave penetrating,
    1.00 is modrate, for decent sail carying and decent wave penitration, and
    1.50 is high, for poor sail carrying and excellent wave penitration.
     
  10. LP
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    LP Flying Boatman

    Hey Bob,

    Actually , my displacement is where I want it. Right now it is coming in at 4200 lbs. I also think that I have a relatively fine hull form. It's just over 3 to 1 L/W ratio with a 25' LOA to 8' beam. Waterline beam is at 7' and that drops to 6' at 15 deg. of heel. The prismatic coefficient of the canoe body is 1.13. I'll leave you to play with that one. While a narrower boat would be fine for performance, I feel I'm compromising interior volume already. I just redrew the lines with fuller forward sections, built down garboards, a foil section in the keel and a gentler rise in the aft buttocks. I haven't redrawn the waterlines yet. I'll post it when it's cleaned up.

    I've got the LCB forward a bit more. Until I do a definitive balance calculation, I'm not going to dink with the hull shape any more. On a craft this size, crew will have a significant effect on balance, so I see an aft LCB as being desireable. The nature of the aft lines gives increased buoyancy rapidly which will compensate varying crew weight and should I encounter a large following sea, the additional reserve buoyancy should keep me from being pooped. Yes, yes, I've read too many heavy-weather stories. The problem is, I still get the same bow down pitch change with the new set of lines. It may be one of those conpromises I have to accept. Thats also where I'll get a lot of sail carrying power.

    I get a 1.14 by your formula. Pretty close to my prismatic coefficient.

    You'll have to convince me of your heft theory. I see wave penetration effected primarily by hull form and inertia (weight). My current boat is unballasted and bluff, but not excessively so. It grinds to a halt pretty rapidly in the right type of wave system.

    Thanks for the input. More latter. See ya.
     
  11. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    Dear LP:

    My 'heft ratio' is not at all related to the Prismatic Coeffiecent(CP). The Prismatic Coeficient is based on your boat's displacement volume/the area of your largest underwater hull section*your boat's waterline length. CP's of more than 1.00 are therefor impossible. A typical CP for a sailboat is between 0.50 to around 0.58. Since your boat has a long keel, which may account for a significant portion of your boat's displacement, it is probably advisable to include the keel in that calculation. It would be interesting to see how others feel on this issue.

    With my 'heft ratio', the entire 'bouyant body' of the hull is included. This is because often the whole hull has to get through the wave, not just the underwater sections. So. For your boat that would mean: 4200/62=67.5. 67.5*20= 1350. 1350/8*8=21.1. 21.1/25= 0.844.

    0.844 is a little less than half way between 0.75, which I consider light, and 1.00, which I consider around average. Not too shabby for a trailer sailer. Compromises. One must also keep in mind that the displacement you are talking about may be the boat sans people and provisions. If so, you can probably add at least 500lbs to your displacement.

    I would be interested to know what the Length, Beam, and Displacement of your present boat is. A friend of mine bought a full ended daysailer. It too pitched a lot when confronted with a chop. But sailing it in a strong wind was a revelation. No sore arm trying to keep it on course like my old boat. I read somwhere that a wave train of the right frequency will stop any boat. I think it works on the principle of 'harmonic resonance' or something like that. I think they said the cure is to angle either into or out of the wave to change its relative length.

    Thanks for your response.

    Bob
     
  12. LP
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    LP Flying Boatman

    Mock ups

    Just for grins, I thought I'd upload a couple (or three) of images. The hull form is essentially correct. The drag has not been added to the keel. Nice to see it in 3D though it is still only a work in progress.
     

    Attached Files:

  13. sharpii2
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    sharpii2 Senior Member

    Looking pretty cool. It would be interesting to see a traditional boat type brought into the 21st century to meet modern needs. It will be interestng to see how it does.
     
  14. Jocko
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    Jocko Junior Member

    envisioning the holes/vents makes me think that I've seen this system elsewhere - submarines. On the same idea - why just a vent? why not a small compressed air tank?

    2 thoughts - a vented ballast tank could/might slosh/fill/empty with the craft's motion/heel

    a shallow boat, as depicted, would benefit from lateral displacement of ballast -> moving water to the high side.
     

  15. LP
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    LP Flying Boatman

    venting

    The single vent would have a valve. If I were to use this system, I'd launch the boat with the vent closed. Once floating free of the trailer, I'd open the valve and move aft, alowing the bow to rise. Once the tank was filled, I'd go forward to the valve and close it, thus trapping the fluid. Since the top of the ballst tank would be below the waterline, it would fill completely if designed properly. In a previous post I also suggested attaching an air compressor at the valve to empty the tank prior to retrailering.

    I'm looking for lo-tech here. One valve and maybe an airpump. Murphey is everywhere, so I try not give him reasons to hang around.

    I'm on the fence right now with regard to a water ballast system. I've not had a very warm reception with my concept, possibly for good reason. It's not a racer, so I don't want to deal movable ballast.
     
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