30' plywood sharpie

Discussion in 'Boat Design' started by davesg, Nov 4, 2009.

  1. gonzo
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    gonzo Senior Member

    The bottom has way too much rocker forward. The rule of thumb is a straight or almost for the first third, then a gentle curve which can accentuate more at the aft part. It will pound and push water forward like a tug.
     
  2. PAR
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    PAR Yacht Designer/Builder

    Now that I've had Jeff (the moderator) move the one thread over here, I'm not so sure it was a wise thing. I'm having difficulty keeping up with the differences.

    Basically, I'm hearing you both feel the need to self design because you're not finding precisely what you want in a stock plan. You'll never find precisely what you want in a stock plan, but it can serve as a strong, balanced base for your dream ship.

    Construction methods aside, the are a few ways around the design spiral. I generally, pick a target displacement range, length and hull shape. Dave your 3,500 lb. boat is certainly possible, but it will require some clever engineering to keep the weight down and you'll be quite limited with what you can do with the narrow interior. From experience I can tell you that a 28' LWL with about 4' 6" WL beam will be in this range, but you'll have very little margin. The 28' LWL NIS is 7,700 pounds with about 3,400 in ballast. This is about right for a moderately dressed cruising vessel of this size and configuration. You could decrease the amount of accommodation, by decreased internal volume which will reduce the displacement, but you lose accommodation space.

    I guess the point I'm getting at is there are a lot of subtle decisions and compromises that must be kept in mind for every design. The sharpie has peculiarities specific to it's type. An understanding of these as well as the design fundamentals are necessary for a reasonable expectation of success. FreeShip (or DeftShip) are wonderful tools, but don't tell you if something is good or bad, just the volumetric results of your efforts. An example would be how do you know what amount of rocker to employ (and why) and what shape. Or do you know why the NIS designs look boxy or high roofed. I understand why Bruce has these this way and it's a good approach, considering what he was after with the design. Even Chapelle (a man I personally knew) understood the limitations of the sharpie and mentions most in he's books.

    I'm not trying to talk you out of a 30' self designed sharpie, but I'm not convinced that you understand the type sufficiently to successfully design a deep water yacht. This makes me considerably more nervous then someone interested in designing an 16' pocket yacht.

    In this vain, it's often much easier to use a stock plan as a base for modification. Assuming you don't move too far out of the box, most things will still be in order. I've seen Ted Brewer's sharpie without the pinky boards and it's a much better looking boat and 5,500 lbs. is the "light" displacement for this craft. In the 3 ton range would be reasonable for this size craft. Knowing you displacement and length, you can develop a rocker to suit the anticipated wave train or use the one worked out in a stock plan. Scantling issues will generally be sorted out. Who would know you used a 28' base (for example) respaced the station molds so it was 30' and designed a new rig, cabin and accommodation arrangements around the new, longer boat.

    If it was me, with limited design experience I'd find a fairly fat 26' or 28' sharpie and respace the station molds. You'd have a leaner ship, with better stability, slightly more displacement, more interior volume and you could go insane designing a new deck, rig, accommodations, making it an all new boat without fear she'll embarrass the crap out of you on launch day (it happens, I can recite names). This is in fact pretty much what I did on my first build. Don't space the station molds more the 10%, 15% at most and you'll be safe, plus enough engineering left over to make anyone bald and wishing for a frontal lobotomy with a kitchen spoon.
     
  3. davesg
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    davesg Junior Member

    I'm a big fan of stock plans, I can find a lot to love in just about any boat, my reasons for drawing up my own really are 'just because.'

    I picked 30' as an arbitrary length to start with, it gives the length for the interior I want without getting completely out of hand. We could just as easily add a transom and lose two feet of the stern and make it a 28 or lose the standing space forward and make it a 26. I am of the thinking that the longer the better, all else being equal, and sharpies are a type that seem to do well long and narrow with light displacement. The displacement I'm shooting for (about 3500 lbs) reflects the intent of this design as a big skiff with two cuddies and room for three to bunk out (not counting the cockpit.) The Norfolk Island Sharpies are certainly wonderful designs, but its the 23 that looks closest to my thinking for this design (stretched of course), other than the 18 I don't see displacement listed for any of them so I don't really know how that relates to my thinking on this one.

    I don't want to get hung up on accommodations here. Sitting headroom and the berths shown are the only sticking points, the large hatches supply the standing space, they are almost more like a sliding top rather than real companionway hatches.

    This absolutely isn't meant to be a 'deep water' boat! Coastal gunkholing in Florida and surround waters for vacation sails is the intended use. From personal experience I would consider the Bahamas and Tortugas to fit into that description as well. This is a fair weather boat, not some survive-anything go-anywhere boat. Like a big skiff with two cuddies.

    Mystic is a great design, but too big, too complicated. Instead of the standing ketch rig I'm interested in a free standing set up, instead of plywood on frame I'd like a stitch and glue type frameless construction. I don't have anything against either of those features, but they aren't what I want this time around.

    This design could just as well be an open boat with three cockpits and sleeping on the seats, the deck houses are meant to enclose that space but not create a real walking around, living aboard type interior, just sitting headroom and standing under the hatches.

    I've restored boats, modified existing designs, and stuck with spec, this time I want to design from scratch. I am lacking experience but in order to gain that experience I am working through this design. I realize this boat will not be as 'good' a boat as a professional, proven design might be, but this isn't about perfection or performance its my first attempt, its about floating right side up and sailing forward rather than backward.

    DelftShip is a really nice program and the main reason I'm using it is for the developed panels it can unfold but I wouldn't expect it to design that boat. I think I understand the design motivation of most of the boats cited, this one is a bit different though, if I wanted a fuller interior then more draft, displacement, and higher freeboard would make sense but I'd really just like a big day sailor with an interior made possible solely by virtue of its length.

    I've read Chapelle's discussions of sharpies as well as any others I could find. (With the glaring omission of Parker's Sharpies book, which is on order.) Monroe's Egret has been an inspiration as well as Presto though of course I don't completely buy into all the gushing praise for those boats I do think they were something special for their intended uses.

    Attached are some updated lines. I reduced the forward rocker, widened the bottom and generally massaged some things. The shear just doesn't look right to me and the bottom doesn't seem right either. Any suggestions?

    [​IMG]
     
  4. frank smith
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    frank smith Senior Member

    This is better , I would want more freeboard . It is starting to look like an Egret . While the flair does provide a cushion of buoyancy I dont think it is good for speed .
     
  5. frank smith
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    frank smith Senior Member

    I also think the bow angle is a little shallow , might have a strong weather helm.
    I try to think of them at 15-20% heel . and look at the chine in relation to a diagonal . But it is basically all in Chapelles .

    Hve you thought about rudder and center board yet.
     
  6. davesg
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    davesg Junior Member

    The centerboard trunk would run from about 10' to 16' aft of the stem. The CLR for the board is at about 14' aft of stem and about 2'4" below static waterline, with board down draft of about 4'6". The trunk is set off to stbd by about six inches to get the main mast step on the centerline.

    I'm really interested in some similar designs that have used a smaller second centerboard aft for more area and balance. I don't think its needed here since I think there is enough area in the main board but its something to think about.

    I had originally drawn in a kick-up dinghy style rudder hanging on the stern. The only real sharpie I have sailed was a friend's 19 footer and the low aspect ratio rudder felt really strange to me but one would fit under the stern here. The stern hung rudder would just be easier to build, use less hardware, and unless someone can tell me why its a bad idea, I think it would perform better as well.

    The question of whether flair is good or not comes down to whether you're comparing it to pushing out the deck or pushing out the chine. To my mind you can make it work wither way, flared like a dory or slab sided like a Bolger design. Personally, I like the flare, but maybe a little more forward?

    Could you explain the 'bow angle' for me a little? The angle in plan made by the sides coming together at the bow? Or something to do with the rocker on the bottom?

    Thanks for the help,
    David
     
  7. gonzo
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    gonzo Senior Member

    It is really fine at the bow and with a lot of rocker. That is a design that will be extremely finicky on the longitudinal loading. Also, running will tend to bury the bow.
     
  8. davesg
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    davesg Junior Member

    Ah, I see exactly what you're saying, I'll work on it some more tonight and post again when I have something new.

    David
     
  9. frank smith
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    frank smith Senior Member

    I'm not sure about the rocker, 3/8" to the foot would give a light craft , and fast ,1/2" to the foot is what is about average, 5/8 works out nicely for a loaded cruising sharpie , it all depends on what you are looking for.
     
  10. troy2000
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    troy2000 Senior Member

    PAR, everything in a well-designed boat is interconnected and balanced. When I've tried just 'tweaking' an existing one, I haven't been happy with the results.

    I'd rather take several boats I like, and turn out a consensus design that's somewhere in the middle of what I like about their plans. And it's worked for me so far, on the few smaller boats I've built.

    I should probably have held off posting until I had something substantial in the way of a design or model to show, but I got all excited when I realized, "my God! There's a place online where I can talk to people about what I'm getting ready to do.":D

    add: I'm not saying I live and work surrounded by fiberglass-worshipping Philistines who have no idea what I'm talking about. But one evening after work a few years ago I was busy knocking together a flat-bottomed canoe, from some scraps of 3/8" CD shear panel, 1x2 Douglas fir furring strips and other miscellaneous scraps, all dug out of a dumpster (desperation; I was 600 miles from home). A framing carpenter stopped on the way to his pickup to ask in an incredulous tone, "yore buildin' a boat outa wood?? How ya gonna keep the worter out?!?
     
  11. frank smith
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    frank smith Senior Member

    How are the sharpie designs going ?
     
  12. YK GEO
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    YK GEO Junior Member

    Dave,

    The basic sharpie design is pretty forgiving, as long as you stay within the general rules recorded by Chapelle, and keep the curves fair and gentle. As to building the boat, have a look at my little boat I knocked together in an afternoon with the only plans in my noggin. It's at the thread "Sharpie skiff - Chapelle's rules". Sometimes we make small boat designing and building way more complicated than it really is.

    For the size you're building, take good advice as to strength requirements. The sharpie hull type racks and twists easily, so you need plywood bulkheads and corner reinforcements to keep it in shape. Also longitudinal stringers to prevent the plywood panels, esp the bottom, from curving in. Torturing in a bit (say 1/2 inch) of transverse curve in the bottom helps a lot. Build it flat, then force the curve in and secure it with framing. If you're building taped seam, force in the bottom curve before gluing the chines together. My current project will be built upright in about 4 female molds with the curve sawn into the molds. I'll screw the bottom to the molds giving me the longitudinal and transverse bottom curves with everything held in place for lining up the sides. That way the interior of the boat will be wide open for taping, installing stringers and bulkhead, etc. Once the hull is all stuck together, unscrew the bottom from the molds, flip it over and do the outside.

    Have you read Munro's book "The Commodore's Story"? Not a lot of pictures and no plans for Munroe's boats, but a good read.

    Good luck with your project.

    Geo
     
  13. gonzo
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    gonzo Senior Member

    The lowest part of the sheer should be about the middle so the bow doesn't look like a needle
     
  14. davesg
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    davesg Junior Member

    I've tweaked the lines a bit, a little more freeboard, more beam fwd. I think there is too much rocker but I'm not sure how to balance keeping the stem and sterns above the static waterlines but still have the depth for the displacement the boat needs. Right now its a bit less than the 1/2" per foot of rocker. Any ideas there?

    Geo - I've read your sharpie thread a couple of times and you make good points on the structure. I've only taken a cursory look at the structure, there are four full width bulkheads sketched in and the interior will all be structural. The biggest problem I see is there is 7' about amidships that doesn't have any full beam structure other than the deck house. I was going to use a mold or two there for getting the panels in the right place but I am a little concerned about torque and bending. I'll know more once I take another lap around the design spiral.

    I haven't read the Commodore's Story, haven't found a copy but I have read 'The Good Little Ship' and what else I could find on Presto and Egret.

    [​IMG]
     

  15. troy2000
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    troy2000 Senior Member

    Well, I went out yesterday and bought a pile of basswood, and some 0.8mm birch plywood. I decided I was wasting my time trying to tell Delftship how the boat should be shaped. I don't have enough knowledge and experience to force a design into it.

    Instead, I'm going to do what I've done in the past with smaller projects: put some wood together, and let it tell me how the boat's going to be shaped. Then I'll start comparing it to established designs, and make sure I'm somewhere in the norm. Since I'm an amateur, I have no burning desire to live or die on the cutting edge of design.....

    Right now I'm just waiting for a couple of Chapelle books to show up in the mail. I have books by other authors, and it gradually dawned on me: since every single one of them references Chapelle somewhere, it would probably behoove me to look up his stuff firsthand.

    I'm wrestling with LOA (a hazard of having too much time on your hands to poke at things...). Given the narrow beam of sharpies, it looks to me like I could really use up to 32' to comfortably stuff in a V-berth, a pair of settee berths, an enclosed w/c and a usable galley. Plus enough cockpit (and supporting hull aft) to let a few friends hang out topsides and enjoy the trip....

    On the other hand, 26' to 28' sure sounds a magnitude or two cheaper to build. Not to mention easier to tow, launch and recover.
     
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