Recreational Rowing Shell

Discussion in 'Boat Design' started by Willallison, Sep 15, 2003.

  1. Tom_McGuinness
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    Tom_McGuinness Junior Member

    Will said: "if I'm worng there are plenty more knowledgeable who can correct me but I think the correct location would be at the LCF - longitudinal centre of floatation..... but unless you're expecting tremendous sideways forces - such as a sail or water skier produce - why would you bother with one at all? Most powerboats turn without difficulty, and installing a daggerboard just increases the possibility of broaching when running down-sea..."

    The human-powered tailboat (www.tailboats.com) has two pectoral fins (ie. rudders) located adjacent to the transom used for steering. Additionally, the single oscillating propulsion fin generates an unbalanced toque which induces some yaw oscillation in the hull. Without the daggerboard, it's about +/-3 degrees or so, which I don't find so objectionable. The daggerboard acting in concert with the pecctoral fins is likely to reduce the yaw amplitude to much lower levels.

    Since human powered boats don't have a lot of power to spare, efficiency is more important than with sail or power boats. The tailboat glides through turns without pedalling...so the further it carries and the less it slips, the greater an angle can be turned through. There's quite a bit of leeway during turns with the existing kayak hull, which I'd like to eliminate. I can do 90+ degree turns with the present hull (which has a built in stern skeg)...but would like to turn further per turn. I'm also looking to reduce leeway during crosswind tracking. A retractable daggerboard seems an efficient way to accomplish this.

    With daggerboard, the tailboat will end up looking like a shark swimming on it's back...which makes a certain amount of anatomical sense.

    Best regards,

    Tom
     
  2. Willallison
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    Willallison Senior Member

    apologies Tom - I was getting your boat confused with another. As you say, you don't have a great deal of power on tap, so as with my row-boat, minimum wetted surface is important....the smaller you can make your daggerboard the better.

    here are the lines for mach 1 hard chine rowing shell
     

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  3. Tom_McGuinness
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    Tom_McGuinness Junior Member

    Will,

    "minimum wetted surface is important....the smaller you can make your daggerboard the better."

    Sounds like great advice.

    Here's another one for you. Are hull design packages such as Prolines or Prosurf capable of automatically minimizing surface area for a given hydrostatics profile? Or is it more of a manual trial and error process?

    Best regards,

    Tom
     
  4. Willallison
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    Willallison Senior Member

    I'm afraid you're aking the wrong guy there. I recall one of our contributors saying that he had a means of inputting various hydrostatic data and having his computer spit something out the other end, but whether it can be done to that extent, I'm not sure.
    In regards to your choice of package, if all you're after is the hull surface, then almost any will do the job for you. My skiff is made up of about 12 control points and three curves (the less points and curves you can use to get the desired shape, the better (fairness))
     
  5. Tom_McGuinness
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    Tom_McGuinness Junior Member

    Will,

    "In regards to your choice of package, if all you're after is the hull surface, then almost any will do the job for you. My skiff is made up of about 12 control points and three curves (the less points and curves you can use to get the desired shape, the better (fairness))"

    I will try to follow your advice to keep things simple.

    Being a bit of a perfectionist, before starting the design process I try to identify the process or sequence of events that will lead towards the optimum design. For the hull, both the hydrostatic design and resistance are very important...and these are all controlled by hull shape/fairness. But it seems to me that hull shape may largely be a manual process...the "art" of boat design, perhaps. While it's easy for me to follow your sound advice on minimizing the number of curves, I'm still at a loss as to determining the precise form of those curves. Perhaps the hull design software will provide various commonly use curve forms and smoothing routines to choose from. All of my numerical routines permit automated multiparameter optimization, which really unleashes the power of the desktop computing.

    Thank you for your patience and sage advice.

    Regards,

    Tom
     
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  6. gonzo
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    gonzo Senior Member

    Tom_McGuiness: What you are saying about design software is my main complain about it. I think it doesn't matter what a program designer decides is well enough. Think first about what material the boat will be built with. Then the style or type of construction. These two parameters will determine to a large extent the shape of a hull. Adapt the possible shapes to the use intended and the owner's preferences, and you'll have a good easy to build boat.
    I looked at the lines, and see no problem developing them. The program may not be able, but experience with boatbuilding materials give you an insight into what they'll do.
    For example, a three ply fir plywood will bend, twist and edge-set more than a five ply mahogany plywood. These details are crucial to design. There are other construction aspects that may be specified by the program but can't be built. A hull to deck seam that needs to be tabbed from the inside could be too narrow for any worker to get at.
    My point, which I always stress, is that the design program is showing a boat that can be developed. Even if it can't.
     
  7. mmd
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    mmd Senior Member

    Not my creation, but the poem is appropriate to the comments by Gonzo:


    THE DESIGN

    The designer bent across his board -
    Wonderful things in his head were stored.

    He muttered as he rubbed his throbbing bean,
    "How can I make this thing tough to machine?

    If this part here were only straight
    I'm sure the thing would work first rate.

    But 'twould be too easy to turn and bore
    It would never make the machinists sore.

    I'd better put in a right angle there
    Then watch those workers tear out their hair.

    Now I'll put the thread holes that hold the cap
    Deep down inside where they're hard to tap.

    Now this piece won't work, I'll bet a buck,
    For it can't be held in a shoe or chuck.

    It can't be drilled and it can't be ground;
    In fact, the design is exceedingly sound."

    He looked again and uttered a cry, "AT LAST!!!
    Success is mine, it can't even be cast!"
     
  8. Tom_McGuinness
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    Tom_McGuinness Junior Member

    Gonzo said: " I think it doesn't matter what a program designer decides is well enough. Think first about what material the boat will be built with. Then the style or type of construction. These two parameters will determine to a large extent the shape of a hull. Adapt the possible shapes to the use intended and the owner's preferences, and you'll have a good easy to build boat. "

    We're looking at designing a lightweight 2-person open-hull boat approx 16'-17' LOA x 30" beam suitable for production in volume at a reasonable cost. It will probably be some type of epoxy or vinylester matrix composite...GRP, carbon, aramid reinforced. We'll probably use the plug and mold build method. There may or may not be a need for a deck insert. Weight is a primary consideration.

    We plan to work closely with the tooling supplier and hull builder to address manufacturability issues at the design stage.

    But, at present, it looks like it's gonna be me designing the hull. I'm prepared to spend as much time as it takes designing the lines by trial-and-error to come up with a design meeting the required hydrostatics with least resistance....but if there's any tricks of the trade I should know about, it seems like this might be a good place to learn about them. (g)

    Best regards,

    Tom
     
  9. gonzo
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    gonzo Senior Member

    The thread started about a one off, foam core boat. Is this going to become a production boat? A solid fibrerglass canoe of that size is around 40Kg. They can take a lot of abuse. There are some considerations that take precedence over "least resistance".
    *stability
    *dryness (how much water you ship in rough weather)
    *carrying capabilities -it is a tender
    *ease of construction
    *ability to beach on a surf
    *size of the cabin top it will be carried on
    *flotation on ends so it won't swamp when launched from a cockpit
    *lift-eyes
    This is my opinion for a tender. It would be different in a competition shell.
     
  10. Tom_McGuinness
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    Tom_McGuinness Junior Member

    Gonzo/Will,

    I'm afraid I've been responsible for a massive amount of "thread creep" here. I think I'd better withdraw now and let you guys discuss Will's most excellent hull design. Sorry Will.

    Everyone's comments are well taken and very much appreciated.

    Best regards,

    Tom
     
  11. duluthboats
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    duluthboats Senior Dreamer

    mmd,

    I have been saying that about engineers for years. They always find ways to make my life miserable and won’t stand for suggested modifications.

    Will,

    I like the soft bilge version over the hard chine. That is a personal rather than a professional opinion. For a one off it would be an easy build using wood strips and glass. It would be one I would like to build.

    Hey Tom,

    I like your idea. Have you seen Thistle the one Harry Bryan did a few years back.

    Gary :D
     
  12. Willallison
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    Willallison Senior Member

    :D "thread creep".....I like that term Tom - and whilst it's true that we've veered a little off my topic, all the information and advice is relevant - and more to the point, welcome.

    Gonzo, Gary, mmd - as always you speak with the wise words of the experienced. I produced my first model to demonstrate the type of craft that I hope to build. There were aspects to it that I thought would need to be changed - it was, if you like at the very beginning of the design spiral. I would like to build the boat as a one-off in foam-core - mainly as I hope (no doubt in the very distant future) to build a 30+ foot cruiser the same way. This - as you suggest, will at least to some extent, determine the vessels shape. Unfortunately, my building experience amounts to precisely zero - so I take my lead on this from you guys. The not-so-subtle hint ;) I'm getting is that to build that boat in foam-core, I'd probably end up with something that would either fall apart at the 1st bump, or be so heavy that my mum would need to be superman to row it.

    Lets just skip the hard-chine boat - I don't like it either!

    So - I'm faced with two alternatives:

    1. Make changes to the design so it can be built out of my desired material. Possible to do successfully?

    2. Change materials. Any building experience is going to be good for me.....

    What say ye?

    oh - one last thing. I may not have made the boat's intended use clear. It is a recreational row boat - not a tender (we have rib's for that) It would be fully decked over, so swamping etc won't be a problem. I bit like this:
     

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  13. Chris Krumm
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    Chris Krumm Junior Member

    Will -

    To give you an idea of typical scantlings for a wood strip boat of this size:

    Canoes or kayaks using 3/4" w x 1/4" t. western red cedar strips with 6 oz glass boat and tooling cloth inside and out. Double layer of cloth at bottom in and out if you're really worried about puncture resistance and/or abrasion from dirty shoes or beaches.

    Graham King, wood rowing shell designer/builder extraordinare in Vermont, has a series of very light round bilged "picnic boats" for 2-3 rowers that use scantlings close to above, as well as some recreational solo wherries and open-water solo shells that use 3/16" thick strips with 4 oz. fiberglass cloth inside and out.

    I designed and built an ~20' LOA x 4" BOA strip planked launch with electric inboard and enclosed foredeck for fishing and camping using 1-1/2" x 3/8" strips and 6 oz glass cloth inside and out, doubled to 12 oz in and out at the bottom. Minimal internal framing - additional hull support by the 1/2 bulheads that formed the seats. Plenty stiff and has proved to be quite durable.

    The strip planking can go really fast if you don't mind staple holes. From experience, I've found building a strip planked boat of that size is as fast as a multi-chine plywood hull, provided you're careful in aligning your strips and don't get lost in making the interior sanding a work of art (sanding concave surfaces stinks...).. I bet your hull in 3/4" x 1/4" strips would be fine. you might even get by with 1" or 1-1/2" strips at the flatter parts, but the tight turn of the bilge will require the narrower strips.

    Tom -

    neither Prolines or Prosurf will optimise hulls based on input for surface area or other hydrostatics. They will generate a "basic" hull shape based on basic dimensional inputs such as LOA, LWL, BOA, draft, etc. I believe the full version of Nautilus ProSurf will generate Lackenby variations of a hull form. You can easily scale hulls along any axis and get hydrostatic reports based on different drafts or displacement, so seeing the relationships between hull shape and wetted surface, for example, can go pretty quickly.
     
  14. gonzo
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    gonzo Senior Member

    Strip plank is fast if you don't want the interior to be perfect. The flats of the planks are just a cosmetic aspect. If the boat is painted, which makes sense if you beach it, the staple holes will never show. I find multichine takes a lot of time with fitting. Particularly the bevel lapstrake type. Stich and glue, with a tortured plywood design can be very fast. The trick is to paint the hull so fairing taks little time. Never admit what's under the paintjob:) . This looks like a surfboat, with a high cockpit sole for flotation. Another idea (oh, no!) :a solid styrofoam core under the sole. This allows a thinner skin, is structural and gives reserve emergency floatation. You fist make the cockpit sole, glue the foam and shape it, then laminate strips of wood or plywood over, and foberglass the whole shebang.
     

  15. Willallison
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    Willallison Senior Member

    Thanks again for your input guys (anybody noticed a marked absence of women on this forum?:confused: :eek: :D )

    Chris - great info. I take it that when you guys refer to strip planking you mean longitudinally run planks stapled (or whatever) to temporary &/or permanent framing. (see - told you I don't know what I'm doing!!:D ) The strips would need to be thin enough to bend around any vertical surface curvature - with the fairly sharp turn I have at the 'chine', is this still possible with the kind of scantlings you refer to?
    Then, how is the outside of the hull faired?
    And does anyone have an estimate of the finished weight (ie kg/m^2 of the timber core + grp skins? And while I'm at it, how about the weight if I were to build using corecell or similar?
    I know there are numerous excellent articles on this type of building - can anyone point me towards one titled "Strip planking for dummies"?!

    Gonzo:

    Funny you should say that - I had wondered about building the whole thing in the same way as a surfboard, but thought that the all-up weight might be too high, though I hadn;t considered using a duble skin of timber then grp....
     
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