Design Issues for small, sail/human powered cruising boats

Discussion in 'Boat Design' started by scotdomergue, Jan 6, 2014.

  1. bajansailor
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    bajansailor Marine Surveyor

    That is an 'interesting' design shown in your attached pdfs Scott.
    What type of rig are you proposing to use?
    I see that the hull form is hard chine - for minimum form resistance, wouldn't it be better to have a round bilge hull form, perhaps more like the Rannoch boats in the link below?
    https://www.rannochadventure.com/boats-2/custom-builds

    I appreciate that you want a long wave piercing type of bow, but your centre of buoyancy is noticeably far aft, and I am wondering about the shape of the waterlines and buttocks aft of amidships, re the hull resistance - might it be better to have the difference between the forward and aft sections a bit less extreme, with the widest section a bit further forward?
    This is a simple question, as I don't know the answer - I would be interested in other opinions on this hull form.
     
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  2. BlueBell
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    BlueBell . . . . .

    Scott, no keel?
    I think we need a bit more info.
     
  3. scotdomergue
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    scotdomergue Scot

    RIG: racing dinghy style Bermuda rig, possibly adding a small 2nd mast aft to help with self-steering. I intend that she perform reasonably well with a single sail ,including with tiny storm main in extreme conditions); ultralight nylon code 0 type foresail for light winds; twin foresails for downwind. Regular main will be fully battened with 2 reefs. All ultralight.

    CHINES: I believe that 2 or more chines provides as good or almost as good efficiency as round bilge. And I can build stitch & glue easily while having no experience building round bilge.

    The SHAPE is an attempt to satisfy a number of goals with minimum compromises If the Freeship software I'm using is as good as I hope, this design will work fairly well:
    - low resistence: I want to be able to row (sliding seat) at between 3 and 4 knots for extended periods of time and to have good displacement performance with a relatively small sail rig
    - adequate stability; at 74 I'm not as athletic a sailor as I once was and am more reluctant about unintended swimming in cold water
    - managing weight balance and trim with sliding seat rowing position aft and inside cabin sitting and sleeping positions forward, a challenge
    - light weight (as light as possible, current version calculates out at under 100 lbs. including sailing and rowing gear, though I suspect it will be a little over that once built and with a small solar power system added.
    - a degree of cruising comfort (minimalist, but more than I used to need.

    No keel, will have lee boards - to facilitate use in shallows and for easy beaching, not to mention towing behind a bicycle.

    This remains a work in progress. I'll post some hand drawings that show more than the Freeship images when I have a chance. And additional details as I do more. Next step is a mock cabin to get a better sense of how I'll feel about spending extended period aboard.
     
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  4. scotdomergue
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    scotdomergue Scot

    PS:

    Enjoyed looking through the Rannoch website. Thanks for the link.

    I haven't yet spent any time on the rig beyond mast location. That will come.
     
  5. skyl4rk
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    skyl4rk Junior Member

    Two-sheet Flywood:
    [​IMG]

    I like the Flywood lines for a sailable, rowable beach cruiser: Two sheet boats http://flo-mo.weebly.com/two-sheet-boats.html

    The Flywood will be more stable than (for example) the Big Guide, due to the flatter, wider hull under the waterline. The Flywood will be a better sailboat, the Big Guide a better rowing boat.

    Two-sheet Big Guide:

    [​IMG]

    If I wanted a long distance sail/row boat with a cabin, I like the Three-sheet Flywood: Three sheet boats http://flo-mo.weebly.com/three-sheet-boats.html
     
  6. peterAustralia
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    peterAustralia Senior Member

    Hi Scott, I looked at the three pdf images that you supplied

    Looks reasonable. Very little rocker, there is no keel. Maybe a low aspect ratio keel say 8 inches deep with a little weight down low, for stability. As to lines, the transition from the sharp bow to the midsection, seems from the topview you have a straight line for bow, meeting a straight line amidships. I would have thought you could bend the foam to give a smooth curved transition as you did for below waterline. I dont like the stern with that underside that could be hit by following seas and push the stern up and bury the bow. Just maybe make the bow a couple feet shorter, and the stern a couple feet longer, giving smoother lines
     
  7. peterAustralia
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    peterAustralia Senior Member

    here is a little doodle,, I made the bow shorter, stern longer. I think foam can be curved in one plane, as per plywood? I have shown a small keel, add some weight down below, like a steel rail, for some stability, 2 inch wide by one inch thick maybe.
     

    Attached Files:

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  8. peterAustralia
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    peterAustralia Senior Member

  9. peterAustralia
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    peterAustralia Senior Member

  10. peterAustralia
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    peterAustralia Senior Member

    bolger dovkie bolger_dovekie.jpg
     
  11. peterAustralia
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    peterAustralia Senior Member

    bolger otter Otter.jpg
     
  12. Milehog
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    Milehog Clever Quip

    Scot, I was gonna make a couple suggestions, but Peter's ideas are more thought out and mature than mine. Your ultra-fine bow will, IMO, negatively affect stability more than your math suggests. Bolger is always worth considering.
    Balance and moderation will yield a much better behaved, more restful and predictable boat, again IMO. Don't discount the difficulties of a cranky, tiring boat.
     
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  13. scotdomergue
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    scotdomergue Scot

    Thanks for thoughts and design ideas. I will think on.

    Boat design always involves compromise. Some reasons for the characteristics of my current design:
    - wide flat bottom: comfort sleeping on the bottom, maximum initial stability [would also contribute to performance on plane if lightly loaded, but this is NOT a significant issue for me]
    - flare of transom: to provide width where drogue lines would run and to keep them away from the rudder [Without this flare, the top view is much more like the suggestions]
    - long, narrow bow, minimal rocker: for displacement speed (rowing and sailing) and light weight; keeping stowed and my weight aft should help with potential bow burying, though this is certainly a concern [according to Freeship calculations, more fore/aft balanced designs end up with significantly more wetted surface, resistance and weight for a given load carrying capacity]
    - no keel: I want to be able to pass over very shallow waters, for a variety of reasons, and want her easily beach-able. I also want her to sit flat when the tide leaves her on the bottom. A reasonable amount of weight on a shallow keel has fairly minimal impact on stability compared to proper loading of stowage and body position (for a truly light weight boat).
    - sharp transition from center to forward at top and bottom of sides is to provide a good surface to attach leeboards that would rotate up along her flat sides.
    - the long straight forward lines will also facilitate maximum strength, minimum weight skins, hand applied using peel-ply. I've never done infusion, and it's way more complicated.

    Note that, at light cruising weights, both bow and stern would be less than 1/4 inch into the water; at maximum cruising weights (still only 700 lbs. gross displacement) less than 2 inches.

    I like Bolger's designs, but want MUCH less weight. Also remember that this is specifically designed as a minimalist SOLO cruising boat, so needs much less capacity. My current designs calculate to 100 lbs. including all rowing and sailing gear. She might be 125 or possibly even 150 lbs. built and including a small solar power system - still pretty easy to manhandle, drag up on a beach, and tow behind my Bike Friday (which could be stowed aboard).

    My analysis suggests that, with all rig stowed below decks, load properly stowed and secured, and me stretched out on the cabin floor (bottom of the boat), she should self-right. I would expect to weather extreme conditions on drogue aft or sea anchor forward.

    And, of course, this would be experimental. It could be very interesting to see how she would perform!

    All that said, I welcome further discussion!
     
  14. scotdomergue
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    scotdomergue Scot

    Just noticed that Bolger's Dovekie appears to have NO rocker.

    Note that most boats designed for or considered for even fairly minimal cruising have gross displacement measured in thousands of pounds, compared to a range of 300 to 700 lbs. for my current design. This makes a BIG difference in potential rowing speeds and size of sail rig needed, not to mention man-handling, etc.
     

  15. peterAustralia
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    peterAustralia Senior Member

    Very difficult set of requirements. Ocean crossable, beach able, very low weight.

    The last doodle I did resembles your marsh duck. A little longer, a few inches wider. Cabin moved forward

    As to an ocean crossable boat. That imho is a heavier, deeper, stronger and probably wider boat. Your hull needs to withstand 50ft seas.

    Very difficult to combine the two into one
     
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