Homebuilt sailing dinghy for children

Discussion in 'Sailboats' started by Ara, Sep 19, 2016.

  1. Ara
    Joined: Jan 2016
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    Ara Junior Member

    Cockpit floor

    When drawing the lines of the cockpit (see post # 3) I intended to use a single plate of 3 mm plywood as floor. The centerline of the cockpit floor between main bulkhead and transom was drawn as a straight line, sloping slightly towards the back to get the water out of the cockpit. While building I changed this plan, because
    a) I doubted that 3 mm plywood would support my weight, and
    b) I doubted that I could apply two types of glue and place and fix the floor to all the bulkheads and styrofoam blocks before the glue was curing.

    The plan was therefore changed to four pieces of 4 mm plywood with scarf joints. The underside was coated with glass (80 g/m^2) while the plywood was bent to shape. Starting from astern the pre-bent pieces were glued in. For the connections between glass, epoxy and wood a mix of epoxy with short carbon fibers was used. The connection between the floor and the buoyancy styrofoam was done with polyurethane foam. Since this foam is curing very quickly, the whole job had to be done as fast as possible. To avoid that the expanding polyurethane lifted up the floor, I used all available clamps, weights, and adhesive tape.

    With kind regards

    Ara
     

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  2. Ara
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    Ara Junior Member

    Foredeck

    The cockpit floor was covered with carbon cloth (linen, 160 g/m^2). Before the front compartment was closed, attachment points for shrouds and forestay made from 4 mm stainless steel were installed.

    The foredeck was glued in place, again with epoxy and polyurethane.

    With kind regards

    Ara
     

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  3. Ara
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    Ara Junior Member

    Rudder

    The rudder has a NAC0012 section, 195 mm chord at the water surface, 185 mm chord at the bottom end, and draft is about 520 mm. The core was cut from yellow styrofoam (30 kg/m^3). After covering the first side with carbon (2 * 200 g/m^2, biaxial, 1 layer +- 90°, 1 layer +- 45°), two holes were drilled into the foam core for the lower pintle. One to take up the pintle, and one at its end, perpendicular to the pintle, to transmit the forces to the carbon shell. Rudder, tiller, and lower pintle were aligned on the gudgeons, and the pintle was glued into place with an epoxy/short carbon fiber-mix.
    Then the second side was laminated with carbon, and the tiller with the upper pintle was glued on.
    Including the bamboo tiller, but still without tiller extension, the rudder came out at 580 g.

    With kind regards

    Ara
     

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  4. JSL
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    JSL Senior Member

    I don't know how old your kids are or their experience but it might be a good idea for an experienced adult to test and trial the boat first.
     
  5. Skyak
    Joined: Jul 2012
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    Skyak Senior Member

    If I was given this boat as a kid the first sail would be a stream of giggles as I zipped off over the horizon.

    A very interesting build -high end material and attention to weight that is not common to kids boats. I wonder what the weight is compared to more elaborate stringer use and conventional glass over ply.
     
  6. Ara
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    Ara Junior Member

    @ JSL: "I don't know how old your kids are or their experience but it might be a good idea for an experienced adult to test and trial the boat first."

    Thank you for mentioning it. The older son has four and a half years, so there is no hurry to let him try sailing alone. And of course I am keen on testing the boat myself...

    @ Skyak: "I wonder what the weight is compared to more elaborate stringer use and conventional glass over ply."
    The hull has 20 kg, which is not that far away from the 45 lbs I found here (http://www.storerboatplans.com/Pdr/PDRscowbalance.html) for a Northbridge Junior built from 4 mm plywood.

    The good thing about what you call "high end materials" is that they are affordable when the boat is so small. I spent less on the complete materials for this boat than I could spend on a set of foils for a more popular little pram.

    The large unknown in the weight/strength/load estimations is the behavior of kids: They have much less weight and force than I do, but nobody can foresee what stunts they will try once they get bored of sailing in straight lines. The OpenBic's here https://www.youtube.com/watch?v=uR4GB2Bia2M suffer from different loads than a Moth sailed by an experienced adult.

    With kind regards

    Ara
     
  7. Ara
    Joined: Jan 2016
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    Ara Junior Member

    Rigging and first trials

    For the beginning I built a simple rig from materials I had at hand. The whole boat building story started about a year ago, when I was sailing with my son and a nice bamboo stick came floating down the river. We picked it up, and it looked just right for a mast.

    My brother, a passionate kiteboarder, donated an old but rarely used 17 m^2 Cabrinha tube kite. The sleeve for the front tube became the mast sleeve, the other tubes were removed, the size was cut to about 3.2 m^2, and my father helped with sewing of a batten pocket. The lines of the kite, made from some low-stretch fibers, became shrouds and forestay.

    Up to now the whole rig is very preliminary, but we wanted to bring her to the water before the end of the season.

    With kind regards

    Ara
     

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    Last edited: Sep 28, 2016
  8. Ara
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    Ara Junior Member

    Choosing a new mast

    For Sunday afternoon windfinder.com had announced 12 knots with gusts of up to 19 knots, i.e. perfect weather for some training on the new dinghy.
    Initially, everything went really well: The dinghy is easy to handle, stable enough even when loaded with two people of 16 kg and 70 kg, the cockpit is self-draining, and it looked like there is some potential for good boat-speed...

    It also looked like there was quite a lot of twist in the sail, the boom was bending, and then the rig came down... Probably not an unexpected happening for readers of this forum. In post # 5 of the thread "Bamboo as mast and spars" PAR wrote: "Bamboo would seem a terrible choice for spars on all but the smallest boats. Even on these you're paying huge weight penalty for the convenience of using bamboo."

    The sailing season is anyway near its end, so I have the time to buy or build a new mast. The overall dimensions of the rig seemed to fit to the dinghy, so I would stay with the same mast length of 3.12 m. The bamboo had a diameter of 40 mm at the bottom and 17 mm at the top, and weight was 700 g. Shrouds and forestay were attached 250 cm above deck, and the batten ended in a "camber inducer" tied to the mast at a height of 220 cm. The mast broke at a height of 190 cm, where the bamboo had a diameter of 29 mm.

    Options are:
    - buy a used standard windsurfing mast (certainly strong enough, probably stiffer than necessary)
    - buy a used children windsurfing mast (harder to find than a standard one, lighter, still strong enough)
    - build a hollow wooden mast, maybe in combination with carbon/epoxy (fun, but necessary dimensions, strength, flexibility are hard to predict)
    - find a thicker bamboo (see PAR's quote above)

    And a smaller sail for really windy days is also an the to-do list.

    With kind regards

    Ara
     

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  9. Petros
    Joined: Oct 2007
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    Petros Senior Member

    looks like a fun little boat, nice work.

    structural properties of bamboo are not very good, I would not even consider using it at all.

    a solid wood mast of clear pine should be inexpensive to make, though a bit heavy, but not that big a deal on a 3 m mast. A hollow one would save weight, but a lot more work to make. it probably is not necessary to wrap a wood mast with carbon/epoxy, a carefully made laminated hollow wood mast should be strong enough.

    Or just find any used mast off of any small sail boat, or sailboard, and cut it down if too large.

    Good luck.
     
  10. Skyak
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    Skyak Senior Member

    In my mind the higher up on a sailboat you go the greater the strength, and stiffness, and the lighter the weight. So on a hull wrapped in carbon I reject lesser materials on the mast just out of principle. A wisely constructed wood mast could work, but a nice used windsurf mast would be bullet proof and support the top notch aesthetic you have going.

    "Too stiff" implies you have designed and tuned the sail to the mast -you haven't, so don't concern yourself. The boat is super light and responsive -it shouldn't have to respond to gusts for you -just communicate them to you.
     
  11. CT249
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    CT249 Senior Member

    In performance craft you don't normally increase stiffness at the top of the rig. It's often decreased to improve gust response, which is actually often very important. The much-hyped Bethwaite flex-tip rig is an example.

    For a kid's boat, gust response could be very important because kids have enough going on just learning to sail, without having to work the boat from gust to gust like you do in something like a Laser.
     
  12. gggGuest
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    gggGuest ...

    Completely in agreement with Chris: not humanly possible to react to gust onslaught fast enough, especially in a small boat with presumably very moderate roll inertia. The rig needs to give the sailor time to respond, and then they can learn to respond correctly.
     
  13. Skyak
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    Skyak Senior Member

    Nobody suggests increasing stiffness at the top -just that material properties increase value.

    So, how does a home builder design a good gust response rig? Where do we find equations for mast stiffness (and position of that stiffness)? He will need equations relating the luff curve and batten stiffness and of course the sail cut. This strikes me as a extra difficult since this boat needs to accommodate a skipper ~50 lbs? and sometimes with his Dad ~175+50lbs? then be optimal as the kid grows each year. Another difficulty is that a good gust response sail shape is crap when reefed.

    Gust response is a fine thing, but how much development would it take? I was under the impression that manufacturers do half a dozen masts and sails even after their modeling to get to what we see. I (as a guy just reading about it) would love to see it, but my best advice would be to make a nice versatile rig that can be reefed to the wide range of winds and weights this boat is designed for.
     
  14. CT249
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    CT249 Senior Member

    Writing "the higher up on a sailboat you go the greater the....stiffness" does sound a lot like suggesting increasing stiffness at the top.

    The only time I created a class for kids, we found that gust response was vital. Until the three of us who were driving it moved over for personal reasons, we were getting good little fleets, and from being out there teaching it was interesting to see how gust response can make the difference between a kid getting towed home and a kid refusing to come in from sailing because they are having too much fun. Reefing doesn't work if the wind is gusting dramatically.

    An '70s-early '80s style windsurfer mast (if available, which is far from impossible since hundreds of thousands were built) is one possibility that could give better gust response. They were bendier than newer masts and would have better gust response.

    A simple small-diameter alloy pipe could well be good enough, though; that's all that the world's most popular class for teens uses. The kid's windsurfer rigs we used to have were also just simple alloy pipes. The amount and position of bend can be adjusted by using lower shrouds to restrain the mid-mast, if necessary.

    In days of yore, Moth masts were tuned by cutting sideways slots in the alloy luff slot, to increase the bend where necessary. Straight alloy pipes are pretty cheap; you may possibly even be able to cut a sideways slot in the back of one, although I'm fairly sure it won't be necessary.

    It doesn't have to be high-tech, just flexible.
     

  15. Ara
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    Ara Junior Member

    Thank you all for the inputs regarding a new mast. You seem to confirm what I also suspected after reading "Higher performance sailing" by F. Bethwaite (Adlard Coles Nautical, London, 2008). On page 284 he describes the benefits of a "whippy bamboo stick that yielded like a fishing rod" on a Moth in strong gusts. This was one of the reasons why I started with the bamboo, although I agree with you, Skyak, that a hull made from modern materials deserves a mast made from light-weight materials.

    Good gust response would also be of advantage because the river banks are covered with trees. Even when the wind direction is more or less parallel to the river, it gets quite irregular down on the water, and in the middle of the river the wind is usually much stronger than on the sides.
    I would already be glad to have a rig working well for a 20 - 25 kg kid, I do not expect that it will be possible to have best performance when the boat is overloaded with 90 kg.

    So my questions are exactly the ones raised by you, Skyak: "So, how does a home builder design a good gust response rig? Where do we find equations for mast stiffness (and position of that stiffness)? He will need equations relating the luff curve and batten stiffness and of course the sail cut." The book cited above does not really give the equations or a recipe how to get the automatic rig with the first try. It rather describes an iterative process involving different masts, sails, trims and a lot of testing on the water by highly skilled sailors. I cannot hope to get the same feedback about gust response of the latest version of a rig from a kid than from a multiple 18 footer champion...

    Your proposal, CT249, to start from an available mast and then to cut slots or to reinforce until it flexes as desired, is probably easier than to do the whole mechanical and aerodynamic equations for the entire rig.

    With kind regards

    Ara
     
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