Dreaming up a light air dinghy

Discussion in 'Sailboats' started by FL16, May 8, 2008.

  1. FL16
    Joined: May 2008
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    FL16 Junior Member

    I would like to share my design ideas with you. As I have no experience neither in dinghy design, nor in dinghy construction every comment as basic as it may be is welcome.

    I am currently sailing a RS800 (http://www.rssailing.com/fleetsnew.asp?fleet=RS800&selection=Details and Spec). It is designed for big sailing waters, waves are no problem but sailing on small inland venue with light air but loads of shifts and gusts is “challenging”.

    So the focus of this design should lean more to the latter conditions.

    The steps could be like this:
    1: Dream up a shape
    2: Verify it by some computations (at least buoyancy…)
    3: Build a kind of prototype
    4: Test if it survives…

    Probably the design will not make it further than step 2 but it is fun and interesting anyway.

    Thoughts that went into the design so far:

    To keep cost for the prototype low I would like to reuse spars, sails and foils from the RS800. Carbon mast is good, foils could be a little bit on the small side. Regarding sails I would prefer one full length batten maximum instead of the RS800 main sail design so I do not need to force them to pop in light air.

    As I do not have the possibility to place a hull somewhere inside for weeks of laminating, sanding and painting and I really, really hate work like that (too lazy???) I started wondering if it would be possible to build a dinghy from coated plywood. At the home depots here you get these sheets covered by a brown plastic layer, some are smooth no both sides, some are “non slip” on one side. They are used for example for car trailers.

    A “hard” restriction is the fact that it shall fit into our trailer so it needs to be smaller than 2 X 5 m, some “safety margin” is welcome.

    Intended crew weight is 120 – 160 kg, many classes claim they can carry 160 kg but it should do it happily not like a 29er that makes you feel like you are sinking in light air.

    The buoyancy distribution should allow to keep the stern out of the water just by sitting face to face (helm windward side, crew leeward) behind the shrouds, crew fighting the jib on the narrow foredeck while helm needs to press his body against the boom to keep crew weight at centreline is not desired!

    Other ideas to increase crew comfort would be off boom sheeting and cleats for Cunningham/kicker right at the mast to keep the cockpit floor uncluttered, maybe with Musto style T-shaped control lines so they can still be accessed while hiking. A rigg designed from scratch would sure have a gnav, the RS800 still features a vang.

    The lines of a hull should be in a way that sailing slightly heeled to leeward to keep boom on the leeward side by means of gravity does not ruin the flow completely, e.g. by immersing a wide and flat stern. Furthermore the hull shape should provide some reserve stability at this point unlike some “box shape” design that go over right away as soon as they are heeled slightly. A light weight carbon mast will definitely help here!

    However if breeze get’s up it is o.k. if the design is slower than comparable dinghies but it should still be somewhat “controllable”, e.g. no “nosediving” whatever this means for the hull shape.

    To keep it simple I excluded trapezes, racks, lifting foils and kites. A pole for the jib is an option but needs to be fixed to the hull as I do not want to put “new” forces on the RS800 mast. For the same reason I intend to keep the distances of mast, forestay, shrouds and lowers the same.

    “Influential” designs are RS800, 505 (both sailed myself) and Merlin Rocket, NS14, RS400 (some pictures/data from the web).

    Can you propose other designs that are worth a look? I have to admit that I do not like scow designs very much, though.

    Starting point is a hull of 4,8 m length, 1,8 m width and 50 cm height. This is lengthwise somewhere in the middle of RS400 and 505, both known as good weight carriers. A shorter hull would save weight but I guess it would make it more difficult to get a nice flow at the stern at low speeds without crew on the fore deck.

    A sailing ready RS800 weighs in at 110 kg. Adding 160 kg crew weight gives 270 kg. The weight for the aluminium racks can be saved but I will for sure not reach the low base weight of a foam core GRP hull so I need to add some margin. What about 300 kg buoyancy if immersed to static water line?

    For safety reasons and being used to it I prefer a self draining design, i.e. false floor. Combined with absence of trapeze wires this poses the question of minimum (vertical) distance between false floor and deck. Do you think 25 cm are o.k. for somewhat “comfortable sitting” on deck? How wide should the deck be if it is a straight horizontal surface and not specifically shaped like a 505 tank? I started of with 30 cm.

    Next question is if the room under the side decks should be closed by a wall at the inside to act as a buoyancy compartment or stay open. While I like the centreboard being close to the water surface when capsized the extra buoyancy should give increased stability when the boat is heeled. Furthermore righting could be difficult it a lot of water rushes from one room under the sidedeck across the cockpit floor to the other side making the boat “wanting” to capsize to the other side right away? What do you think about this?

    So how does the design look like now?

    The hull basically results of three panels, a flat bottom and the side walls. A Flat bottom would make building way easier, I do not know at all if it is possible to come up with a decent hull shape like this.

    Fore deck and side decks as well as front half of the bottom panel are strictly horizontal. The second part of the floor panel rises from 50 cm below deck to 30 cm below deck. The bow is vertical so both start at the same point of length.

    The false floor is 25 cm below deck so the height of the stern is 5 cm. This allows to mount a bung on each side to drain the hull.

    The idea is to have a inner “skeleton” of which the member that runs all the way on the centreline creates a independent buoyancy compartment on each side.
    The “skeleton” further consists of bulk heads under the mast foot, the shroud fittings and wherever else needed, especially so the crew does not break through if they step somewhere on the cockpit floor or deck 

    Front floor panel and fore deck would both feature a more or less triangular shape to ensure a fine entry. The back part of the boat is a more roundish shape. Maximum width of the floor is 80 cm while maximum width of the deck is 180 cm again which should give a nice flare of the side walls and a maximum waterline width of 130 cm.

    Mast is deck stepped at 200 cm from bow, the false floor starts right behind. At the stern the hull narrows down to 60 cm at bottom panel and 120 cm at deck height would give the same flare angle as at maximum width. Perhaps too narrow in regard to righting moment down wind when the crew moves aft, though.

    Using similar shroud positions as the RS800 forces a minimum beam of 150 cm at half length.

    Bottom and side panels could extend 1 or 2 cm aft of stern to ensure the water flow splits of the hull nicely.

    The initial design in the attachments got the widest section pretty far aft (about 70 %). Due to keeping the angle of the side panels constant at 45 °C this moves the rise in the bottom panel pretty much to the stern as well.

    Mast should be further forward to allow crew move forward but that would mean a smaller jib.

    So please tell me what you think? How weird is it?

    Attached Files:

  2. FL16
    Joined: May 2008
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    FL16 Junior Member

    Additional pictures

    All drawings created in PowerPoint...

    Attached Files:

  3. FL16
    Joined: May 2008
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    FL16 Junior Member

    The material I am thinking of seems to be called "phenolic resin coated plywood" ("Siebdruckplatte")
  4. Ike
    Joined: Apr 2006
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    Ike Senior Member

    why don't you just use exterior grade plywood? It has waterproof glue, and cost a lot less than marine ply, and you can get it at Home Depot. The unfinished side can be on the inside of the hull so no one will see it.
  5. PI Design
    Joined: Oct 2006
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    PI Design Senior Member

    Take a look at the Flight 4.3, available in Germany through landenberger sailing. Its based on a NS14 but with bigger sails for the light winds you get there. The video here shows them going pretty fast in light airs and the sails look cool. The jib is set on a boom and the mast is an over-rotating wing mast. Apparently it keeps pace with a 505 around a course, depsite the fact it has no kite.
  6. bistros

    bistros Previous Member

    It seems you are a reluctant Ferrari driver wanting a comfortable homebuilt Infiniti G35 Sport Coupe. The RS800 is a full-on production twin wire skiff design that is far easier to sail full bore than it is to sail at 50% of it's potential. Your design brief seems conflicted - light weight high performance bits on an easy-to-build, comfortable light wind optimized design.

    Many designs done more than 30 years ago are a better fit for your requirements than the RS800.

    There is a new stitch and tape plywood design being designed locally that fits your brief quite well - it will get it's first public outing in early June and I'll post pictures and info about it as soon as the designer and design sponsors okay release. 120 pound hull, asymmetrical option, designed for comfortable hiking and strong performance in a light wind venue. The sponsors and designer are targeting this as a kit built boat. CNC cut panels for accuracy.
  7. FL16
    Joined: May 2008
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    FL16 Junior Member

    @IKE: Though I am not really sure what exterior grade plywood looks like, I guess that the difference is that the other one provides a decent smooth surface without sanding, fillering, sanding, painting, sanding, painting ...

    The thicker even bring a non-slip surface on one side for cockpit floor with them.

    @PI Design
    With a lot of phantasie you may see same similarities between NS14/Flight 4.3. Someone took a Flight to England and puts all kinds of texts and pictures on the web to promote it.

    We intend keep the RS800 to sail it full bore at venues like Hayling Island, Lake Como, Travemünde or Carnac - that's what's life about anyway.

    Just looking for some alternative for the week-ends and afternoons sailable around here.
    Your design sounds interesting!

    I am aware that something like described by me will never reach the refinement of a RS 800, not even close. It is just fun and a challenge.

    Some quick excel indicates that center of buoyancie will to be too far forward, i.e. close to the mast. Let's see...
  8. gggGuest
    Joined: Feb 2005
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    gggGuest ...

    You really don't want the flat bottom, wetted area and all that...

    For ergonomics take a very good look at the 59er because Frank Bethwaite did a lot of work on that. You very easily do a lot worse than to be "inspired" by all those internal dimensions.

    Fr light airs performance in general take a very good look at the Thames Raters, especially for rigs - the hulls are turn of the century designs...

    Light boats are more easily homebuilt better in foam sandwich than plywood.

    For your light airs requirement I'd strongly consider a raked bow or the jib on a short bowspit in order to still have a reasonable sized jib and have the weight far enough ahead of the centre of buoyancy - look at the way National 12s have gone for very small jibs and masts near the bow which helps get the weight forward.

    Length is going to be your friend for speed...

    Rocker aft kills top speed but can help with transom drag and control.

    I might draw something like this, but will admit much of the underwater shape is stolen...


    Some thoughts on why it is... You want more upright topsides lower down so that you get clean release from the chines when upright and lower wetted area when heeled. The flare is of course for righting moment, but it needs to be high enough up so that you can roll tack. The U sections mean better moderate speed and lower wetted area. The shape this boat was evolved from, BTW, was built in plywood, its not such a nightmare to get that much curve in as you might think. Rocker distribution is pretty even, so that you can get weight in front of the rocker, but not hopefully taking too much of a hit on top speed.

    There ought to be plenty of room in the cockpit without one of those ghastly push kicker things that stuff up the sail and mess with the mast bend. What to do with downwind sails is a real quandary. Small asymmetrics you hardly need bother because its faster to drop the thing and square run with the jib goosewinged half the time, big ones mean all they will ever do is run, and you're stuffed square reaching. Pole kites, well, I really don't know that I could cope with using one regularly any more, tho' an ocasional trip out is nostalgic fun... You might actually be better with an N12 style dangling jibstick. On the rig I'd want gust response, gust response, gust response... Really quite a bendy rig. Having a NS style over rotating mast would certainly add a little extra for the crew to do if there's no kite. If I did have a pole kite it would be on a seriously long pole, 3 metres say.
  9. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    Forget exterior ply!
    Sure the glue is waterproof, but its only holding knotholes together - thats if they join at all.
    You cant get it thin enough for a lightweight dinghy anyway - because it is so rubbishy you would put your foot through it in 1/8 or 3/16", while marine ply is usefull at this thickness.
  10. gggGuest
    Joined: Feb 2005
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    gggGuest ...

    Its not all as bad as that... Its doing just fine on my IC decks. The prices marine ply is in the Uk you have to be seriously waving the cash around to buy it...

  11. FL16
    Joined: May 2008
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    FL16 Junior Member

    Gust response: I would have thought that gust response is great if you are already twin-wiring flat out. But would you allow the sail to flex while you are trying to catch the one and only gust on the current leg during a light air race?

    A raked bow would reduce water line length at a given overall length, a bowsprit that can be taken off adds additional complexity for the structures up front. Hm...

    On the other hand a RS800 jib is fully battened so it cannot be used in a overlapping kind of way. Looks like a smaller jib would be a good idea as it would at the same time bring sail area/righting moment to a more common ratio.

    Based on our specific settings for the equalization system of the RS800 this (hiking) design would only give about half the righting moment.

    I remember one light air race when I left the kite ashore as my crew had little sailing experience and I was afraid of dousing going totally wrong in a bunch of boats at the leeward mark. By pushing the main out, sometimes goose-winging, we could follow 29ers doing kind of 170° gybes easily and without the hassle. That's why I would rather try a jib stick if it can be fixed to the hull. If it get's boring a bagged symmetric could easily be added in regard to the hull but the mast is definitely not made for the forces introduced by a kite pole.

    The sections drawn by guest remind me pretty much of ICE int14 designs - not the way I would like to go. I guess the extra bouyancy of the flares high up will kick in rather late and then slow down the boat harshly. (At least the latter is what our racks do now...)

    Loosing some top speed due to more rocker aft is not an issue as top speed will be limited due to lack of righting moment anyway. Introducing rocker upfront will help loosing buoyancy there but will make the flat bottom even more of a problem?

    I expect a foam sandwhich meaning a lot, lot of work untill you have a decent surface finish on the hull outside.

    Anyway, as much as I believe that low weight helps planing and top speed, I am unsure if a extremely low weight helps in light air. Peak speed will be higher but for a full lap around the course with many gusts, shifts, manouvers etc. some "dampening" weight in the hull seems to help. 505's for example do not seem to suffer so much from their hull weight although you could probably build them to half weight by using all the expensive stuff.

    For a hull of this size 100 kg might still be o.k.

    Maybe at this point I should stress again that I am no professional and all this is guessing work just based on my limited sailing experiences!
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