Sailing Dinghy Design

Discussion in 'Sailboats' started by Tim B, Mar 12, 2003.

  1. Wardi
    Joined: Nov 2003
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    Wardi Senior Member

    If you are going to design a 14'skiff, why not go to minimum beam and surface area.
    For a Moth this is about 320mm waterline beam which is the basis of all new designs over the past 10 years and is much faster than the fat skiffs in all conditions.

    At present I would regard 14's as fat skiffs and would guess they should be around 430 mm waterline beam or so! Stability may then be an issue...but you want to go fast right?!!....perhaps the rudder foil can be used for horizontal stability as well as lift!
     
  2. Gavin
    Joined: Jan 2004
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    Gavin New Member

    Hi Mad,

    This is my first post on this forum though I've been reading for a while. I'm not really qualified to comment on design or statistical issues, but a little local knowledge may be helpful.

    I suspect that I14s are showing up as slow is the way Portsmouth numbers are calculated, not the speed of the boats through the water. Portsmouth numbers are calculated based on empirical data recorded during mixed fleet racing. This is reasonably accurate for most classes, but for 14s it produces skewed results. 14s sail against each other in a few tight knit fleets and most 14ers (myself included) who sail in mixed fleets just aren't competitive when we all get together. So Portsmouth numbers are taken from the times of average to slow 14 sailors. This may be the same for other classes, though I doubt the tendency to concentrated fleets is quite like it is with 14s. Also, with other classes the difference between average and fast is nothing like as great as it is with the 14.

    I can attest to the inaccuracy of the Portsmouth rating. In winds up to 12 knots I generally beat better sailors than myself on adjusted time(i.e. in winds when I'm less likely to go swimming).

    Hope this is helpful.

    Gavin
    I14 US 1071
     
  3. CT249
    Joined: May 2003
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    CT249 Senior Member

    I think you'll find thre I-14's "problem" is its lack of length. It's REALLY important; as a few smart guys have said.........

    "Length" wrote Olympic gold medalist and champion 18 footer designer/builder/skipper Peter Mander decades ago, is “not quite everything. Say ninety-nine percent”.

    Phil Morrison says length is viery important even in skiffs - "especially upwind and in waves - maybe not much when being blown down wind like a leaf behind a large asymmetric. Wavemaking still comes into it even for faster classes.”

    “The biggest limitation to the speed of a 14 upwind stems from the fact that it is a short boat with a sailing displacement to length ratio similar to that of performance keelboat” writes Paul Bieker. “Even with its great power to weight ratio, the 14 spends a lot of time at transitional planing speeds upwind where wave drag is a large proportion of total resistance.”

    The 14 is not just short, like most skiff's it's heavy 'cause it stuffs 2 guys into a small hull. The DLR is 89 or so.



    “The 49er is quicker upwind (than the I-14), because it’s two feet longer" - Dave Alexander (maker of top I-14 sails).
     
  4. spoedvraat
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    spoedvraat Junior Member

     
  5. gggGuest

    gggGuest Guest

    The trouble is that sort of number is of limited value without the other info, what immersion you are running at the transom etc. Prismatic varies loads as you change pitch angle as you will know if you've played with it. Numbers I've seen quotes for 14s and Cherubs by people like Bieker, Paterson, Stimson vary between 0.6 and 0.65, but over .63 is never going to be appropriate unless you have very special issues to consider. And on the other hand Julian Bethwaite considers that the prismatic coeff is not at all helpful when designing a dinghy, but then the Bethwaites have a huge body of numerical design data to work from, and no-one else seems to have that sort of level of homework.
     
  6. spoedvraat
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    spoedvraat Junior Member

    I can see why Cp is not all that counts. The factors I am also looking at and trying to get a handle on, is effect of rocker and longitudinal distribution of volume (curve of areas ???)
    The Cp falls out of the equation the moment you start planing, and is thus very important to guarantee that you have enough power to get planing, after that it is the center of pressure vs longitudinal weight distribution that becomes important.
    Please correct me if I am wrong.
     
  7. CT249
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    CT249 Senior Member

    I'm ignorant of this (and many other things). Why is the relationship between CoP and weight dist. important? Some fast boats (12' skiffs) plane bow-up. Some (49ers) plane almost level. SOme (Moths) seem almost bow-down. Doesn't the CoP move back and forth dramatically, and if so when should it be in what relationship to the weight? Why not adjust it with crew weight?
     
  8. gggGuest

    gggGuest Guest

    Well the prismatic is a way of numerically describing one aspect of the shape of a boat, and because its dimensionless it has some relevance in comparing between different boats, even different sizes. And unlike most measurements you take, like rocker for instance, it does relate to the whole surface of the boat. I think to say its irrelevant once planing is partially to miss the point. The volume distribution as an aspect of the shape of the boat is still relevant when planing, especially at lower planing speeds.
     
  9. spoedvraat
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    spoedvraat Junior Member

    Well, I'm glad that rocker is mentioned, because I'd like to try and understand that a little better as well. How is it defined, measured, and does anyone know what the typical values are for the little boats?
     
  10. gggGuest

    gggGuest Guest

    Rocker is normally defined as being the offset from a notional straight line between the transom and the stem knuckle.

    Values are enormously variable. I've come across reasonably sucessful 12 foot boats varying from about 8 inches to about 1 inch. The general trend since the Antipodean design revolution of the early 70s has been for low values, but its definitely possible to go too low. You usually find out you've got too little rocker when you fly through the air as your bowsprit goes spear fishing for flatfish on the sea floor...

    There's an obvious link between rocker and prismatic, in that all else being equal a lower rocker boat will have a higher Cp. Of course all is most definitely not equal...
     
  11. spoedvraat
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    spoedvraat Junior Member

    Thanks for the answers on rocker.

    My new boat has 4 inches of rocker, boat is 16.5 ft long. Who knows what it will do ??
     
  12. gggGuest

    gggGuest Guest

    ]No idea, I have zero experience of a boat that long. You might find some of the information at this page useful though.

    http://www.javelins.org/Technical/virtualJavelin/virtual_index.htm
     
  13. Ian
    Joined: Apr 2004
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    Ian Junior Member

    Cruising dinghy 'rightability'

    I am designing myself a cruising dinghy and have some questions on 'rightability'. I realize that this is a racing-dinghy thread, but it is obvious there is a lot of very knowledgeable dinghy people watching this space and I would be very appreciative of any advice. The boat will be sixteen foot long and somewhat similar to a Drascombe Lugger, though I hope it will be better looking and somewhat faster. My experience is with keel boats and have not capsized and righted many dinghies in my time. I notice that Drascomes have fairly slim low bouancy seating. Is this necessary to prevent the boat from being too stable upside down?
    I would like more water-tight storage than that and am considering making only one side low bouancy and having part of that seat free-flooding (which would be fine for wet storage). I will have integral fresh-water tanks amid-ships in the self-baling bottom which will double as ballast.
    Is there a optimum amount of bouancy that should be provided for in the side seats? Should the boat, when over on its side with the mast in the water, be floating with it's center line at the water level (so the mast is flat to the surface), or is it better for it to float higher than that? I can see the possibility that a boat that floats high when on its side might be easier to right from that position but harder to right when totaly inverted. Is this true?
    I could really use some advice on how best to go about making a dinghy easy to right by one person in bad conditions and very cold water. I would like to cruise up to Nova Scotia and possibly further.
     
  14. Ian
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    Ian Junior Member

    rocker

    Generaly, if you want to be able to load a boat varying amounts at different times and don't want the drag of a submerged transom when heavily loaded, you will have a transom that is well above the waterline at average load conditions. This will give you a lot of rocker and prevent the boat from planing. This fine in a rowing dinghy that won't be going that fast. If you want to a higher top speed, but don't want to drag the transom when rowing, design the the bottom of the transom right on the waterline in average load conditions. This will give you medium rocker. If you want to really go like hell, plane early and very fast, design the transom to be below the waterline at average loading. This will produce very little rocker. Depending on the boat, you can possibly lift that transom clear of the water in light airs by moving crew weight forward. Naturally, the weight of the boat and the beam affects the amount of rocker that is associated with having the transom located where you want it.
     

  15. spoedvraat
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    spoedvraat Junior Member

    Hi Ian,
    It seems we have similar goals. Although I'm a bit apprehensive of suggesting it, why don't we flash up a new thread to keep things clean. Something like cruising/camping dinghy design.

    I include my "specs" herewith, and will try some images of what I have done as well.

    Requirements for the Dayboat:

    Where:
    The boat will be sailed on inland and fairly rough coastal waters. The worst case scenario is probably the same as for an oceangoing boat.
    I will try to apply sensible safety factors when designing critical components.

    How:
    The boat will be singlehanded often, but will have to carry 4 to 5 people on day-sails.
    This means I must be able to launch, rig, de-rig and retrieve the boat alone.

    What:
    Various options exist for construction, I prefer strip-planking and glass/epoxy reinforced approach currently. I haven’t done this before, just stitch-and-glue. I have enough basic knowledge of composites to feel confident I can do the necessary work.

    When:
    The boat will most often be used for daysailing during the weekend, and might have to do a three day trip once a year.

    Broadly stated imaginations……:

    I want to have a boat which can be used to chase the local racing dinghies, which are mostly 505’s, FD’s, some local classes, and a smattering of 49’ers (which I won’t try to catch.) I thus require a light boat which is powerful enough to plane quite easily, with crew on trapeze if required.
    However, come the summer holiday I want to use the boat as a basic cruising dinghy, on which I can cover some distance in marginal conditions, which means high stability, adequate freeboard, predictable (even docile) performance, reef-able sailplan and enough storage for the “stuff.”
    The concept I want to attempt is probably not a new one, but currently it revolves around the idea of having two options of centerboard/keel, and two of rig/sailplan, thus:

    Sailplan/Rig: Keel/Centerboard:
    Racing mode: Powerful, large roach Unballasted foil
    Cruising mode: Smaller/docile Ballasted Keel

    In cruising mode there must be a headsail furler, lazyjacks and reefable main.
    The boom is higher up the mast, and the roach of the cruising main is much less. The mast can be raked back to balance the helm, or we go to decreasing the size of the furling headsail as well to balance.

    The “furniture” inside the boat to make it nice for cruising must be removable for racing, and this is probably one of the hardest things to do elegantly.

    Further ideas:

    I like the concept of an open boat which has a double bottom, so no self-bailers etc required. Water coming on has no option but to disembark, pronto. This is nice and safe.

    Double rudders are probably equal to singles all things considered (except cost…,) and leave a nice space for outboard engine mount in the center of the boat.

    A bimini is non-negotiable in the summer down here.
    An easy arrangement for a boom-tent would be very convenient when in cruising mode.
    Further we have thought of using rolled-up (lengthwise) inflatable mattresses as buoyancy under the seats, to double as beds when unstowed and unrolled for the night.
    A battery is required when cruising, as we need to drive the VHF, GPS, tillerpilot, running lights and fishfinder as a minimum.

    And as I start churning numbers, the following seems to emerge:

    Loa 5000
    Beam 2000
    Sail area 14 m² (white sail area) (main 9.5 m², jib 4.5 m²)
    Mass 120 kg (all up, excluding crew, racing mode)
    Mass 80 kg (hull only, racing mode)
    Construction: 6mm Cedar strip with 1 layer 300 g/m² plainweave either side. Resin:glass = 1:1

    Cp 0.6
     
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