Sailing Dinghy Design

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

  1. shu
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    shu Junior Member

    A while back a couple of interesting I14 hull designs were floated on this thread. Are you guys happy with those hulls and ready to look at blades or rig, or is more work required?
     
  2. Tim B
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    Tim B Senior Member

    In a way, I'd like to dosome more design work on the hull, but I feel it's time to get the foils done, to keep the project moving.

    Cheers,

    Tim B.
     
  3. Guest

    Guest Guest

    planning hulls

    sorry for the side tracking of the subject,
    but i remeber you were talking about how hard chines help the the water flow some how when planing?
    because i have been resently thinking designing a flatbottomed cherub dingy (Austrailan) like the "frizz" skiff moth, on the skiff moth site. but wider, i was thinking of having very little rocker like only 30mm for flat out planing speed. But i was just wondering would the straight sides on the bow coming to a right anlge at the bottom make the boat extremely uneffeciant when not planning and moving weight forward to lower the wetted surface, or when wave peacing.
    also does anyone no of any calculators or programes which can tell the lift given from a certian area at a certian speed?
    thanks heaps for your help.

    yours sincerly
    Bolts
     
  4. mad engineer
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    mad engineer Junior Member

    Silly Idea of the Week

    That mid-length rule on the 14 is a pain in the backside but as far as I can see it there is still a loophole if anyone is daft enough to try and exploit it...it would probably only serve to get the loophole closed but would be an excellent test of teh Moth approach versus planing skiff approach at another scale.

    The rule gives a rise of floor measurement at midlength, and the flat pan test at this point to limit measurement bumps. It does not give any restriction on the shape of the hull above the measurement point or in my opinion the shape of the hull beyond 6" either side of the measurement point.

    So we could design and build an ultra narrow 14, but with wings, of 12" chord to extend the hull skin out to the measurement point as in the sketch below. I think it could be done so the wings are above the static fully loaded waterline position, but the dynamic waterline and the effects of waves would be a bit unpredictable.

    ANyway - shoot it down in flames folks....
     

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  5. Guest

    Guest Guest

    i don't know about in I14's but in cherubs (same midlength problem) it must be fair and continues so there can be no hollows or winglests like in your design.
     
  6. mad engineer
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    mad engineer Junior Member

    The Australian Cherub rules require a chine - the I14 rules don't.

    The rules at this section are...

    Rule 3 Shape and Depth of section 2134mm aft of bow

    At this section:

    (a) The outside of the skin shall not be higher than 200mm above the outside of the keel at a beam of 1100mm.

    (b) The top edge of the hull or gunwale assembly shall not be less that 508mm above the lowest part of the hull.

    For the purposes of this Rule, any hollow in the keel or keel band shall be bridged by a straight line from which the measurements shall be taken.

    At these measurement points the hull shall pass the following test:

    A pan-shaped instrument consisting of a flat disc of 305mm diameter with a vertical lip of 13mm depth internally shall, when placed on the hull, touch at two or more points on the lip and nowhere on the disc. This restriction shall not apply to boats first registered before 1st January 1991.

    Rule 4 Sheer

    The sheerline, defined as being the top outside edge of the hull including the deck and gunwale in profile from the side, shall be straight or a fair continuous curve.

    Rule 5 Flare and Tumblehome

    A taut tape on the outside of the hull containing the vertical transverse section of the hull below the rise of floor measurement point defined in Rule 3(b) shall nowhere exceed 19mm from the outside surface of the hull.


    I have no doubt the establishment would object and see it as flagrant disregard of the spirit of the rules - especially if it turned out to be faster!
     
  7. shu
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    shu Junior Member

    I'm not so sure the I14 "establishment" would throw it out. They allowed horizontal wings on the aft mounted rudder, which is really just a cheater way to get additional waterline length. There is also a rule which requires the sheer to be fair and without hollows, minimizing the ability to incorporate wings to facilitate hiking/trapping. Yet hiking racks have been sticking out of the sides of I14s for several years now. I'm going to look at the rule again, this may just meet the requirements.
     
  8. shu
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    shu Junior Member

    Correction on the above. The rule states the gunwale shape must consist of one continuous fair curve. So fair hollows would be permitted, but it still limits wing-style extensions.

    My review of the rule indicates the wings would meet the hull rules, but violate the hydrofoil rule, which limits the horizontal foils to one, and limits its area. The single foil may be divided by its support. I don't think the hull would meet the classification of a support.
     
  9. mad engineer
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    mad engineer Junior Member

    I looked at the hydrofoil rule -

    "Only one hydrofoil in contact with the water and configured such that it can develop a vertical dynamic lift component while sailing with no heel may be fitted. This hydrofoil shall comprise only one lifting element, which may be divided by supporting (non-lifting) structure. The sum of the areas of the lifting elements of all underwater appendages not lying in a vertical plane shall not exceed 0.14 m2. The hydrofoil shall be symmetrical about the longitudinal centreline of the hull, or foil to which it is attached, at all times while sailing. If such a hydrofoil is used in one race of an event, it must be used in all races of the event."

    The area rule applies to "underwater appendages"...As I see it if the winglets are clear of the fully loaded static water line then they cannot be classed as being "in contact with the water" nor as "underwater appendages"... but then again I am no lawyer or expert on the application of the measurement rules.
     
  10. Neat idea, Mad, but I think it's outlawed.

    You wrote "I am no lawyer or expert on the application of the measurement rules." I'm a lawyer (although sometimes my boss isn't so sure, and I'm sure I'm not a GOOD lawyer) but I'm not expert on measurement rules. Evading the letter of the law is not often enouogh - if unclear they will be normally read according to the "mischief" rule ie if it's obvious they were put in to stop "mischief" they will be interpreted in that way.

    However, surely the rule "only one hydrofoil (of) one lifting element, which may be divided by supporting (non-lifting) structure" knocks out the idea; you have two, separate and supported by the hull. The hull is a lifting, supporting structure through both dynamic and buoyant lift. Furthermore, if the foils are that close to the water won't they be continually breaking the surface, causing drag and then "crashing" in a way that will make the boat very hard to sail and perhaps slow? Isn't this why Moths put the foils on the centreboard? Air entrapment on foils is a disaster AFAIK and it happens when they are close to the surface.

    Without a long look at the rules I think a hull-mounted foil may also fall foul of the "pan rule";

    "A pan-shaped instrument consisting of a flat disc of 305mm diameter with a vertical lip of 13mm depth internally shall, when placed on the hull, touch at two or more points on the lip and nowhere on the disc. This restriction shall not apply to boats first registered before 1st January 1991."

    The Hutchinson 1 (I think) 14 had measurement "bumps" that looked like 1/4 of a dinner plate stuck on each chine at the measurement point. They would be outlawed under this rule but the boat was "grandfathered" in. Yours wouldn't be.

    Re prismatic; I know what you mean, but I have a gut feeling that in skinny boats the entry angles are often narrow enough already. Imagine, say, a Moth drawn out to 30' by extending the bow to a needle-shape point. Sure, the entry will be fine, but the wetted surface will be incredibly high. It's a balance, isn't it, between wsa and entry angle. The very fine bow tends to drive so low through waves that the WSA bounces up 'cause the whole topsides are underwater. Maybe with narrow boats, with their already-fine bows, we've gone too far?? Just a thought - but look at an IACC bow, or even a Winder Fireball....
     
  11. Tim B
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    Tim B Senior Member

    I feel we are flailing somewhat, and stand a chance of making ourselves look less clever than we actually are. Could everybody read pages 1 to 7 on this thread. Whilst it takes us back to June/July this year, I think you'll find, that many of the discussions we have now, are much related to those in the previous pages of the thread.

    Could I also suggest, that since we all have a cad package, access to a cad package, or a pencil, paper, and calculator, that some of the suggestions raised are actually drawn. See if they look sensible.

    In addition, there is no point, especially on a single thread, wasting a lot of space with blatently impractical discussion (I'll pay for that, I can tell). Ideas can be shared between classes, of a similar size, but it is pointless to do a straight scale between different sized boats, because one ends up with an excessively high stability/speed ratio.

    Cheers, and I am working on the I14 (v4)...

    Tim B.
     
  12. Phil S
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    Phil S Junior Member

    scaling

    On Tims point about scaling:

    A week ago I say Bruce's scaled up moth racing. It is 16ft with moth wings, trapeze and 12 sq m of single sail.
    It was fast at times, but as Tim suggested stability is the problem, and the number of capsizes were more significant.

    It certainly was not as fast as moat of us expected.
    It was beaten convincingly by Bucko's 14ft singlehanded spinnaker boat and other MG14s. When our courses coinsided Bruce did not look any faster than the mid fleet moths. We suggested he was where Bucko was two years ago, still learning to master the beast.

    But as Tim suggests scaling is not apparently effective in this case. I would prefer to see an experiment where only one parameter is changed at a time and I am still seriously condsidering a longer hull to bolt under all my moth gear.

    As for scaling concepts up and down. Narrow hulls with fine entries are now popular all the way from Radio Marbleheads at 1.25m up to maxi yachts like Shockwave at 27m.

    Where they are not used are where the rules ban them or where existing rig or crew practices make them impractical.
     
  13. shu
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    shu Junior Member

    Bravo, Tim B.
    I am looking forward to seeing your latest revision.

    If someone can point me in the right direction (are there instructions on this site that explain how to attach/insert graphic files?), I'll post a few interesting (to me anyway) ideas here as well.
     
  14. Tim B
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    Tim B Senior Member

    Not strictly a revision of the I14, but interesting none the less in my opinion. There is, on the web, a free yacht design package called SDN. Translated from spanish, it is a good,useable package, there are a few quirks, but nothing which would cause a significant problem. I have spent about 2hrs learning it now, and I seem to be getting the hang of it. It may be worth looking at, as it does hydrostats as well. However, it neither draws as well as Rhino, nor does hydrostats as well as hullform. It is, though quite a useable piece of software... the hull below is after 2 hrs work...

    Cheers,

    Tim B.
     

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  15. b14maniac
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    b14maniac Junior Member

    Hey tim,

    Looks good. I've been looking for an alternative to Hullform (without having to spend many hours and much $$$$ to learn Rhino or having to spend much $$$$ to get a copy)... do you have a link for it there so I can give it a whirl?

    b14maniac
     
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