Sailing Dinghy Design

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

  1. Guest

    Guest Guest

    great work everyone. as a 14 owner (B3 Hull and foils, CST mast, irwin sails) i have to say that you are tackling a huge hurdle (to outdesign bieker and join the fray in the "next generation" of 14 hull design) and you are to be commended for a great job...it's good to see. for bieker's latest generation in hull design (the B4) see: http://www.i14.org/ovi/

    used hulls from oz and the us will be available after the worlds Aug 23- 31 for a good price. worlds '05 is in NZ and the boats that are racing now will be plenty competetive then (carbon rules!) now lets get some of you in the class!

    anyway, for those who stated that they would like something outside the I14 (but not a moth, not an IC) here is another fourteen foot "skiff", box rules boat....except it is singlehanded. based on the uk cherub. jim c is more than helpfull in sharing ideas and his own perceived pros and cons of the current design. he would undoubtedly be excited to see some others rendition and interest in the boat.

    http://www.cherubpres.f9.co.uk/plusplus/

    keep up the good work, as i said, it's good to see. and remember, lead is for fishing, and life is too short to sail slow.

    cheers,
    kml
    ..._/)..._/)...
     
  2. Tim B
    Joined: Jan 2003
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    Tim B Senior Member

    Marketing

    Sorry I've not contributed for some time, but I've had a full time job and no useable CAD package. Still, I see things looking up. Would anybody object if the I14 were marketed on my homepage and others (by which I mean yours) and a small fee requested to cover administrative costs (and possibly a small comission split equally for the major parties involved in designing the boat)? It would certainly boost the profile, and since I presume we can't raise the money to build it ourselves easily (which I should have thought of but never expected the design to get this far), it may help to have a request from an external benefactor.

    Tell me what you think,

    Cheers,

    Tim B.
     
  3. b14maniac
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    b14maniac Junior Member

    Tim,

    Whose design are you meaning by "and I mean yours"? Where is your website?

    Cheers,
    b14maniac
     
  4. Tim B
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    Tim B Senior Member

    Other websites, b14. And I was thinking of marketing my design, I just thought that since you lot had helped in the conception and refinement, I should let you have a say.
    My site is http://uk.geocities.com/tbmarine2003/
    As far as I'm concerned, we can market it all we want, as long as we all sing off the same hymn sheet so to speak.

    Incidentally, the last I14 I posted is rendered as near as makes no difference to it's true waterline.

    Cheers,

    Tim B.
     
  5. b14maniac
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    b14maniac Junior Member

    Sounds like a good idea to me!
     
  6. Tim B
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    Tim B Senior Member

    Anybody else?
     
  7. Phil S

    Phil S Guest

    Dinghy Design

    Lets get back to the design subject rather than flogging someone's favourite.

    A few entries back there was talk about an ideal single hander.
    I am biased because I sail a moth, but at 53 years old and 80kg it is not as easy as it might be.

    What might be of interest to moth critics is that the real instability in a moth is not lateral but longitudinal. Most bad capsizes are not sideways but nosedives. It also does not carry weight well, so now most of the top performers are smallish people.

    My ideal boat would be a moth type about the size of the IC.

    About 5m long, 10 sq M rig, wings about 2.3 wide so you can transport it and weighing about 45kg all up. Preferably one sail for simplicity. And this paragraph is about all the rules you need.

    I think this boat would be balistic. After all at 11 ft the top moth sailors can beat everything up to 16 ft at least some of the time.

    Even an old fart mid fleet mothie like me can beat Bucko most of the time over the last three years, although he is getting an edge lately.

    Overpowered fat boats are not the way to go. Low resitance efficient design is the future.
     
  8. BrettM
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    BrettM Senior Member

    Hey! Somebody who thinks like me!
     
  9. mad engineer
    Joined: Jul 2003
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    mad engineer Junior Member

    You can't have too much lard...

    Interesting debate and true up to a point...

    Just for fun I have put two hull designs into Michlet and into a very basic Savitsky spreadsheet to have a look at what comes out for moth style versus planing skiff style boats.

    Both hulls are 4.27m long (14ft) and have a displacement of 140Kg all up (I nkow this is overkill for moth style but it isn't too far off for a planing skiff style boat).

    Up to hull speeds of about 15 - 20 knots it is true that a moth style hull is dramatically lower drag and faster. But from 15 kts plus, planing hulls are definitely the way to go...if you can develop the power to get there.

    The attached graphs should explain it - in the black graph Hull 1is the planing hull, hull 2 is the moth concept.

    This is in agreement with what Frank Bethwaite writes in his book about cats versus dinghies - OK I think of a moth as an 11ft single hulled cat - in moderate winds they are superb, but they cannot get tot he same top end speed as a plaing hull because they stick in the water.

    Now the question is how to get the best of both worlds...

    By the way anyone is free to build / advertise / sell / finish designing the I14 hulls I put up, I almost certainly won't in the next few years...

    Tim - what does your final design look like?

    Cheers,

    Jon
     

    Attached Files:

  10. re

    "This is in agreement with what Frank Bethwaite writes in his book about cats versus dinghies - OK I think of a moth as an 11ft single hulled cat - in moderate winds they are superb, but they cannot get to the same top end speed as a plaing hull because they stick in the water."

    The argument about whether Moths plane is still going on; some of the top designers reckon they don't, others reckon they do. But regardless of that, even if Moths don't plane they go really, really fast for an 11 footer. I've seen Moths blow away the 3 time world FD champion and the former national Sharpie champion (a 19 footer similar in speed to a 505 or FD) on a reach in 15 knots plus. If an 11 footer can do that, who cares if they plane?

    The Moth's problem is actually square running where it cannot sail as deep as 16' skiffs, Sharpies etc.That is more evidence that it's not the design's high-speed planing that is the limit.

    As 3 time world Moth champ builder/skipper/designer Mark Thorpe says, people have the impression that the Moth is a light-air flyer but in fact it's better in the heavy stuff - when it's alleged inability to plane should slow it down.

    Re Frank B; he has done much great work, but he is known to have a major tendency to ignore facts that don't fit with his arguments. He still thinks cats don't go , boards can only reach, and the 1975 S&S maxi ketch Kioloa is as fast as a modern maxi. He's wrong.

    And finally, regarding the Swift Solo; Kerrist it's heavy (way heavier than a timber International Canoe) and since the Canoe gets fully powered up quite easily with a smaller rig and the sailor on a dirty big plank, I wonder how the Swift would survive in a breeze. Guys like (world Open 14 champ) Bucko and the Viz and Musto designers have gone for much smaller rigs and they are not exactly idiots. There was a comment in Wooden Boat about how novel a jib was on a singlehanders; true, ICs have only had them for 70 years.....
     
  11. Andy
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    Andy Senior Member

    There may be some answers to many of the questions just posed...if I may hazard some topics for discussion!

    Re Moths and planing: without going into the science of it too much, I would suggest that moths do NOT plane, at least not without exceptional circumstances. 'Planing', as we know it, is the situation where the boat has enough speed coupled with a suitably shaped lifting surface (ie the shape of the bottom) that the resultant dynamic lifting forces take over from the buoyancy forces which stop the boat from sinking, and support the hull with no need for buoyancy forces at all. The transition period is where these forces are both supporting the boat, to a greater or lesser extent. the proportion of these 2 components and their changing drag characteristics determine whether its a smooth ride into planing or if there is a hump in the resistance charts that Bethwaite identified.

    Heavy, wide hulls create a high bow wave and a deep wave trough at the stern as displacement speed is approached, and the hull is thus working against gravity as it takes on an uphill struggle against its own energy sapping wave system.

    Light boats create a much shallower wave system, and therefore have less work to do against gravity to break free from their wave systems. the dynamic lift of skiffs and so on helps keep the wave system small as planing speeds are approached, and allow them to break through the barrier with varying degrees of ease. there are many other details i know, but lets keep things simple for now.

    The moth hull does not benefit from this dynamic lift, at least not until much higher speeds are attained. Its mechanism for achieving higher than displacement speeds is therefore somewhat different. If you watch a moth sailing, the narrow beam and light displacement mean that they seem to cut through the water with very little wave creation. Therefore they dont have the attitude problem with the bowwave and stern trough that other hulls have. Instead they just get faster and faster, and although dynamic lift does gradually appear, it does so slowly and is hardly needed to help overcome such a small barrier. Any thin and streamlined body should display similar attributes - if you drag a small foil through the water at 30 knots, it is not planing in the traditional sense, although it is clearly moving much faster than displacement speed.

    But skin friction resistance will always be present if the boat is not able to lift out of the water - part of the reason why the dry running foilers are sailing almost twice as fast as the boats stuck in the water. This may partly account for why many moth sailors find it difficult to sail much faster than about 16 knots, whereas boats which have been bodily lifted from the water (49er at al) carry on accelerating to silly speeds. The speed of the moth is therefore exceptional in aound the buoy races as it should have less problem with getting faster than boats which plane, although in higher winds and at certain angles they will find themselves blown away by things like cherubs and so on.

    The acid test for all this theorising would be a tow test of something like a hungry Tiger design. Does anyone have such data?

    As for the best of both worlds, Frank Bethwaite may have already found it with his 'free flowing' (dynamically humpless) hull, as this has the same effect as a possible complete lack of hump in a streamlined hull like the moth, even if the drag due to wetted surface and form drag is higher at displacement speeds.

    Incidentally, has anyone considered a tortured ply stitch and glue hull like the stealth design by Richard Woods ( http://www.sailingcatamarans.com/ )? This is how moths used to be built till quite recently (with a bit of fibre reinforcement) so a singlehanded 14ft, homebuilt boat with blistering performance could happen for comparatively little money. A simple open rule with length, chine beam, overall beam, hull weight, sail area and material restrictions could allow everyone to play at great speed for little dough...

    Only my tuppence!
    :D
     
  12. Tim B
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    Tim B Senior Member

    Phil S,
    I am in no way trying to flog my favourite design, and I appreciate the comment, I have become largely stagnant because the only CAD package I have is old, on an archaic machine. When I start to progress, you will hear about it, but I need a new CAD package first to be practicle.


    Mad Engineer,
    See the I14 published on the 6th July (somewhere in this thread, or in my gallery).

    Andy,
    I was sailing a LARK (2-man racer) against a moth and 2 Lasers at the weekend (I was sailing single handed). Therefore, all the boats were of a similar weight (to within a tolerance) the end results depended more on strategy than boat-speed (it was a light wind day). I suspect, that on a marginal day for the LARK, it would have lost out to the Moth, whilst gaining over the Lasers. Further to that, though, I suspect that on a full planing day (where the LARK would carry both helm and crew for righting moment), the LARK and Lasers would be close, with the Moth being somewhat left out. I maybe wrong.

    My reasoning is this, to all intents and purposes, the LARK and Laser are fairly similar. The light air performance I quote as a 2/4 and 3/5 both times beaten by at least one Laser. In marginal planing conditions, I suspect the slenderness ratio of the moth to come into play, and thus for it to do well. In full planing conditions, though, I know for a fact that the Lark is faster than the Laser (we've tried it), and by virtue of reduce drag, I would expect it to beat the Moth. Of course, hull shape is a determining factor, but I'm working on the moth I raced most recently, which was probably 1995 or there abouts.

    Cheers,

    Tim B.
     
  13. Stephen Ditmore
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    Stephen Ditmore Senior Member

    On the subject of plywood Moths: the "Mistral" design that is currently dominating the Classic Moth class in the U.S. is entirely developable from sheet plywood. Since its midsection is nearly V shaped there is no need for a hard chine, and nearly all are built entirely of 1/8" ply. See photos on page 4 of this thread. Plans are available from the class at www.mothboat.com for $20. Note also that the Moth is a development class. If its shape is optimised for light winds it's because they predominately race in light winds. A wide flat Moth would be completely legal.
     
  14. Boy, Tim, I think you'll be waiting for a long while for a Lark to beat a good Moth in planing conditions. The Lark has a PY of 1073, the Moth is 1000, that's a fair bit faster - and the UK Moth PY seems to think the boat is a bit slower than we do here.

    Put it this way - how does the Lark go against a world class (ie at the front of the world championship fleet, not an also ran) Flying Dutchman, Contender or 505? The top Moths will beat or push the 3 time world champ in FDs and world class 50s, the two time world Contender champ will not see the way the Moth went. I've seen it happen, in planing conditions, consistently and in open water. That's one helluva lot faster than a Lark, which is slower than a 420 in a breeze!


    re tow tests and planing.

    A few of us have talked about tow tests here in Sydney, but they are a considerable pain to organise. Anyone got any ideas about how to get reliable numbers? I've seen Frank B's rig but it would be beyond me to organise.

    As for planing, Andy, as you said it occurs when dynamic lift "stops the boat from sinking". one mutiple world Moth champ says "they do plane, but they plane low"; he's now sailing Formula Windsurfers so he's familar wit the ultimate flat planing shape.

    SO the question is, is there any reason that a boat supported by dynamic lift has to rise to the top of the water? For example, some narrow-tail sailboards plane quite low in the water; they were designed to do that in the days of less efficient fins. Yet these boards are definitely planing; they'll go downwind with an 18 or Tornado. I used to have a 12" wide 8'6" long speed board with very, very sharp rails. It would float about 20-30 kg, so basically anytime it moved it was planing - yet it often planed quite low in the water. So maybe there's a difference we must make, between planing lift that stops a boat from sinking but does not lift it, andf planing lift that physically lifts it above the displacement waterline.
    I know the latter will reduce wetted surface, but at what cost?

    Where did you get the 16 knot 'speed limit" for Moths? I wonder if it's not a result of the short length rather than hull design; I mean, length is important (vital) even in planing skiffs, and the Moth ain't much longer than a Cadet. If an 11 footer can blast past a 19' Sharpie or 20' long FD on a fast reach it ain't doing too bad.

    One of the big lessons in design in recvent years is that planing is not necessarily the answer.

    The older Aussie Moths were flat-bottomed 60 kg (rigged, apporx) scows. They were definitely total planing hulls, similar to a Contender in pace at the end of their era, normally comfortably faster than the "light but normal" 11' Europe. Then the UK fat skiffs came out; they were like skinny, light winged lasers and were similar in overall speed to a scow in our windy conditions, faster in Europe. They do plane - but they are much slower than the skinny modern boats in all but very light winds. So the Moth has moved away from the ultimate in planing hulls, and it's become faster. A scow has no chance in any conditions according to the expert Mothies I have asked. So where is the problem in not being a conventional "planing hull"?

    A few of us have talked about two tests here in Sydney, but they are a considerable pain to organise. Anyone got any ideas about how to get reliable numbers? I've seen Frank B's rig but it would be beyond me. Could we just tow one boat at a time at a set throttle setting and use a GPS for speed numbers???

    By the way, look at sailboards. The old round bottomed "displacement" Lechner olympic class is faster around a course than the flat bottomed planing Mistral up to 14 to 16 knots. That's the estimate of Larks Kleppich, Barcelona medallist in Lechners and 2000 world champ in Mistrals. The Lechner does plane, but it's not what most would think of as a fast shape - yet it beats the Mistral most of the time. So once again, the "ultimate planing hull" loses out.

    Frank's "humpless hull" is a matter of considerable doubt among designers I have spoken to. The old Aussie designs (seen in Ice and Wedge I 14s adn the Tasar to some extent) had fairly flat panels giving a lot of planing area, at the cost of more waterline beam (ie wavemaking drag) and wetted surface (as the vee gives more surface to volume than an ellipse). So they did have a fair bit of drag at high displacement speeds. The whole idea, tho, was to generate lift to plane before other boats could.

    The newer boats, though, are narrow, so they have lower wetted surface and wavemaking, and generally have a keel-line flat for better dynamic lift than the old vee shaped boats. Even the guys who design the (45 kg/70 m2 of sail ) 12 foot skiffs say they are "displacement boats", becvause they are designing for low wetted surface and low wavemaking drag. The result is such low drag at displacement speeds that there is no hump. They may plane later, but they go faster until they plane and when they plane. This is more the Moth route.

    The Bs go about the hump problem a different way, it seems. As Frank says, he has been following his own design theme for 40 yeasrs and he doesn't look much at other people's work. They stick to the vee shape abandoned by most other designers; because the 9 ers are so long and light they don't need the volume created by elliptical sections, they have big rigs so wetted surface is not so vital, and they are therefore quick so the superior high speed control of the vee shape works well. They don't have a hump because they are long and light. It's not a secret. BTW, Julian says he looks to Moths as a design leader, as do most of the Aussie designers.

    Interestingly, Frank uses the Canoe as an example of a boat with a hump, but I can't feel one on my Canoe. I also used to note that even the Flying 15 has very litte "hump" effect; it's 330 kg but 20' long so the DLR is lower than many dinghies. That seems to be the crux. Frank B's flat vee Tasar, tho (which I love) has a severe hump.

    Or that's the general viewpoint after interviews/emails with the Bs, Bieker, Nash (NS 14/skiff), Morrison, Patterson (Cherub/Moth), Walsh (12' skiffs) Thorpe, Clarke (ICs, Vanguards, C Class) , Drake (NACA/windsurfers) etc etc etc.

    And finally, Phil S does make tortured ply narrow Moths, and they are quick! Just a couple/few minutes off the pace ofthe HT is my info from a former wold champ; that will put you in the top 25% of the world's hotttest Moth fleet.

    Chris (not a designer, but merely someone who talks to them)
     

  15. Andy
    Joined: Aug 2003
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    Andy Senior Member

    Chris -

    you make some very good and interesting observations, many of which highlight for me the philosophy of design at its current state of the art. I fully agree with most of your points, but if i may be permitted to add some comments of my own...


    You're correct with your belief that hulls do not have to bodily lift from the water to be planing / supported fully by dynamic lift. But it is more efficient at high planing speeds to reduce the waterplane and wetted surface area, which is why I feel moths are more limited in their ultimate top end speed than, say, a 49er, which does lift more. I haven't sailed a moth (although I have one in the works...) but Yachts and Yachting magazine did a review of the class last year and one of the most prominent sailors in the class reckoned that 16 knots was about the limit for sustained top end speed, whilst 14 sailors seem to talk more in terms of 25knots plus in perfect conditions. Much faster than any of the conventional boats you mentioned anyway!


    To quote your message: "I used to have a 12" wide 8'6" long speed board with very, very sharp rails. It would float about 20-30 kg, so basically anytime it moved it was planing - yet it often planed quite low in the water". If you had a total board plus pilot weight of 80kg (a guess), and 30kg of that was supported by buoyancy forces, then you would need enough dynamic lift to support 50kg to achieve equilibrium. You would not be planing though, if we accept the definition of planing to be a hull supported fully by dynamic lift.


    "One of the big lessons in design in recent years is that planing is not necessarily the answer". The distinction here is not whether or not the boat planes, but how it does so. The unfortunate fact about lift is that it is always accompanied by drag. Modern dinghy designers are learning more and more to appreciate the subtleties of shape, to maximise lift whilst mimising drag, throughout the the speed range. This is the essence of Bethwaites 'free-flowing' hull shapes - the way the drag is managed throughout the speed range is very smooth as the lifting forces move from buoyant to dynamic lift. This is manifested in different ways - in the 49ers case its a lack of a speed bump.Older designs may or may not have that bump, but they can still be less efficient for other reasons such as the total drag over the range can be higher.


    Moths, on the other hand, seem to have a lower resistance over most of the speed range, but are dynamically not as suited to ultimate speed as boats designed for a different mode of operation (much more lift at the expense of proportionately more drag until waterplane area is reduced substantially). So overall, I'd have to suggest that moths are more efficient for most of the time, but the super fast planers are a good deal more efficient at very high speeds.


    The question of efficiency (lift versus drag) goes some way towards explaining why the Lechner may be better than the Mistral as a board shape in many conditions. The lift advantages of a very flat shape can be largely negated by the proportionately higher drag in most conditions than a more rounded, compromise shape with better lift to drag ratio. Most flat bottomed boats are 'one condition wonders' - a balanced lift to drag ratio over the speed range with low overall resistance will be better almost all the time. So i agree with your comments that designers are "designing for low wetted surface and low wavemaking drag. The result is such low drag at displacement speeds that there is no hump. They may plane later, but they go faster until they plane and when they plane. This is more the Moth route". But it is my opinion that the moth is still more efficient at these speeds but becomes less efficient in comparison with the other boats when they begin to lift out of the water.


    Of course the foiler moths are using a different mechanism to achieve similar results, and no-one can say they are not a good deal faster than conventional moths...!

    I would contend your comment about the 49er that "They don't have a hump because they are long and light". Displacement and its longitudinal distribution does of course determine the wave making resistance, but as I said earlier humps (disproportionate increases in resistance) can appear from the dynamic lift induced resistance problem experienced by boats of certain (very flat) shapes.

    Humps on conventional boats can be very subtle - one of my mates recently sailed an RS800 and proclaimed that until he experienced that boat, he had never noticed the hump in my FD. The magnitude of the bump in many cases is probably thus fairly small, and will have more of a difference on your average lap times than on any feel you may experience from the boat. The tow test or a tank test would be the only accurate means of determining this. one would need a spring balance to measure resistance, a fast towboat with a side arm, a gps and flat water!!!


    About Phil Stevenson's moth designs: here is someone to listen to!!! A cheap, easy and effective method of home building, with plenty of advice for cutting the cost whilst staying fast. Everything I have seen of his work chimes with what I feel a low cost development class should be, and perhaps a new singlehanded 14ft class built on the same ethos is what the world needs. It's just a shame there aren't more high performance builders like him!


    Incidentally, if you want a scientific look at moth design, check out http://www.zingads.com/doug/moth/

    Good hear your opinions,
    Andy:)
     
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