Sailing Dinghy Design

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

  1. tspeer
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    tspeer Senior Member

    Perhaps Leo will weigh in on this, but I'd be a bit careful of Michlet predictions for a wide hull like a planing dinghy. It definitely can't handle the dynamic lift in the planing regime.

    Michlet is really best suited for slender hulls, like kayaks and multihulls. But it's about the only thing out there for the price!
  2. Phil, I agree that the Tasar/NS comparison shows that the DLR is not the only criterion of a humpless hull. It's just that AFAIK there seems to be two ways of achieving it - the Nash way, and the ultra-long and light way. With the latter, the whole idea is that when the boat is very long, the loading per area on the hull that is required to plane is vastly reduced. At the same time, the hull speed of tjhe longer boat is higher. Before the boat reaxches the
    high-drag part of the hump near 1.5 sq r LWL, it's already achieved the dynamic lift to plane. Doesn't matter how high the drag of the hull woujld be near 1.5, if it's planing at 1.0 or thereabouts.

    Actually I think the Merlin Rocket guys may have been where Nash was, years before. Look up Keith Callaghan's Merlin Rocket designs, his boats of the early '70s look more closer to a modern NS 14 than the modern NS 14 does to a Dribbly or Carrack to my eyes.

    Hey B 14, can we get some more detail on Julian's remarks about old 16s? In 1975, Frank B wrote that because of their weight, the 16s had two totally separate design themes, the "planing boat" and the fine, deep "Knifer" displacement boat, and that victory depended on condiktions. I wonder if Julian was referring to the "knifers"??

    I know a while later they went to a real "speedboat" planing hull before Nash and ASEA or whatever it was showed them all about skinny boats.

    The little info I have on the '60s 16s shows skinny but pretty standards flat vee hulls, like a Tasar with less rocker but maybe more vee. The real deep vees may have been fast, but they were also hard to sail.

    "the traditional Bethwaite hulls lack the ability to use the fine veed bow sections in any sort of a breeze due to the way that most (pre-9er) Bethwaite designs have a poor weight distribution (all down the back) which counters the volume in the bow sections".

    What do you mean having the weight down the back? Boat weight, crew weight, volume??? I think the Tasar balances about half-way along its length, with crew I'd assume the c of g is 60% aft. Both seem normal to me AFAIK.

    The Tasar trims with the bow knuckle in the water upwind in a breeze when pointing, but from memory (mine's been in the shed since Christmas and not raced much for a while before that- crew problems) the knuckle is about water level when planing upwind and obviously well above the water downwind at speed. Is that what you mean by not being able to use the bow?

    There was a note by Mark Bethwaite, about the Medium Dribbly era, that a curve of volume that started very fine, maxed out well aft and dropped abruptly was faster than the more constant curve. I can't find that note at the moment, tho', to check on it.

    Interestingly, the 505 is designed so the bow lift out at speed, and the B 18 does the same doesn;'t it?

    Please note I'm not knocking the concept, just looking for more info.

    Also; just noted the Canoe drag curve in Marchaj. The "hump" is very, very flat. Also p 184 of a French or Swedish "Erikson's Prin ciples of yacht design" (can't recall which, and my notes are buried) shows a graph of hydo v buoyancy lift with a nice cross-hatched area at the "rise of cg" point, which seems to be to indicate that around the "hump" not even the experts want to preduct anything!

  3. astevo
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    astevo Junior Member


    i dont think that we should be drawing such a big line between displacing and planing. there is no one point where the dynamic lift increases rapidly,it is related to speed and so dosent have the sudden increase that is talked about. by planning bouyany lift is reduced but never totally removed.
    from observations the boats which have been refered to as humpless in this forum all seem to plane lower in the water, than either their earlier development forefathers or similar boats of the same size.
    lets think about the boats we call humpless: the ones that have rated a mention are the 9ers, new ns14's, canoes, moths and 12' skiffs.
    these boats are all narrow waterlines compared to their predecessors (or similar sized boats), they also plane at lower angles and heights than boats of similar size.
    when we compare the boats and the can make well based observations based on the changes in shape.

    the tasar (and early ns's) is V'd in shape and flares out to a wide stern and has a heap of rocker, the new ns14's (tequila 99 and aero 11's) are narrow on the water line with a flat area forward and and more vertical sides forward. and lots faster.
    the 29er is longer and v'd compared to the cherub, it is humpless but lacks the speed when going really hard that the cherub has, despite this the good 29er beats a cherub. thew 29er is also over a foot narrower than an aus cherub on the water.

    all these boats are faster than ther eraly versions.
    the conculsion are obvious, when we design with in parameters length is the most obvious, so the disp/length ratio thatmakes 29ers go is not reall at the core of what we are looking at here. it is clear that the narrow hull as more humpless than wider equivalent, thereis really no secret, whilst a wide planning hull may have faster all out speeds than a similar narrow boat the narrow boat will be faster round the course in a fair course throughtout the wind range

    i think the large hump is caused by trying to generate too much dynamic lift at the expense of mid range speed. look at the conditions we actually race in. most races are in the 10 to 15 modeate range. where the maximum dragracing speed dosent really matter. why compromise light air and upwind speed just for 15kt and over downwind speed.
    if a 11ft moth can max at 16 point something knots measured by rohan on a moderate day at easter how much faster is a plaining boat going to go down hill, remember this limit is also a non kite speed. i want to know how fast skiffs and kite boats will do with no kite.
  4. mad engineer
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    mad engineer Junior Member


    If I can try and pull together a few of the threads from everyone else's intelligent comments (as opposed to my approach of make a sweeping statement based on very little - but at least this gets the intelligent comments into the open...)

    1) Moth's are incredibly fast - but especially for an 11ft boat.
    2) They are incredibly efficient in most sailing conditions.
    3) The design is predomiantly displacement but may benefit from some dynamic lift at the top end.
    4) They do have a top end speed limit that is probably somewhere around 16 to 20 knots.

    5) 12 foot skiffs are also designed to minimise drag in displacement mode. They are very narrow waterline (750 - 850mm wide or so I believe)
    6) the 12s have the benefit of unlimited sail area to which gives them huge power and the ability to plane inspite of the narrow waterline. Top end speed is definitely in excess of 20 knots.
    7) Cherubs have hard chine hulls and go extremly fast in planing conditions (best is reputedly 25kts plus) but are very definitely slow in displacement mode. (I did hear a rumour elsewhere that a Cherub might be able to out-drag a 12ft skiff when it is howling.)

    8) Hump-less performance is a big bonus in a planing hull as it improves performance at speeds around hull-speed.
    9) Humpless hulls follow 2 approaches - very narrow and low drag and fast in displacement mode so they plane later but with a smooth transition (12 ft skiffs).

    Or there is the Bethwaite approach which to date has not been explained by the designers but my interpretation of the cause and effect is this - remove volume from the bows to make them sharp and pointy and cut through waves easily, this results in the dynamic lift coming from the back part of the hull which is big and flat to maximise the lift generating efficiency. Therefore as the boat transitions onto the plane, all the lift is coming from the back end, the bow stays low and therefore the boat doesn't "sail uphill" so the drag is reduced and the transition to planing is therefore smoother and earlier than in Approach 1.

    So my thought now is this - and this all started from the thought of a 5m long Moth -

    If a Moth was scaled up by 50% to be 16ft long, and 18inches wide, with an all up weight of say 100Kg including the sailor, would it generate enough dynamic lift to break free and plane freely? (by plane freely I mean be be at a point where it is most efficient to trim the boat such that the wet length of the hull is reduced thereby gaining the benefit of reduced wet area - I don't believe this is generally the case with Moths)

    How much power would this need?

    If you can generate the power and the hull could break free and plane freely, then I would agree that it would be a winner in all conditions....

    Now over someone who knows what they are talking about please....
  5. mad engineer
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    mad engineer Junior Member

    Just another thought on the 5m long Moth concept...

    If it is 50% longer, but with the same displaced volume, then the wetted surface is going to increase. Beyond hull speed, I believe viscous drag becomes dominant over wave-making drag. If the wet surface is greater, the viscous drag will be greater...

    I quickly put an 3.35m Moth and a 5m version into michlet just to see what would happen - both displace 100kg and are about 350mm wide at the water line...

    The graph is attached - Hull 1 is the 5m Moth, Hull 2 is the standard length. Interstingly the standard moth is lower drag at higher speeds due to the lower viscous drag. Obviously this ignores the benefit of the longer length in pitchpoling. The longer boat is faster at low speeds.

    Food for thought.....

    Attached Files:

  6. mad engineer
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    mad engineer Junior Member

    "faster at slower speeds" - like that makes sense!

    What I meant was the longer moth was lower drag than the 11ft moth at lower speeds...

    (speeds on the graph are in metres per second - double them to get knots - roughly)
  7. astevo
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    astevo Junior Member

    is it possible to run a series of hulls throught the program and see what the optimum might be? im suprised by that result.
    also reduce the waterline beam to 320 as that is what we really sail at and see what happens.
    is it possible to determine the components of drag due to wet surface and due to wave making (i know when you get down to it is sort of due to the same thing ( viscosity and no slip conditions...), but work with me people).
    also there appears to absolutley no bump in the long boat.
    does this program simulate planning too? and how accurate could it be trying to simulate a turbulent flow?
  8. Guest lurker

    Guest lurker Guest

    Some thoughts from an interested but technically challenged observer of this topic, if you'll indulge me.
    At planing speed dynamic lift will be produced equalling the weight of boat and crew.
    Efficient planing will occur when an increase in speed will have a corresponding reduction in wetted area. (double speed =half wetted area)
    Will all shapes of hull that plane follow this rule of efficiency or is this where a wider hull do better than a narrow one.
    Is there an optimum angle that a particular shape should plane at to achieve the above condition?
    Is this why a narrow shaped hull stops accelerating at a particular speed eg the moth at 16kt approx .
    I have played with some hull shapes derived from a Javelin using on older version of maxsurf and very quickly found that trim angle had a huge effect on results.
    Is it is hard to simulate moving crew weight in vpp programmes.
    Is the Bethwaite style hull then really two shapes blended into one? A narrow front end with sufficient rocker to keep the flatter aft section largely out of the water until the hull digs itself a hole near displacement speed. At this point the aft section can develop enough lift to plane . This then being a sort of crossover point where resistance is equal in displacement or planing modes.
  9. I have to say, Astevo, I think you got it exactly right from what I can see.

    The only thing I wonder about is "when we design with in parameters length is the most obvious, so the disp/length ratio thatmakes 29ers go is not reall at the core of what we are looking at here."

    If someone was designing a boat without class rules, isn't the easiest path to speed making the boat longer? Lots of designers say that nothing is as important as length; particularly when it comes to all-round performance.
  10. Phil S

    Phil S Guest

    A couple of thoughts on the recent posts. And some other ramblings.

    The graph showing an 11 ft narrow moth having less drag at high speed than a longer version is confusing. If we look at A Cats as the closest thing in terms of weight and length, I think they would have a much higher terminal velocity due to lower drag.

    An A weighs about 150kg with crew, a moth about 100kg, I think the big moth concept would be say 125kg counting a heavier crew. So I would think the big moth would perform a bit more like an A than a Moth.

    There is one big moth in existance in Sydney, but I have not seen it yet, I think there are some teething problems still.

    After some discussions over the weekend I can add to some of what I and others have posted. One young sailor who I have great respect for and who was one of the first to buy a 29er expressed the thought that the 29er reaches a speed limit when the nose starts to go down. This is even with the lift from the kite. So if the supposedly humpless plaining boat has a speed limit too where does the high speed of planing boats come from?

    After the 29er the same guy tried a 12 and expressed the opinion that it did not plane until the big spin was up. So maybe the 12s do have a hump too.

    The really fast planing boats all seem to need some volume up the front. This tends to be at the expense of all round performance. The Cherub is the prime example. By the way the observation of Peter Moor about relative top speed of the Cherub and 14 was based on a windy Nationals week in Perth about 6 years ago, which was before the 14s put foils on the rudder. At that time they were serious nose divers.

    Yesterday we followed a V8 powered hydoplane down the highway. It was one of the old traditional designs with forward sponsons which at speed carry all the load. These are located well forward of the weight of the engine. All these types of power boats plane on flat surfaces at the front, not flat surfaces at the back. Ocean racing power boats are different they need to survive.

    The concept that our sailing boats plane on wide sterns in out of date. We plane on the flat midship area, with initial lift from the bow and mast sections. The old fashioned pictures of boats planing with the bow high in the air is due to the excessive spring these boats had, and which was needed to carry their excessive weight in displacement mode. Modern light boats do not need that spring and so do not need to lift the bow so high to get a positive angle of attack between the planing suface and the water.

    The pictures you see of 12s almost out of the water is about sail power not hull shape. If you put a big sloping luff sail far enough in front of any hull it will lift it out of the water. I guess that is what makes those narrow bow 18s controllable.

    Latest news on Jim Buckland's BUCKO 14ft skiff singlehander is he is going pretty fast downwind, cleaning up a mixed fleet regatta on the weekend. We were sailing moths on a different course so no speed comparisons this time.
  11. astevo
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    astevo Junior Member

    all i meant by that is that, on the whole in this forum we have been talking about designing within a set of rules. moths i14's ns14 etc, all of these are fixed in length so while we could make a boat faster by simply being longer, it isnt really on the cards.
  12. Tim B
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    Tim B Senior Member

    Spot on astevo. Length serves for a lot, but the efficient solution is never to go bigger and put in more power. 14 feet is a nice size, it is big enough to provide good speed, but small enough to be sailed by flyweights like me.

    Guest lurker, good to have a post. have a read through the last couple of pages.

    Incidentally, the rudder's going back together nicely, just goes to show how fast these things actually are.


    Tim B.
  13. Guest lurker

    Guest lurker Guest

    Thanks, yes I will reread the last pages carefully, I must confess I didn't initially see the answer to the question of planing at high speed.
    Regarding the hydroplane concept, surely they invented the bump, at least casual observation of them accelerating from a standing start would suggest that. I realise some of that is the very high pitch propellor stalling at low speed.
    Wouldn't the reason for the forward position of the sponsons be to get some of the boat in the water ahead of the centre of mass to assist with turning. Some I have seen have vertical foils attached to the inside of the sponsons to give extra bite in a tight turn. They still look to me as though they plane on the back end of each sponson and the main hull.
  14. Just a few quick words about high performance sailboats, from an interview today with designer who knows about Moths and current 18s probably better than anyone....

    "I think the 18s are way behind the Moths, at this stage of the game".

    So how narrow would this designer go in an 18, if he wasn't restricted by class rules?

    "I don't know, that’s the fun of it and it’s going to be completely a function of the skill of the guys that you are dealing with. But my first guess is to take the Moth and scale it up and add a little bit, just for safety. The problem is that if you make it so narrow that the guys spend the whole time swimming, then you don’t move forward because they don’t get time actually sailing the thing."

    This designer is sure that the Moth does plane, accepting the classical naval architecture definition of planing as being the state where the centre of gravity is higher than if the boat were at rest. Note, this does NOT mean that the boat has to be planing high and bouncing across the water like a 49er!

    I think it should also be pointed out that if we blow up a skinny pintail Moth to about 18' and give it an 18' style rig (3 sails, assy) and stick the crew out so their feet are 3' from the gunwhale, we end up with basically a slightly skinnier, lighter International Canoe.

    The lesson is simple - you should all just go buy canoes!!!

    And I'm not biased at all.


  15. Phil S

    Phil S Guest

    Some more thoughts on what part of the boat it planes on.

    Basic physics says that the support must be under the centre of gravity, give or take any other moments applied.

    Most sail boats have the forward force say a third way up the mast, and the drag somewher under the water. So there is a moment trying to push the bow down, and so the lift, be it buoyant or planing, must be in front of the weight or centre of gravity. Most boats can not move the crew far enough to get the centre of gravity back over the flat so called planing area. This effectivly proves that we plane on the middle of the boat.

    Power boats have the thrust under the drag so have a nose up moment. At their speed this is accentuated by windage. So they do plane on the area aft of the centre of gravity. (I am not sure about the three point hydroplane though.)

    I can think of only a few sailing exceptions where other forces change this position. 12s 18s and 49ers have very long poles and the spin has such a sloping luff it generates a huge bow up moment. Some also have frames which allow the crew weight to go behind the planing area. Hence the bow high attitude when under spinnaker. But also consider that these boats are a real hand full bearing away around the top mark, when the narrow bow wants to go down because the lift from the spin is absent.

    Extending the same logic.

    The pin tail shape of modern moths will plane because there is adequate flat area in the middle. Mine is 375mm wide and I know it does lift my 80kg on reaches. In stonger winds it does nose dive.

    Also if nose dive susceptible boats are provided with enough bow up moment they will go faster than the present nose dive speed limit.

    This is proven in the case of the 12s, 18s, 49er and the Tornado and other cats since they gained the spinnaker.

    The first stage in making a moth type go faster is to make it longer, extend the bow and get more boat out front, like catamarans did 30 years ago. This is my 21st Century single hander canvassed a few pages back.

    The nexts step is to put a long pole and sloping luff spin on it and now you have that fantasy 18 Chris was discussing in his interview.

    People still look at moths today and say it is impossible, then old farts like me show it is not that hard. The same was said when the Windsurfer first came on the scene, and it started the greatest increase in sailing activity ever. We just need some conservative minds to open up and try some new ideas and we will have some amazing boats for this century instead of staying in the last one.
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