Why a sailing dinghy's hull is so much heavier than a windsurf board?

Discussion in 'Sailboats' started by xarax, May 4, 2009.

  1. CT 249
    Joined: Dec 2004
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    Location: Sydney Australia

    CT 249 Senior Member

    "Why should a dinghy has to have a greater freeboard than a windsurf board"

    Because it's faster on a dinghy - proven fact. High freeboard is fast on an all-wind board, but all the board classes have maximum limits on their depth/thickness - and all of the modern gear is made as thick as possible.

    What works on a boat may not work on a board, and vice-versa, because of different dynamics. But surely you don't think the dinghy designers haven't been trying to optimise freeboard height on a dinghy do you? Talk to ex-world Moth champ Mark Thorpe or the R Class (12 foot two man boats weighing about 27kg IIRC) about their experiments about freeboard height; they've spend a lot of water time and time on computers borrowed from America's Cup teams working on this issue.

    Hell, Mothies have repeatedly tried semi-submersible boats, with a torpedo-like underwater hull and a rig and wings suspended above it on pylons. They have negative freeboard - that's much more adventurous than anything longboard designers have created for racing.

    With respect, have you ever spoken to Bieker, Bethwaite, Thorpe, Brown, or Morrison about dinghy design? I've been lucky enough to speak to them and to almost all of the top Raceboard designers of my day (ie Helmut Kirner from Hi Fly, Rick Naish from Mistral, Hughes de Turkheim from Tiga, and I'm sure that the dinghy sailors are better educated about design and more knowledgeable, because of the decades of experience they can draw on.

    "Dinghy sailors can also wear wetsuits as the windsurfers do, cant they ?"

    Huh????? Since when did dinghy sailors NOT wear wetsuits?

    "But I was referring to the structural characteristics of the hull, the way the whole thing is holding together, the distribution of the load forces. not the shape of the hull s skin."

    Xarac, the dinghies have been deeply concerned about structural characteristics of hulls for many decades. Do you seriously consider that people who built boats with 1.2mm thick skins 20+ years ago were not concerned with weight? My first boat didn't have bungs or hatches (to dry the hull out) because they were too heavy - we just used stick-on plastic. If you stood on the wrong bit of cockpit (no one ever thought of standing on the 1.2mm thick foredeck) you would go straight through.

    With respect, it is just not logical to assume, as you have done, that the Admiral's Cup builders and designers and Formula 1 engineers who build dinghies have discarded their immense knowledge when they build dinghies and skiffs.

    Dinghies were into foam construction before boards were even invented. The reason they are built differently is because they are built BETTER. The most basic engineering (beam theory etc) shows that it is better to put stronger material on the outside of the hull and air inside, rather than put a whole lot of foam near the centre of the hull where is does basically nothing except add weight.

    As Bistro says, there has been enormous development in dinghy designs, which is why a dinghy like a Moth is now FASTER around most courses than a board.

    I would agree that windsurfers have a different mentality. Windsurfers these days buy what the industry sells them. Many dinghy sailors (like Bistros) would prefer to innovate themselves. Within about 1km of my mooring, one professional engineer (former creator of champion Skiffs and A Class cats) is building yet another foiling Moth at home, a boatbuilder is modifying a 12 Foot Skiff (carrying about 70m2 of sail on a 50kg hull!) while another designer is probably sketching yet another winning carbon 12 Footer.

    Contrast that to the board classes, where these experienced engineers would
    be banned from even entering with the product of their decades of thought and experience! The boats are MORE innovative and highly developed than the racing boards, which are comparatively staid and restricted.

    Yes, you can fit detachable wings; I had them on my first boat. But they are often a hassle; they can jam with salt or sand JUST LIKE SAILBOARD MASTS AND MAST EXTENSIONS DO. It's not as if the boards can get around that problem!

    Fabric-covered dinghies are an ancient idea. There used to be strong classes of them here in Australia in the 1930s, racing alongside the Skiffs. However, they died out.

    Sorry, but a skinny short dinghy is a slow dinghy. You need length to go really fast (apart from planing downwind under a big spinnaker at certain angles in strong winds) and you need beam to achieve sail-carrying power. A skinny short low-freeboard dinghy is more like a Sunfish (fun, but much slower than a Laser) than a high-performance boat.

    BTW Bistro, a good course-racing board is a lethal machine around a course. A few years ago, for interest's sake I took my Lechner (13' long round-bottomed board with 7.3m sail) in a few races against the #1 and #2 Moth in the world. In those light winds, those narrow carbon pre-foil HTs would have been faster than a Bladerider, but the Lechner was just as quick (and I didn't pump).

    So boards don't HAVE to be one-dimensional; that's just a part of the one-dimensional marketing that has seen the sport shrink so much.
     
  2. bistros

    bistros Previous Member

    Agreed. Our local boardsailors cluster around one beach to the leeward of the prevailing winds and reach back and forth all day. We never see them more than 300 meters off the beach. (I think the scantily clad ladies on the beach may be a factor as well.)

    All you've said (quite eloquently) mirrors my experience as well.

    Xarax might want to look at GA Boats - they have got well tested ultralight fabric skinned boats that certainly meet his criteria.

    At some point though choices have to be made as to the design priorities:
    ultra light weight OR performance. I'd have to say that excepting using more expensive and better materials, my boat is about as light as you can get for a low tech homebuild that can perform at top levels. There is nothing there that isn't needed for strength and rigidity, given the materials used. We added strategic re-enforcement in places preliminary testing showed to be understrength. Surprising but much boat strength is needed for dolly movement, trailer travel, shore storage and operator mistakes - not just on-the-water sailing strength. I found that a design optimized for only sailing was not strong enough to deal with things that happen outside of sailing.

    We could have built the same hull shape using carbon/kevlar hybrid over foam core and vacuum bagged it to save about 15 pounds more. The cost delta would have been about $7,000 - and we would have still needed to build the first hull to use as a mold anyway.

    Hull weight also gets to the point of diminishing returns as it drops below operator weight. In my case I can lose 15 pounds easier and cheaper than I can take 15 pounds off the boat. I weigh a lot more (dressed in wetsuit, trap harness, rash guard, pfd) than my fully rigged single hand skiff.

    Where you sail is a huge factor as well. Many venues demand a serious strong hull and the ability to deal with wind, wave and weather that will crush ultralight boats designed for gentle venues and use.

    --
    Bill
     
  3. xarax

    xarax Previous Member

    Thank you again CT 249,

    "Why should a dinghy has to have a greater freeboard than a windsurf board?
    Because it's faster on a dinghy ."
    I would be glad of a much lighter and little slower dinghy !

    "Dinghy sailors can also wear wetsuits as the windsurfers do, cant they ?
    Huh????? Since when did dinghy sailors NOT wear wetsuits?"
    It not a question of time but of space...:)
    In my neck of the woods, as well as in many other places a little far from the poles, at summer and much of spring and fall, they almost never wear a wetsuit, and, unless there is a global freezing ( less probable climatic change nowadays, you would agree...), they will never wear a wetsuit ! I am talking about the average weekend sailor sailing mainly at summer and some part of spring and fall, not the member of the sailing club competition team. I want to say that the high freeboard tradition comes mainly from the designer’s task of protecting the sailors from the elements, sailors wearing normal cloths, not of achieving a higher hull speed . It may well have been discovered recently that a lower freeboard is detrimental to the maximum speed of a high performance acrobatic dinghy, but when we count the watersport participants, this is a market a little bigger than the foilers market, i.e. tiny !

    "With respect, have you ever spoken to Bieker, Bethwaite, Thorpe, Brown, or Morrison about dinghy design? I've been lucky enough to speak to them and to almost all of the top Raceboard designers of my day, and I'm sure that the dinghy sailors are better educated and more knowledgeable."
    With respect, I consider myself a generally educated and knowledgeable dinghy although I have nener spoken, and probably I will never speak, to " Bieker, Bethwaite, Thorpe, Brown, or Morrison", but am free to discuss such matters with connoisseur people that have, like you. We are not participating in any education and knowledge competition show here, I guess...:) I respect the - more educated and knowledgeable than me - people s thoughts and opinions, so I ask them to be kind to share them with me, but I also consider myself free to talk about my thoughts and opinions, even if they are half digested, probably completely useless or wrong, but not offending anybody.

    "With respect, it is just not logical to assume, as you have done, that the Admiral's Cup builders and designers and Formula 1 engineers who build dinghies have discarded their immense knowledge when they build dinghies and skiffs."
    I think that they don’t apply this immense knowledge to build a truly light dinghy, simply because they do not care about it, they function into the strict requirements of another market, that of competitive high performance sailing. They don’t care building a slower, simpler car toppable dinghy, because they think that there will be not a wide enough market for it. But we have to admitt nobody of them discovered or participated in the evolution of the windsurf neither, right ? Windsurf sailing could have been discovered some centuries later than it actually did, isnt it that so ? :)

    "The reason (dinghies) are built differently ( than windsurf boards) is because they are built BETTER."
    Give me a lighter, slower, WORSE build dinghy, please ! :)

    "Windsurfers these days buy what the industry sells them.
    I caught you here ! Simply wrong statement, if you are talking about high performance windsurfing as you are talking about high performance dinghy sailing. But who cares? I mean, this is totally irrelevant with the point I try to make, we could not dismiss windsurfing even if we had wanted to. I simply want a lighter sailing dinghy, even if it is SLOWER than a high performance foiler, skiff or world speed raceboard !

    "Fabric-covered dinghies are an ancient idea. There used to be strong classes of them here in Australia in the 1930s, racing alongside the Skiffs. However, they died out."
    Thanks for this information, too. May be they could have done better if they have been tried in our age of high tensile carbon fibres. But again, they could have died simply because they were not the faster boats around, which are irrelevant to our post here, I have to repeat. But there are millions of young boys out there, some of them will be hooked to the dinghy sailing easier and earlier in their if they will have the opportunity of buying a very light dinghy sailboat.
     
  4. xarax

    xarax Previous Member

    Thank you again Bistros,

    '"At some point choices have to be made as to the design priorities ultra light weight OR performance."
    I think that A LOT MORE thought has been given to the performance priority than the ultralight priority !

    "Surprising, but much boat strength is needed for dolly movement, trailer travel, shore storage and operator mistakes - not just on-the-water sailing strength. I found that a design optimized for only sailing was not strong enough to deal with things that happen outside of sailing."'
    Exactly ! That is why if we eliminate trailing and facilitate operations, we will come up with a lighter design.

    "Hull weight also gets to the point of diminishing returns as it drops below operator weight. In my case I can lose 15 pounds easier and cheaper than I can take 15 pounds off the boat."'
    Lucky ! I can not ! :)

    "Where you sail is a huge factor as well. Many venues demand a serious strong hull and the ability to deal with wind, wave and weather that will crush ultralight boats designed for gentle venues and use."
    Give me a lighter boat for gentle venues and use that most people, Eskimos included. are interesting in ! :)
     
  5. bistros

    bistros Previous Member

    It always never fails to surprise me that extracting the real priorities from clients come during conversations, not their initial design brief. I design systems for clients in the banking sector, and to be honest I get more honest feedback from a rapid prototype presented for criticism than I do from a client's design brief. People are far better at expressing what is wrong than they are at telling you what they really want.

    I tend to call this "ready, fire, aim" development where the rapid prototype is "fired" and I use the feedback to "aim" the real development effort once the client has something to tell them what they don't like.

    In your case weight, portability and simplicity of getting on the water are trumping speed, ultimate performance and suitability for a wide variety of venues.

    I would not dismiss trailers as a concept entirely - my boat (very light and easily cartoppable) is much cheaper to trail than to rooftop - gas mileage really suffers (minus 25-30%) with a boat up top, versus the tiny trailer that sits in the wind shadow and adds only about a 10% penalty.

    It isn't the weight, it's the windage. Trailers are also nice in that all the gear ends up under the cover, rather than in the car. When traveling with family, dog and whatever the trailer makes the difference between bringing the boat and leaving it home.

    My trailer is a combo nesting trailer/dolly and I have to say a dolly makes launching/retrieval easy - and it keeps the road bearings out of the water. The good part is I built it from cast off parts for a total investment of $400 - and new comparable versions sell for $2,500. I've found I have to run the trailer wheels under recommended inflation by 20 PSI - because the boat is so light it doesn't affect the 600lb design suspension at all.

    I sail out in Lake Huron every summer vacation - and it is oceanic in size and conditions at times. The shortest crossing distance across from where I am located is 80 miles (130 kilometers) to Michigan. Shores are all rock, and safe harbors very infrequent, so a strong boat is necessary. My regular venue is a stretch of major river 40x3 kilometers.

    I did a bunch of courses years ago in the Canadian military reserve with Inuit people (called Eskimos by others) from our far north - unfortunately kayaks, biadarkas and the like are not in use any more. Freighter-style square stern canoes and aluminum boats with outboards have replaced native skin on frame boats. Native kayaks have been relegated to an artform and historical craft.

    Cheers,

    --
    Bill
     
  6. xarax

    xarax Previous Member

    But where do the trailers themselves end ? Obviously you are not living in an appartment in the heart of a city struggling for garage or parking space, as millions of not -so -lucky people like me do...:) Not mentioning the struggle of towing the trailer far from the road on a rocky or wide sandy beach...Can I order something that does not need a trailer, a crane or a weightlifter, please ? :)
     
  7. gggGuest
    Joined: Feb 2005
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    Location: UK

    gggGuest ...

    If you're taking the Pico as an example then you're obviously not very au fait with leading edge dinghies. The Pico is an entirely different game. The hull construction is chosen with minimum maintenance and cost of production as primary aims, not (light) weight and hull stiffness. Those were low importance to the designer due to the market they have to serve. Every design is a compromise, balancing different factors, and those are the ones that are important for that particular task.

    Any of the leading edge dinghy builders in UK, AUS, NZ or USA could build a Pico shape/sized boat for I should think about half the weight with increased stiffness. But the result wouldn't be nearly as resistant to cack handed handling, sailing full speed up the beach and the like, and would be a lot more than twice the cost. So no-one would want one!

    There is no particular difference in the skill levels between the top board designers/builders and the top dinghy designers/builders. Indeed they are often the same people!
     
  8. ancient kayaker
    Joined: Aug 2006
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    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    Xarax, you seem overly concerned with the question of freeboard in a dinghy; it's not there just to keep the folks on board dry, it's to keep the water from getting into the boat. That will slow down a boat better than anything except hitting a rock! :) Even a sit-on-top fully decked boat experiences extra drag if the sheer goes to far under, the water that comes on board has to be either shed quickly or accelerated up to boat speed. My canoes have about 4" (10 cm) freeboard which is adequate even in choppy conditions but far less than a heeled dinghy would need. Also some hull shapes sail better heeled due to reduced wetted surface.

    I can probably design and build a significantly lighter single-handed dinghy using wood, flush top, wings, the works, maybe 30 lb/14 kg. However to be that light it might be too narrow for performance, I would think, and would be very uncomfortable at my age (69). :eek: However, my design expertise is more for building methods than design. It's an interesting thought though.



    I see you are living in Athens, I would love to see that city. You may not know that those whom we call Eskimos prefer to be called The Inuit. The kayak is a thing of extraordinary beauty created by the Inuit and other far North peoples from whatever washed up on the beach, walrus skins (usually) and incredible skill. I've had the privilege of examining several close-up and I find it hard to believe what can be done with one's own teeth, sinew, and bone knives and needles. At the seams, the skin is split across it's thickness, the first layer is sewn then the second layer is pulled over the seam and sewn again; the sinew thread does not penetrate entirely through the skin's thickness. Completely waterproof and as hard to describe as it is to believe. I keep trying to get a boat museum to accept responsibility for a 1960's recreation currently stored in a barn, but I suspect that the average curator is only interested if the boat is a hundred years old and decrepid: this one is almost immaculate and could be paddled. Every detail of its classic construction is clearly to be seen. Sigh! :(
     
  9. xarax

    xarax Previous Member

    Beautiful, that is the one word description of ancient kayaks!

    P.S. 1. I dont mean to offend Inuit people calling them "Escimos" of course, as I am used to say "Japan"and not Nippon. All people should be called by others as they call themselves,( if this is not anybody else s name) . I will try to remember it next time.
    P.S . 2. Preventing water go over the board, resulting in greater wetted area and less speed, may be achieved by a higher bow, the suitably formed " nose' of windsurf boards. The freeboard is a part of the boat that needs to be strong enough to resist many operations in or outside the water, as Bistros has remarked. Ct 249 says that : "A skinny short low-freeboard dinghy is more like a Sunfish (fun, but much slower than a Laser) than a high-performance boat.'' Well, I have not said anything about a high performance boat in this thread, did I ? On the contrary, I believe that there exists a limit of weight that can be saved from a high performance dinghy due to stiffness considerations. So I would be glad to have a light slower-than-a-high-performance-dingy ! Moroever, I will become very rich if it has the commercial success of the Sunfish !:) And if we had a dinghy like this, then we could evolve it and see how fast it could go. Nobody, ( not even "Bieker, Bethwaite, Thorpe, Brown, or Morrison",:) ) would ever think or predict, in the beggining of the windsurf evolution, that these crafts would hold the worlld speed record on water for so many years ...So let us try to copy some things from the windsurf world, as they did from ours !
     
  10. bistros

    bistros Previous Member

    I keep forgetting that European folks just don't consider distances in North America. To get to where my parents live on Lake Huron, I drive for eight and one half hours non-stop (except for fuel & coffee) at 100 kilometers per hour. This would be the same as crossing three complete countries in Europe. And with my drive I will not cross even one third of the width of the province of Ontario. Fuel costs here are a serious matter - even if they are cheaper than European prices.

    I live in the city, with a suburban home on a narrow lot - I store my trailer vertically between the houses in about three feet by 6 feet of space.

    --
    Bill
     
  11. xarax

    xarax Previous Member

    I live right in the heart of the historic centre of a city, less than 5 minutes away from the sea, but on a pedestrian, small, curved, stone paved road where a vertically standing trailer would serve only as a monument ! :) I store my board on the balcony, trying to cover it from the pigeons...
     
  12. ancient kayaker
    Joined: Aug 2006
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    ancient kayaker aka Terry Haines

    So what you are looking for is a light folding sailboat. Light enough to put on top of a car single handed with ease, and folding to permit storage in a small apartment. You care less about comfort and performance.
     
  13. BWD
    Joined: Aug 2006
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    Location: Virginia, US

    BWD Senior Member

    Thinking about this overnight, I found a CAD model I did of a windsurf hull, 255x87cm, and decided to transform it into a tiny dinghy, to use the CAD data to compare weights and other properties.

    For simplicity, I assumed the foam was all the same density, beam would be 120cm and keel to gunwale height of 40cm was appropriate.
    I paid no attention to hydrodynamics, just made the board model into a boat shape of the same length to generate some numbers. The exact shape of the boat of course could be adjusted for stability, etc. without change in amount of materials.

    The results, in the attached picture, illustrate some reasons boats are rarely built this way. Weight is the first reason. Adding just a little freeboard gives about twice the surface area, and almost 4 times the volume, compared to a board shape.
    So the boat needs a lot more materials, as has been said already, leading to the second reason:cost. Labor and/or machining/molding cost for a foam boat core would also be large, and so would the cost for so much foam and glass.
    But the result would be as delicate as a windsurfer! (third reason)
    For contrast, I also estimated weight for a glass-skinned plywood version. As you can see from the picture, wood would be lighter by 7.6 kg even allowing 7kg for reinforcements and bulkheads, and using the shape that was chosen to use a small amount of (expensive) foam. A shape designed for wood, decked over and with small cockpit, would save even more weight, as has been written by another poster (my example of wooden construction may is a little overbuilt!).

    Wood would also give better puncture resistance. Something to think about.

    There are lots of boats, plans and kits for little sailboats, under 3m long, and under 45kg in hull weight.
    Most are plywood.
    But if you want something plastic, there are lots of options from optimist to bug, mostly polyethylene.

    I have visited Athens and I think if I were there I'd try to find a sailing club or something to keep a slightly bigger dinghy for sailing.

    It would be cool though, if you could hang a dinghy from a balcony on davits, and lower it onto the car -a lifeboat to escape from city life! (Or hang it above the balcony, to give a roof...)
     

    Attached Files:

  14. xarax

    xarax Previous Member

    Thank you BWD,

    We have pinpointed a basic reason for dinghies being so heavier than boards in this thread, freeboard. I have not realized, till now, the correct magnitude of this! So my first idea of dispensing with the freeboard altogether looks pretty right. Down with the freeboard !

    "It would be cool though, if you could hang a dinghy from a balcony on davits, and lower it onto the car -a lifeboat to escape from city life! (Or hang it above the balcony, to give a roof..."'
    ...to some homeless dinghy sailor whose house was taken back by the bank, liquidated and paid as a bonus to a windsurfer greedy manager ! :) Down with the windsurfing managers !
     

  15. BWD
    Joined: Aug 2006
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    BWD Senior Member

    dark humor.... I was assuming the balcony was safely above the level a person could reach from the street. ;)

    If you want to look at eliminating freeboard totally, it can apply to boards as well as boats, but it doesn't always make it better. ... ....
    For example, this is what I am building today, a wooden surfboard, copied after the ancient style, it is only a plank. I think it will work (design proven for 100s of years), but not better than normal modern boards. Not lighter either! But cheaper, and fun.:cool:
    And the freeboard should be about 3mm (but less than zero with rider.)
     

    Attached Files:

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