Sailing Dinghy Design

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

  1. Guest

    Guest Guest

    Hey Andrew

    Re "it is not really accurate to use the flat plate approximations for the modern type low planning skiffs, as streamineflow is maintained more than in olderstyle hulls."

    Can you provide more detail about the difference between old and new hulls? I seem to recall a pic of the Woof hull showing a very defined spray root and therefore, I assume, a stagnation point.

    Tim, the current anglo-saxon way is to limit overall length. The traditional European way was to limit sail area and often have no restriction at all on LOA (see Z,M and N class Renjollen). Saying that working on a fixed length is the only way to refine things - why? It can lead to you bigger rigs, and bigger rigs are often more expensive than chucking another metre on a hull. Furthermore, limiting LOA offten leads to handling problems, whereas limiting sail area has less of a tendency I think. The European Renjollen are roughly as fast as a skiff with a similar size rig (I think, I'll check it) yet they were stable enough to sail on those freezing lakes before wetsuits were invented. For many people that's important. Look at the success of the RS 800; it's a very popular skiff-type boat which is very un-skifflike in having a small rig and that makes it much more acccessible than the 49er which is about the same length.



    Just a thought.

    A cheap skiff class could have a one design rig that everyone can get secondhand- any ideas? Although I must say in the slight chance I build it will be too long for the class; it would be a skinnier, lighter Canoe.

    Still can't work out about the code. In what class is there THAT much difference between the settings that smart people with years of empirical experience (ie the winners) use, and the theoretical settings from the code? How does a code account for changes due to dirty wind, a hole at the top mark, a luffing match, powerboat chop that requires you to change into a lower groove?

    Ever race against Frank Bethwaite? He has a vast theoretical knowledge yet, with respect, he's never been the winner that someone of his experience and ability should be. As I know (from a good friend who sailed with him for years, and having raced against him often) he's too concerned about theory. Sailing continually deals with turbulent flow that no computer can model; we are always trimming on the edge of turbulence, our foils and hulls are in a turbulent stream.

    One of Frank's many contributions was to note how much the real wind varies, and how it's the people who adjust second by second who win; that's not something one can code for. Best trimmers I have sailed with may be Skip Lissiman (Australia II) or Ian McDiarmid (world J/24 and Soling champ) or John Bertrand (very briefly); all sailmakers, none of them concerned with theory once on the water; much more reliant on educated seat of the pants.

    BTW, actually comfort isn't the reason I prefer the Tasar to the Taipan; the cat is TOO comfortable and that's the problem - it all feels so remote when you're just standing in trap while the boat carves along. Inthe Tasar you're working through every wave, main in and out, travellier and vang up and down, tacking every minute. It's so much more involved; the cat is like a fast freeway drive in a BMW, lots of pace but no sensation of speed.

    Exscuse my long-winded raves, it's a help to sort out my limited understanding of these things.

    Chris
     
  2. Tim B
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    Tim B Senior Member

    Chris,

    Fair comment on the last point.

    I was, in fact, referring to hull refinement when I said the way to understand how to go fast is in fixing length.

    As for the code, re-read the suggestion, and you'll see that I do not suggest modelling everything, but interpolating onto an 'optimum' solution from true measurements.

    As for your comments on computer modelling, tread carefully, because Computational Fluid Dynamics is becoming very powerful. I suspect that in ten years from now, most turbulent flow and free surface questions will be soluble.

    Cheers,

    Tim B.
     
  3. mad engineer
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    mad engineer Junior Member

    A few thoughts that have been developing...

    astevo - you said your ply Moth was off the pace when you had too much rocker in the back end and then went much faster when you flattened it? I was wondering if this wasn't to do with dynamic lift but more to do with the changes in the way the boat makes waves above hull speed. At hull speed, there is a wave crest under the or just behind the transom. As the boat goes faster, this crest moves further back behind the boat and the water gets flatter as the bow and stern waves move out of phase. So, traditional displacement designs with lost of stern rocker work up to hull speed because the curve is matches the water surface profile / water circulation within the wave more closely. But because Moths go masses faster than hull speed, the flatter stern becomes faster because it becomes a better match to the water surface profile / water circulation pattern of the now flatter water...

    So there is a difference in the least drag design for boats that work at speeds above and below hull speed...

    Chris - you were saying that someone was contemplating designing a planing A-class cat...our local Taipan sailors claim that the taipan is a planing cat already. I have never seen that mind you.

    and a last stupid idea which must have been tried before but if it hasn't it would be easy to fit to a moth if you can cope with the ridicule... If we want dynamic lift then, whilst a flat plate is better than a curve bottom on a boat, you would get more lift if you could turn the water that is flying upwards along the side of the boat and then force it to go down again with a sort of half pipe along the side of the hull. It would allow the boat to stay very narrow on the water line but still get lots of lift. something like this....
     

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  4. Phil S
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    Phil S Junior Member

    Yes, Andrews Bent and Twisted Moth did show the wave characteristics you have described. It meant it was very good a slow speeds but not so good once every one else was going over hull speed.

    I think that this boat and our logic here has shown that the straight run aft is not just a benefit to planing boats but to narrow boats as well. I notice in multihulls the stern rocker like the Tornado has diasapeared from the big French ocean racers and now the lines are very straight.

    I would have immagined that the Taipan and most cats normally have a little too much bow down moment from the tall heavy rigs to actually get the bow up and plane. That was until they began putting spinnakers on long poles. Now there is no reason why they should not lift the bows and plane just like the narrow bowed skiffs.

    The same should apply to a long narrow dinghy, especially with a spinnaker on a long pole.
     
  5. Phil S
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    Phil S Junior Member

    I forgot to comment on the turned down flaired gunwales.

    These look a lot like the wide moth flares which have virtually disapeared in the last few years. Similarly they were on the bows of catamarans in the 1960s, but disapeared before 1970.

    While in theory they might offer lift they are much more significant in providing heaps of drag from wave impact and windage. They slow the boat so far that I believe they cause the boat to trip over them.
     
  6. Mad (and everyone), as Phil says the pipes look a lot like the bow spray rails on old cats (Hellcat etc?) which caused too much drag. Talking to the designers it's obvious that most are turning away from the search for dynamic lift and more into reducing drag. I know dynamic lift "eventually" reduces drag but why not reduce drag in other ways ie cutting down beam, moving from flat sections to elliptical to reduce wetted surface etc? Using energy to lift the whole bloody boat is not particularly efficient, it's all redireceted from going forward.

    There's an analogy from windsurfing, in that it's dead easy to rake the rig miles to windward and cause a large component of upward lift, enoughy to lift the whole board out. It's also dog slow; in the final analysis we are trying to go forward, not up, so why go the long way around?

    None of the good Taipan sailors (we have # 1 and 2 in the nats in our club, one was last year's world A Class champ too) are sure they planewithout the kite AFAIK. They do feel to be sitting higher when you're well back and the boat is moving fast, but maybe that's cause then you're on the more bouyant sterns. perhaps it's also lift from the windage of the tramp- whaddya reckon Phil????

    Some good guys reckon they may plane with the kite, as Phil says. I have yet to try one with the kite but there's no doubt there's a big lifting force but not as big as the skiffs perhaps due to the cat's shorter pole.

    I have dibs on a ride on Stalky's (last years world A champ) boat which should be fun!
     
  7. Tim B
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    Tim B Senior Member

    Another guest,

    I guess this is predictable, but this point should be made. One of the conditions of a dinghy is that it floats with the gunwhale above the water (sometimes a freeboard is specified in the rules). The second is that it moves as fast as possible. Now, the problem with a narrow beam is this, since we know that the upward force on a stationary hull is equal tothe weight (and thus proportional to volume) of water shifted. Therefore, if length is fixed, beam decreases and draught increases. This is exactly what we don't want. Case being that the wetted surface has gone down, but the coefficient of drag has gone up. Therefore, by keeping the beam wide, and keeping the Cd low, we can attain the best of both worlds, in that we meet a transition, where WSA begins to decrease as the speed builds. What's more, it does it very quickly. Lift an inclined plate out of a bowl of water, note the change in WSA with height, now put the plate on edge and lift it out, look as the change in WSA. So that's the reason It's been done for so many years, the question now is, can we make a fast hull over the whole speed range, that uses the same theory?

    Cheers,

    Tim B.
     
  8. mad engineer
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    mad engineer Junior Member

    As i thought - stupid idea!!
     
  9. Tim, I do in fact understand the concepts. We may just be coming from different backgrounds when we differ as to whether we should be going towards planing hulls, or further away from them
    . Your starting point may be a Lark, mine's a board or a lightweight planing hard chiner.

    Chris
     
  10. Tim B
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    Tim B Senior Member

    I'm just somewhat concious that there is a minimum weight for a sailing dinghy, and thus a minimum volume, so narrowing the hull cannot be used on it's own. have a look at the difference between the I14 on page 2 (ish), and the ones posted later. The change is not to go faster, it's to accomadate the rules... and that's our other problem. Steve Baker's moths, presented earlier on this thread are worth a look as well.

    Cheers,

    Tim B.
     
  11. SailDesign
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    SailDesign Old Phart! Stay upwind..

    "Steve Baker's moths, presented earlier on this thread are worth a look as well."

    I posted some Moths? Wow!! ;-)

    There were some nice Moths back a few pages, though, weren't there? Or was there another Steve B? My forgettery gets better by the day.

    BTW, Tim B, what part of Southern England are you from? I lived around Chichester harbour for 20 years (mostly Hayling Island and Emsworth)

    Steve
     
  12. astevo
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    astevo Junior Member

    remember we are takling about design within rules. the international 14 has the rule at 7ft back (at 200up -1100 wide) and are therefore always going to be wide boats.
    The moths dont have any rules as such. the difference between the wide moths and the new narrow ones is in the order of 20% faster round a course in all conditions. what the differences' are between the us moths and a narrow skiff, i've no idea but start at around 30% faster at least.
    while the moths are getting lighter. the 14's are staying the same in fact the aus 14's have got heavier.

    Even the 14's are going narrower. Or are at least trying to . i read an article where Paul Beiker said abouthis 14's that by increasing the volume down low the boat floats higher and has narrow waterlines as a result.
    if wetted surfce was that big a consideration why is it that every development boat you see is getting narrower on the waterlines
    it isn't just dinghies either. the yachts are becoming narrower with slab sides. i just dont see a problem with the hulls sinking deeper into the water and those who design boats professionally don't seem to either.

    anyway a while back chris asked me to clarify something.
    >>Re "it is not really accurate to use the flat plate approximations for the modern type low planning skiffs, as streamineflow is maintained more than in olderstyle hulls."

    all i meant by that was that the hulls we all consider to be humpless(nothy, moth the ninerrrrrrrrrrggggggggghhhh 's)
    tend to sail round with the bow in the water (or at least pretty close) with the bow in the water there is no stagnation point.

    Im not sure about the 12. its pretty obvious that it planes and by having the forward spray must have a stagnation point, but im not really sure about how to think about the 12's . they certainly are not any kind of feat in efficiency.
    i spose we can see how the kiwi nuplex goes next interdom. remember that is was almost new and on waterloo bay last time it was still compeditive.
     
  13. mad engineer
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    mad engineer Junior Member

    And we are back to the go over the water or through the water argument...

    The key difference though between the Moth designs and the I14 designs is in the rules. 14s have to meet the mid-point width and rise of floor rules which drives the boat into a planing approach. Moths don't, and they have shown that it is faster for their combination of sail area and beam restrictions to slice through the water.

    If you had a totally free hand though, what would be faster??

    But anyway, I was reading Australian Sailing recently and saw an article about the NS14's and the start of a trend towards having wider bows and pintail sterns as this was closer to the standard aerofoil shape that seems to be used everywhere else for low drag rather than the more typical back to front aerofoil shape of a modern NS14's waterlines.

    I was wondering whether it would really work as I can't help feeling that the impact of having the water surface must change things. Normally, aerfoils are fully submerged in a fluid - eg air or water. The pressure over the surface changes as you go from the leading edge to the trailing edge but there isn't any wave making effect (is there??)

    But, we know that for rudders, they pierce the surface and the drag for a surface piercing foil is much higher than one that is fully submerged. So, if we look at a dinghy hull, it's span is in effect the draft, which in comparison to its length (chord) is very very short. Which must mean the surface piercing effect (ie wave making) must be comparitively enormous.

    Anyone else got any ideas?
     
  14. astevo
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    astevo Junior Member

    i agree the blunt bow would realyy slam into any kind of chop.

    there were some pretty basic errors in that article, i thought so anyway.
    the moths are now narrower in the bow than ever contrary to what nash sugested.
    i also think the new northys are narrower forward than most . (maybee not as narrow on the water as the aero 9 but by reducing the topside flare the effect whilst sailing through waves would ammount to narrower waterlines.)

    id say the wide bow is doomed to fail.
    drag of a surface peircing foil, hull or whatever,i would imagine would have alot to do with the angle of entry. keep in mind i no next to nothing about this sort of theory. Much as it was found that pointy boats were faster way back in the day. Remember bethwaite talking about how the finn went well due to its narrow bow, in his book.

    foils are shaped as they are because they are needed to generate lift. they also do not have to worry about wavemaking drag. and the suface peircing whilst a factor isnt anywhere near the top of the priorities of a foil. lift of auch a narrow aspect foil(a hull ) could never be significant due to massive problems with the short span. as for what is gained from this sort of concept compared to normal hulls (whatever normal means) i think would break down like so.
    1- lifting area forward.
    2-possible lift to windward from hull acting as centerboard. and as a result reduction of wetted surface by reducing foil size.
    its all pie in the sky stuff i cant help but thinking it will never work.

    has anone out there had anyting to do with design of keel bulbs? im wondering why there is so many variations in their design. just have alook at the last americas cup boats.

    in terms of start from scratch hull design. i think the narrow shapes are always going to be fast but there is a question of control at high speed. im of the opinion that long narrow hulls are always going to be fastest throughout the range.

    a while back chris raised the topic of sail area only restricted boats. im curious to know how they developed. in size shape and if one type of boat proved itself better in all conditions(that is does the winner depend greatly on wind strengh)


    one thing is for certain the international canoe stlye rudders are definatly the way to go. the fact that the surface is not peirced reduces drag and the effeect of the hull as a cavitation plate mean is can be much smaller so there is less wetted surface. definatly something to go into a moth in the nerar future. the only problem is we are so short that you loose steering moment.
     

  15. ggggGuest

    ggggGuest Guest

    Some observations...

    Humpless Hulls
    When considering humpless hulls you need to have a very good look at what the wave systems round the boat are doing IMHO. With a narrow fine angle entry boat (with a bow shape not unlike a 49er) in moderate conditions I've noticed the affect that the bow wave is apparently imvisible/non-existent, compared to the big wave system a blunter bowed boat creates.
    This humpless effect is occurring around the speeds where wave generation is at its greatest, and wave generation represents hydrodynamic drag as I understand it. If some factor with some hullshapes is radically modifying this wave system something very different must be happening on the dragfront.

    Scaling up and down
    Dinghy hulls between different sizes scale *really* badly. Some very comptetent designers have come badly unstuck. Between 12 and 14 feet for example there's good empirical evidence that the wetted area/prismatic/form drag etc tradeoffs are *very* different.
     
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