Sailing Dinghy Design

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

  1. Andrew, I'm still trying to get good English language info on the classes with only sail area restrictions but even basic Babel Fish translation says that the weather played a big part in determining with winner. However, they may not be a result of the class restrictions; look at skiffs at some earlier periods, or even Moths in the fat skiff/scow days.

    Re "If you had a totally free hand though, what would be faster??" Andy Dovell knows Moths and 18s and he says an unrestricted 18 would be Moth shaped (ie 18' long, skinny, pintailed, but with 3 sails - sounds like an Assymetric Canoe with 1' extra....).

    Andrew, are you sure those figures on scow and fat Moth performacne are correct? Thorpey and Chris Dey reckon the Moth yardstick should be 98-100, if your figures are right the fat skiff would rate equal to a Europe (yardstick 120) when history tells us that they are much quicker (they blitzed the 2 of a kind on yardstick 108 or so by minutes in light airs).

    The Europe is similar in pace to the US skiffs I think as the US Mistral design was allegedly slightly faster but more tippy than the Europe. Also the Scows were competitive on a yardstick about 9% slower than the narrow skiffs, when the scows were sailed by the best guys. Somwhere I have a chart by Wardy indicating scows had a yardstck about 108 which ties in with my recollections and info.

    In the UK, where yardstick racing is much more important thanm it is here, Colin Newman has collected the figures over the years.
    He reckons they go - current design (HT)=1000, last fat skiff (Magnum 8)=1060, 1985 fat skiff (magnum 5 & 6) 1100, & the Europe is 1139. So that's about 10% difference to old Magnums, which out here were (generally) similar to scows.

    Methinks there's a big effect from the fact that the guys now in scows and skiffs are less expert and are sailing in bad wind.

    As Phil says there is a stagnation point with airflow around aerofoils, and they are totally immersed in the fluid; therefore surely there's a stagnation point with a boat even if the bow is down. Also look at the normal boat; the IC planes with the bow down, unpleasantly so at times, and the vaunted humpless boats also have the bow dcown when they start to plane. Finally, hydrofoils are fully immersed yet they by definition are creaating dynamic lift. The same with a sinker sailboard, they will sail with the bow underwater yet rely largely on dynamic lift.

    The whole planing effect AFAIK is not the over-played Bernouilli effect but the rather simple physics of Newton, isn't it??????
    So therefore can we say a boat that trims bow-leve is not planing? ?????

    Underhung rudders are very nice to sail with, but to my horror some of the top UK sailors use stern-hung rudders on their ICs, as does Steve Clark (IC world champ and owner of Little America's Cup winner Cogito) on his "junior Canoe". The only conclusion I can draw is that they don't make a huge difference BUT that's on a canoe stern where the flow oonto the stern-hung rudder is smooth, and I think it sits in a rather high-pressure area rather than in the "hole" behind a normal transom. The IC rudder is a glorious thing to use, interestingly they haven't gone to much bigger rudders as so many other classes have.

    Mad, the day after I first read about that NS14 theory from Aus Sailing I came across a mention of a WW2 fighter plane with a fuselage designed arouond the same theory. The plane was the Brewster Buffalo, the mention was in a book about disastrous aircraft......that seemeed significant to me. The same "aerofoil" or "cod's head and mackeral tail" shape was seen in yachts until the first "America" came along.

    Then again, the world's fastest course-racing singlehanded mono AFAIK (Lechner D2 board) is actually the teardrop shape. I think the point is that it's skinny (about 500 mm waterline beam on 3.9m length) and therefore, like 12 metres, IACC boats and Sharpies, the bow entry angles are very fine even if the bow is comparativley blunt (in terms of the BMAX being forward and the waterlines convex rather than straight or concave). I don't think an NS (900mm waterline beam (?) on a 4.3m (?) length, more beam in waves) would be skinny enough to benefit from this.

    Chris
     
  2. astevo
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    astevo Junior Member

    you are probably right, about the old moths. none of what i said is actually calculated properly.
    since ive been about the old styles have never really been well represented. and are seemingly always in dirty air as yuou said. if we got some of the good guys back into them im sure they would improve a lot. the fact remains that they are still slow by the standards of todays moths. that said i read in the results for the perth worlds that greg hilton(ex multiple national champ) got some top 10 finished on his bunyip scow in the windy stuff. good effort indeed.
    i like the idea of a big skiff type thing but really i look this sort of discussion for ideas and ways of improving within existing classes.
    in terms of the wide front type of shapes the gee bee pylon racers of the thompson trophy in the 1930's were very tear drop shaped if you look around the big radial engine. sort of looked like a tapered coke bottle with wings. like the big skiffs if you strap 1000hp on to anything, it will go fast. however they did kill people....but somebody once thought it had low drag. not sure if they really did anything serious annalysis of it though.

    i would like to hear more about keel bulb design. does anybody out there know anything about it. i feel this is somewhere where we can learn about how narrow hulls behave. this is because of the reduced bow wave and wave peircing effects in narrow hulls.
    also chris in th d2's how did they handle a big chop.ie did the bow slam alot.
     
  3. Tohbi
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    Tohbi Senior Member

    the ac boats have addressed the question of dynamic lift by adding wings to the keel.

    we've been discussing the differences between shalllow, planing hulls and ac type displacement hulls in the "sail area vs ballast" thread. in recent years the trend has been to the all out speed potential of the planing designs and it is becoming difficult to find sailors who have experience on the older, narrow, heavy-displacement designs. "another guest" gave an interesting observation of his experiences on that thread that bears reading.

    i think we're missing something when we opt in favor of a planing dish simply for a few seconds more speed. of course, three of my four sailboats are planing designs but i like em best when they're heavily ballasted and handle like sailboats should, not like skimming dishes, imho and limited experience [i can imagine ole man herreshoff saying that]. maybe. i was born in the wrong era.
     
  4. Andrew, as far as I can recall the D2s kicked the Mistral type by even more in chop upwind than they did in flat water. Downwind, they were hard to handle, particularly under the old "soft"`pinhead long boom sails when we square ran and you'g catch a wave, surf faster than the apparent, and suddnely you'd be standing on a semi-circular log with the rig blowing backwards into your face...

    Boat-type bows were tried. One won the '85 lightweight worlds, when it was still a development class. The Oz Sailing report said the boat-style bow was fasrt in the chop but I dropped in to that regatta on the way back from the Raceboard/F42 worlds and don't recall anyone raving about its pace particularly; womens and heavy divisions went to the conventional spoon bows that dominated then and later. I think the conventional boards that won that series of the next were brought out to Oz by Zeke and Zali Steggel (world cup winning snowboarder and Olympic medallist skier now) but tghey were nothing unusual.

    In Australia there was a boat-bow D2 but while it was very good in light air, it was slow in a breeze. The mid-section (too fat) and stern (too narrow) had an effect but the same manufacturer later built a faster high-prismatic spoon bow shape.

    The entry angles on these skinny beasts gets so low so easily, that I wonder if you can't get too low a prismatic too easily if you go fine in the snout. Bill Beaver, IC and Moth sailort and naval architect, reckons the IC has too low a prismatic ( I think it's about .62???????) as a result of rules.

    I agree with what you said about Bieker's 14s, he's following the same sort of route as the NSs with the narrow waterline and high chines leading into elliptical sections with a planing flat underneath. It packs lots of volume into a narrow, low wetted surface shape. Lots of the 14 guys really reckon Bieker is outstanding.

    I wouldn't like to bet on a high-prismatic I 14 (unless the loophole the Hutchinson found inthe mid-girth is still open) because I think it would be too fat, but Snubby's high prismatic Moth may be very, very quick.
     
  5. astevo
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    astevo Junior Member

    we have talked about the spoon bow idea before but i dont really see what we can gain from it abart from having low bow entry angles and a quick transfer into the round parallel shape.

    id say the 14s are already very high prismatic and have been moving in that direction even further of late(from what little of the boats ive seen and photos of new ones, this was what i observed, can some of the 14 guys here confirm this ).

    we have got this far and conversation has stagnated a bit.
    i think we have talked over what ideas are present in existing boats but we have had very few new ideas. neither have we made any conclusions that we wouldn't have known previously maybee it is a question of how we adapt ideas from one class to another???
    ive no idea and if it were that easy come up with new ideas to apply id imagine we would be in the shed building better boats rather than discussing it here.
    before we start trying to pull it all together we really do have to have a look at the boats we have all talked about. discus their performance across the range around courses. and of course agree on what we think the significant features of the boat are.

    a while back the aerofoil stagnation point was discussed. this point is at the very front point of the foil where the point id pirpendicular to the air flow. sure they are totally imersed but remember at the corresponding point of a hull is the very pointo ofthe bow where the flow is split. you can really say that a hull is planning just because it pushes water.


    ive got my own opinions but what definitions do you have of planning? it a word that gets used to often given that we all have slightly differing definitions.
     
  6. shu
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    shu Junior Member

    My father and I are building what is essentially 1" shy of being an I14 hull out of tortured plywood over frames and stringers. It started out as a 1983 Farr Javelin, but I didn't like how wide it was forward, so I made some changes. The end result is about 1 inch narrower at the midship measuring point than an I14. (That seems to be the sticky point for this rule). All other measurements are ok I think. But we were already building it when I read the rule. Besides this is going to be a single trap dinghy with about 75% of the upwind SA of an I14. If someone can tell me how you put .jpg images into these posts, I'll show some of the construction photos.

    One of Beiker's boats (I think it was a modified B2) was extra narrow and sported some short longitudinal rails to make it meet the midship measurement requirement. Apparently, they were long enough to avoid disqualification by the dish measurement. The boat was quite fast, and may have won the worlds recently, if memory serves. Does anyone know if this feature was grandfathered in, or if it's still legal?

    I hate to prolong the beam debate, but here goes: Someone said earlier that all the classes without beam restrictions have gone narrow. Here is an exception: The open classes that are used for the singlehanded ocean racing events have no restriction on beam (other than being sort of self righting). Yet, the designs have evolved to be quite wide, and they're always dealing with waves. Perhaps it is because they reach or sail downwind most of the time?
     
  7. ggggGuest

    ggggGuest Guest

    AIUI The Open 60s are so wide because of a side effect of the rules. I think its that they are restricted to the ballast inducing something like 10 degrees of heel. You cant your keel shedloads more sideways if its a wide boat than a narrow one for x degrees of heel.

    If my Farr Cherub of that era is any guide going skinnier on the front is a good call by modern standards. Maybe a tad less rocker too...
     
  8. astevo
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    astevo Junior Member

    the open 60s are also wide so that the water balast is moved off centre. they mainly sailed downwind in wild conditions during the race so the sled type hull shape is better. i wouldnt be suprised if some of the narrower comparable size boats may actually be quicker round the tracks.
     
  9. SailDesign
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    SailDesign Old Phart! Stay upwind..

    You would be surprised at how little downwind work there is in a typical RTW race, and how much upwind stuff. Reaching there is, but not all of it is broad-reaching. Also surprising is the fact that the fat boats go upwind like crazy.
    A lot of the downwoind sailing is in heavy stuff when it seems better to head off and try not to break the boat.
    Steve
     
  10. astevo
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    astevo Junior Member

    i always imagined that the race would be very much a downhill slide, but ok, ill take your word for that. Just thinking about the boat they are very heavily water balasted and canting keel boats. is it the stability that makes them go? also whenever ive seen them sailed they seem to be way over on their ear, maybe this leads to narrow waterlines. My judgements are mainly based on what ive seen of 'Xena' a lengthned open 60 which races seriously on the aussie east coast, out of sydney. it will go downwind with the best maxis but struggles up hill. they recently did the moololaba race with only 6 on board. so they clearly must benefit from being so light.

    maybe having such light weight means that without the 10 ton of water on the rail it floats high enough to benefit from the flare in the hull creating. narrow lines as a result.
    i dont know much about these sort of boats but on my experience (and im heavily biased this way) the narrow hulls always seem to faster particularly uphill. this is of course in dinghies. you get all sorts of different things happening in yachts and in this particular forum it isnt really what we are concerned with.
     
  11. The word from Scott Jutson (deisnger of the boat that holds the Open 50 round the world record, True Blue), Sean Langman (owner of Xea Grundig), Andy Dovell (designer of Xena) and sailing on (once)/against the boat is that the Open boats are reaching/running demons and low and fairly slow uphill.

    Xena, says Sean, is too fat to point, and the water ballast means you're restricted to two-tacking beats. If you get a slower, high-pointing boat to leeward off the line you're in trouble, even a 40 can take a long time to get past if you have to pinch too much.

    The guys reckon a conventional skinny boat without water ballast is better around a windward/leeward or triangle. The fat boats also stick in light winds; too much wsa.

    Does anyone have more prismatics for dinghies?

    My #s go like this; Moth .68, NS .62(?) thanks Stevoes; Canoes .51 to .54 with no direct correlation between Cp and performance (due to mid-length rules), UK National 12s recently .6 to .52 with the latter allegedly "being a little undercooked".

    IC sailor/14 sailor/naval archiutect Bill Beavier says .67 is best above 6 knots boatspeed for an IC.
     
  12. SailDesign
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    SailDesign Old Phart! Stay upwind..

    astevo,
    Coyote, an old (and now departed) Open 60 would sail fastest at 23.5 degrees. We didn;t believe the VPP, but went and tried it anyway. Yes, narrower waterlines. And she would definitely plane uphill, in anything over about 12 knots, and tack through about 95 degrees (not ideal, but better than some...)
     
  13. SailDesign
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    SailDesign Old Phart! Stay upwind..

    astevo,
    Coyote, an old (and now departed) Open 60 would sail fastest at 23.5 degrees. We didn;t believe the VPP, but went and tried it anyway. Yes, narrower waterlines. And she would definitely plane uphill, in anything over about 12 knots, and tack through about 95 degrees (not ideal, but better than some...)
     
  14. ggggGuest

    ggggGuest Guest

    Prismatics

    Recent UK Cherubs vary from about .64 to .68 according to designers, but its *so* variable with the amount of bow/transom immersion you run the figures with for I'm not sure how useful it is anymore!
     

  15. mad engineer
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    mad engineer Junior Member

    Just a thought on prismatics - isn't going for a higher prismatic coefficient only likely to work where the minimum beam is not restricted?

    If you can reduce the beam, then moving volume to the ends of the hull, and thereby increasing the prismatic coefficient, will be an advantage as the maximum cross section of the hull is reduced for the same overall displacement.

    However, if you have a minimum beam restriction, then as far as I can tell going for a high prismatic gives you the disadvantage of a blunter entry without giving you as much benefit from reduced maximum cross section...
     
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