4 Meter mono foiler project

Discussion in 'Sailboats' started by wind_apparent, Apr 27, 2008.

  1. wind_apparent
    Joined: Apr 2008
    Posts: 257
    Likes: 6, Points: 0, Legacy Rep: 96
    Location: boulder colorado

    wind_apparent wind driven speed addict

    You have to remember there are foils under the thing adding their own plot to the story. The flatter wide hulls (49er, 18fter) are great for plaining, but a foiler lifts off before plaining speeds are achieved, so we are dealing in a sub-plaining area here. The game is changed to the fastest hull shape in a closed 0-7/8 kt window. My reasoning behind the wider hull was this, 1. I was interested in testing a theory i'd been throwing around with a NA friend: If you take 2 moth hulls that are the same weight, one skinny and deep and one wide and short, which one is faster? Went back and forth on it a while, but everything seemed to come down to which is worse, the parasitic wave making drag of a current skinny moth hull, or the wetted surface area drag of a wider low freeboard hullform with less wavemaking drag? Since I'm not a CFD wiz, and I wanted a moth anyway, I figured I could test it out. 2. I am no lightweight by any means, (185lbs) I was thinking as the skipper weight gets heavier, the skinny hull sinks more and more, causing more drag for a longer period as the sailor weight goes up. They already have a disadvantage when it comes to take off speeds, so maybe heavier helms would benefit from a wider hull. 3. I'm just learning, so stability might be nice. 4. I had all the hull parts sitting in the shop ready to cut down and glue together, which would get me foiling much quicker by using some off the shelf moth kit.

    We'll have to wait and see what happens, or as doug said, if it happens. (I know, I couldn't believe he of all people said it either.) Anyway, hull-shape is not even relevant if your fois are ****. Right now I just want to put a boat together so I can go sailing instead of building. Off to the shed.
     
  2. waynemarlow
    Joined: Nov 2006
    Posts: 412
    Likes: 36, Points: 28, Legacy Rep: 134
    Location: UK

    waynemarlow Senior Member

    I think we need to totally rethink the way the moth hulls need to be thought of, they are only there to get up enough speed to let the foils work, what they don't need to be is displacement type hulls with lots of energy wasted pushing the water aside.

    I know that canoe body shapes have the least resistive shape of all and now with the Moth jockey being more and more in one central position then it too must have merits.

    Would love to get involved in the Moth class but at 95kgs it would be a problem, pity they don't have a form of equalisiation of some kind, but that in itself would create so many problems. Perhaps just for fun.
     
  3. Doug Lord
    Joined: May 2009
    Posts: 16,643
    Likes: 315, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    ==========
    Less wavemaking drag than a skinny hull of the same length? Really?
     
  4. Doug Lord
    Joined: May 2009
    Posts: 16,643
    Likes: 315, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    ---------
    Wayne, the Moth can carry the weight-just doesn't do so well in light air or marginal foiling conditions.....
     

    Attached Files:

  5. Doug Lord
    Joined: May 2009
    Posts: 16,643
    Likes: 315, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

  6. CT 249
    Joined: Dec 2004
    Posts: 1,701
    Likes: 78, Points: 48, Legacy Rep: 467
    Location: Sydney Australia

    CT 249 Senior Member

    The 49er is a narrow hull by comparison to older skiffs - all of the skiff classes have moved away from a planing-style hull to a narrower "displacement" hull designed to reduce WSA and waterline beam. Bieker, Morrison and all the Aussie skiff designers are all on record as saying this.

    The flatter and wider a sailboard is then the slower it is in light winds. At the ISAF trial at Garda a few years ago, in light winds the very fat and wide (7'6" long x 3' wide) Formula boards could not get to the first mark before the longer and skinny (12'6" x 2'3") Raceboards finished the two-lap race. That's despite the Formula board carrying 11-12.5m of sail compared to the 7.4 to 9.5 of the Raceboard!

    Similarly, a round-bottomed 12'9" "Division II" board, which has a narrower waterline beam and a skinny cigar-shaped hull, is even faster than the Raceboards in light winds. Before foils, a Division II board with 7.3m sail was as fast as the world's best Moths in light winds, whereas a "tunnel hull" Raceboard was definitely slower than a Moth.

    The "tunnel hull" Raceboards are distinctly slower in light winds than the "boat shaped" round-bottomed Division II windsurfers. And of course tunnel-bowed scow Moths are vastly slower than skinny displacement-style Moths in light (or heavy) winds. However, some do say that in a drifter an older and wider but very round and veed Duflos-type Moth is faster still.

    The proof on the water, tested time and time again, is that the narrower shapes are quicker in light winds than the tunnel-hull shapes.
     
  7. CT 249
    Joined: Dec 2004
    Posts: 1,701
    Likes: 78, Points: 48, Legacy Rep: 467
    Location: Sydney Australia

    CT 249 Senior Member

    The Moths, when foiling, are very close in performance to Formula Boards, which are the fastest windsurfers around a course in such conditions. In non foiling conditions the Moths are much faster than the Formula Boards.

    The Moths are much quicker in foiling conditions than a long "semi-planing" windsurfer, if by that you mean a Raceboard-style hull like the former Olympic Mistral IMCO. In lighter winds the Moth probably still has the advantage.

    When you say "perhaps its time to have a more careful look, anybody have any data on drag resistantance of a displacement hull such as the Moth to that of a tunnel hulled windsurfing board" it seems that you don't think that Moth designers have thought of these factors. In fact, many of them have a vast amount of theoretical and practical back-up - for example, the Hungry Tigers were partly designed on the computers of an America's Cup designer, who also gave some input. A very careful look has already been taken.
     
  8. Doug Lord
    Joined: May 2009
    Posts: 16,643
    Likes: 315, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    =============
    The 49er hull and other modern hulls have not moved away from planing hulls-they have moved away from "humps" in the drag curve common to almost all planing boats-more so before the early 90's.
    Frank Bethwaite in Book II* says:" In the years since 1990 one of the principal design advances has been first the recognition of the dynamically humpless hull-the realization that hulls exist that do not baulk as minimum planing speed is approached-and then the exploitation of this understanding in the design of useful boats"
    Of course,THE principal design advance in dinghies since before the 90's is the bi-foiler, as demonstrated by the Moth, R Class ,Mirabaud and other one offs. The drag reduction/speed increase related to the adoption of the bi-foiler configuration is far and away the most important single advance in dinghy** design,probably, since the days of Uffa Fox.
    --
    The humpless design can be important to a foiler too if the onset of planing tends to occur prior to liftoff-a humpless design would be a clear advantage in that case.
    In the case of the Moth with a 10/1-11/1 or so L/B ratio(at the waterline) the onset of planing occurs substantially after liftoff so a humpless design is of no consequence.
    As the L/B ratio gets smaller(the hull at the wl gets wider) the consideration of a humpless design becomes more important for a foiler in order to facillitate early takeoff-depending on the waterline length of the boat. A "hump" in the drag curve just before takeoff is something to be avoided.

    * "Higher Performance Sailing"
    ** for my purposes, dinghy= small sailboat
     
  9. CT 249
    Joined: Dec 2004
    Posts: 1,701
    Likes: 78, Points: 48, Legacy Rep: 467
    Location: Sydney Australia

    CT 249 Senior Member

    I used the term "planing-style" and "displacement" as some dinghy designers do - as shorthand for the conceptual difference between a design intended to maximise planing lift (i.e. "Wedge" style Aussie 14, classic Aussie Cherubs, etc) and a design that chases lower drag at medium and low speeds - in other words something like the "humpless" hull. Of course all these boats plane, but the "displacement" term merely reflects a boat where there is a lower priority on maximising planing lift and a higher priority on reducing drag when in the displacement regime.

    Paul Bieker described what I was calling the "planing style" like this in a piece on '80s Aussie 14s; “Most of the Australian hull designs have very flat bottoms, with a little rocker distributed evenly over the length of the boat, low chines, and fine bows. This style of boat is potent in flat water and 12 knots or more breeze, planing earlier and faster than other designs. However, in light air they seem to be relatively slow, due primarily to high prismatic coefficient and a greater tendency towards transom immersion.”

    The "displacement" hull (using a term from 12 Foot Skiffs etc) has higher (if any) chines, a narrower waterline and more U-shaped sections, often with a narrower stern. It therefore has lower wetted surface area and lower form drag, at the expense of losing planing area. To quote skiff designer and NA Rob Widders, this style is “more efficient upwind and downwind in wind speeds less than 20 knots due to the lower wave and frictional resistance as a result of form and wetted area factors”. The lower form drag allows these boats to go faster in light and moderate winds. They have less of a "hump" than earlier designs because their WSA and waterline beam are narrower. Some of these features were seen many years before on MRs and N12s. OF course, the lower drag at displacement speed increases speed which then increases dynamic lift and therefore allows the boat to plane early despite having less planing area.

    Some say that the 9ers adopt a different approach to achieving the same effect. That is, because they are not constrained by class rules they can be considerably longer and/or carry more sail, thereby dramatically reducing their DLR and form drag and increasing their hull speed and reducing the drag hump around the forced regime. The fact that 9ers are free of class rules means that they don't need the same high-volume sections to achieve the same result, like shorter and heavier skiff types do - however there remains the same intention of reducing drag at lower and medium speeds rather than on increasing planing lift at those speeds. Julian for example referred to the development in his 18s as moving more to a "displacement" shape and away from the Tasar hull that could be called more of a "planing" shape.

    The May/July 2007 report from Simon Watin confirmed that the 49er DOES have a hump when trimmed with crew weight to achieve its maximum performance. The humpless drag curve came when no such trimming was done - the boat is faster WITH a "hump" caused by a reduction in drag.
     
  10. Doug Lord
    Joined: May 2009
    Posts: 16,643
    Likes: 315, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    =================
    A "hump" caused by a reduction in drag??? CT, forgive me but that just doesn't make any sense: a "hump" is, by definition, excess drag that occurs just prior to planing!
    Can you provide a link to the report by Simon Watin?
     
  11. CT 249
    Joined: Dec 2004
    Posts: 1,701
    Likes: 78, Points: 48, Legacy Rep: 467
    Location: Sydney Australia

    CT 249 Senior Member

    Imagine a boat that has the following drag figures;

    1 knot - 2 lb of drag
    2 knots - 4 lb of drag
    3 knots - 6 lb of drag
    6 knots - 12 lb of drag
    8 knots- 16 lb of drag
    10 knots - 20 lb
    12 knots - 24 lb
    15 knots - 30lb etc

    Plot that and you will see a straight line.

    Imagine a second boat that has the following drag figures;

    1 knot - 0.5 lb of drag
    2 knots - 1 lb of drag
    3 knots - 3 lb of drag
    6 knots - 12 lb of drag
    8 knots - 16 lb of frag
    10 knots - 17 lb of drag
    12 knots - 20lb of drag
    15 knots - 22lb of drag

    Boat 2 has a planing hump but has much lower drag at most speeds and is therfore a better boat in that respect.

    The hump in the Watin tests came because drag at lower speed ranges was reduced by crew positioning - plotting that lower area on the drag curve created a "hump" at higher speeds.

    Given the slurs you have made at me I see no reasons to send the Watin information - to ask for something from someone whose honesty you have repeatedly impugned seems rather odd.
     
  12. Doug Lord
    Joined: May 2009
    Posts: 16,643
    Likes: 315, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    ===================
    I made no slurs-absolutely false-I merely questioned a couple of things you said-and I still do:
    1) "all of the skiff classes have moved away from a planing-style hull to a narrower "displacement" hull"

    2) " the boat is faster WITH a "hump" caused by a reduction in drag."
     
  13. CT 249
    Joined: Dec 2004
    Posts: 1,701
    Likes: 78, Points: 48, Legacy Rep: 467
    Location: Sydney Australia

    CT 249 Senior Member

    (1) is a simple fact - refer to the Bieker quote, for example. There is a distinct difference in shape and philosophy between a Bieker and a Wedge, or a Nash and an O'Mahoney, or a Brownie and a Bethwaite - or a 49er and an earlier Bethwaite. Talk/correspond to Bieker, Bethwaite, Nash, Morrison, Walsh, Moore, Stephenson, Brown, etc and study designs and the pattern becomes obvious and is well known. I can't help it if you don't want to know it.

    (2) is obvious - if you reduce a boat's drag below planing speed, then a low spot in the drag curve will appear at those speeds. If you have a low spot you inevitably have something of a "hump" next to it - but if the hump is caused by REDUCING pre-planing drag then it is obviously a good thing.

    And you have repeatedly insulted me and my integrity in the past, so you shouldn't be surprised if I don't hand over stuff sent to me by a designer.
     

  14. CT249
    Joined: May 2003
    Posts: 1,230
    Likes: 68, Points: 48, Legacy Rep: 215
    Location: Sydney Australia

    CT249 Senior Member


    Quoted to use in another thread
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.