Skinny Class 40

Discussion in 'Boat Design' started by Paul Scott, Jan 16, 2023.

  1. Paul Scott
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    Paul Scott Senior Member

    Spent some time trying to find an existing thread on this, but no joy, so…

    now that class 40 design has reached a point where

    -sailors are afraid to go outside while sailing,

    -slamming is terrifying,

    With the 90 degree test for righting moment based on a narrow kg strap strain righting test, would a narrow hull design make any sense, with

    a blended hull optimized for reaching, and a daggerboard for the light, and upwind(I’m basing this on the 2 moveable foil part of the rule, which might be stretch).

    The advantage might be a lighter, smaller boat than a scow, that could, a la Bieker (Beiker?) be heavier at the waterline and below, much lighter above the waterline, same insanely huge rig, less wetted surface, slamming, green water over the whole boat,and no alarmingly huge amount of hull hanging out in space when going upwind, wagging the whole boat around with every wave, etc.

    A skinny boat might be better upwind, in light wind, soak better downwind, have a kinder motion comfort, things which the current class 40s have given up on. Move the mast a bit forward to allow sailing downwind in the big stuff without jibs etc.

    I’m basing this on the following experience:

    -My boat, Amati, 40’, granted, a coastal cruising sled, which is 10’ beam, 8.5’ at the waterline, a mere 8.5 feet deep, lighter than a class 40 at ~ 9600 lbs, D/L static in the low 90’s, goes upwind like meter boat, can go north of 20 knots downwind with main (<500 sq ft!) only, etc etc. She is more like an open 40, but hey….

    -sailing an E scow in 15+ knots in a chop- given the right combo of gust & wave upwind, an E scow can be easily knocked flat, and at least the bow gets blown/slapped downwind, and the boat staggers in waves. I guess a keel would help that? ;)

    I was drooling at pics of Jack Setton’s 30’ toothpick, Paulo Bua design, which doesn’t pound and zips downwind. Her D/L is a lot less than a class 40 though … ( Maybe the class 40 minimum weight is typeforming?)

    When I look at scow class 40 race trackers, in the big stuff, they seem to chug along mostly at 10-12 knots, suck in light winds, and are anemic upwind in any wind strength.

    Since more than a couple of class 40 designers were looking at windsurfer design for design cues, D2 hulls, which are skinny, worked with both pointy and scow (rounded) bows, and everybody was vanging the hulls (heeling) upwind anyway. I think it’s pretty accepted that D2s were great in the light, upwind, ok in heavier stuff, but not as effective in the heavier stuff as race boards, although the Davidsson and Crit 650 did a bit better in more wind than the rounder D2s.

    There’s more, but I’ve got some non boat stuff to do. Sigh……

    So would a skinny hull offer any advantages, other than avoiding post race laser surgery for detached retinas?
     
    Last edited: Jan 16, 2023
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  2. skaraborgcraft
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    skaraborgcraft Senior Member

    Big fan of ULDB types as well as Colin Archers.....each have their place. Skinny boats like the Van De Stadt Royal Cape One Design and "Black Soo", have shown their worth. They just lack "form stability" compared to a fat sterned modern sled, but a deep canting keel could help.

    I crossed the Atlantic on a smaller but similar type of chined hull, and it would, if one is brave enough to pile on sail, easily surf on moderate waves. Upwind, for a light boat, the chine really helped by digging in.

    If i was the winner of the Maine lottery, I would be building a 40ft version of "Black Soo", with canting keel, carbon spars and modern sails.
     
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  3. Paul Scott
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    Paul Scott Senior Member

  4. wet feet
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    wet feet Senior Member

    I have the feeling that the designers currently operating will have run through a range of possible permutations before sending their designs into the world.I'm not familiar with the state of the art with respect to VPP software and how well integrated it might be with foiling hulls or even Class 40's.Which might be unfortunate because running a comparison inside a box on one's desk is much less expensive than building the real thing and the result is known in a fairly rapid manner.So my question is what is the destination of this thread?Are we simply discussing permutations of shape,estimating performance or is there an attempt at a real design coming at us for comments and maybe,a potential build?From a competitor's point of view it would be a bold individual who built a design that strayed too far from the currently successful crop-unless there is a clear and demonstrable advantage.I doubt that they mind too much that the features that make the boats fast compel them to remain undercover,first to the finish is what matters and then the discomfort is over.I don't think there is any particular desire on the part of any sailor to be compelled to go on deck if the systems allow the same job to be done while sheltered.Going back in time I remember Mike Richey taking great pride in being able to cross the finish line of the OSTAR without having to set foot on the deck of Jester throughout the race.
     
  5. skaraborgcraft
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    skaraborgcraft Senior Member

    Skippers sailing skinny boats have always said the same thing when competing against beamier rivals, "we dont have the same power (righting arm)" . Skinny boats can not deploy water ballast tanks in the same way.

    Just watched the start of the Ocean Race from Alicante, those foiling boats are doing 35kts and no skinny hulls to be seen. I still like them though. Not much boat to build, less sail area to power, though wetted surface might be an issue. Im no racer, but i do appriciate an easily driven hull.
     
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  6. Paul Scott
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    Paul Scott Senior Member

    Idle curiosity more than anything, given I have a boat designed and built before the class 40 keel hauled the open 40 scene, claiming it was an affordable & wholesome alternative. Still, since windsurfers were involved: (but you are right about fa fa fa fa fashion…)

    74745046-9725-4772-9846-01EDD4C69798.jpeg
    and an open 40, for good measure

    A612F3AB-2FF0-4FE4-B269-A226E91053BC.jpeg
     
  7. Paul Scott
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    Paul Scott Senior Member

    but as foils are developed, will heavy wide aircraft carrier hulls be necessary? (Look at foiling Moth hulls these days….)
     
  8. skaraborgcraft
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    skaraborgcraft Senior Member

    It is beyond my area of interest really. I think at present its the area of the transition zone between sailing and foiling and those usual wind and sea conditions that still require a "normal" hull to operate. No reason a skinny hull cant compete "if" the foils were the same distance from the mast as the wide body boat, as that lifting arm would other wise be reduced. The moth style from what i understand requires skillfull body placement and mobility to stay foiling, not sure how that might work with a larger boat, though no reason why not in theory......just instability from a single foil source rather than two?

    I cant really imagine foiling in excess of 20 knots and actually sleeping, bad enough hitting a log at 4 knots......
     
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  9. wet feet
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    wet feet Senior Member

    The comment about foiling Moth hulls can be taken in either of two ways;the hull itself is about 18 inches wide and I'd call that fairly skinny.The wings,which Moths have featured for more than fifty years,extend to about 7'4" and exit for the maximising of righting moment,they also serve as the attachment point for the shrouds.The mention earlier regarding righting moment is at the core of the speed enhancement that has occurred in classes that were formerly dependent on lumps of ballast slung beneath the hull.The combination of canting keels and water ballast has given what were pure displacement hulls the opportunity to get through the water more rapidly.The more effective location of the mass that provides the righting moment has permitted the displacement of the boat to be reduced and consequently they have to move aside a smaller amount of the ocean as they pass.Which leads to the conclusion that a skinny hull won't generate the righting moment,but may have advantages in very light winds due to it's lower wetted surface.A Moth in low flying mode isn't affected as much because it already has a hull with a tiny amount of wetted surface,but it demands a very high level of fitness and competence to make the most of the speed potential.
     
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  10. Paul Scott
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    Paul Scott Senior Member

    I get that- the question I have at the center of this is, given the porky weight limit of these boats, would there be any advantage to a smaller spiral of sorts-

    A smaller hull (slender) needing less material, possibly shaped something like a d2 round board, which might make rule bound building materials easier to work with

    so excess weight could be concentrated at, or below the waterline,

    which might reduce or eliminate the keel bulb, which, along with a smaller hull, would reduce drag,

    Which means smaller sail area (easier sail handling) or in this case, more effective unlimited sail area downwind (sail area to hull surface area).


    I think most agree that in Archimedean mode, and in waves, this could be faster, less frontal area meeting waves.

    The crux seems to be in forced mode, between Archimedean and planing, which is where the open 40s seem to live downwind. The only rule test for stability seems to be the 90 degree test, where the mast has to exert a pressure upwards on a strap between certain values, so everybody plays in that sandbox, the main difference being how much hull your dragging around in and out of the water. So my question is, how much is the drag differential between scow and skinny, both static and dynamic? Computing hull behavior/ drag in forced mode and planing is notoriously difficult and beyond what I have at hand.

    But for example, consider the pic of Robbie Naish above, and look at the hull in its waves- the hull isn’t riding on top of the water, the stern is lower that it would be at rest, but the release is clean, and the bow is up a bit, but how much wetted surface friction drag is actually reduced vs how much wave drag is creating drag? When I model scow vs skinny, wave and skin friction goes waaaay up for the scow vs skinny. I mean waaay up- so the scow seems like a brute force approach. But maybe the rule is type forming that way, since it was developed from a French offshore point of view? On the other hand, if enough % of the weight is down low, how much stability would there be for less, but more refined force along with less drag? More than a few NAs ugly there are advantages to skinny (like Tom Wylie).

    what’s going on in the class 40 off of Brazil is informative, as well as the beginning of the Route de Ruhm, I think- how much does being in front after light air make up for coming onto heavier air with brute force? Maybe a different routing strategy? Differently shaped sails?
     
    Last edited: Jan 17, 2023
  11. myszek
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    myszek Junior Member

    Something like this?
    vds40.jpg
    It was drawn several years ago, when Class 40 rules allowed the use of DSS foil:
    vds40dss.jpg
    and the estimated performance was really good.
    Unfortunately, this changed: foils are explicitely forbidden in the present version of class rules. As well as daggerboards and canting keels.

    I played with various hulls for 40's. With a heel of 30deg, a skinny hull provides the righting arm about 0.7m, while the 4.5m wide one gives 1m, and the scow-like one 1.2m.
    On the other hand, the drag of the skinny hull can be lower by 20% than for wide one. As a result, the VMG windwards for a skinny boat is lower by about 0.5kt, according to my simple VPP. When reaching and running, the difference vanishes.

    Foils or canting keels would obviously reverse this result, but they are not allowed. Currently, the only possible way to provide the additional rigrhing moment is the use of paravanes (see for example ) - until they are forbidden, too. Anyway, nobody tried paravanes in ocean racing sailboats, somebody should at last...

    regards

    krzys
     
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  12. Paul Scott
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    Paul Scott Senior Member

    very cool! Intriguing stuff, especially offwind performance. ( Or was this with DSS assist?) Couple questions-

    how much would a flap on the fin keel change upwind performance between scow, mid, and skinny hulls? ? (I think keel flap is allowed?) While a skinny hull loses VMG to a scow to windward (I think I have that right), how much does that account exploiting the ability of a skinny hull to point higher and then soak for speed? Or steering through waves? (This is what I’ve noticed with Amati.)

    any idea of drag differences in waves?

    Would placement of hull construction (more weight at waterline and below) in a skinny hull be moved down enough to affect bulb size? Affect strap test?

    At 90 degrees, what effect does a skinny hull have on the strap test vs a scow hull?

    Would the skinny hull have a higher ‘hull speed’ in reference to forced mode? (I’m trying to remember at what point 1.3 * square root of the waterline moves towards 1.6 * square root of the waterline.)

    Thanks again
     
    Last edited: Jan 17, 2023
  13. Paul Scott
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    Paul Scott Senior Member

    C28F340E-CD1E-4287-AEBF-FCE773E6944E.jpeg
    I should have typed lowrider skinny Bambi in Headlights type Moths. My apologies.

    There is the Merlin Rocket approach- above

    Water ballast on the edge of a similar flare on a class 40? IIRR, Callaghan found a balance between weight and rocker depth / shape that was effective leveling things out between different weight crew, and this was for a planing dinghy… although some new 30s are looking like they are trying that approach, and using flare as immersed hull when heeled- Hans Genthe’s Aolos 30 kind of has that approach, I think. A bit less D/L than class 40s.

    It might be useful to compare low rider scow moths vs skiff moths, I suppose- the D/Ls weren’t that different from the class 40’s, setting aside restricted sail area for the moths.
     
    Last edited: Jan 17, 2023
  14. skaraborgcraft
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    skaraborgcraft Senior Member

    From my own experiments, flatter buttock sections going aft and keeping low displacement allows for exceeding the traditional sqrtWLx1.3. Wetted surface area plays a big part in the forced hull speed in displacement mode, but a skinny hull will knife through chop that will cause a wider hull to slam. Though given the Mini scow performance against "normal" bow minis in the Fastnet one year, it did very well upwind, but more of an endurance test for the skipper and boat to which breeakes first.
     
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  15. skaraborgcraft
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    skaraborgcraft Senior Member

    Something like that, yes. I go sailing to get away from other peoples "rules".....so one has to be committed to a race class to follow said rules. When things get that serious, people stop having fun. Each to their own.
     
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