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Lifting bows with foresails >
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Lifting bows with foresails

Backview

Lifting the Cherub«s bow

For those of you who followed the "Lifting bows w/foresails"-thread in rec.boats.racing: Here«s a computer run for the Cherub-dinghy, with an asymmetrical spinnaker in an apparent wind of 20 kn at an apparent wind angle of 70¡.

I only give a short explanation below. You can figure out the rest from the listing at the end of the article, if you«re interested. A very good, more in-depth discussion on the subject can found at Jim Champ's Cherub pages.


View from behind & below

Asymmetrical - section forces [N] & moments [Nm]


Section DriveF SinkF HeelF HeelMom YawMom PitchMom 1 24 -13 12 3 1743 -19 2 143 -97 141 151 5116 77 3 182 -122 153 343 2646 332 4 171 -140 124 408 -1247 533 5 123 -161 73 311 -3031 585 6 26 -53 10 48 -558 174 Total 669 -586 513 1265 76 1682


  • F stands for force (in [Newtons]. 1 N = 0,1 kg = 0,2 lbs)
  • Mom stands for moment
  • Section 1 is at the foot of the sail, 6 at the top (every section between the red lines).

The third column, SinkF, is the sail force component perpendicular to the sea surface. Negative sink is LIFT, so the kite is lifting the boat at a force of 586 N, or about 59 kilos. The asymmetrical is driving the boat at 67 kilos, so the sail is lifting the boat nearly as much as it is thrusting it forward.

Considering that the boat weighs (incl. crew) some 250 kg, the lift of the asymmetrical is important in terms of reducing displacement. No wonder Cherub sailors will tell you their boat is merely skimming on the surface, leaping from wave to wave.

 MainSail - section forces [N] & moments [Nm], including mast


Section DriveF SinkF HeelF HeelMom YawMom PitchMom 1 46 -5 38 31 -2691 42 2 78 -9 61 102 -5615 142 3 73 -11 56 145 -6003 207 4 65 -12 49 174 -5760 250 5 60 -15 46 205 -5461 293 6 35 -12 23 125 -2052 211 Total 356 -65 273 781 -450 1146


Even all sections of the main are lifting (negative SinkF). I have assumed a realistic bow-up trim of the boat (from photos) - the mast leans so heavily aft that the main is actually working like your book cover opened. Nevertheless, both sails are pitching the bow down (positive pitching moment), the asymmetrical even more so than the main.

Together, the lift of the main and the asymmetrical corresponds to approximately 30% of the boat's displacement.

Sideview
Sideview showing the typical bow up trim of the Cherub.


MACSAIL DATA SHEET: Asy.10xaero & Main.10xaero» AWA:  70.0¡


* INPUT VALUES *

Rig measurements
      I= 5.650     J= 3.000     BAS= 0.600     Rake at Fstay= 1.100

Sail scaling
      Headsail luff= 6.500      Mainsail luff= 5.480

Mast diameter
      Up to hounds=   0.000     At mast head=    0.000

Apparent Wind
      App. wind angle= 70.0     App. wind speed= 10.0 VelScale= 20.0

Sheeting angles
      Mainsail=         47.0    Headsail= 49.0

Boat attitude
      Heel angle=      0.0      MRPL(%)= 10.0
      Headsl tack h.= 0.250     Freeboard h.= 0.5

* PANEL CONFIGURATION *
      CutNumber= 6

Panels in Headsail
      Segments= 6      Sectors= 10      Total panels in Sail= 60

Panels in Mainsail
      Segments= 6      Sectors= 10      Total panels in Sail= 60

Wake alignment:
      HeadSail Steps: 3, Steplenght (m): 0.1000
      MainSail Steps: 3, Steplenght (m) = 0.1000

* AERODYNAMIC COEFFICIENTS *

* HeadSail             Area = 13.0  m2

  HeadSail - Shape description

  Section  Height   Chord    Girth   Camber    Twist  Entry/CL
      1    0.184    3.229    3.629   21.5 %    -11¡     86¡     Fris 22-16
      2    0.886    2.938    3.341   22.7 %     -6¡    -82¡     Fris 55-14
      3    1.929    2.604    2.948   22.3 %     -2¡    -76¡     Fris 55-13
      4    3.005    2.119    2.432   23.5 %      2¡    -70¡     Fris 51-13
      5    4.219    1.307    1.535   25.6 %      7¡    -61¡     Fris 46-13
      6    5.258    0.400    0.482   27.6 %     12¡    -54¡     Fris 41-14

  HeadSail - Lift, drag & flow

  Lift coeff. CL  = 0.993     Drag coeff. CD = 0.335
  Drive coeff. Cx = 0.818     Heel coeff. Cy = 0.654

  Section  Section cl         Luff flow               Leech flow
      1       0.28            luffing                 Sep. at x= 0.74
      2       0.85            luffing                 Sep. at x= 0.80
      3       1.16            Ideal - low             Sep. at x= 0.84
      4       1.22            Ideal incidence         Sep. at x= 0.80
      5       1.11            Lee bubble - short      Sep. at x= 0.79
      6       1.23            Lee bubble - long       Sep. at x= 0.74

  HeadSail - section forces [N] & moments [Nm]

  Section   DriveF   SinkF    HeelF   HeelMom  YawMom   PitchMom
      1       24      -13       12        3     1743      -19
      2      143      -97      141      151     5116       77
      3      182     -122      153      343     2646      332
      4      171     -140      124      408    -1247      533
      5      123     -161       73      311    -3031      585
      6       26      -53       10       48     -558      174

  Total      669     -586      513     1265       76     1682

  HeadSail - Center of effort distance from mastfoot [m]

  Vertical CE      Horisontal CE      Pitching CE
  -17.386 m           0.064 m           1.340 m


* MainSail             Area = 8.2   m2

  MainSail - Shape description

  Section  Height   Chord    Girth   Camber    Twist  Entry/CL
      1    0.339    1.795    1.809    5.5 %      1¡     59¡     Fris 21-20
      2    1.157    1.786    1.813    7.5 %      2¡     66¡     Fris 22-19
      3    2.116    1.724    1.764    9.3 %      4¡     73¡     Fris 23-18
      4    3.077    1.577    1.622   10.3 %      6¡     78¡     Fris 24-18
      5    4.044    1.299    1.337   10.6 %      8¡     82¡     Fris 26-17
      6    5.019    0.633    0.651   10.6 %      9¡     84¡     Fris 27-16

  MainSail - Lift, drag & flow

  Lift coeff. CL  = 0.859     Drag coeff. CD = 0.270
  Drive coeff. Cx = 0.715     Heel coeff. Cy = 0.548

  Section  Section cl         Luff flow               Leech flow
      1       0.77            Lee bubble - short
      2       0.88            Lee bubble - short
      3       0.85            Ideal - high
      4       0.81            Ideal incidence
      5       0.90            Ideal - low
      6       1.03            Lee bubble - long       Sep. at x= 0.25

  MainSail - section forces [N] & moments [Nm], including mast

  Section   DriveF   SinkF    HeelF   HeelMom   YawMom   PitchMom
      1       46       -5       38       31    -2691       42
      2       78       -9       61      102    -5615      142
      3       73      -11       56      145    -6003      207
      4       65      -12       49      174    -5760      250
      5       60      -15       46      205    -5461      293
      6       35      -12       23      125    -2052      211

  Total      356      -65      273      781     -450     1146

  MainSail - Center of effort distance from mastfoot [m]

  Vertical CE      Horisontal CE      Pitching CE
    3.755 m          -0.716 m           2.725 m


* Combined coefficients, both sails together

                        HeadSail       MainSail       Combined

  Lift coefficient       0.993          0.859          0.941
  Drag coefficient       0.335          0.270          0.310

  Drive coefficient      0.818          0.715          0.778
  Heel coefficient       0.654          0.548          0.613

  Heel.moment coeff.     5.035          4.948          5.001

  Ratios

  Lift/Drag ratio        2.965          3.179          3.037
  Drive/Heel ratio       1.251          1.305          1.270
  Drive/Heel.momnt r.    0.163          0.144          0.156

  * Forces & moments in apparent wind 10.0 m/s,    70.0¡, no leeway

                        HeadSail       MainSail       Combined

  Driving  force         0.650          0.357          1.007 kN
  Heeling force          0.520          0.274          0.793 kN

  Heeling moment         1.265          0.781          2.046 kNm
  Yaw moment             0.076         -0.450         -0.374 kNm
  Pitching moment        1.682          1.146          2.827 kNm

  Center of effort distance from mastfoot [m]

                        HeadSail       MainSail       Combined

  Vertical CE          -17.386          3.755         15.116
  Horisontal CE          0.064         -0.716         -0.207
  CE for pitching        1.340          2.725          1.687

  * Forces in apparent wind    10.0 m/s,    70.0¡, leeway= 4¡

  Driving  force         0.685          0.375          1.060 kN
  Lateral force          0.473          0.248          0.721 kN


Aerodynamics calculated in 172 seconds
                        


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