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.

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 showing the typical bow up trim of the Cherub.

```MACSAIL DATA SHEET: Asy.10xaero & Main.10xaeroª 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

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

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

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]

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
```