Canting investigation – A numerical simulation with DB32 Jean-françois Masset May 2018 [email protected]

DB32 is a numerical boat inspired by Melges 32 and firstly used to investigate the Dynamic ballast issue, see details of the boat and of the VPP used for this study in : https://www.boatdesign.net/threads/dynamic-ballast.59729/

1. RM curve with keel at 0° and at 45°, for DB32 at 1952 kg displacement inc. 3 crew sit windward

RM (kN.m) vs Heel (°)

DB32 Disp.1952 kg inc. 3 crew sit windward Red : Keel 775 kg at 0° ; Blue : Keel 775 kg at 45°

25

20

) 15 m . N k ( 10 M R 5

0 0 5 10 15 20 25 30 35 40

Heel (°)

>>> the RM30° increase by +24%, so do the strengths on the rig and the concerned structure

2. Comparison upwind and downwind with same sail surface

We just add the wetted surface of an extra needed to provide the leeway resistance that the keel at 45° no longer can provide efficiently : a daggerboard surface estimated at 80% of the keel wing for upwind sailing, 1/3 of this daggerboard for downwind conditions.

Upwind conditions includes waves for a Raw estimated to 8% of the cam water drag.

No added waves resistance is taken into account for the downwind conditions. Upwind with twa 42° - Boat speed (Knts) vs true wind (Knts)

DB32 Disp.1952 kg inc. 3 crew sit windward, same sail surface Red : Keel 775 kg at 0° ; Blue : Keel 775 kg at 45° + daggerboard

8,0 7,0 6,0 ) s t

n 5,0 K (

d 4,0 e e p

s 3,0

t a o 2,0 B 1,0 0,0 0 2 4 6 8 10 12 14 16 18

Wind (Knts)

>>> the speed advantage really starts for a wind of 8 Knots, of about 10% to 15% + the ability to sustain the speed for 2 more wind knots before the first reefing (i.e. before reaching 30° heel). Below 8 Knots of wind, it is better to keep the keel at 0°, and even a bit negative (leeward side) in very light winds to reduce the overall wetted surface and help sails to keep a good shape.

Upwind with twa 42° - Boat heel (°) vs true wind (Knts)

DB32 Disp.1952 kg inc. crew 3 sit windward, same sail surface Red : Keel 775 kg at 0° ; Blue : Keel 775 kg at 45° + daggerboard

40 35 30 25 ) ° (

20 l e e

H 15 10 5 0 0 2 4 6 8 10 12 14 16 18 20

Wind (Knts)

>>> the gain in heel is mostly in the 20° to 30° heel range, for wind > 11 Knots. Below 8 Knots of wind, it is better to keep the keel at 0° anyway. Downwind with twa 140° - Boat speed (Knts) vs true wind (Knts)

DB32 Disp.1952 kg inc. 3 crew sit windward, same sail surface Red : Keel 775 kg at 0° ; Blue : Keel 775 kg at 45° + 1/3 daggerboard

14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 0 2 4 6 8 10 12 14 16 18 20 22 24

>>> the speed advantage really starts for a wind of 12 Knots, up to 18% for wind 18 Knots + the ability to sustain the top speed for 4 more wind knots before the first reefing. Below 12 Knots of wind, it is better to keep the keel at 0°.

Downwind with twa 140° - Heel (°) vs true wind (Knts)

DB32 Disp.1952 kg inc. 3 crew sit windward, same sail surface Red : Keel 775 kg at 0° ; Blue : Keel 775 kg at 45° + 1/3 daggerboard

45 40 35 30 25 20 15 10 5 0 0 2 4 6 8 10 12 14 16 18 20 22 24 3. Comparison upwind and downwind with a 118% sail surface

The heeling arm is also increased accordingly (Zsail is increased by 1,18^0,5).

Upwind with twa 42° - Boat speed (Knts) vs true wind (Knts)

DB32 Disp.1952 kg inc. 3 crew sit windward Red : Keel 775 kg at 0° ; Blue : Keel 775 kg at 45° + daggerboard 100% sail surface Blue dashed line : Keel 775 kg at 45° + daggerboard + 118% sail surface

8,0 7,0 6,0 ) s t

n 5,0 K (

d 4,0 e e p

s 3,0

t a

o 2,0 B 1,0 0,0 0 2 4 6 8 10 12 14 16 18

Wind (Knts)

>>> The speed advantage is noticeable only in light winds, about 8% for wind speed < 8 knots. For stronger winds, the extra sail surface does not give more top speed, due to the extra heeling arm and drag due to more heel. Actually, the speed curve shifts to the left, demonstrating that for light winds the propulsion key parameter is the sail surface while for strong winds the propulsion key parameter is the RM.

Upwind with twa 42° - Boat heel (°) vs true wind (Knts)

DB32 Disp.1952 kg inc. 3 crew sit windward Red : Keel 775 kg at 0° ; Blue : Keel 775 kg at 45° + daggerboard 100% sail surface Blue dashed line : Keel 775 kg at 45° + daggerboard + 118% sail surface

40 35 30 25 ) ° (

20 l e e

H 15 10 5 0 0 2 4 6 8 10 12 14 16 18 20

Wind (Knts) Downwind with twa 140° - Boat speed (Knts) vs true wind (Knts)

DB32 Disp.1952 kg inc. 3 crew sit windward Red : Keel 775 kg at 0° ; Blue : Keel 775 kg at 45° + 1/3 daggerboard Blue dashed line : Keel 775 kg at 45° + 1/3 daggerboard + 118% sail surface

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0 0 2 4 6 8 10 12 14 16 18 20 22 24

Same comment as for upwind : more sail surface has the effect to schift the speed curve towards the light winds without giving more top speed for stronger winds.

Downwind with twa 140° - Heel (°) vs true wind (Knts)

DB32 Disp. 1952 kg inc. 3 crew sit windward Red : Keel 775 kg at 0° ; Blue : Keel 775 kg at 45° + 1/3 daggerboard Blue dashed line : Keel 775 kg at 45° + 1/3 daggerboard + 118% sail surface

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0 0 2 4 6 8 10 12 14 16 18 20 22 24