# Drag Coefficient versus Size

Discussion in 'Hydrodynamics and Aerodynamics' started by Inquisitor, Sep 15, 2020.

1. Joined: Feb 2013
Posts: 74
Likes: 2, Points: 8, Legacy Rep: 26
Location: Virginia, USA

So I am barely even qualified to have an opinion and hesitate to open my mouth. But, I might learn something. So, in this academic callisthenic and in the context of flat vs cupped wouldn't the cupped create more speed through push and pull? Based on the fact that the air would be moving faster and cleaner across the leeward side of the sail with less buffeting and possibly a bit of forward lift pulling or at least mitigating some drag? This could all be basic sailing knowledge that I don't have. When I mated on 12 meter catamarans we would always pull the battens as tight as we could to create the curved foil.

Forgive my ignorance if its too much.

2. Joined: Feb 2020
Posts: 12
Likes: 1, Points: 3

### Tedd McHenryJunior Member

@khaos :

I'm not sure exactly what you're describing, but it's important to distinguish between sail lift and sail drag. Lift is force generated perpendicular to the bulk wind direction and drag is force parallel to it (i.e., force pointing downwind). When close hauled or in beam reach the driving force is lift and the sail is acting like an airplane wing. In those situations, it's important to keep the flow smooth over the lee side of the sail because if the flow separates (becomes turbulent) you lose lift and gain drag--the exact opposite of what you want. But when running downwind you're driven purely by drag. You don't want to smooth out the flow because smooth flow creates less drag, and aerodynamic drag is what's moving you through the water.
So, on a downwind run, if tightening the jib or genoa increases area and creates more turbulence that's a double win.

3. Joined: Aug 2017
Posts: 529
Likes: 224, Points: 43
Location: Littleton, nh

### Will GilmoreSenior Member

The shape of the curve is more important than the depth of the camber. If the curve of the sail foil is too deep, the shape moves too far aft. This means the lift-force increases, but adds more to the heeling force than to the driving force. In that case, flattening the sail will allow for a better angle of attack that gives some forward drive. Of course, that's when on a reach. If sailing DDW, a nice big fat belly is best.

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.