I've been watching the America's Cup preliminaries and noticed that some teams, especially Alinghi Red Bull, have a lot of wrinkles in the bottom of their mainsails.
Although you often see wrinkles on sails, mainly radiating from the tack or clew, surely this can’t be good for airflow, power or drag.
What does anyone think?
Wrinkles
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John Perry
- AYRS Chairman
- Posts: 105
- Joined: Tue Nov 08, 2016 6:39 pm
Re: Wrinkles
Hi Chris
Here is a diagram from the late Tom Speer, an AYRS member who worked professionally in aerodynamics and who played a large part in the development of wingsails for the Oracle Americas Cup team. Unfortunately I do not know where this diagram was published.
This diagram is clearly for a single skin mainsail on a rotating mast but it might give us a clue.
At the bottom of the diagram you see the edge of the boundary layer to scale. The wrinkles you see on video of AC75 sails seem to be mainly in the aft third of the sail, towards the leech, and fairly close to the foot of the sail. In this region the boundary layer on the leeward side of the sail is quite thick (say at least a few inches) so the wrinkles would not penetrate through that boundary layer and I would not think they would matter too much. On the windward side of the sail the boundary layer towards the leech is much thinner so I would guess that the wrinkles in that area will increase drag. The boundary layer is very thin around the mast on the leeward side and any roughness that penetrates the boundary layer there may promote separation so I guess it is particularly important to keep the surface smooth in that area and in the case of the AC75 mainsail to minimise any step where the leeward mainsail skin is joined to the Dee section mast.
Here is a diagram from the late Tom Speer, an AYRS member who worked professionally in aerodynamics and who played a large part in the development of wingsails for the Oracle Americas Cup team. Unfortunately I do not know where this diagram was published.
This diagram is clearly for a single skin mainsail on a rotating mast but it might give us a clue.
At the bottom of the diagram you see the edge of the boundary layer to scale. The wrinkles you see on video of AC75 sails seem to be mainly in the aft third of the sail, towards the leech, and fairly close to the foot of the sail. In this region the boundary layer on the leeward side of the sail is quite thick (say at least a few inches) so the wrinkles would not penetrate through that boundary layer and I would not think they would matter too much. On the windward side of the sail the boundary layer towards the leech is much thinner so I would guess that the wrinkles in that area will increase drag. The boundary layer is very thin around the mast on the leeward side and any roughness that penetrates the boundary layer there may promote separation so I guess it is particularly important to keep the surface smooth in that area and in the case of the AC75 mainsail to minimise any step where the leeward mainsail skin is joined to the Dee section mast.