Mast rake: explain please!

29ers have a fixed length forestay, but you adjust the rig tension/rake by the shroud adjustment as it gets windier.

Originally posted by Daniel Holman In aerodynamics, a swept aerofoil cannot derive as large a lift coefficient as an upright one, so when not overpowered you get more "bite" from an upright rig.

Hi Daniel

Out of interest is the lift coefficient really that applicable compared to the change in helm balence by moving the centre of effort...?

Originally posted by Daniel Holman Hi - TBH the CoE move is opposite to what you'd want - as you feather a sail, the CofPressure moves back giving weather helm which is slow - unless this is all matched by moving CLR back by raising dagger or centreboard. Can't give numbers off the top of my head but have a gander in any of the basic aerodynamics texts on stall angles for finite aspect ratio wings at different sweep angles.

Not sure I have ever read many that relate to low enough wind speeds that would be apparent.

That said Bethwaite Snr showed some interesting effects of wind sheering between 1m and 6m above water at low speeds enough to disprove that coefficient effects only apply with greater force.

The theory you mentioned earlier completely stands up, be nice to see someone take this up as an experiment.

By the way I would say that feathering and moving coefficient giving you weather helm is due to the drag of feathering not applicable by rake.

Raking gives an efficient aerofoil reducing drag, increasing lift but also displacing unwanted high up the mast force. Centre of Effort moved aft in this case reduces helm. Think windsurfer more than laser dinghy.

Edited by NHRC - 03 Dec 14 at 10:57pm

Not sure if it's relevant here but it always strikes me that sideways bend is too often ignored in the models and descriptions of how rigs work.

I'm not totally convinced by that though. Yes, I know that lift is reduced with sweep (subject to taper ratios etc) but... Many boats actually have fwd sweep of the 1/4 chord point with a near upright mast so a bit of rake would help improve the sweep angle and anyway even when raked the sweep angle of most rigs is not more than a degree or two. So it's hard to imagine the effect is significant enough to be all of the answer all of the time.