International Mirror Class Association of Tasmania.

Toll Tasmania: Supporting Tasmanian Mirror Sailors




Mast Efficiency

Subject:   Mast efficiency

Hi Martin,

Before I provide my comments on mast aerodynamics I would like to commend you on the standard of debate that has taken place over the past few weeks.

Regarding the effect on lift drag ratio in the gaff-mast overlap section I think it is very obvious when you consider that the mast and gaff are not ever in line with each other in the direction of wind flow – approximately 40 to 45 degrees upwind and greater for reaching.

This is because the gaff pivots only a very small amount when pulled up hard as it normally is.   Our assessment is that the gaff adds approximately 40% to the effective mast width.   And the effect on the lift forces is further magnified by the fact that all of this width increase, or more, is on the leeward side where the greatest “damage” is done because of the well known fact that the normal low pressure forces on the leeward side of the sail in this region are much greater than the forces on the pressure side of the sail.

Regarding the effect of the gap with lacing, I referred this again to Patrick Couser together with the reference to the Bethwaite chapter.   (We both have copies of his book).   The relevant part of his reply was as follows: “……. referring to the particular diagram of interest on P.192, I don’t really see that there are any conclusions that can be drawn with regard to the mast-sail gap.   There is no comparison with a mast and sail with a gap.   It’s well known that the presence of the mast greatly reduces the suction, but there is still a suction and, if there is a gap, air will be drawn through the gap and reduce the suction further.   It’s the pressure difference between the high and low pressure side which cause the driving force and as explained by Frank later in the section it is more advantageous to have as much pressure difference in the forward part of the sail than further aft …..”

Patrick concluded by saying it would be interesting to talk to an Ian Campbell of the Wolfson unit (whatever that is) to get their ideas, and he offered to contact Ian who he knows well, if I would like.   I have accepted this offer and will let you know when Patrick reports the outcome.

Could I also offer a suggestion about the proposal to alter the mast step position for the Bermuda rig.   It is my opinion that the effect on sail efficiency of a small increase in mast rake would be very small compared to the effect of the Bermuda rig as detailed above.  

There would be less deterrent for those contemplating changing from the Gunter rig if they did not have to change the step and it would be best if the mast step was left unaltered.