Aspect ratios

I have been reviewing the information that you provided about the force of the wind at heights above the water at which Footys operate, and have a request for you. I am somewhat old-fashioned and find the ‘font’ used to denote the various mathematical functions that you use in your table as well as in the previous posts somewhat confusing. It is a shame that the standard computer keyboard does not have all or most of the standard math symbols.
Could you review those posts and repeat the equations etc. with conventional symbols? Without wanting to be needlessly instructive, could you use a program like ‘Paint’ where the symbols can be written by hand?

This discussion about lift and aspect ratios in the theoretical is fascinating, but, the boat has to perform on off-wind legs as well where a higher span will drive the bow down further than the sails that have evolved in this class to date. Off-wind bow down sailing trim is endemic to the class and reducing this tendency with the goal to eliminate it in all but the most hard pressed conditions is the biggest challenge to the Footy designer. It is also the least discussed on this forum.

I have found that lighter boats handle better off the wind,with reduced tendency to dive.
Longer boats also seem more resistant to diving.
Once you start building boats with similar displ/length ratios to larger models you start to see comparable handling.

Hi Rod,

Sorry for the laziness.
The equation editor in either MS office or Open Office does the job.
Please see attached pdf.
We are currently making some different sails.
There is obviously always a compromise.
We’ll post some templates soon.


Sorry - I’ve been away playing the two unlovely games of Earning a Living and Footy Class Politics – so back to the real world.

I haven’t had time to read all the latest contributions in detail but three things strike me as of particular interest (I’ll get the names wrong, so I’ll leave them out.).

My first major point. Actually a rag taggle of bits relating to wind velocity. First, the surface roughness coefficient z0. One of the problems is that it is a codge. First, so far as I am aware, it is totally empirical and the physical conditions to which it applies are a bit vague. Second, this is not completely surprising. So far as wind over water is concerned, it is not a constant but a variable. The action of the wind on the water surface itself affects the roughness, which affects the action on the surface … In other words, a sea builds up and the roughness changes. Third, the edge of a pond is more or less by definition a line of transition of surface roughness. I think I can get my head round what happens in reality, but mapping that reality into the model offers causes steam to come out of my ears!

My second major point. The distribution of speed v. height is mapped as a simple log function. Where any such function tends to zero, tiny overall differences can have a big effect on the curve at its 0 extremity. The method suggested is an empirical one for civil engineers who generally have very little interest in wind pressure a foot or so above the ground. Accordingly some time ago I started looking for people to whom it was of interest. Unfortunately I was hospitalised at any early stage in my researches and cannot find my notes – including the names of a couple of contacts. Various things did emerge. One is that there is a highly developed (albeit low presence) science in this area. Most of it is proprietary to Monsanto or one of their competitors and relates to pollen dispersion, etc. Numbers cost $$$$. Soil scientists/erosion people are also interested but, again, so far as I can see numbers cost. The most intriguing thing came from John Amoroso (Tallastro) who came up with a paper on radar observations of butterflies migrating with or against the prevailing wind. The butterflies seemed to regard an altitude of 80 cm as a magic number. The local flora was grasslands in the SE USA.

My third major point. I have long held that what is called in the USA the ‘storm’ rig was no such thing. With my class management cap I have held this up against both the Kluxers (So your boat is too big and heavy to exploit it. So make your boat lighter and more easily driven) and the Hedgers And Ditchers (You think this cuts costs and eliminates extreme sail plans. Let me present the 7-masted 305 mm rig). It is gratifying that the engineers are now looking at such rigs from an engineering – rather than purely rule driven – point of view and coming up with some very interesting answers/questions.

Keep at it people. And let’s have some boats THAT ACTUALLY TURN UP AT RACES.


Hi Angus,

There is a formula for calculating Z0 for uniform distributed roughness elements. However there is a wide variation on the values give for water because, as you point out, water is very variable. Generally the calculations are used for larger heights than what we are interested in but there is some work out there and the results over water stand a better change of being correct than over rougher surfaces.
I wasn’t necessarily looking for an imperial answer rather than a comparative tool. The actual power values were just for interest and the efficiency of the sail needs to be taken into consideration.
With the spreadsheet it is possible to compare any sail area distribution over all of the likely Z0 values.
It is just a starting point before actual experimentation.