Footy design - thoughts

I have been involved in the footy class for good two years now. till today i've build some 10 footys.
In 4 design shapes: Spade(4boats, 9hulls), Karo(4boats, 2depron ,2carbon+1 carbon hull), Duck(a Gary Sanderson design) and a "proto" the newest model.
The participation in two footy goldcups gave me the chance to try other peoples footys, amongst which are the infamous ICE, Steri, Ranger etc.
Alongside with footy and other sail class activities i've been studying naval architecture by myself. Finding resources in books like Principles of yacht design, Elements of yacht design(skene), Ted Brewer books, and all the literature and technical papers i could find on the web.

This is where my thinking stands today:

Lenght,

The advantage in lwl gain by positioning boat diagonally into the measurment box is minor, not worth trying if technical difficulties appear with generall arangement. But if it suits the boats concept then the boat should be as long as possible.

Width,

Generally there are two types of boat: a narrow one and a wide one, not many footys are found in between.

Imo a narrow boat should have a beam of 70-100mm, less is to little, as sail carrying ability is reduced. A very narrow boat will suffer in gusty conditions as it will be hammered by the gusts(if the rigg is set for the luls) or left behind in the luls(if the rigg is ment for gusts)

A wide boat, 135-153mm of beam. These boats are very "powerfull", they carry lots of sail, and usually sport a ballast of around 250g or less, because more ballast doesn't seem to improve righting moment enough to beat the added weight/drag.

All-up weight,

I went as low as 300g with spade models, but hit problems with tacking, similar problems have been reported by other builders.
The black KARO weighed above 520g, and is the heaviest footy i sailed beside the ICe(can't remember the weight, but it was above 500), and this karo suffered from unresponsivness, and dynamic sinkage due to the forces of sails. With a different hull a heavy boat might perform.

My conclusion is that it is safe to stick below 500g and above 350g, offcourse the boat should be build as light as possible, and ballast should put the boat up to it's full design weight.

Freeboard,

At stem should be as large as possible(7-10cm on boats like SliM, ICE, Steri, Urca and my designs)
at stern it should be 0, or as small as possible to fit the boat in the box with the maximu possible draught.(stern freeboard: 3mm-4cm on my designs and ICE's, urca, steri, duck..)

Hull design details,

The prismatic-coefficient should probably be around 0.6, because this suits an overopowered sailboat.
No speed/angle measurments were made with footys, the only refference could be the internet course, but the conditions are varying and the angles sailed are questionable.
Volume in the front third of the boat is key to downwind performance. Three types of bow are found on footys.
One type has little volume, and a high freeboard, and is designed to be pushed down and cut the water when overpovered downwind(Slim, Karo, Ranger). This type gives stable downwind sailing and is probably just as fast as the moustache found on ices.
Then there is the moustache, seen on ice, awk and bug footys. This bow is pushing the water away from the hull creating lift.
Finally there are footys with lots of volume, these are trying to stay above the water, but usualy don't.

The most effective seems to be the high bow with lesser volume.

The moustache is also succesful on british designs.

Ballast(ratio),

Currently i am experimenting with ballasts of 200, 250, 280, 300, 350g, trying to see how they work on different types of hulls, sometimes the all-up weight is to large...
..al bulbs have cg on the same chord lenght of the finn and approximatelly the same shape(something like a naca 64 xx)

It seems that 200-250g work well on wide boats like spade and proto, with the 300g bulb there is already a significant loss in performance upwind, tho the two boats are much more stable when sailing downwind with a heavier bulb. Note: spade was designed for an all-up weight of 380g with a 200g bulb, protos all-up was to be 420 but works best at 375g with a 200g bulb.

The 280g bulb works best on the KARo the 300g bulb can be used for waves and stronger winds, the 350g bulb is to heavy for all of my designs and is waiting for use in a future project.
250and 200g are to light as the 2000sqcm a rigg heels the karo way too much, and tacking in waves is compromised.

Ballast ratios for footys seem to reach from 40% to almost 70% of the all-up weight, light footys are build just as light as the heavier ones, and that is the reason for their lower % of ballast, they simply have less lead.
Generally 50-65% of ballast/all-up weight is a good result.
Two exaples:
Karo, all-up:500g ballsat:280g ratio: 56%(depron boats had a better ratio due to lighter hull)
Spade, all-up:380g ballast 200g ratio:52%
Data fot further comparison here
Note: often it is better to add some weight in order to make the boat more user friendly, e.g. hatches, switches instead of tape.

Draught,

should be maximized, the theories about lower wetted surface with a shorter finn don't work in the real world, footys are all about POWER a.k.a. sail carrying ability.

Rudder size,

I got good results with 50sqcm and bigger rudders. Some footys use smaller rudders with a very high aspect ratio, and they work well, but if the boat is not perfectly balanced they stall in caotic conditions and the boat loses control.
Karo is sporting a 75sqcm rudder. It's big, bigger than needed but reliable when tacking, waitng at the start etc.
All in all a big rudder does no harm, it reduces leeway, makes handling easier and does not slow the boat down much because of all the sail power available.
Another tip is to have a rudder that is at least 15cm deep(from wl), so when the boat pitch-poles downwind, 2-3cm of the rudder stays in the water and control is not lost.

Keel finn,

Due to the low Re footys experience the profile should have a fine entry, the max thickness should be 10-12% of chord, less seems to be risky due to the big leeway angle when accelerating/drifting in light airs.

The planform area on my designs is about 60cm^2(325 size heli rotor blade), this is also the lowest advisable area, less than this does not provide enough sideforce. Leeway increases.

For non chined hulls or hulls with flat sides which provide less "grip" under heel an area of up to 90cm^2 can be used.

A big finn reduces tacking ability, so a boat should have either a small rudder and a small keel finn or both of a bigger area. 

The keel finn shoul be set far aft to move the CE back. The bulb CG must match the LCB of the hull or be set for max 15mm aft of the LCB on the vertical axis(the -15mm option will increase weather helm and reduce nosediving down-wind.

Rigg sizes,

The rigg size is increasing from year to year as the boats are refined.
at the moment the sizes are: A+(light air rigg) 2000-2700sqdm, A 1400-2000sqcm, B 900-1400.
c-rigs are rarely used but generally measure for 500-900sqcm, stormriggs are below 500sqcm.
Going above 75 cm of rigg height above deck rises the CE to high, so that should be a good upper limit for all but the A+ rigg.
The sails should cover as much of the boat lenght as possible, meaning that the whole rigg should not be 20cm wide but more like 50cm wide, this balances the boat better for upwind sailing(it gets more directional stability, and is not as twitchy)
High aspect ratio should be kept if possible!

Servos/Rx/Battery,

Most of footys use 9g servos for rudder and sheeting.

For the rudder a 4.5g servo is more than enough, saving 5g of weight.

In the past standard servos were used for the sheet, but there is no need for such a heavy servo, aspecially if the boat is sporting balanced una or swing riggs(a nominal power of 2 kg will do)

With classical riggs a "2.5kg" strong servo should be used.

The above is my experience with a 6.5-7cm long servo arm and direct sheeting.

The Rx should be as light as possible to save weight and space.(weight under 10g is exelent, under 25g is good)

The battery can be a 1s lipo of up to 1000mAh, weight approx.: 26g

Four AAA are also commonly used but are abit heavier.

A on/off switch is welcome, to improve user-friendlyness.

To sum up, the above are just some measurable parameters that work with footy class sailboats.
At the moment i am not sure what is the fastest footy type.
Speed seems to come from the overall package. Such a package can be achived using different approaches:
Like ice(design by Roger Stollery), a relatively heavy and beamy design, which handles downwind with moustaches.
The rigg is a swing, with enough power to overcome the high weight

Or SLiM, a Phil Tyler designed and build narrow boat, of medium diplcement. Going downwind with a narrow cutting bow and a efficient una rigg.

More here