Precision of foils

Everyone keeps telling me that at Footy sizes the sectional shape of foils does not matter - just take a flat plate and sharpen the nds a bit.

Does anyone actually have any theoretical or properly controlledc experimental basis for this?

Just a guess - it’s possible that Lester Gilbert :graduate: might have something???

Lester is prepared to spend megabucks on hyper-ptecise foils

footies are relatively small and slow, so how would you make a foil that is of use? Or rather, what would bl of foils on a ?Footy? Reynold’s numbers, viscosity and drag, need to be compared to the drive available from the rig & boat.

I think you almost have the nub of the question. The one thing that does not scale with the size of the boat is the reaction time of he driver. The conttrol loop of a Footy is therefore highly prone to over-runs - over-steering, over- sheeting. under-steering, under-sheeting.

This would suggest foils with higher thicness ratios and lower aspect ratios (if posible without other sacrifices) toproduce a more ‘forgiving’ foil. But this has nothing to do with precision of design or manufacture.

i garee that a flatish, “low tech” foil may be better for a footy… it should be noted that the blue jay, lightning, snipe, finn, comet, and star boat all have flat - or near to - lifting surfaces… it is not a horrible way to go. also, a footy is going “slow” enough that what we are really looking for is lateral resistance, which creates the lift we need… also, theoretically it creating a “perfect” foil, works, but practically, - working with low speeds and high reynolds numbers - nothing is to scale. also with the high pitch-to-length of a footy, there is no way to keep laminar flow in a predictable pattern anyway. in my mind i would say that there is more benefit to having a “flat” foil, that keeps roughly the same flow/efficiancy over all angles than a wonder keel “uber precise” one that only does its job in a perfect, unchanging world…:graduate:

I take issue Sir!

Fine, Finn, etc use flat plates. That surely is not necessarily because its a very goodshapebut because it was an easy shape for Rikard Sarby to make i post-war Finland and has since become entombed in the rules.

Yes, a stalled foil with big arrea and low aspect will produce much more lateral rresistance than a small, high aspect one: taking an early '70s $&S boat off a mooring and getting it wrong is likely to have muh less catasstrophic consequences than its Farr equivalent, which will go sideways at a merry rate. But this is not a performance consideration.

Yes, strange thins happen. Tess with a Kiitiwake and a caluclated full draft fin suggest that stall is almost inevitable at harmonic pith frequencies to windward (this is not knoocking he Kittiwaye: the same applies to all or most Footys). If the boat is ‘out of the groove’ to windard, flow reveral over the botom part of he fin is perfectly possible.

I’m not sure how Mr. Reynolds reacts to this - certainly not out of the tetbook.

Don’t forget that you can scale a boat and the appendiges down, but you can’t scale the water down. You’ll have to create some kind of fudge factors to make any formulas work. Either that, or just throw away all the big-boat design principles, and make things up new in your own way. :graduate:


No Tomo, for the most part the scaling princioles are well understood - albeiy somewhat complex. And there’s nothing in them, as far asI know, that says hat a Footy’s a special size. Hence my enquiry.

You may think that you understand things, but I’m still working on them- Footy-wise. I rarely put much thought into hobby tasks- I prefer to just have fun with the boats & leave the calculator in the desk at work when I leave.

this is the foil section i’m gonna try first. i’ll go from there as to sharpened flat, or different foil.

No ofence Tomo! I make my living by words, so the calculator’s ny home toy. And no, I don’t think I understand these yhings: I might understand enough to ask some sensible questions.

and the rudder.

Angus, though the scaling principles are basically known (not by me, however) my understanding is that the AmCup designers have been using tank models over 8 feet long to avoid some misleading data from the days of 3 or 4 feet long models (what was the one with the blunt stern bustle?)

That seems, IMHO, to indicate that when you get down to one foot, the data just isn’t available to be sure of much. I also think that when you factor in choppy water, it probably gets even more difficult to really know what’s going on.

I’d be very pleased to see a couple of Brits with good understanding of design and mathematics get together and figure some stuff out for us. That’s assuming you and Lester speak the same language! :smiley:

Hopefully, we are getting enough different Footys in the water to get some empirical data this season. If everyone is willing to share their ideas about what works, or doesn’t, we’ll start to build a body of “Footy Class Knowledge.”

All that said, I shape my foils with reasonable care, given that I’m carving them by hand and eye. I doubt that a nicely shaped surface could be any worse than a flat one, and they look prettier to me. I envy the carving, molding, and layup skills of folks like Nigel.

I was recently wondering if I could develop foils in Freeship and have plugs milled from the Freeship files. Haven’t pursued the idea, though.


i should say, i, like graham shape my appendages by hand and by eye, because i do think that having some shape is better than none at all… i have done enough flying to know that! :devil3::scared::graduate:

sail fast!

I’m new to model yachting and the associated hydrodynamics, especially at these scales. But I do have rather more experience with model aircraft, and particularly very tiny ones. Seeing as Footys hold a similar position in the nautical world, maybe they have some things in common.

There’s been a vast amount of research done with high speed / large scale / high Reynolds numbers aerodynamics, and you can look up almost anything in the textbooks and get it right first time. But as soon as you get into very low reynolds numbers all that knowledge starts to fall apart. Conventional aerodynamics seem to hold very little water (sorry, bad pun) at those scales. Remember the bee that technically couldn’t fly according to the very best aerodynamic knowledge available? And you only have to ask the guys who fly those delicate rubber powered indoor models which have Reynold numbers almost in minus figures that it applies to aircraft too. The air is almost like treacle at those scales and airflow doesn’t work the same as at the larger sizes. If you took one of the aerofoils used on those tiny models and scaled it up to a decent size it would look horrific and create so much drag you’d give the computer a panic attack!

What’s my point?

I guess that at Footy scales the Reynold numbers and forces acting on the foils are so small and under-researched that conventional theory has little useful information to offer. And there are so many variables that it’s almost impossible to create a particular foil that will be reliable and at its best in all conditions. And even if you could create such a foil, would it be possible for the average coffee table builder to make?

The exciting bit is that develpoments of Footys can be a really useful way to learn more about such small scale hydrodynamics. Time I started making mine eh?

Thank you Martin, that is a very good answer to my question. It seems to me quite reasonable to suppose that we do not accurately understand all the scaling factors affecting form drag and induced drag at very low Reynolds numbers - far more reasoable than to say that they do not matter. Of course a Finn is a very good boat but that is not grounds for saying that its centreboard is some sort of optimum - except in he contect of Abo in 1951 when the materials were (relatively) easy to obtain and work.

Angus - The Finn is a one design class, and as I think you mentioned before the flat centerboard shape is “entombed” in the rules. Has there ever been an experiment outfitting a Finn with an airfoil section centerboard to test performance head-to-head with a standard Finn? If not, what bearing has speculation about Finn class performance in relation to their centerboard sections have to the add to the discussion of the development of Footy design? The Finn is a good, technically challenging boat to sail, and it competes against other Finns in one design racing.
In my limited experience with the Footy (and who among us has enough experience to claim expertise?) they don’t behave much like the M class boats that I’ve designed over the past 30 years, or for that matter any of the sit-on-boats I’ve sailed. So, while discussing big boat performance characteristics is interesting I think that it is beside the point.
I am beginning to rethink the torpedo shape of the lead bulb. The nose down attitude of most Footies downwind, even in light air is probably more a source of drag than the flat plate versus airfoil keel shape issue. I have tried mounting the bulb at a 6 degree nose up pitch in order to “average” the pendulum effect induced when the short waterline boat encounters out-of-scale waves in short frequency. I saw no empirical performance change. The “average” setting doesn’t seem to do much to offset the extreme trim changes a Footy goes through.
I bring this up to emphasize my long held position that foot long boats are different from anything else that sails. As has been pointed out in this thread not much research has been done on this end of the sailing spectrum. There has been no commercial interest to spur investigation. We are on our own. I have argued from the onset of the change in Footy rules to the box rule that fashioning the Footy after larger class boats was premature since there was not enough understanding of the performance characteristics to make the assumptions that governed the box dimensions. The discussions that appear on this forum reenforce my view.

Niel,it was not I who brought up the Finn but 420saailor and may pointaboutithas been precisely yours.

I too am looking at variable bulb cant and lowdrag/low lift bulbs with interest and no positive results even on the calulator.

One thing that suggests that the normal principles of naval architrcture (or at least some of them) do continue to apply is that a much lighter and much narrower and lighter than nprmal Footy - free sailing one - is markedly faster than the conventional offering.

But I repeat my question. Does anyone have any theoretical basis for this alleged change in the rules at some point between100 and 30 cm length?

I belive a precise foil will be better than a rounded off flat plate for example.

Look into the tailplane sections designed by Mark Drela for model sailplane use.
The claim is they can cope with super low reynolds numbers.

Niel mentions the shape of bulbs and the range of fore and aft trim a footy goes through,He is quite correct in this observation.
Firstly the ammount of fore and aft yawing can be contolled quite well by my new impoved Una rig.
(details in another thread soonish)
And secondly I notice the Balmain bug evolved to have near Spherical ballast bulbs.Thus having the same drag at all yaw angles.(alot probably!! but the same never the less:) )