How to make fins and rudders

Footy Class General Discussion (Footy)
1

I don’t know about anyone else, but I do not have the skill or the eyesight to make high quality Footy foils to the sections specified in the books by normal ‘whittling’ modelling methods. A short review of published data shows that very small diffeences in proprtion/radius can have very large effects. At Footy sizes the tolerances are tiny - a few thou (tenths of a millimetre to anyone outside the USA).

I recently disovered that a friend of mine who teaches Computer Aided Engineering at the local Technical College has a ‘rapid prototyping machine’ that will build up a precision shape over a core very quickly and cheaply.

Even better he has a shortage of ‘real’ projects for students so (within limits) anything he does is cheap or free.

This should be worth looking at. It is all very well to say that Footy foils need development but if these super foils cannot be manufactured accurateky, they are mere junk.

To the Cub Scout brigade (with whom I have great sympathy in principle), I would suggest that since said cub scouts will spend their lives in a world dominated by such machines, they should get into them now and that one of a good scout’s attributes has always been to wheedle and beg (see Baden-Powell).

Comments?

A.

2

Angus,
I like to hear more about the prototyping machine. Any details?

On a somewhat different subject (with no disrespect intended to you or the Footy class), I’m a bit curious why are intent on taking a “high tech” approach to this class rather than something like the IOM class? That being said, I have rather enjoyed your stimulating posts.

Regards,
Bill K

3

I’ll try to get details. As I understand it the machine builds up a powder shape electrostatically and then resinates the result - or something like that.

As to why I’m involved in Footies and hi-tech, it is actually the cub scout approach. I challenged my 15 year old nephew to a match race in home designed and built Footies as an entertainig engineering project. I have a strong view that the young should be taught to use modern materials and techniques rather than things that were industrial practice in 1920. When did you last see an industrial product made out of pieces of brass soft-soldered together?

And, yes, my other project is a Phidget, a new class being officially launched on Sunday by Lester Gilbert, Graham Bantock and friends. It is essentially an IOM without the length restriction and the limitations on materials. I am the proud possesor of hull number GBR 1, so I’d better do something about it pretty soon!

A.

4

Remember the law of diminishing returns! At the speed a Footy travels, there is no point in engineering a fin to the tollerances you mentioned. A simple fin, even a flat plate with rounded ends will work. With such small tollerances, the gains would be miniscule compared to gains or losses by missing just one windshift.

I am building a Triple Crown IOM at the moment. My fin is a sheet of thin ply sandwitched between two sheets of thin balsa. Total cross section is about 7mm. It is amazing how stiff is the sandwitch compared to the original components.

I hand sanded the balsa a bit at the leading edge and more at the training edge, and a bit off both sides towards the bottom. Sighting frequently down the foil keeps things about equal.

Next step will be to cover with light weight fibreglass cloth and epoxy. The end result will be strong, light, stiff and cheap!

This technique should produce a competitive Footy fin. and well within the skills and resources of a boyscout.

5

Tell us more about Phidget Angus?

6

On Monday when it’s fully official Brett!

A.

7

THIS ASSERTION IS TOTALLY WITHOUT ANY EXPERIMENTAL EVIDENCE. IT IS INTERESTING THE VERY HIGH PERFORMANCE MODEL SAILPLANES, WHICH OPERATE AT REYNOLDS NUMBERS NOT THAT FAR REMOVED FROM THOSE OF A FOOTY FIN REGARD THE SECTIONAL SHAPE OF THEIR WINGS TO BE THE MOST CRUCIAL ASPECT OF THE WHOLE DESIGN

Angus

PS I didn’t see a single IOM at the Championship with a fin of the type you suggest - even ones from some relatively impoverished places like Croatia were very precisely made carbon, mostly from Sails Etc or David Creed. This may of course be the herd instinct of the sportsman, but I have a suspicion that if EVERYONE is doing it, it might just have a grain of truth in it.

8

I have had some experience with rapid prototyping in my work as an architectural model-maker and prototype modeler, in both resin sintering and stereo lithography. Mostly these technologies are used for product design refinement or as the basis for creating molds for the manufacture of components. The rapid prototype plastics that I’ve worked with have very little inherent strength, are soft and heavy, and generally need surface finishing. A fin created through the current rapid prototyping technologies could function as a plug to make a mold for a fin, but the materials that are used in the process do not fit boatbuilding criteria to be used directly. Also, very high end rapid prototyping (read very expensive) could produce a keel shape that was highly accurate. But I would be skeptical of a product produced from a school level project, only because I would imagine that their product would be produced on lower tier machines with inexpensive resins. Distortion, both twisting and arcing, have been common on prototyped parts we’ve gotten, particularly on parts with an uneven crossection. However, the quality of these prototyped parts has improved dramatically in the last five years or so (you should have seen the crap I had to work with then), and I expect that fairly soon top quality parts made with an assortment of materials will be within reach of model boatbuilders. I would think that having a keel plug carved by a CNC milling machine would produce a more usable part in the here and now.
Although I disagree with a boy scout being the litmus test for what is acceptable in the Footy class, I tend to agree with Angus that those boy and girl scouts should be exposed to the machines and technologies that will be prevalent in their world. I also think that they should learn the motor skills to not need these machines at all.
Hi-tech building should be part of the Footy Class, because even as a restricted class it is a design oriented one. There are plenty of conventional one-design or proto-one-design classes out there at one meter or less in length to choose from. The Footy is unique in its radical rethinking of sailing models, materials and components. There will be room in this class for beginning boatbuilders and low-tech boatbuilders, but it is an experimental class and the amount of hi-tech materials incorporated into a footy hull is minimal and well within reach of all but the most destitute. I just don’t see the point of putting to many limits on the creativity of the folks who are out there thinking up the better mousetrap.

9

“Although I disagree with a boy scout being the litmus test for what is acceptable in the Footy class, I tend to agree with Angus that those boy and girl scouts should be exposed to the machines and technologies that will be prevalent in their world. I also think that they should learn the motor skills to not need these machines at all.
Hi-tech building should be part of the Footy Class, because even as a restricted class it is a design oriented one. There are plenty of conventional one-design or proto-one-design classes out there at one meter or less in length to choose from. The Footy is unique in its radical rethinking of sailing models, materials and components. There will be room in this class for beginning boatbuilders and low-tech boatbuilders, but it is an experimental class and the amount of hi-tech materials incorporated into a footy hull is minimal and well within reach of all but the most destitute. I just don’t see the point of putting to many limits on the creativity of the folks who are out there thinking up the better mousetrap.”
Today 06:04 AM

Well said, Niel. Though we don’t always agree on those limits, we are on common philosophical ground here.

Bill H

10

Hi All

Marine Modelling International, through its Editor, Chris Jackson, is sponsoring a new class, called “Phigit”. Temporary details at

http://www.onemetre.net/OtherTopics/XYZClass/XYZClass.htm

We are waiting for the “announcement” and article in the next issue of MMI, due within a few days, and then we will “go public”. The Web page mentioned above has the MMI article to download for your information. In the mean time, I’m certainly interested in what y’all think! BTW, the “we” is Charles Detriche, Graham Bantock, and Chris Jackson, with me acting as administrative assistant (smile).

11

What was the catalyst for coming up with this concept? As someone who likes to design I find this rule (as it stands currently) disappointing. Without crunching some numbers, I would say lwl of 1.1 to 1.2 m and a loa of 1.5 to 1.6m would be just about ideal. Because of the rules simplicity, I think these boats are going to reach an optimum shape very fast, and then stagnate.

12

Lots of design latitude and performance challenges in the Footy class, though, Dan. Time you stopped lurking around here and gave one a try:)

Bill H

p.s. If you do, maybe I’ll reciprocate and try a 3R.

13

Hi Dan

It was something that Charles and Graham came up with while doodling on a drawing board in an idle moment… (smile) Well, Charles is an amazingly prolific designer of RC yachts (around 50 I think, and still growing), and his Estrellita design caught Graham’s eye, so the two of them thought they’d see if anyone else liked the idea.

“There might be interest in a class which uses freely available IOM sails and rigs but in which there were few restrictions on the hull.”

14

I’m not a Footy sailor just yet (smile), though I look and listen, but Bill’s question caught my attention.

It seems to me that one can treat the Footy as a toy boat, something the kids find captivating, and so of course a bit of aluminium plate would make a fine fin, and so on.

One could, however, take it seriously. It could be an International class, and the class could hold World Championships. (No reasons why not!) In this case, it seems to me that its rules place an absolute premium on precision engineering and master craftmanship. Only exceptionally talented builders need apply, preferably builders with access to research facilities to explore regions of aero- and hydro-dynamics that as yet have barely been scratched – the ultra-low Reynolds numbers regime, and the boundary layer regime just above the water surface. I’d guess that in due course a World Championship Footy would easily cost as much as a World Championship Marblehead (smile).

15

Well said lester,
I agree on all points.Its a bit of a worry this monster that I have helped create:)
Most of the boats built so far are fairly low tech in both the design and construction.,This is slowly changing and when racing starts we will see the game rapidly change.
My reason for building low tech etc at the momment is I am still finding the type of hull/rig/appendage/displacment/beam etc etc package that is going to work best.Ive never been so confused in all my life after 4 years and about 20 models I am only now starting to get a picture in my head of what I think the final answer may be.and even then I can easily change my mind tommorrow.

This new class with the IOM rigs sounds like fun,they will be nice looking boats with good manners…

16

I totally agree with Brett and Lester.

You will forgive me, gentlemen, but almost every Footy design I have seen (apart from boats that are self-evidently toys) strikes me as an attempt to stuff a fairly ‘ordinary’, ‘nice’ little boat into this rather unlikely set of design parameters. Many of them succeed - ones that spring immediately to mind are Bobabout herself, Razor, Halfpint, Kittiwake …

There are, of course, perfectly legitomate reasons for this - some commercial, some to get the class going without frighening the punters off by ‘weird’ designs with unfamilar aesthetics, some because that’s the kind of boat the designer wanted to build and so on.

Most use fairly low-tech construction for ease of building or (and it is not quite the same thing) ‘dad and kids access’.

So where do we go from here? I remain totally unconvinced, as I argued above, that crude design is any less harmful at this size than it is at any other. The fact that the gains in drag are tiny in absolute terms is offest by the fact that the amount of power available to overcome that drag is also tiny - and the windshifts are just the same.

My calculated length of a Footy internet course differs from Brett’s slightly - he has added in a fudge factor and I have not, but we are broadly agreed that it is about 600 feet - 600 boats lengths. In an America’s Cup boat this is something in the order of 50,000 feet, so knocking on for 10 miles. Taken in those terms (literally) microscopic differences start to have an effect. (If these figures don’t seem to stack up, remember that hull speed is proportional to the square root of the waterline length - twice the length doesn’t give you twice the speed).

If anyone recalls, my first posting to this site was about ‘Chequebook Racing’. My contention was that hi-tech materials did not matter as far as costs were concerned because the abolute quantity of material involved was so small. This is partly true: minimum order quantities mean that setting up to build your first hi-tech Footy can be quite expensive - but you have an awful lot of material left over to build the next dozen!

Like Neil and others, I do not believe that the bot scout should be the ultimate litmus test of appropriateness of a construction technique. Like them, I also feel quite strongly that the use of high tech should not be viewed by anyone - dad, doting graandfather or (probably least likely) the boy scout himelf - as a bar to the boy scout. However, as design advances cost will become an issue.

In order to keep costs down and move forward away from the ‘toy’, we have to get our acts together as a group - and one of the areas in which this can be done is foils, hence the title of this thread. A persistent teenager with good eyesight, or a trained model maker like Neil may be able to make accurate foils. I cannot - and neither can most of us.

I am investigating whether it is possible to bring together two resources to which I have access (or at least a degree of contact) to see whether it is possible to make pulltruded carbon fins available at a reasonable price. I guess (and it is very much a guess) that the wholesale price of the pulltrusion is likely to be around USD 25/metre if the die can be be made at little or no visible cost. On the basis that Reynolds numbers are not dissimilar to those used by model sailplanes we would use a section that is conservative in that type of application. One thiing that is encouraging here is that comparability between compressible (air) and incompressible (water) fluids improves as the Mach number goes down - and the Mach numbers of Footies and sailplanes are both tiny.

I have no interest in making any money out of this. Neither have any desire to upstage Brett’s investment in fluid dynamics know-how. On that basis, is anyone potentially interested?

OR, EQUALLY IMPORTANT, DOES ANYONE THINK IT SHOULDN’T BE DONE? - UNWANTED HI-TECH, TENDENCY TO FREEZE DESIGN ???
Angus

17

For the record,
All I have done so far is commision a leading airfoil and glider expert to design section shapes for Footy sized yachts.
I still don’t have absolute values for planforms…though I am getting closer each day.
Off course once I make just one of these and sell them my secret is out…
No problem,thats how model boats are.
I have no problem with any group effort into moulded fins,get to it guys.
Maybe I could be a customer.

BTW…I made a nice fin last week from Airex foam with just one layer of 50gms
cloth each side,light and no deflection with even a very heavy ballast bulb.
I won’t disclose the thickness.

A cheap pre made fin won’t halt development in this area in my opinion.They are stll so cheap and easy to make and so many things to be tried.In all likelyhood a mass produced fin won’t suit all hull designs.
After you have carefully carved and sanded a lot of fins and rudders you get pretty good at it,just like a HLG builder.
stiffness isn’t as big a deal as it is with other classes,hence building is much simpler.

18

Angus, your idea has been used by a club in my area to produce fins and rudders for their US1m boats. They invested in a machined mold so they can lay up carbon-fiber foils accurately. Because they spread the initial cost over a number of people, and recoup part of that investment with the fee for each foil, it has worked very well.

In spite of that, I don’t think I care to participate. Nothing personal, I just prefer the freedom to play with each new boat. I’m starting to standardize things like mast box dimensions, fairlead locations, rig attachments, etc. so that I can “plug and play” with different fins and rigs. But having “mass produced” foils moves the whole thing a bit too close to one-design classes for my personal taste.

I’m sure there are many others who prefer to minimize building time and use tried and true designs and products, though, so your concept should appeal to them.

Bill H

19

Angus has pointed out that the primary drag on a Footy will generally be due to wave formation - hull speed determined by the square root of the waterline length.

Once I get control over problems with my Footy’s tendency to bury its bow, heel excessively, hobby-horse in chop, fill with water because the hatches leak when the boat heels, etc., etc., then maybe I’ll take a look at improving the foil sections of the fin and rudder. I don’t expect that time will actually come, of course, but I also suspect that on a closer look I’ll figure that the difference in form drag between the perfect foil and a thin section with pointy leading and trailing edges is negligible compared to the wave drag (which sailplanes never see), especially in the typical pond chop with the typical eddying pond winds.

Now if someone were to set up some sort of deal whereby we builders could convert a Freeship or Hullform design into instructions for numerical control of machining, which we might send in with a nominal fee of around $200, say, and receive a male or female form, that sort of high tech I might take a real interest in - although primarily for classes other than the Footy. Right now I don’t have a handle on what a Footy hull should look like, much less a design I’d spend $200 on just to get a form to work with.

Mike Biggs

20

I’m inclined to agree with Dan, after a few design iterations the hull will probably settle down. I’m starting to form the opinion that if you want innovation in design you need a rating rule of some description.

The 3R looks very promising.

Cheers.