[QUOTE=bcarter1234;43219]Hello,
Stumbled across Footy’s about a week ago. My first is proceeding… well…afoot.
I’ll have lots of questions but this forum seems to have lots of answers.
Here are the particulars:
- Razor hull and deck from 2mm depron using foam safe CA and hot glue.
Good
- I plan to cover the hull and deck with So-Lite film.
John Donnelly in Scotland is experimenting with kayak-type sructures.
Find him on UK Footy Locator map: http://maps.google.co.uk/maps/ms?hl=en&ie=UTF8&msa=0&msid=113677077282091778067.000436ba48564c1eeba70&ll=55.838314,-3.603516&spn=3.924479,7.69043&z=7&om=1&iwloc=00043d40da76e4affe775
- Internal structure is a mix of 1/16" ply and balsa.
You shouldn’t need much internal structure. A 2 mm depron monocoque is pretty strong. Use low density, high permeability timber infused with epoxide resin. Make some test samples and see how the resin soaks in. If you need more soak, add a little alcohol (meths, surgical spirit) to the resin. This will make it runnier and delay gelling. Effect on cured strength is purely academic.
- Fin is 1/32" ply bent over a thin carbon rod to form a cateye section.
Crazy engineering – strong material on the inside, weak material on the outside: sandwich inside out. I can say this without being accused of being big headed since it was Brett’s idea, but Moonshadow’s fin is a marvel. A hand shaped western red cedar fin (high resin absorption), resin treated as above. It’s incredibly stiff. See below on foil shapes.
- Aft hung rudder 1/32" balsa bent over 1/8" aluminium tube.
As above for material but probably use balsa. Set a (2mm ?) carbon rod into a slot in the blade and fir in with epoxide+ microballoon. 1/8” is almost certainly too thick to get within a sensibly shaped blade of suitable size.
So much for the prelude. Now the questions:
- Is there a preferred fin length? My blank would allow about 7-1/2" extension below the hull. What in the world do you base this figure on?
If I knew the answer, I’d be rich! The trade offs are less depth, gives less righting moment (=less horsepower), but also (probably) less tendency to hobbyhorse, less wetted surface and less tendency to nosedive on the run. No single answer: has to fit the rest of the design.
- Is it best to glue the fin in or should it be easily removable?
Given the difficulty of answering the last question, there is a lot to be said for making the fin removable. Also make the bulb removable from the fin. That way you can re-jig fin geometry with little trouble. This may also be an easier way of correcting hull balance than moving the mast. There is a very small weight penalty, but realistically the 3 or 4 g involved are unlikely to make the difference between your first boat being an unbeatable winner and a total dog.
- How do I waterproof the balsa without harming the depron?
Epoxide. But ALWAYS regard the result as a sandwich structure in which much of the real strength is in the epoxide. Yacht varnish is lipstick: epoxide is structural.
- Are there advantages to placing the rudder as far back as the rules allow?
Yes, big ones. Almost all Footys with underslung rudders are either pre-box rule hangovers (max length is 12”, period) or diagonal boats (Moonshadow)
- I would appreciate any guidance regarding rudder size, shape, thickness and crossection.
The planform is not particularly important. Size depends on the overall characteristics of the design. Broadly speaking, a narrower, more easily controlled boat can get away with a smaller rudder for the same helm effect. Section shape is very important. I suggest a thick NACA section – say 12%. For fins, don’t be lead astray by boats like IOMs where thicknesses as low as 6% are fashionable. Their problems are different. I think that if you go looking under files in USA Footy you will find an NACA section calculator that I put there some while back. If not, look for an NACA section catalogue on the web and go from there.
- Can anyone who has deck mounted servos show me or explain the structure involved?
No!
I’ve replied at length to this partly because Brent wants answers, but partly in order to demonstrate that the new plastic/carbon weight –controlled Footys are actually not much harder to build (if at all) than the more traditional ones. There is admittedly a switchover expense: the bin under the bench now contains miscellaneous lengths of carbon rod, not brass. But after that the techniques are no more difficult, just different.