Has anyone measured the bulb for a Footy? I have access to a lathe, and would like to make a plug to cast some Footy bulbs. No offshore tackle shops within what- (500 miles?) of Ohio…
I think I just need the length and diameter. the length of the taper would be good to know, and the shape.
i have a turned epoxy one here, that i was going to do the samething with.
i could mail to you if ya want it. i was going to offer 2 halves in composite, so that people who did not want to muck with smelting lead, could jus mix up some shot, with epoxy, fill the two halves, then mush’em together. i have not the time to do this at the moment, but it sounds like you do.
yers if ya want it.
I forgot to mention I was after a 200g (c. 8oz) weight. 8 ounces is actually 226g tho. that’s a 12% difference. Ian HB used a piece of brass tube, which would makethe calculations easier
Nigel-
You hang on to your plug, I want to try this one myself- I just would like some rough dimensions. The calculator says I’ll need about 19.7 cc ( density: 11.3 g/cc)
I’ll get you a set of offsets tomorrow. Actually you can make the bulbs in three sections to any weight within reason. Ellipsoidal nose, cylindrical centre section, parabaloid tail - just adjust the length of the cyclindrical section.
Design is taken from the maths of an MIT tuturial paper on ‘slender body theory’ i.e. low drag, low lift.
Difficulty with that is that you can get some very funny flow directions as the boat yaws, pitches, etc. We, like Boeing, Airbus etc. with fuselages are pretty determined that the right answer is to minimise drag, which is relatively simple over a wide envelope, than to get improperly understood lift that may or may not be acting in the right direction at any given time.
As a matter of fact, it does: http://www.onemetre.net/Design/Bulbcalc/Bulbcalc.htm . There is a download there of Marko Majic’s awesome Bulbcalc program, which has more than enough information to REALLY give you enough experimental work to last you a lifetime!
As I understand it, Marko Mjac’s bulb calculator works for NACA aerofoil sections, not for ‘slender body’ type bulbs. In the former the object is to generate high lift at minimum drag. In the latter, the object is to generate minimum lift at minimum drag. Since the direction of flow over the tip of a model yacht kerel is a pretty variable (if not unknown) quantity, the lift may actually be counter-productive. Hence my preference for the slender body solution.
This has the concommittant advantage that the centre section of the bulb is a pure cylinder - and carries the majority of the volume. It is therefore possible to make families of buyulbs of different weight my making standard nose and tail sections and matching them to centre sections of different lengths.
How can you have a keel bulb (or fin) that “generates lift” when it’s not always in the same attitude, in relation to the hull, esterline, etc. On one tack it would be lifting in one direction, say down, to help the boat right itself, and on the other tack, it would be lifting up, to create more heel. Only when the mast is vertical, will “lift” from the keel bulb be useful, to lift the hull straight up & out of the water ( like foils will) and conversely, when heeled, will only help push the boat to leeward.
And the same thing for the keel fin.
So then one must conclude the best shape for the fin is one with the most area and least drag, and for the bulb, the most weight and least drag.
But then is all this really necessary for a Footy? The speed is relatively low, and, IMO, reducing leeward slipping is of most concern?
Bulb sizes Dimensions in centimetres
Family 1 – Standard Footy
Keel family consists of eliptical nose section, cylindrical and supraparaboloid tail section. (I think the last word is correct - seen in profile a parabola whose origin is at the after end of the cylinder so that the line is tangential to the cylinder).
The nose and tail sections are standard. Weight is adjusted by altering the length of the cylindrical sentre section.
Each weight comes in two versions. One with a through slot for the fin and a transverse hole for a single pivot bolt (adjustable bulb cant), the other as the plain shape. The weight of the bolt is INCLUDED in the weight of the bulb.
Radius 0.76 cm. Nose Length 1.95 Tail Length 2.90
Fin Chord 3.60 Fin thickness 0.325. Keel bolt 4 mm S/S
See what I mean about slender body: just like a jet airliner fuselage - and for essentially the same reasons. If anyone likes their bulbs longer and thinner - just ask.