Keel Bulb dimensions

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.


What weight(s) do you want?

10^3 pardons Angus,

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


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.

Hope this helps

Thanks, Angus. I’ll drink a pint or two in your name, mate!:zbeer:

I like to use a flat tail, like a duckbill.

Personally, I would think that a slender body would give me a ‘high lift’ IF you know what I mean… :wink: :smiley: :lol: (I can say that because I’m a younger guy.)


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.


maybe post the slender body bulb shapes here Angus?
handy info to have around.

I’ll bet you that Lester’s website has this stuff on it.

As a matter of fact, it does: . 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!



Marko’s program has a minimum bulb weight of 1 kg. I wonder if he would be interested in expanding it?


I kind of assumed Angus and Lester might already be aquainted. Maybe not; I don’t know that much of British geography.

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.

Over to you, sir!


You had the same idea I always had, Angus,

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?


Yes, of course it’s important. What matters is the RATIO of the propulsive and drag forces, and that’s much the same as in any other boat.

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

Nose/Ord x y
0 0 0.00
1 0.33 0.53
2 0.65 0.71
3 0.98 0.82
4 1.30 0.90
5 1.63 0.94
6 1.95 0.95

Tail/Ord x y
7 0.00 0.95
8 0.48 0.92
9 0.97 0.84
10 1.45 0.71
11 1.93 0.53
12 2.42 0.29
13 2.90 0.00


Cylinder length 9.55 with slot
Cylinder length 8.61 without slot


Cylinder length 9.93 with slot
Cylinder length 9.00 without slot


Cylinder length 10.34 with slot
Cylinder length 9.39 without slot


Cylinder length 10.71 with slot
Cylinder length 9.77 without slot


Cylinder length 11.12 with slot
Cylinder length 10.32 without slot


Cylinder length 11.49 with slot
Cylinder length 10.54 without slot

Pretty picture follow tomorrow.

I get dizzy looking at your chart!


Sorry. It started life as a table in Word - looked much prettier. E-mail me at and I’ll send you the civilised version.

I drew one out,
I got this…seems about right:)

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.