bulb shape for a footy

Footy Class General Discussion (Footy)
1

Which would be a better bulb shape for a footy – a long skinny thing, say 8" long but only 3/4" diameter or so, or something considerably shorter, but considerably fatter, say 1.5" or so?

The reason I ask is that it seems the ratio of ballast weight to boat weight is important, and if you can move several ounces of boat weight into ballast weight it would presumably be a good thing – so why not put your batteries in your ballast bulb? They’d obviously have to be rechargable AA’s, and it’d probably be a PITA, but it seems like it might have merit.

-Rick

2

I think if you have a look at the rules you find that while the rule states that “anything not expressly restricted or prohibited is permitted” putting the batteries outside of the “hull” is specifically off-limits. Its a great idea, and one that has been tossed around before, but, like the multiple rudder question and the smaller battery question, its a rule that i don’t forsee changing in the near future.

Keep up the good work though! its thinking outside the box like that makes footys such rewarding boats to play with!

3

I think the smaller diameter/longer ballast is better for 2 reasons:

If you look at it mathamatically, the CG will be lower as the CG will be closer to bottom of box…

The drag will be less because of the reduced cross sectional area.

If the boat is highly angled in the box, the longer ballast can present a problem.

4

The problem of bulb optimization is a very interesting one but much more complicated than it seems at first glance.

A ) for a bulb shaped body there are two kinds of optimization :

a1:minimum drag for a given sectional area ( such as a engine nacelle fairing on a multiengine airplane )

a2 : minimum drag for a given total volume of the body ( this is a problem that was deeply analyzed to improve old airship performances )
This is our situation.

B ) for a given volume :

b1: increasing lenght to diameter ratio, wetted surface is increasing and drag coefficient is decreasing ( a slim body is more “slippery” )

b2: decreasing lenght to diameter ratio, wetted surface is decreasing and drag coefficient is increasing ( a blunt body has more drag )

total drag depends both on drag coefficient and wetted surface

Somewhere it should be a L/D ratio that is the best one

C ) for a given speed

c1: increasing bulb lenght, reynolds number is increasing , and frictional coefficient is decreasing

c2: the opposite is true decreasing bulb lenght

Just to make things more complicated this effect is variable with speed, it is cross connected with point B, and there is also an abrubt change of coefficients with transition from laminar to turbulent flow

D ) for a given volume and lenght

bulb cross section can be modified from height to width ratio 1 ( a circle ), to something that is wider and lower.

d1 :a low and wide bulb has more wetted surface but lower center of gravity
d2 :a “rounder” cross section has less wetted surface and higher cg

E )Last but not least, a bulb shape and size is affecting keel lift changing its effective aspect ratio due endplate effect

F)Looking to improve ( at least on paper ) keel performances, it should also be considered that when not going downwind, few degrees of leeway are to be expected and for this reason flow will not a perfect axial one

G)If you are in the wake of somebody else, or just due to wave turbulence water flow on the leading edge of bulb and kell will be not a perfect one, and unknow amount of existing eddies are not to be forgotten

+++

Conclusions

looking at americas cup very different solutions on boats designed by very skilled naval architects, for very well defined conditions ( valencia ) , it seems obvious that a final word on bulb shape optimization doesn’t exist
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My way

after a careful analysis of existing experimental data , and an optimization worksheet, it seems to me that for an “average” footy ( 200 gr of lead @ 0.6m/s ) drag is decreasing going up to a L/D ratio around ten
exceeding ten, the curve is almost flat and is not worth to have a longer bulb

somewhere I have plotted a graph L/D versus drag ( given speed and weight )
I will try to do my best to find it ( among an unbelivable amount of other stuff ) in order to publish my data on the forum

Flavio

Folgore ITA-5

5

Brilliant info sheet Flavio, thank you!

6

I agree totally with Flavio’s analysis. What this ‘nice man from Italy’ does not tell you is that he is a professional naval architect. Both his general training and his maths are infinitely better than mine (law student + linguist turned wannabee engineer).

For what it is worth I attach a spreadsheet which will calculate the offsets of slender body bulbs. I can’t finf the MIT paper that started me down the slender body road byt try this

http://www.myoops.org/twocw/mit/NR/rdonlyres/Ocean-Engineering/13-49Maneuvering-and-Control-of-Surface-and-Underwater-VehiclesFall2000/45EBA09F-66B5-4BD6-9123-B1A1FF7A590B/0/chap9.pdf

As a comment on what Flavio is saying, unless I ave been inattentive or unobservant, the evidence of the America’s Cup is that tear-drop shaped “high lift” bulbs are a no-no. The resan for this is fairly obvious: at the end of a very deep fin the angle of attack will vary wildly with pitch and heave. Lift is very nice if it’s going your way - but is it? In my early days with Footys Graham McAllister and I carried out an experinent. We stimulated a Kittiwake (a very moderate design) into the highest amolitude rhythmic pitch that did not take a huge anount of force (measured subjectively as pressure on Graham’s finger that the mast head. We recorded the frequency. We then applied that frequency to the boat saling at a speed/length ratio of 1.2 (seemed a good guess for windard speed). WE discovered that, although it was not possible with a Kiiitiwake (less than max draft, probaly quite good pitch damping, it was not impossible that a boat with poorer pitch damping and maximum draft might actually experience flow reversaal over he bulb at the mid-point of the pitch cycle.

Just a thought (?:graduate:?):zbeer:

7

Barrett - Albatross: what a rilliant notion. Not the Foort you bring to theEuro GP in you hand-baggage. This one goes round your neck.

Sorry mat< I couldn’t resist it.

:devil3::zbeer::devil3:

8

What we do know is that Footies are always readjusting themselves as they move, they are not “solid” feeling as an M Class boat or other large class. So all the movement they experience should pretty much make negate many of the common assumptions of form or surface drag, lifting shape or not. The bulbs just don’t stay in one attitude long enough to establish consistent flow.

For those proponents of long slender shapes, bear in mind that a good deal of the time your boat sails downwind she will be bow down. That long bulb will be showing a lot of her topside as she sails through the water. Even upwind, most Footies dig their bows in. So, perhaps we should be concerned with finding an average angle of attack that works for different bow down attitudes. That is should the nose of the bulb be angled up so that as the boat trims out bow down the bulb assumes a more level trajectory? This would probably favor a shorter, more stout bulb profile.

Flavio? Angus?

9

Not at all my friend, not at all…

10

If you look at the bulb calculatior (which I have finally remembered to attach), the system alows for the belb to be attached by means of a transverse pin through a tongue at the bottom of the fin so as to the cant of the bulb to be djusted. There is a formal paper given to a Woolfson Unit sminar by Graham Bantock and Lester Gilbert on bulb cant in IOMs. I do hae a cpy but I suspect haqt Uni of Southampton think I should have paid a lot of money for it, so I’d better not cast it around to widely. Suffice it to say that the assumptions are njot vry sophisticated, but they do sugest that angling the bulb upwards at the nose has potentially big payoffs and that these get bigger as the aspect ratio of the bulb increases.

However, if you look at Lester’s website, there is an article about bulb cant from which I interpret the results of field experiments to be entirely ho-hum.

Over to you.

:zbeer:

11

Without doubt remarks from Niel and Angus are correct.

To summarize :

a ) footys are rolling, pitching and jumping much more than their bigger sisters do.

b) extreme bulbs ( very long, very wide, very strange ) are potentially dangerous if you are not sailing downwind, straightline, with no waves.

c) “fishing lead” bulbs are without doubt too crude : Lenght/diam ratio is around 3, they are symmetrical fore and aft ( good if you are going backward), and their noses are usually too sharp ( not “softnosed” enough )

d) very slender bulbs ( L/D around 10) “seems” to best suited to reduce drag in almost ideal condition.

Conclusions:

L/D less than 3 : the bulb is acting almost as a brake

L/D between 3 and 5 : is a very conservative and “mid range” solution

L/D between 5 and 10 : is an extreme choice, suited to fair wind, and almost no wave situation, as well to a race tactic based on good speed instead a “tacking duel”

L/D more than 10 : most probably is a too extreme solution, drag reduction is only marginal, but a lot of negative side effects are to be expected

So ,keel bulb optimization ,is based also on the way you prefer to race, as well to your race are environmental situation

( As well everything else affecting boat design. )

Flavio ( naval architect & marine engineer )

Folgore ITA-5

12

Hi Neil, yes bulb angle does bear consideration. From the early Kittiwakes I realised that there was a significant difference between the static waterline and what I have come to call the ‘working waterline’. I took many photos of the boat sailing and looking at the ones of it working at decent speed on the beat or a reach came up with a position for the working waterline on a Kittiwake. The Siren was the first of mine to benefit from this working waterline and it is a part of the new K2 kit design.

My feeling is that at slower speed or ghosting along with the bulb now nose high the speed related drag component has fallen low anyway. While on a bow dipping run the bulb will still be nose down but to a lesser degree than a bulb set parallel to the static waterline. There may be some reduction in the bow down effect due to bulb dragging on the full run too I think.

Graham