FOOTY Performance.

Footy Class General Discussion (Footy)
21

Instinctively Bill this feels right …

… a Footy designer has the option to add as much horsepower as is required to drive a hull … an ULDB may be the most competitive in light air where hull speed is not always achieved … once up to speed however, the extra mass carried by a heavier boat will carry it through tacks and windshifts with greater momentum, thus providing an advantage …

… to the contrary, if the skipper is clumsy or speed comes off for whatever reason , the ULDB will then have the advantage …

22

Yes Barrett, of course it was you I was getting at. Only go for the best of the opposition.

You force me further out of the closet than I intended.

Surely the point is that there is nothing for nothing. The fat boat gains in stability, but does its drag go up faster than the resulting gain in horsepower? To me the evidence is that it does – unless you have moveable ballast. A pencil tips over less easily than a dock: it also takes a damn sight less power to drive it – so it does not need so much stability or sail area. This incidentally has its own side benefits – the rig can have a decently high aspect ratio without its CG or CE going too high, pitching moment is reduced, and so on.

Back to the main point: we are not looking for stability per se: yacht design is a matter of ratios, not crude numbers. What we ARE looking for is the optimum ratio between power and DRAG (not weight). The mystical magic number here is ‘form drag’, which Every Gonzo’s Book of Naval Architecture generally dismisses in a couple of paragraphs adorned (if you are lucky) with a few coefficients that ultimately hark back to Nathaniel G. Herreshoff’s hilariously funny speech at the works Christmas party in 1904.

At this point, let us agree with MechDoc that ‘hull speed’ is a myth – an arbitrary point on a rapidly steepening curve of resistance/speed. It has everything to do with form drag. Wave drag is being distorted by considerable amounts of (viscous) form drag. My gut simplification (and heaven knows I am anything but a professional fluid dynamicist) is that, assuming you do everything round reasonably nice ‘streamlined’ curves, the form drag is in proportion to the amount of water you push out of the way and how far you push it. The proportionality is at a power higher than 1 – which explains why thin boats are generally fast even with less sail area and/or stability. However, I think that there are substantial differences around the ends of the displacement speed regime and around the ends of the boat.

Given that you are aware of my obsession with Mariner, you might care to work out the next piece of the jigsaw for yourself!

23

Just a thought in passing.

I hate going back to 12 metres all the time, but they do represent a uniquely finely tested set of data under a stable rule over some time.

Remember Nefertiti - designed by Ted Hood. She went for non-rule-minimum beam by about a foot - about 13’2", I think. The idea was not to increase stability - she had slacker bilges and cnsequent lower girth penalties to compensate, which gave her rather more sail area, the same ability to carry it and probably a touch less wetted area. What Ted Hood ws trying to do was to increase the efficiency of the rig by increasing the sheeting base of the genoas.

She was not successful. So even a bigger, more efficient rig and the ability to carry it could not compensate for a proprtionately quite small increase in beam.

Whether this means anything, I’m not sure. The trick is in the ratios and they are pesumably locked away in some archive in New England.

24

Angus, you make me smile!:stuck_out_tongue:

you may be very right, i am starting to feel that the footy model may end up being a narrow, slab-sided, relatively heavy displacement boat. your point about the drag going up as the wetted surface goes up is a very good one. so before i launch off into the last of my defenses of the “wide” boat i want to make it clear that i do agree with what you are saying, and that to an extent am playing devils advocate…:devil3: of course, to another extent:

i do think there still is something to the idea of a wide hull. you are right that the “wide” boat finds its roots in attempting to get more leverage for the ballast, be that human, or water. however, the need for that disappeared with the advent of the canting keel - all of the sudden you could have a very narrow boat, and still get the stability you needed, without any extra width. yet, designers stayed with wide boats. [i am thinking of the open 60’s, VOR boats, open “xyz’s”] i figure this to be because of the fact that as i have said, when you heel a wide boat out of the water, the wetted surface goes way down, almost to the point where the boat attains catameran sailing qualities. if you think about the pictures of the Vendee boats, they are often planing downwind, or over on their ear, on a reach, or more rarely, on their ear upwind. when they are in these latter two modes of sail much of the boat is out of the water - low wetted surface. much lower than say one of the new 40 foot canters, that are very narrow, which heeled over have much the same wetted surface as when upright. that said, as i noted in my last one of these things, this design philosiphy may not have a place in the majority of footydom, simply because there is not a lot of reaching done by these boats, and so, spec-ing a boat out to be a rocket ship across the wind may not be a “fast footy” however, in the few times when the boats go a distance on a reach, or have a triangle course, where it is a an upwind, reach, reach format, i think a wide boat would not be as penalized as perhaps we are thinking… of course, i have not had the chance to test this in real life…:rolleyes:

to quickly add something totally off topic about the 12s, is it just me, or should we drop the new AC boats, and go back to the 12s!? that i think would be the best thing that happened to the Americas Cup since being allowed to build boats out of aluminum/composites…!

anyhow, as is becomeing popular to say, “just my .02!”

barrett

25

Gee,

Where’s tetrax & waboats now? How come they aren’t mixing it up with their off-topic theories and irrelevant dictionary definitions (of “skinny,” "wide, " or “best” :stuck_out_tongue: )

THIS is where any discussion should be happening, not the other stuff that had been defined since the beginning ( I won’t mention what.)

Cheers

P.S.

Barrett, Thanks for not say “my .02 cents,” which is actually $.0002 or 2/100 of a penny!
2¢

26

yay! no dreaded “R” word!:smiley:

27

Rumble seat?

28

Speak of the :devil3:

Because “skinny”, “wide” and “best” are not amenable to being settled by dictionary definitions; “skinny” and “wide” are relative, and the “best” boat is the one that wins… or the one that sails utterly reliably while you sit in a deck chair passing the hours… or the one that does whatever it is that you want a boat to do.

This is NOT where discussion should be happening - people should be making boats and racing them against one another, not talking about it. You can’t “discuss” a boat into being the “best”.

As for rules… to be honest, it’s all fine and dandy with me. Your rules do not “define” what you think they define - if anyone was considering RUNNING a Foot event, I would recommend they didn’t; I certainly wouldn’t - I wouldn’t want to be the one who gets screwed by the guy who enters a 2-battery boat, for example. But if you guys are all going to play nice like nice kids, and take the rules to mean what you want them to mean, it’s fine. Just don’t cry like kids when that guy does screw you.

29

Tetrax. If you want to play Intervarsity moots, that is your business. I used to find it quite fun and was reasonably good at it. These days, I prefer to be a nice kid and have good-hearted fun with my friends at the lake and on the internet.

If that is what you want, welcome. Come in and contribute. If it is not, I suggest that you go and play somewhere else. You cause a lot of people here a great deal of annoyance and are unlikely to find any great meeting of minds.

30

This is EXACTLY the place to discuss ideas. No, you can’t define ‘best’ but you can discuss hull theory & design and any other thing. Then build your idea and test it, and share your findings. That’s why this website was created.

As for rules… if anyone was considering RUNNING a Foot event, I would recommend they didn’t…; I wouldn’t want to be the one who gets screwed by the guy who enters a 2-battery boat, for example.
As part of the race notice, the RC will first announce that the ISAF RRS will be used, and would probably be followed by any special things or ( for a Footy regatta) anything special to say about Footy specs. You can bet on having certain things spelled out to avoid confusion; including contact information to get any specific quesyions answered before race day.

thx

31

Not correct as I see it.
If you want to race in all conditions at some stage you will have to change to a smaller rig.Under the rule the only other rig avalible to you is one 305mm high.

If you want to compete in an event with a building breeze for example (very common situation) you will need to have a boat fast in the 2 rigs chosen right across the spectrum.

To me this means you probably don’t want to big a change in areas between rigs.

When starting a new design it is the windspeed I want to use for the “B” rig crossover point that i would be looking at first.

Again a pure internet race boat can disregard that…if you like waiting for your “design” conditions that is.
I guess we should make it clear just what sort of Footy is being discussed here.

32

i woudn’t worry about talking about the two “different kinds of footys” here, but we may want to specify at the top of our message what type we are about to launch into thoughts on, so everybody else has some context…:rolleyes::p:p:p

33

I’d still contend horsepower isn’t limited … which isn’t to say sail shape / aspect / rig type won’t be; they certainly will …

… guess my point is/was that within practical boundaries, a 30cm b rig could be designed to drive a 1000 gm boat, from which follows the design of the A rig … and which then allows the mass acceleration/deacceleration arguement to follow … as long as we’re not skirting the extreme edges, horsepower should be available to drive most any hull (A or B rig included) …

34

I’m not at all sure I buy the deceleration argument. In a drifting match rapid acceleration gets whe whole rif going - gets some shape into the sails. Of course the boat isn’t ‘making its own wind’ but it is making what airflow there is over the rig easier. Hence he acceleration and deceleration curves are not symetric.

35

i agree with both of you, i think brett is probably right, you don’t want to go from a 20" rig to a 12" rig because the breeze went up just enough that the 20"er strated to loose control, the 12"er is going to be under powered. But, i also think that as tmark said there is a way you can make the “B” rig big enough to not have that happen. however, i have another thought. most well designed sails can have their shapes changed to accomidate changing wind conditions… with good outhaul, cunningham, and in somecases, mast crane/rake control you can totally alter the sail’s wind range… [to a point, i don’t want to be misunderstood to be saying that everything can be fixed by use of sail controls, but the length a sail can be used in a certain wind regime can be lengthened by the controls used to shape it…] also, could we reef? =P

i agree with you angus that when there is no wind, the boat that can accelerate into what there is and move with it fastest will have the edge, but, in a tacking duel, a wind shift, sudden manuver, the law "objects in motion stay in motion, unless acted upon by an equal and opposite force-- a heavier boat, with more enertia will require more force to stop it… it also will retain much of its momentum through a tack in medium air, and -this is just a conjecture- but i would imagine that a heavier boat would not get tossed around as much as a light boat.

that said, there is a reason lasers are faster in light air than say, a bullseye, or a big plastic boat, they move more easily with less force… but once you take the air away suddenly, a big heavy boat will drift further…

just my .02! :rolleyes::graduate::cool::stuck_out_tongue:

36

Problem with all that Barrett is that you have to make the rig automatic - and by that I mean clever geopmetry, not Heath-Robinson arrangements of craks, levers, claws …

Incidentally, I think that the 'toss around in a slop number is something like the waterline beam/waterline length ratio or possibly waterline beam to displacemernt. The waterplane coefficient probably comes into it somewherte too. I will ask someone who probably knows, at least tht the supertanker scale.

37

didn’t “waboats” claim to know something about oil tankers? :p:rolleyes::rolleyes::stuck_out_tongue:

as for the sail controls angus, i had thought they would be the kinda thing you pulled up to the dock between heats for… although, some sort of rupe goldberg kinda thing might at least afford some laughs…

38

I think that was the fleet of mighty 20’, kite-assisted Zippo fuel tankers that ply the Swan Rivr by day and by night bringing solace to the somkers of Perth.

I was more thinking of the ‘19 controls on one radio channel’ schoool.

39

well, if someone can fit 19 functions on a channel, please tell me, cuase then, we have a breakthrough…

as for the 20’ zippos… i always knew there was something funny about the “i konw all about tankers with kites” story!:rolleyes:

40

actually, i would be interested to hear if someone got 2 funtions on a channel… i.e. to functions out of one servo, etc…:cool: