## Re: [FootyUSA] Re: ideal footy displacement

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• One thing we often forget to do is realize that the physics don t scale down 1:1 when you get as small as the footy. Things like Reynolds numbers work
Message 1 of 34 , Sep 28, 2009
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One thing we often forget to do is realize that the physics don't scale down 1:1 when you get as small as the footy. Things like Reynolds numbers work differently when you get as small as footys are. That said I like this type of work and I think it benefits our class a lot. Build one and see what imperical data you can get to add to your theory. Who knows, you could be on the cutting edge!

Allan

----- Original Message -----
From: mudhenk27
Sent: Monday, September 28, 2009 7:37 AM
Subject: [FootyUSA] Re: ideal footy displacement

LR, that's a good analysis. I hope you'll build one and keep us informed. I think you might find that a boat that light could have some trouble in choppy conditions.. .you can't scale down the wave factor. I could be wrong, though, since the lightest boat I have is 340 grams.

Bill

--- In FootyUSA@yahoogroup s.com, "LR" <lincolnr@.. .> wrote:
>
> For further comparison, a full scale Soling is probably around 140. I think the one meter model is around 165.
> --- In FootyUSA@yahoogroup s.com, "LR" <lincolnr@> wrote:
> >
> > I may be missing something, but it seems to me that footies as they exist are very heavy. If I understand correctly, the battery restriction is now gone, so it ought to be possible, with care, to build a very light footie. There is a ratio generally called the DL ratio, even though D/Lcubed would be more accurate. There's a neat calculator here:
> > http://www.sailingu sa.info/cal_ _dl_ratio. htm
> >
> > Seems like most footies are close to a pound, or even heavier. One of the neat properties is that, for a footy with a one foot waterline, this ratio is very close to the weight in grams. So a typical footy might be 450. Now, we'd like fast boats, right? A J24 is around 180. A light US1M might be around 130 or so, although I guess some heavier boats are somewhat competitive at 150 or higher. To get a footy in that range you need to get it less than a third the displacement of a typical footy.
> >
> > I think this could be done if you used one of the new 2.4 gHz radios from an indoor flyer, and maybe an HS-55 (!) for the sheet, while using the built in linear servo for steering. It would be radically different, but I bet it could sail a LOT faster in many conditions.
> >
> > Maybe it would be worth it to go even lighter. I dunno.
> >
> > This probably wouldn't require any exotic materials, just thinking and care. At one foot I think the weight is probably going to be determined often by the lightest gauge materials available, rather than strength or even stiffness. (Ok, this is intuition as I haven't built one yet.) And those indoor planes are pretty cheap. The receiver is only \$60 by itself, or you can get the whole thing with tx for \$120 if you don't mind throwing the plane away. If you're really sneaky use the motor to make a hidden auxiliary, but don't tell anyone.
> >
>

• I have upgraded from the futaba 3115 to the futaba 3102 as well. metal gears enough torque...it is also the rudder servo for my footy, s1m, victoria, and odom.
Message 34 of 34 , Nov 5, 2009
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I have upgraded from the futaba 3115 to the futaba 3102 as well.  metal gears enough torque...it is also the rudder servo for my footy,  s1m, victoria, and odom.  and I run a 2 cell lipo with no regulator  with no burnouts yet...

I run a sloop rig

marc
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