Carrying only 1 bottle during 70.3?

What about calories?  I'd need about 3 gels per hour, so that might add up to 150 mg if I drank 12 oz of gatorade an hour.  You could also try salt tabs

That said, I wouldn't want to do a HIM on gels alone.

I do almost all my races on gels and water, including a HIM which I have done twice (will be my third time again this year).  It's in June, in Kona (), but I'm also from Hawaii so I'm accustomed to the heat.

I carry two bottles of water for the bike, and while I could possibly make do with just one, I wouldn't want to risk it.  There are ample aid stations on the bike, but lots of times when I'm drinking, not all the water goes in my mouth.  Sometimes I purposely spray water over my chest, neck, and back to keep a little cooler.

Also, the bottles they handed out at aid stations did not have a sealed top...so some water would leak out if you had it mounted horizontally.

The best way to determine this for you is to test it during training.  Go out on a ride and force yourself to ration your water in a way that would simulate only having one bottle.  Just make sure to take into account that you would have just swam 1.2 miles before, and you still need to run 13.1 after.  Nutrition/hydration on the bike is crucial...because you can't take anything in during the swim, and it's usually too late to "catch up" on the run.

I didn't rely to much on gatorade last year for calories.  I only drank about 18oz of gatorade on the entire ride so that is only about 120 calories.

I didn't think about this.  I will have to experiment and see if the bottle will ride without spilling out.

This is what they were handing out at my HIM (and are pretty common at WTC races).  Some of the volunteers would rip off the top cap for you.  Not a problem if you're using frame mounted cages where the top stays pointing up for the most part, but about 1/3 of the water will leak out slowly if you put it between your bars without the cap.

If I got two bottles without the cap, it wasn't a problem for me having two cages.  I'd stick on in my downtube cage, then spray myself down and drink from half of the bottle before putting it between my bars.  And I still had 1.5 bottles with me before the next aid station.

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I had no idea that they handed out that kind of bottle...Seriously?

Last year I volunteered for the Grand Columbian Ironman and Super Tri and they had actual biking water bottles.

I guess with 1000+ people you kind of have to do that...but that seems cheap.

Yea that is what they handed out last year at Redman 70.3 too.  I guess I need to experiment and find out where the water level is on the bottle before it stops leaking.  Because if all I had to do each time I got a new bottle was a squirt on the head and a drink and it wasn't leaking anymore than I would probably be good. 

I have the original speedfil and won't give it up! I really like the refill capability.

I only carry one bottle for an IM....

...At a time.  Reload at the aid stations and try to finish at least 75% of if before the next aid station.

I have the A2.  Love it.  Tried the frame mount and didn't really like it

This.  I raced rev3 Knoxville last year with a 1 bottle setup.  Dropped a handup and was lucky to grab another before the end of the aid station.  x2 to the leaking also, plus they squirt some out when you grab them from the volunteer.  I think I ended up under-hydrating on the bike that race.

I didn't think about this either.  You guys are making good points that are shooting some holes in my plan.

I've never tried a Speedfil but I have a Profile Design Aerodrink and I can't stand having the straw in my face.

Not a huge concern.  I would like to be as aero efficient as possible plus I really like the look of a clean bike frame.

I don't like the hose in my face either, but it's minimal annoyance to deal with for being able to drink in aero.

most have abandoned the original Speedfil (from what I can gather) but I cling stubbornly to mine.  it has a large reservoir and refill feature is pretty handy...

I read an article on that and the picture they posted was an aero bottle.  I have a hard time seeing how a big bulky cylinder shaped bottle would make you more aero but I could be wrong.

I couldn't stand Ironman Perform, so I lived on water and solids for 140.6.  It's fine - just be trained for it.  I did start with one bottle of gatorade G2 that was gone probably within the first 40 miles and popped an electrolyte at T1, my special needs, and T2 (and my run special needs).  That was Arizona 75-80ish degrees.

Do a sweat test in the heat, then you know how much you want to drink.  (not that you should try to replace EVERY pound of water you lose)  I don't recall the numbers off the top, but you can find how much sodium you should take in per liter of water.

No, a bottle inbetween the aerobars is more aero than no bottle.

According to cervelo.

http://www.cervelo.com/en/engineering/ask-the-engineers/hydration-and-aerodynamics.html 

I have the profile design aero bottle and agree with you on the straw in my face.  After my first race with it when i got home I cut the straw down even farther.  Now to drink i have to drop my head down a little ways...still staying in aero but don't have the straw in my face either. 

I used two bottles on my last HIM.  I had the Profile bottle and one bottle on the seat tube.  I replaced the bottle on the seat tube at the aid stations...I kept my carbo pro mix in the aero bottle and used the seat tube bottle as water only....

My thought process on a HIM race...better to have an extra bottle than not have enough.  It is such a small amount of weight/aero that it will never out way the possiblity of becoming too dehydrated and bonking on the race.