Does doubling the ingredients apply to yeast?

I have a 5 gallon recipe I want to make 10 gallons....... I figure I will double the ingredients......Do i need to add 2x the yeast or just add the same amount and wait a little longer?

at $9 a pop I would like to conserve if possible

Not sure if it matters but it is the YEAST STRAIN: 1335 | British Ale II™
http://www.wyeastlab.com/hb_yeaststrain_detail.cfm?ID=141

 

Short answer is yeas, you should pitch more yeast, but no, you do not need to buy two packets/vials. Read up on yeast starters.

 

I think the danger with underpitching is that the longer lag time 'till the yeast gets going can mean more risk of an infection, and the yeast could also produce off flavours. Do you normally make a starter with your liquid yeast? I would think making a bigger starter and starting it a day earlier to give the yeast more time to reproduce would give you enough for a 10g batch. I've also seen threads on here about doing a couple of steps to work up to a starter that's big enough for 10g.

I guess it partially depends if you have the activator or propagator size of yeast. The activator claims to be enough to pitch directly into 5g (though some would debate this), so I'm sure you could pitch it right into a 2+ liter starter. With the smaller propagator, I might do a day in a 1 liter starter, then move it up to a larger one for another day before pitching into your beer.

 

You determine your pitching rate to decide how much yeast you are going to pitch. The general rule is, 1 million cells per milliliter per degree Plato (~4 gravity points). So if you have 1.050 wort, here is how you calculate your pitching rate:

50 gravity points/4 gravity points per degree Plato = 12.5 degrees Plato
There are 3785 milliliters in a gallon * 10 gallons = 37850 milliliters

37850 * 12.5 * 1000000 = ~473 billion cells needed

So you would need 5 smack packs to ferment that wort of yours assuming you're at 1.050. I would recommend making a large starter instead with one or two packs instead of buying 5 packs. Hope that helps.

 

Yes, you need more yeast for 10 gallons. Making a starter is a little extra work, but much cheaper than multiple pacs.