Drift Sock?

[vancouvercanuck]

What's the average speed a drift sock will slow your boat drifting?

Say a 18' boat on average with 20-30 mph wind.

[eyedoktr]

There are way to many variables involved for an accurate answer. It depends on sock size, boat size/weight, current, etc.

In the winds you ask about (20-30 mph) I use two socks on my Lund 1800 Fisherman. I can't remember what size the socks are but they are larger than the package recommends for the boat.

[Barleydog]

VC-  I have an 18 ft. V-hull bay/river sled/boat which I use 2 smaller socks, (say 3 ft. diameter gap), cleated off to both sides of the transom.  This seems to give the wind less surface area to push...  Anyways, I never tried using them in any winds above 20 knots.
Here's a quick guide:
10 knots = 1 - 2 mph
15 knots =  2 - 3 mph
20 knots = 3 - 4 mph
This is just a wag using the fishfinder and 2 socks.  I had my driftsocks made by a boat top manufacturer in Oregon.  He makes them 3 or 4 foot at the mouth with a 6" opening at the back.   The funnel design makes them easier to remove from the water, fast!  
I have yet to use them since I bought a new 4 stroke 9.9 Honda.  This little gem keeps me in place without the fumes and noise.  Hope this helps?   -Barleydog