Your hypothetical question is predicated on a false assumption:
i.e. "sheltered anchorage - calm day"

I think that one really needs to be solving this for the worst case
scenario, not the best case scenario. In calm, protected water anything
will hold you. The real issue is when the winds/waves pick up.

A better approach to this question might be to ask what the appropriate
anchor weight and rode for your boat weight should be, or the
appropriate anchor style to bottom type (this is a religious question)
and which direction the wind/waves are likely to force your vessel.

In some cases, the best solution might be putting out a second anchor,
running the motor to reduce strain on your ground tackle, and being
ready to head to deep water at a moment's notice. Thinking one can
simply lay out a magic amount of scope is perhaps wishful thinking; a
more prudent approach requires understanding the environment and how it
could potentially change, and then preparing for that.

I think the 3 to 1 ratio a bit chancy myself. I'd stick to 6 or 7, 10
if possible. More importantly, I'd put as heavy a primary anchor down
as I could possibly afford, and as beefy a chain as possible. Also,
using highly elastic snubber lines from your bow to the chain will
greatly reduce the shock loading on the ground tackle.

You might also consider setting your GPS or Radar alarm to sound if you
move position or get too close to something else.

Hope this helps.

Robb

cvj wrote:
I am getting different advice on the recommended scope when anchoring.
Chapmans says 6-7 times the depth, whereas my Danish friend (Very
experienced sailor) says 3 times depth is sufficient, especially since I
have a heavy anchor and all-chain rode.

In the following hypothetical situation, what scope would the group
recommend:

38 foot sailboat
35 lbs anchor (Plow type)
all-chain galvanized steel rode (300')
mud bottom
sheltered anchorage - calm day - lots of other boats at anchor
High tide mark at 20 feet above bottom.

Appreciate any feedback!