On a single-hulled boat you can tell you need to reef when you're heeled over too far (unless you're going downwind). Often the correct choice to reef will actually make you go faster. Obviously it depends on the conditions. With multihulls you don't heel (much) so it's harder to tell by feel. You need to know a few things:
1. What is the maximum rated force of the weakest link in your rig. (probably the sails)
2. What is the minimum force on the mast
required to capsize
the boat? - This is a difficult one to calculate as when it starts to lift
, it offers less sail area to the wind
etc... the manufacturer should be able to give you some guidance.
3. What apparent wind
speed does the smaller of (1) and (2) equate to with a full sail? (some basic mechanics required here). What apparent wind speed does the smaller of (1) and (2) equate to with 1 reef, 2 reefs
4. What would be the maximum apparent wind speed possible in the current
conditions (ie if you change your point of sail)
If (4+15%) > 3 for your current
sail configuration, then you need to reef.
If you want a bigger safety
margin, use 20% instead of 15%.