Yes, that's normal for a boat
of this particular design idiom. Never sailed a Mirage 30 but I sailed the 27s a lot. The seller was pulling you leg. Heeling to 30º is absolutely counterproductive in any boat designed in the modern idiom. 12ºs would be common enuff because that's where the boat sails
best. 15º on the clinometer is an indication that it's time to shorten sail. 18º of heel means you'd better get on with the job before you broach as you apparently did.
All well-designed boats will have just a touch of weather helm
when heeled to 12º. That is indeed a safety
feature because if you take you hand of the tiller, the boat will stand up on its feet and luff with no input from you. In addition to being a safety feature it also permits you to know what the boat is doing just by the slight pressure of the tiller against your hand, and by the "thrumming" the water
flow over the rudder
transmits to you hand.
in a boat as small as 30 feet is not desirable because that "feel" for the boat is destroyed by the wheel's linkage to the tiller.
When you heel a boat of this particular design idiom - it's not just the Mirage 30, it's all modern cruiser/racers - the rudder
pulls out of the water
either partly or completely due to the boat riding up on its midships bilge
, so that you can no longer control the boat, and it broaches wildly as you have now experienced.
An aggravating factor is that boats of this type sail to a large degree on their headsails, and people therefore tend to carry something like a 135% or 150% genny long after it's safe to do so. If at 12knots of wind
you've come down to a 100% jib
and full main, at 15knots a 70% jib
and a single
reefed main, at 20knots a storm jib
of about 50% and a double reefed main, you can sail comfortably and safely because the boat will not broach. You'll find also that you will go FASTER through the water.
Always consider when you take a boat for sea trials that the seller will know no more than you do, and may well know less (that might be why he's selling!) so don't ever believe anything the seller says unless it can be independently corroborated :-)
Welcome to our forum, and be assured that any question you may have can be answered by the "old hands" here.