The boom vang will stop the boom rising. If you are ddw your boom as you describe it maybe at say 100 -110. Altering course 10 gives you 110 -120, whereas the wind must be at >135 to give a gybe. A minimum change of 15. You could get some movement from a wake or very likely flukely winds in the light which could set off a gybe but in the light it wouldn't matter too much but it can be disconcerting. You wouldn't need a preventer then.
In fact what can easily happen with minor wind fluctuation and steering wanderings is that rather than being ddw you are sailing by the lee and and a further small turn that way is added to by the resultant change in apparent wind easily accumulating in total to put the wind ahead of the boom and gybe you.
You would be better off not sailing ddw in those conditions but bearing off say 20-30 and gaining boatspeed and vmg. Then you would find that having the boom to leeward gives you some extra degrees of margin, a minimum change of 45, making a gybe unlikely.
Last edited by chris_gee; 02-17-2007 at 10:30 AM.