Hey guys turns out from what i've read that your boom hight should be around you shoulder (don't no if that's when your powered up in the sailing position or when just standing on the board ) however i've been riding with it between my chest and belly button and that now feels comfortable . today i moved it up to shoulder hight and it made leaning back in the powerd up position easier, however i felt in the lulls and whilst gybing i had much less control . should i continue with my comfortable low position. or is it better for me to raise the boom and spend a week or two getting used to it are there and real disadvantages to having a low boom hight

Low booms (~nipple height) + seat harness + short lines used to be fashionable. This setup works all right for flat water. It pushes your harness lines forward a bit and shortens the distance between the foot straps and mast foot a bit. It's fast and it makes water starting a tiny bit easier but it has a few faults imho.

1) It makes hooking in and out a little less convenient.
2) It makes the ride a little harsher and tends to shake your eyeballs a bit more.
3) It's more difficult to hold downward pressure on the mast foot while unhooked with a low boom.
4) With long lines the board will run freer in pitch and you will find it easier to hold your head still and feel more comfortable and you will find it easier to swing your weight in and out with long lines.

Whether you use a seat or waist harness is a matter of comfort.

Take a look at Guy Cribbs technique articles (www.guycribb.com)

For boom height he recommends "About Neck Height, or a fist or a few fingers off off the back of board"

TECHNIQUE => QUICK TIPS => BOOM HEIGHT shows some pics that clear up the description.