If the 'soot' has become ingrained so as to render most 'normal' detergents ineffective then you may have to concede that, at some point in the not too distant future, the boom may require a re-spray (depending on how long the soot has been allowed to accrue).
As a temporary measure (in order to 'restore' the whiteness) mix a tin of Brasso metal polish (liquid form) into a bucket of warm detergent - the soot will be removed! But .. with it will go much of the clear coat (if you have any on your craft). Once you've used Brasso you will need to keep the tail clean on a regular basis in order to avoid further applications of the 'gritty' solution.
Good luck .. and send us a photo once your tail is clean!
