Does anyone know of a rule of thumb type formula for calculating any increase in draft caused by adding gear? I suspect it is not linear and will have to include beam or bow angle or something. Any help would be appreciated.
Not sure about rule of thumb for yachts, but on larger vessels, if you get the details from your builder, by applying simpsons rule you come up with a TPI ( ie how many tons are needed to increase draft by 1 inch) I believe computer programs have now replaced simpsons rule but I find you sometimes get a better understanding of a subject by applying first principals or am I just an old greybeard.
For a quick rough approximation, multiply waterline length by waterline beam (in metres) and find three quarters of the product. Divide the load in tonnes by this figure to get increase in draft in metres. This assumes that the cross sectional area of your boat is approx three quarters of LWL multiplied by WL beam.