prop size realation in twin vs single engine

[stefano_ita]

Hi all

i was wondering, in a full displacement hull (trawler) wich is the relation of the prop size in case of 1 or 2 engine?

same diameter and almost half pitch (let's say almost, not to have a 200% more power)

or

half diameter and same pitch as the single.


acording to me the second one, because in the 1st case half pitch would be double rpm, no sense during cruise, and a very strong pushing power from stop to 2-3 kts that is meaningless, not being a tug

what about you?

p.s. someone know a site where giving diameter, pitch and rpm it calculate the required horse power?

thanks