I have a 5 HP Honda on my 18' Grumman SF boat.
I'd say the boat with the 70' Rude comes in at @ #1200 er so.
The 5 HP will push her along top speed about 5-6 mph, 2 adults all our gear, fuel in tank, etc.
It'll troll all day at 2-3 mph.
This is on the St. Lawrence with or against the current.
I had a 22' Silverline Cuddy and a 9.9. HP, that boat went for @ #3600.
That 9.9 pushed that boat @ the same speed.
I've seem smaller outboards with those HUGE pushing props push heavy boats along................... I dunno how you figure out what size/pitch in those props, but they look cool!!
Just my 2 cents!
Joe
Always take any advice I give with a can of beer er 2!