As you say, lots of info but the only speed reference I can find is Alan Winnet's reference to 4-5 knots with a 5 hp engine. Basically I would like to get a minimum of 6-8, (and in a dream world 10) knots out of it because sods law, if it's going to, the main engine will break down half way to Cherbourg. At 6 knots at least I can get back to some land, somewhere in 5 or 6 hours, (carrying 9 gallons of auxiliary petrol). However, if a 10 hp, or even a 15 is still not going to get me that 6-8, then there's no point in going to the extra weight or expense. Might as well get a 5hp and shout help when the scurvy sets in!
According to EP Barrus, the 34 kilo, 15 hp Mariner should, on paper, push my new rib along at "a good 10 knots". But paper doesn't always tally with reality. If someone has practical experience of that size engine, (8,10 or 15 hp) in a 6-7 meter Rib, that would be wonderful.
Then I have to figure out where to put it, and whether I can lift it!
Maybe I should order the scurvy tablets now!
Life is like a box of Leggo,
The last bit's always bloody difficult to fit!
(With apologies to GC)