"This is basicly two complete
"This is basicly two complete different engines designed for different use.
The 1,7 is for a relatively flat bottom boat with not much demand of torque for getting in plane. It is vulnerable to load, one of the reasons of the piston problems.
The D3 volvo, which has absolutely nothing in common with the 31 series except for the fuel,has at least a bit more torque and the possibility of the duo prop option which may be useful in an installation like this.
Most ribs needs the 'planning torque' all down to 20% of the rpm range. They are heavy to get planning, but then hardly any power needed for speed encrease.
Honestly to me putting a diesel sterndrive in such a small rib seems to be a totally waste. With the limited rpm range available (4000), the wheight of the package and the cost I can not quite see the advantage. I have owned, sold and been running several ribs in the past, this is to me the least appealing of solutions. Power vise a 125 HP (gas) inboard eaquals something like 85Hp outboard in acombination like this. I did some tests for Volvo some years ago. With todays technology in 2 stroke outboards E-Tech/Optimax (not 4-stroke!) the total cost of buying, running 10years, and service will probably be far in the favour of the outboard, unless perhaps You are planning to cross the Atlantic!
And do not forget the fun of running a sporty rib in rough conditions. Your solution sounds more like a ferry ride on the river Thames, but perhaps that is what You are after.
Anyway, good luck."