Re: correct props for 570 and DP-A outdrive
I did some analysis while experimenting with different pitch propsets on my 570/DP, and calculating what the pitch was. The performance I get from my B5 set puts it as a 22-23 pitch. Each step up from B5 to B6 then B7 seemed to be between 1.5-2 in pitch. Working down the line to a B4 then a B3 I think the same step increment would apply, 1.5 to 2 pitch difference.
Putting your numbers in (4100rpm, 1.95 ratio, 21 pitch, 35mph) gives a 16%slip. Seems high but we will use that as a constant and look at what a move down to the B3 will show. We will use 19 as the pitch for the B3.
Using 35mpg, same slip, same ratio, the engine will be at 4500rpm. This seems about right to me for a heavy boat. The condition of your engine relative to beaing able to generate the original horsepower specs is also an important factor.
The 2inch pitch per step in the B3-B4 is not from Volvo but an estimation on my part regarding real performance specs from my experiments, but it is close enough usually to help in this type of comparison.
If your engine is healthy and in a good state of tune then I think you will get about the same top speed but now at the top rpm in your engine's range, and see better acceleration and be able to keep on plane at a slightly lower speed than now.
