tamd40b

I have a 40 ft vista.. (k a shing) with twin 165 hp volvo.
With ms3c 1.1.53 Trans
Is a 4 blade 20 x 22 better than a 3 blade 20 x 21?

Welcome aboard. You wont 'know' until you try them but there are prop calculators available on-line at sites like boatdiesel.com and Michigan Wheel's site that may get you closer. My guess is you will need to go down in pitch to 19 or 20" with a 4 blade, not up.

Props are not good or bad. They all have characteristics that must be matched to your specific boat, not just your year make model. A common and acceptable way of estimating how a prop will work on your boat is to first measure a known props performance on your current boat. You will want to measure the boats maximum capable rpm with the current prop along with the boats speed. Both need to be measured accurately. From there, a different prop can be guesstimated from its blade count diameter and pitch. There is a lot more to props than this, but those are the basics that get you into the ballpark. It often takes a couple shots to nail a desired outcome. Which is also to say that there are trade offs to be made, so no prop can ever be chosen without knowing what you are trying to achieve. You will need to determine and state that.

Which is as concise as I know how to put the basic notion that your question needs a lot more detail. Props is an area that requires quite a bit of investment to even ask a basic question. Otherwise, you are going to get imprecise answers that are nothing but guesses, or worse are specific answers to a whole range of potential questions unasked.

thank you