Gear Box Ratio and Prop RPM DE 38 Engines and Such Forums

I am about to buy a new inboard to replace my Volvo Penta 2003. I also have installed a new prop shaft and prop, cutlass bearing. At the moment I am looking at the Yanmar 40hp common rail that has great reviews of the Beta 38 hp. I am a bit concerned about lightning surges blowing the electronics of the yanmar but I love how efficient and quiet they are. Any input to help with this decision would be welcome.

The reason the this post is that I need to work out the required prop speed WOT. Each engine has different RPM and I need to work out the gearbox ratio required. I hope someone can help me. I have owned this boat for a year now, the honey moon is over and we are still in love. Hence treating her to a new shiny engine. See you all out there. R.

Also what is S/30 A/30 and R30 on the engine spec sheet? Thanks.