I have the volvo and use the 14.5 x 19" 3 blade alum prop now for boarding with the ballast in. I also have looked at the 4 blades - wallet doesn't support it yet. The more power you have at low rpms will help keep the speed. If you can over RPM the engine at WOT alittle its ok because you can chose NOT to push the throttle that far. Add the ballast first - run with 19" prop and WOT the engine making sure you DON't go over the MAX rpms. If the rpms at WOT are slightly about max rpms for your engine then the 19 - 4 blade should be the ticket. If you have alot of throttle left at WOT then you might want to seek out a higher # pitched prop.