kymudder

Hello my name is Bob I'm new to the forums. I have a late eighties islander 180 with a mercruiser 3.0 and was Hoping I could get some advice on a couple things. My tach doesn't work any ideas on what to check first. What should my top speed be with a 3 blade 19 pitch prop? And finally I have a nice teak swim platform off my old boat its 72 inches wide and 18 inches deep an fits perfect to the islander but I never see any similar models with swim platforms will I be OK to.mount this in line of where my ladder is now after i remove it? Thank u )