Hi. I'm sorry I know you've probably heard a million questions just like mine, but please help. I need a new prop for a boat I just purchased, it's in bad shape, about 6 in of skeg is gone. I have a 1986 Sea Sprite Continental 19ft runabout. With a mercruiser 170 alpha one. I cannot find any information on it of course for proper replacements. I do have the #off the prop on there,#48 78120 a40 19p but i'm not sure that it is the right size. I took the boat out for a test run yesterday but only hit 35mph at best. That cant be right for this boat, is it? Also can somebody explain to me the function of the skeg other than to protect the prop? Thank you.