It's the weight, not the hp that's the problem. To run the heavier (than OMC 75) shortshaft Mercury 85 in GP class Paul had to make a bottom modification with Bondo to get the tail up. The V-4 with its 20" downhousing would be a handling nightmare because of both weight and height. Far more sensible would be to put an OMC SST-60 motor on the boat. In any case, the pad and turning surfaces adjacent to the pad must still be good enough not to flex, and must be straightedged. A flexing bottom will kill the speed regardless of hp.
We and others who were competitive in GP Class never ran an 85 Mercury on the 13' boat, we used the 14' model, some (Joe Burgess) ran the 15' model with success. Some few may have run the heavy 85 shortshaft Mercury on a 13' Delta or Critchfield, I don't remember anyone running anywhere near the front with a 13' boat, most others ran a 14' Hydrostream (those boats wouldn't turn). Actually, I generally added weight and stepped up to run GP with my smaller and faster OMC 75, and was always competitive.
Do I know you?!
Joe
Thanks for your input. Cant think of many people who could offer a more informed opinion on this matter than you. Your advice is VERY much appreciated.
How heavy of a motor can the the 13 footer handle? Is it strictly the weight or is it a horsepower issue also? I know of one in Houston thats about to get a highly modified 3cyl. It should be well above 100HP.
Mine is an earlier 1970 model with a wider pad,approx 10"wide. Its in the process of being fully re-cored and a new transom and stringers.