Re: Mercury 50 vs mercury 70 ?
I don't know about Merc, but I had a 1976 15' tri-hull, originally with a 50 HP evinrude. The hull was rated for 70 HP max. with the 50 on the back, it required me to go to a 15" pitch prop, plus smart-tabs, in order to plane reliably. Even then, there was not enough power for any kind of tow sports other than a tube at slow speeds. Time to plane was in the +10 second range, and make sure you weren't heading into the wind. Top speed at WOT was about 23-25 MPH.
I replaced the 50 HP Evinrude with a 70 HP Johnson when I ended up blowing a piece of piston skirt through the crankcase. The 70 HP was the right motor for that boat. I ran the 70 with a 19" Pitch stainless steel prop, and it came on plane quickly and reliably. Enough power for a kneeboard (a little harder to get a skier out of the water, but with an experienced skier we were able to do it) or for a fun ride on the tube. I could load the boat with two adults and three kids with little loss of performance. Top speed was 32-35 MPH. Fuel consumption was about the same as with the 50 HP i.e. about a gallon an hour.
In my oh-so-humble opinion, if your boat is rated for a 70 HP, and you have the ability to put a 70 HP on the back of it, go that route rather than be disappointed at the marginal performance of a slightly under-powered boat.
Rgds
Eric
