considering a 1975 sceptre

[Fr. Frank]

Twin inline-6 Mercruisers? Or twin OMC V6 motors?

The twin inline-6 Mercruisers were generally 250 ci 165 hp models. (Mercruiser actually produced a 250/250 hp version for a couple of years, I think '69 & '70) At any rate, with twin 165's you should cruise around 28-30 and top out around 42-45. They are very reliable engines when maintained properly.

If you have the twin OMC V6 sterndrives, plan on changing them as soon as possible. They were the "stringer drive" models, the old Buick 225ci V6 block, and available as 165, 175, and 185 hp models. Their weakness was the big rubber boot around the sterndive. If that gets torn in any substantial way, the boat can sink in less than 5 minutes. Been there, done that.

Performance-wise, the OMC is less than the Merc. The 185 hp OMC model will provide less thrust than the 165 Mercruisers.
That's why the old OMC sterndive earned the nickname "Slow MC"