TOPIC: Changed Out Drive. What Prop do I need?

I have a 1967, 165 Hp Mercruised straight 6 (250 cubic Inch) in a 23 Ft. Dorsett Cabin Cruiser weighing about 5500 lbs (Empty weight 3600 lbs). bought it in 72 & use it a lot. I munched the original OD and replaced it with a used pre Alpha with different gearing. With the old OD it would get up & plane with a normal load & I could run it at about 3400 RPM & 3 or 4 inches of vacuum. I took it out the other day & with the new (used) OD and 2 adults, ½ tank of gas, water & minimal equipment on board it maxed out at 2800 RPM was definitely Not on Plane & zero inches of Vacuum. The prop is a Mercruiser 48-79570-15, 15” pitch & I think 15 ½ inch diameter.
I’d like to get the performance back to where it was and at this point I’d rather change props than gears.
Anyone have a good formula for calculating a different pitch prop?

Hi John,I run a 165 L6 on a 20'tri hull MC1 drive the stock ratio is 1.165 with a 14.25,17p this does 62kph at 3900rpm the boat weighs 1450 kg 60lt of fuel 2 people,these motors at WOT max at 4600,I tried a 19p it maxed at 3200,there is no such drive as pre alpha,either Type 1,R,or MC1,then Alpa 1,depending on the ratio a 15p should rev it's a-s off,maybe you need a motor tune,hope this helps

A few points to talk about.

1) There was an inline 6 prior to 1967 that was rated at 140 HP then 150 HP. These were not 250 blocks, they were 194 and 230 if I remember correctly (all 3 were GM bocks & look similar externally). Redline was 3900 on the I-6 140, & I don't have the 150 numbers in my head. (There was also a 292 rated at 200 HP that used a MerCruiser 2 drive for 2 or 3 years in the late 60s).

2) The 250 block was used starting in 1967 and was rated at 160 HP for 1967 thru 1969 and some carry-overs into 1970. 1970 also saw the introduction of the 165 HP. The difference was the manifold. 160s had a crescent shaped top to the manifold that was removable with about 12 bolts. The 165 manifold was a one piece casting. The 160 & 165 HP were rated at 4300, not 4600 as stated above. Gear ratio was 1.65:1 on all 160 & 165 units.

3) I can only assume you have swapped a V8 lower unit onto your boat if you lost RPM. There were 2 additional ratios used that would drag your RPM down. Those are 1.32:1 and 1.5:1 Go down in prop pitch at least 4" and try that.