140 w/ 470 drive RPM & MPH #s

[Tycer]

This is a follow up on this thread: https://forums.iboats.com/threads/merc-140-motor-470-outdrive.722537/#post-5478672
Thanks again for all the replies. With a bit of calculamating and trials I’ve gotten the disparity sorted.
The boat is an Aristocraft Nineteen, about 1800 pounds and these numbers are from a normally loaded boat with a full tank of gas two adults, a dog, food and beverage. Not glass smooth water and variable wind around 5 mph.
‘85 Mercruiser 140 motor with 470 outdrive. Quicksilver aluminum three blade 15.5”x18p

2500=20mph
2800=22
3100=26
3200=27
3500=29
4000=34
4500=39

 

[Tycer]

4500 is WOT

 

[Scott Danforth]

that is about all that the 3.0 has. best speed I have ever hit with a 3.0 was in a 16.5' boat and we hit 42mph.

 

[achris]

As long as you're getting to maximum recommended RPM at WOT (for a 1985 140, that's 4200 to 4600) then everything else is academic. You're getting to 4500rpm, so all good.

You have negated the lower reduction ('470' drive has a reduction of 1.65:1, 140 used a 1.98:1 drive reduction) with a smaller (pitch) prop, so as far as the engine is concerned, no harm, no foul...

Chris...

 

[QBhoy]

Good going. Most I’ve ever seen out a 3.0 on a normal 18ft bowrider was 44. Something mph GPs with a laser 2 on it.
exception was from an old cvx16 with the old 140 3.0 and a pre alpha and ballistic prop. That was just shy of 50 gps. Not your average boat though.
generally 40 would be the average and good going on these boats. Your prop sounds like an odd one. I’d try a black max standard 19” 3 blade on there. Would lose an inch at least in diameter over your current prop and might just be good. 50/50. 17” black max be worth a try too to see how it goes. Might be happier all round with that, but keep an eye on the rpm.