prop and rpm

[jody]

Hello all.
I'm new to this group and new to boat ownership. I have been around boats and driven them just never owned and had to maintain them.
Just bought a 2003 1900sr with the 4.3mpi and alpha 1 out drive.

SORRY about the long post.

The original problem... The boat had now power and was very slow like less then 10-15mph.(the tach and speedo are both not working) As a retired master auto mechanic i did some test and maintenance to the boat just to get it to a known good base line.(plugs , cap, rotor, wires, oil and filter and fuel filter. also did a comp. test. 137psi cold with closed throttle within 3psi difference. also noticed the prop was pretty beat up so using the mercury prop selector put on a new 3 blade 14.3dia 21pitch prop. says i should get around 49mph at 4600rpm.

Well went out for another test drive and the boat is much better.now using a digital shop tach hooked up to #1 plug wire and a gps sppedo app on my phone(which i verified in the truck on the way to the ramp) the boat got to 37mph at wot but the rps went to about 55-5600 rpm, which i know is high. this doesn't make sense to me.

the stats. 2003 4.3 mpi, alpha1 out drive with a 1.62 ratio 2 adults in the boat with roughly 30-35 gal of fuel.(i think)
the numbers do not add up and my brain is having trouble comprehending how a prop will change the rpm.

[mmwjr]

I would think a 15.5” x 19” prop would be a better selection. Not sure what you entered into the calculator but the diameter is to small IMO. A 4 blade may also be a better selection but other with a similar boat to yours will chime in.

[Phillbo]

Call Hill Marine. They will dial you in.

[jody]

thank you for the info.

[mrt214]

According to Maxum the 4.3L 190 was equipped with a 14x23 prop.
The 4.3L 220 was equipped with a 14 1/4 x21.
You can find this info at Maxum Customer Care

[jody]

Thanks mrt214. thats what i came up with too, but at WOT. it seems to be over revving a little and not reaching the speed that the chart says it will. I'm not sure if i need more or less pitch to decrease the RPMs. That is what i was trying to ask originally. lol.

[donald]

An increase in pitch will lower RPM's.
An increase in DIA. will lower RPM's.
And vise a versa.
Speed is relative as there are to many thing that effect it, you want to be in the max RPM band for your engine
Don

[gamoooos]

I have a 2001 1900SC 190hp 4.3L and I just went to a 4 blade 14x21 and it made a huge difference to the ride quality. I lost a little bit of speed, but the boat rides so much smoother in rougher water and well worth the switch.

[jody]

Thanks for the advise and inputs guys.
Donald, that is really what i was looking for. thanks