View Full Version : Sweet 16 RPM question

05-26-2012, 11:18 PM
New to a Donzi 16.
The boat is running great, motor is smooth, accelerates well, engine does not run rough at any point.
My question is what should my RPM be at WOT.
1990 4.3 OMC Cobra, 2bbl Rochester, completely stock, original factory 3 blade prop. aftermarket Bennett trim tabs. Engine only has 25hours on it.
Thank you in advance for the info.

Kickin Dinghy
05-30-2012, 05:22 AM
My 1990 sweet 16 with the same set up except for the trim tabs would turn 5200

05-30-2012, 10:49 AM
Nice boat! I have the same year and set up also :) According to the manual, optimal WOT is 4200-4600rpm. I turn about 4400 with a 21 pitch prop and a top speed of 50.

05-30-2012, 11:15 AM
Donzi Dan, does that drive have a 1.5:1 ratio? 14+% slip seems mighty high.

05-30-2012, 12:50 PM
I believe it's 1.86. How'd you come up with 14%?

Just Say N20
05-30-2012, 01:00 PM
Cool Prop Calculator: http://www.rbbi.com/folders/prop/propcalc.htm

Fill in 4 to get the 5th.

The 16 V6, 4.3 2-bbl that I have run, and belongs to a friend of mine, runs mid-50s @ 5,200 rpms. I don't remember his prop pitch, but I'm thinking it is a 21" stainless. I don't know his outdrive ratio.

05-30-2012, 01:32 PM
DonziDan, I guessed that it is a 1.5:1 ratio. If I use 1.86 as the ratio it says that your slip is negative 6% meaning that you have a magic prop and it's going 6% more than 21" on a full rotation. I doubt that's the case though.

Here is the calculator. Kind of fun to play with: http://www.rbbi.com/folders/prop/propcalc.htm

05-30-2012, 08:13 PM
Looks like I'm dyslexic...double checked the manual and the ratio is 1.68:1. This still comes out to only 4%...so perhaps my rpms are higher than what I remember. No magic prop for me :frown: