Re: Bravo 1 to Bravo 3
Once you get the lower bolted on, You can easily determine your ratio by simply removing the spark-plugs, put a wrench on the engine and turn it.
While turning it place it in either FWD or REV (don't put it in gear unless you're turning the crankshaft)
Once in gear, turn the engine 10 turns and have a helper count blades. try to estimate the last turn of the prop (it'll likely be fractional 1/2, 3/4 etc)
Or, just measure the last turn of the prop in degrees. (90, 120, 180 etc..........a fractional turn would be 180 degrees divided by 360 = 0.5 turn etc
Then use your calculator to determine ratio if it's not an easy fraction.
I.E. if 10 turns of the crankshaft produces approx 5 1/2 turns of the prop ==> 10 divided by 5.5 = 1.81:1 ratio.............
10 turns : 5 turns===> 2.0 to 1 ................
10 input shaft turns : 4 1/2 prop turns ===> 2.20:1 etc etc.
Once you know your ratio, you can determine *approx* the prop pitch needed to give you max RPM at WOT.
to do this you would use a prop-slip calculator such as
Prop Slip Calculator
You can change parameters as needed to estimate........leave one blank to calculate it.................... In a planing type hull you would use somewhere around 10-15% slip. Then use the approx speed you know the the engine (@WOT) should push the boat lightly loaded.
Since you're using a 7.4L engine, you're likely at 4600 RPM at MAX power. So if you know the approx full speed, say, 57 MPH (with the Bravo I) Then use 4600 RPM, 52MPH (because you'll probably be around 3 mph slower) and about 12% slip. when you know your drive ratio, you can get some ballpark numbers by playing with variables. (prop pitch)
The boat is certainly NOT going to go any faster (& likely slower) with the Bravo III so if you calculate using prop pitches that give you much higher speeds you'll know that they're not going to work.
let me also add that I just noticed you're working on a Searay 250 Sundancer. Since that boat might be a little heavier than other faster boats, your slip numbers might be a little higher.
Also, larger (slower boats) will have higher ratio Bravo III drives.
My 7.4L +1.81:1 Bravo III came out of a 24ft (cuddy) Bayliner. According to the previous owner, with the 24p props, it did about 50-55 mph @ 4600 RPM.
