Mercruiser WOT?

[HT32BSX115]

since your motors aren't stock, most of the guidelines are no longer valid

Wouldn't you agree that once the "new" (build) max RPM is known, Then select props that will make that RPM at WOT? (since you wouldn't [or shouldn't intentionally] operate above that RPM)

I would also suggest that particular parameter still generally applies regardless of the "build" since the "new" (higher or lower) RPM is where maximum HP might occur.

On the other hand, Most (stock OEM) drives and props (excluding Mercury Racing, IMCO, etc) are designed for a rather limited range of eng RPM.......... run them way out of the range (high) and you start having all sorts of problems (heat, wear, cavitation etc).

A 270 Amberjack isn't really a "Go-Fast" hull........With the extra HP, it's going to have really great holeshot.

Now having said all that, I did a little calculating with http://www.go-fast.com/Prop_Slip_Calculator.htm

48GPS MPH in a lightly loaded cruiser type boat with twin engines should yield somewhere around 15-20% slip

I assumed that you have 1.50:1 ratio Bravo I drives.

Using the prop-slip calculator 1.5:1, 19p props, and 48MPH, It took 5000 RPM to do it. 6300 RPM with same parameters yielded 61MPH

It might be helpful if you post your drive ratio and current prop pitch.

If you don't know your drive ratio (and you don't believe the placard on the drive) you can determine it experimentally by removing the spark plugs, put the drive in gear and turn the engine by hand. for 1.50:1, 3 engine turns will produce exactly 2 prop turns.

You can see it's imperative to know the RPM and drive ratio accurately. If you have 1.81:1 drive ratios, the above numbers (at 6300rpm) produce about 50mph etc.......

 

[tank1949]

Thanks to all! Thanks to all that replied. Flowscan RED switch was set on #2 and should be set on #3. I have contacted former owner and asked what was in boat prior to strokers. He may have had 6 cylinder (s)????After strokers were installed, he admitted he was having problems getting autopilot and shore power to work.

He may have "bumped" the red switch by accident but I am still in total confusion about his battery switch wiring. With both switches set to OFF, I am still getting power to Helm's "accessory" switch.

He also didn't have OD trim working correctly for both OD. He ran with ODs in full down.

In addition.... Unless acceptable practices have changed, both battery switches set to OFF should restrict battery power to everything, except a fused connection to bilge pump float switch.

Another problem to solve......