Boat/prop max speed type question

Unlikely because the increase pitch would lower rpms. The only as to get more speed is to get more lift. Most bay boats just don't lift enough to make a huge difference esp with less than a 200 on the transom. Bass boat is another story.

6350 is already hot. If you lower the pitch, you'd be putting a lot of strain on your engine if you dropped the hammer.

I mean, pretty sure you can make assumptions and just calculate it.

I'm rusty, but I'm assuming with the 17" pitch prop, you get

>17/12= 1.42' advancement per revolution at 100% efficiency.
>At 6350 RPM, that's 9017' per minute or 1.71 miles per minute
>1.71*60= 102.5 MPH.
>So that must be a prop efficiency of (45.7/102.5=) 44.6%.



Working backwards, assuming same prop efficiency (which is obviously not the case),

>50mph/.446= 112.1 MPH at 100% efficiency
>112.1/60= 1.87 miles per minute-> 9873.6' per minute
>9873.6/6350=1.55'
>1.55*12= 18.6" pitch.


So I'm rough calculating that you would need 18.6" of pitch to hit 50 mph at 6350 RPM. To do it at lower RPM, you will obviously need more pitch than that.

I don't really know why I did this as it's not reality in practice, I was bored. Not even sure I'm calculating it correctly. Just going off of some info I remember in the past, but it's been a long time since I've looked at this stuff. Plus it's only a small piece of the pie. That extra ~2" of pitch is going to bog down the motor and not allow it to ever hit the 6350. Your hole shot is going to suck. Also, the efficiency is not going to be the same (and 45% efficiency doesn't sound right, so I may be doing something wayyyyy off). It really does take trial and error, and you can just hope you can find some data from other people who did the trial and error for you.

pitch up 2... 6350 should drop down to about 5950 to 5900rpm, and if you stay same speed and drop rpm's raise dat motor up a little at a time till you get dat rpms back up and you'll increase dat speed. just watch you water pic up ports. may need to tape off top 2 if you get too high.

Has anybody in this thread seen the power curve for that engine? The worst thing you can do (when you have a rev limiter) is overwheel.

I've seen people spen tons of money looking for speed out of a boat. Setup on the boat can make a huge difference beyond what horsepower can do. X-dimension , setback, trim tabs, weight, and most importantly hull design will have the biggest things to do with speed.


If those are not right you are wasting your time and money with props and horsepower. Other than the prop the entire lower unit is nothing but a big drag in the water. On a lot of go fast boats/bass boats the x-dimension is basically even with or slightly above the deepest part of the transom.

They should publish a WOT operating range for the engine, meaning the biggest prop you can swing is the one that puts you at the lowest allowable WOT RPM. You likely have some wiggle room there, but if you overwheel you'll significantly accelerate engine wear.

Call Mercury and talk to an engineer. They have to have a tech line.

I think I recently mentioned it in another thread. I talked to a powertech prop engineer about my boat (Ranger Banshee flats boat that I was trying to squeeze a little more out of a 4 stroke 60). I probably chatted with that guy for about 45 minutes. Really nice guy. I wish I knew his name to pass on. Even he will tell you that it's far to complicated to spit out an exact answer, but with their experience, he can put you in a 5 MPH range of where you will likely be. He was immediately familiar with my boat and the 4 stroke 60. He spat off figures that were exactly where I already was and said that's the best he could think of. We spent the rest of the time looking at options to help hole shot in shallow water, blow outs on turns, etc.. Was a really educational call.