Hello! I´m wondering is there any rough formula to calculate prop pitch vs engine or shaft HP?
I´m going to add 256 blower on my 454mag and wondering how much pitch can add to prop?

Specs now: 350HP max 5100rpm and top speed 65mph with 23"mirage, Bravo 1 1.5:1. I´m happy with current acceleration and would like to keep it about same with blower...

Is it possible to calculate HP per pitch for example 350HP / 23"pitch = 15.21HP per pitch?

If adding 100 HP with blower could it be 450HP / 27"pitch = 16.66HP per pitch?

I undestand steeper prop the more HP needed but is there any way to calculate?

Plug in some numbers on Bam's site http://www.go-fast.com/boat_speed_predictions.htm

bam/Marc put the Merc calc on his site.

Here is our calculator.

http://bblades.com/propeller-slip-calculator/

Back in the day, we also wrote a equation to determine the potential speed per the HP to weight ratio based on the efficiency of the particular hull. This is different from the slip calculator.
Call us and I'll give you that number and better answers.

Not the best way to calculate without experience but it is a way to estimate............ we basically use 1" of pitch per 100HP gained but,,,, every application is different....every application is unique.

B

Both are good tools to come up with the numbers your after. I've found BAM's speed and slip calculators to be pretty freaking close. Especially if you have current engine data, GPS speeds, along with an accurate boat weight. Then just plug in the changes to power and pitch to see what fits.

Just used Bam's model it came within 1mph...pretty cool app there.

Yep, calculating with those I came up with 26"- 27" pitch.
I´m struggling between props Dia 15 1/8 26p or 14.5 27p. Both 3 blades...
Never had slip issues so would`t like to increase dia. I believe my present mirage is 14 5/8.

I have pretty light hull, small boat (regal medallion 18ft). Weights about 2800lbs. I tried 15 1/8 28p 4-blade last summer WITHOUT blower and the top speed remained same but acceleration was like driving with grandma... also top rpm lowered to 4000. So bad memories of too steep prop.

I´m definately going to run first with present prop and testdrive engine with blower on and see how it sounds.

It depends on where the new motor makes its power and how big the torque curve is as others have mentioned. For example a new motor may have a similar torque curve but makes peak power 800 rpm higher. You actually may not even go up in pitch but simply run the same prop at a higher rpm. Just some food for thought.

Edited: Ooops, saw 350hp and small boat and assumed mild small block. My bad.

It revs up to max (5100) with present prop easily so definately need to add some pitch with blower mounted. Just the question is how much I believe blower will add about 100hp? 454 mag, Gil offshore alu exhaust, double 600cfm holleys and weiand 256. Sounds reasonable?

Doesn't matter where it currently revs up to, but rather where will the new motor make peak HP. You will need to prop to the new motor. Don't get super focused on pitch alone. You might need blade surface area (bigger 'ears' or more blades) or perhaps additional diameter. Now I'm not saying pitch change won't be required but there are other factors in the prop that may optimize overall performance.

If I were you, I'd start with your current prop and run it with your new motor and get a SOLID BASELINE. Record in 200 or 250 rpm increments from idle to WOT the rpm and GPS (I say again GPS) steady state speeds. Compare your theoretical speed to your actual speed to get a feel for slip and efficiency. Once that is done you will have a much better grasp on which direction to go in prop selection. Now there is a fair chance you may be bouncing off the rev limiter with your current prop and new motor. So be it. At least you have a baseline from which to compare.

I wish you success in your project.