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TonyS

I own a '95 Captiva 232 with a 7.4 liter Mercruiser in it. I'm running a Mercruiser 21 pitch 3 blade prop on it. I get around 51-52 mph on perfect water. I've had guys tell me with the same setup and a 24-25 pitch prop they're getting around 65 mph out of it. Could changing the pitch really get me 12-14 mph more? Without changing anything else?

Mike_G

What RPM are you running at WOT (wide open throttle)?  Also.. which 7.4?  Carb, MPI, MAG?  -=Mike G.

TonyS

In running around 4800-4900

Mike_G

How much fuel?  How many people?  Air/water temp?  Sorry.. I know a bunch of questions, but it'll help figure out where you're at prop wise.  -=Mike G.

TonyS

Carb

Mike_G

You're a bit high on the RPM.  That motor is happy between 4400 and 4600 RPM.  That's where it makes the most power.  What is nice though, is that with the prop you have, you can load the boat up with a bunch of people/fuel/etc. (more weight) and it would probably be pretty close to the correct prop.

At 4900RPM, you are leaving speed on the table and running your motor a little high.  A 23 pitch will get you more speed.  A 25 might be too much prop, especially if you load it up with some weight.

Anybody else want to chime in here?  -=Mike G.

TonyS

Any advise would be appreciated. New at owning a pleasure boat. 

TonyS

A couple adults and usually a full 40 gallons of fuel.

TonyS

Air temp is in the high 80's and low 90's here. Don't know the water temp. Comfortable. I'm guessing mid 80's. 

TonyS

Would love to run in the 60's. Can a change of prop make that happen?

Mike_G

In my opinion, consistant 60's is going to be pretty difficult, especially in warmer air/water temps on a stock 310HP 7.4.  Engines make more power with colder temps.  Might make it there once in a while, but it would have to be perfect conditions, just the right drive trim, trim tabs all the way up, etc.

My experience comes from myself having the same motor in my 24' Sunsation Rocket 2400.  That was a 23.5 foot "offshore" boat with a 6 foot beam and a pad at the rear of the hull.  A "purpose built" race/speed boat that is narrower and weighs less than your boat.  I could get low 60's on hot days.  My best was 65ish mph (GPS speed, not dream-o-meter speed which read almost 70mph), on a mid-60 degree day... no waves just 6" of chop, 1/4 tank of gas, by myself, with a tail wind and farting towards the back of the boat.    I'm not sure how the 232 hulls run, but I would think they wouldn't run as fast as my old Sunsation.

Again... I'm sure someone will chime in.  Let's see after some other people give this thread a read tomorrow.  -=Mike G.

TonyS

What do you think I would gain with a 23 pitch? 

TonyS

Or maybe a four blade?

TonyS

Looking for more speed without taking all day to get out of the hole. 

jme097

Thats part of the sacrifice you make when you start changing props. Either you get overall better hole shot and lower top end speed or you take longer to get out of the hole and higher end speed. I would be thrilled with 50mph on my 280. I think asking for 60mph is a bit much but others will chime in for sure

Mike_G

A 4-blade probably won't give you more speed.  A 4-blade will give you more lift at the rear of the boat.  That's why you often times see a boat with a stepped hull that is made to ride more flat in the water (vs. bow up) using props with more blades.  There are many more benefits of a 4-blade prop such as planning, cruising fuel economy, heavy load carrying, etc., but speed usually isn't one unless the hull "needs" the lift in the rear.

It's always a trade off.  Out of the hole will normally be better with a lower pitch prop, but again, with those RPMs you are under-propped for the load you are carrying.

It's tough to get much more detail without knowing the gear ratio of your drive, calculating prop slip., how many hours on motor/how "healthy" is it, etc.  Many factors come into play when it comes to "tuning" the prop size/pitch correctly.   Honestly it will start to get above my knowledge and experience to give much more advice than I have.

Are you running a Mirage 3-blade?  If it were me, I would move to a 23 pitch Mirage of the same size/# of blades and expect 3ish mph of speed increase.  If you get more speed, great!  If you want to try a 4-blade you can, but a 4 blade with the same pitch is a little harder to turn than a 3 blade.  Generally, people go down 1 in pitch when moving to a 4 blade from a 3 blade.

Here is a good article about the 3 blade vs. 4 blade prop - https://www.boattest.com/view-news/3276_which-is-better-4-blades-or-3-blades

Do some Google searching as there is LOTS of info out there.  Many people have been in the same situation.  You will probably find results that would apply to your situation.  -=Mike G.

howardrams

Increasing pitch by 3 or 4 probably won't get you to the 60's though it might add 5 mph.  But don't forget it will slow your out of the hole take off a bit which might impact how quick you can get 2 skiers out of the water.  It's a trade off.

Lake_Bum

I'm not sure how much you played with prop size this summer, I see this thread is several months old!  A tool I used, when boat shopping recently, was the Mercury prop selector site. It is VERY accurate, and it was spot on, when it came to the size prop I needed for my boats engine/outdrive. 

https://www.mercurymarine.com/en/us/propellers/selector/#/step-one