Prop for 26' cruiser / Bravo 2

[transamconvert]

I'm looking for a bit of help upgrading my prop. Boat is a Crownline 250CR, 26' LOA about 7000lb dry. 7.4L with a Bravo 2.

Present prop is an aluminum 17.75" x 21P 3 blade. Goes just over 40MPH @ 4500RPM which is in spec. Holds plane at 23MPH @ 3400.

I often have 6+ people and their gear aboard and the boat works harder than I like to get on plane loaded like that. I would also prefer if I could cruise on plane at a lower RPM.

I *think* I want a SS 4 blade. Powertech makes an 18.5" version but I don't know what pitch to go to given the change in blade count, material and diameter. Open to other ideas too. Appreciate any help.

--Mark

 

[MikDee]

A 4 blade is the right idea! 18.5 is that pitch? If so it sounds about right.

 

[transamconvert]

18.5" is the diameter. You can get any pitch you like. They also have a 17.25" diameter series. They say the bigger one is for 'heavier hulls' and my gut feeling is that I want the bigger diameter.

 

[alldodge]

I'm looking for a bit of help upgrading my prop. Boat is a Crownline 250CR, 26' LOA about 7000lb dry. 7.4L with a Bravo 2.

Present prop is an aluminum 17.75" x 21P 3 blade. Goes just over 40MPH @ 4500RPM which is in spec. Holds plane at 23MPH @ 3400.

I often have 6+ people and their gear aboard and the boat works harder than I like to get on plane loaded like that. I would also prefer if I could cruise on plane at a lower RPM.

I *think* I want a SS 4 blade. Powertech makes an 18.5" version but I don't know what pitch to go to given the change in blade count, material and diameter. Open to other ideas too. Appreciate any help.

--Mark

Not knowing your serial number or year I will guess you have a 300 or 330 HP 7.4 engine. Your turning 4500 RPM which puts you in the sweet spot of your WOT area. With that you have the right pitch prop. To increase torque and better planning time you could prop down from a 17.5 down to 16 or so but then you will need to watch you don't over rev. Increasing pitch will reduce your WOT and a 4 blade while increasing a bit more lift you load is just to heavy. I had similar issue with my cruiser, it came with a single 7.4 330 HP engine and weighting in at 10K, putting anymore then 2 people caused it to have trouble. My fix was changing the motor to a 415HP 502 and now it will run. You to need more HP to run that kind of load. IMO the only hope is the lower pitch prop

 

[MikDee]

I had a 15.5"x17" Black Max (Merc prop) on my 79'-24' SeaRay cuddy, Merc 350/260hp and it worked fine, smaller diameter props didn't, but I don't know if you need an 18.5" diameter prop on there (the bigger the diameter, it scrubs speed). Pick your prop by the pitch, not so much the diameter as long as it's 15" or more. I'd say you need approx. a 15.5 x 19" pitch 4-blade SS.

 

[transamconvert]

It's a '95. Regular 7.4, I think 300HP rated. I'm getting people confused with diameter and pitch. Prop now is 21 pitch. I was just noting that different props have different diameters too and I didn't know how that would factor in. I was planning on dropping pitch to the 17-19 range with the 4 blade realizing that this will cost some top speed. A major engine upgrade would be suh-weet but isn't in the cards at the moment. Surely I can make some improvement by re-propping, no?

 

[alldodge]

It's a '95. Regular 7.4, I think 300HP rated. I'm getting people confused with diameter and pitch. Prop now is 21 pitch. I was just noting that different props have different diameters too and I didn't know how that would factor in. I was planning on dropping pitch to the 17-19 range with the 4 blade realizing that this will cost some top speed. A major engine upgrade would be suh-weet but isn't in the cards at the moment. Surely I can make some improvement by re-propping, no?

The diameter has as much if not more impact then the pitch. The B2 turns a large wheel as you know. If you reduce pitch or diameter, this will help you come out of the hole with more ease, BUT it will also increase your RPM. Your engine will work easier with the higher RPM do to less load, but (that word again) your engine will burn just as much fuel.

As before reduce pitch is the way to get it up on plane, but you will gain only that. Need a B3 and better yet more HP.

 

[jestor68]

What is your gear ratio; 1.81, 2.0, or 2.2?

Mercury offers their Bravo II props in 3 blade al and SS in each size. No 4 blade is offered because their tests showed it didn't work any better(or as well).

You need to prop it for whatever the "normal" load is. If that's 1200 pounds of people/gear, a 19 pitch in SS might be in order.

 

[transamconvert]

I was able to find out that my ratio is 2.00:1 and confirmed that WOT should be 4200-4600.

I talked for a bit with a rep from powertech. The understanding I got from him was that either diameter would work and that I could run a smaller pitch in the larger diameter without over-revving giving me two options with the same WOT rpm - one faster and one with better holeshot. If I were staying in a 3 blade he recommended dropping to a 20P. In a 4 blade he recommended 18P with the smaller 17.25" diameter and 17-18P with the larger 18.5" diameter.

My feeling at this point is to give the 18.5" x 18P model a try. At some point you just have to try something to eliminate some of the variables. Unfortunately that looks like its going to cut my top end to about 36MPH, though to be honeest I rarely run flat out. What's less clear is what it might do to my cruise RPM which I'd love to lower.

The biggest question is the diameter.As I understood it PT will exchange pitches but charge $100 restocking to change diameters.
I did some benchmarking and the only comparable 4 blade for the Bravo 2 I found is the "big cow" which is an 18.5" diameter. I also noticed that in the pitches I'm looking at (17-20) the 3 blade mercury props are all 18+ inches in diameter. Plus bigger is better, right?