Prop Sizing

[johnnjerri]

Mazum Guys, Have a 2004 31' twin 5.0 MPI 260. Prop question, has 2.2 gearing I believe and turning 24 pitch Bravo III. I am at 3700 feet elevation and only getting about 4400 rpm. Before I spend a bit of cash, would changing to 22 pitch make that much difference in getting some revs and better acceleration? Thanks

[shrew]

Welcome Aboard johnnjerri!!!

Hijacking threads is forwned upon, so I moved your post to it's own thread.

Acording to Mercruiser, the 5.0 MPI should have a MAx RPM range of 4600-5000. At 4400 your a bit under the max range. I would have guessed a 24 pitch prop would have been overpropped for that setup, and the number you're reporting would seem to confirm that.

Take a look at the Mercruiser Prop Calculator:

http://www.mercurymarine.com/propellers/prop-selector/#

I don't have enough information When use the 'basic' option for length it shows either the 22.5 or the 24 pitch. I would strongly suspect you may need to go to a 4 blade front, 3 blade rear 22.5 pitch. Run through the calculations yourself and let us know what you come up with.

[pascavone]

since you are new here I will offer some simple suggestions before you re-engineer your propulsion, as the factory equipment is installed for a reason:

1. When your running at 4400 rpm, uses your finger and tap the tach a few time and see it the needle changes.... they do stick....
2. On plane, trim up your outdrive and watch your RPM go up, and your MPH, hopefully go up.
3. If your diggin in with tab down, six fat chicks, a full tank, then yes, you might be a bit sluggish.
4. In your in the tide, or with some chop, or wind, it will hold your down also.
5. Seams to me, all these 30's will only do about 34mph to 37 mph with twin 5.0, even the twin 7.4 are similar.
6. WOT ( wide open throttle), the boat once on a calm day, and bench mark the RPM, my 7.4 is 5,500 and that 37mph.

[johnnjerri]

Thanks for the info. I did purchase a set of new 4x4 from Hill Marine, spendy!!! Anyway went with 22 pitch. Getting on plane was better, they are quieter and smoother on the vibration from what I could tell. I was doing 30 mph @ 3500 rpm loaded pretty well. engine full down and tabs trimmed up. I have to keep the engines trim all the way down, raise it a little maybe but do much more and it cavitates. Wide open throttle is about 38 mph @ 4200 my elevation is 3800' though. I guess that is not bad Should I go to a 20 pitch prop setup? It seems to cruise well and not sure going to a 20 pitch is worth it.

[mmwjr]

John been almost a year since the last post on this, I hope you changed the prop out back then and are reporting on a season of use and not just getting to it now. Anyhow glad it worked out