What prop after a repower?

[CAGE RATTLER]

Hoping to get some ideas of what size prop i should try with my new repower.

My boat is an 85 AJ 255 w/10 ft beam. She's pretty heavy. I was told it weighed 9000 lbs when they had it on the sling when it came out this past year which shocked me since dry weight is supposed to be under 6000 lbs.

Im replacing a 2003 350 MAG MPI (300 HP) with a new 496 MAG MPI (375 HP).
Bravo 1 Outdrive.
When i got the boat it was turning a 19 P prop and it would not get to WOT rpm's. It was replaced with a 14.5" 15 P and i liked the responsiveness but i think it was just slightly under propped but i left it that way and just watched my rpm's. I think a 16" 16 P may have been a perfect choice but planned on waiting till this year to replace it. By the way .... these were all aluminum 3 blade props.

Not sure if any of that helped ................ but what size do you think will get me close to optimum with the new 375 HP 496 MAG MPI??

Any thoughts would be appreciated. Unfortunately none of the props i have are in very good shape anymore so just trying one to see where rpm's will be wont work so i have to buy new and want to try to get as close to optimum as i can.

Thanks

 

[CAGE RATTLER]

I just found the 19P yesterday and its in better shape than i thought so i guess i have something to start with.

I just figured that someone here would have a similar size boat with a similar powered engine that could get me close in case that 19P is to far off. This engine has a WOT of 4400-4800 and a rev limiter at 4950 so i'll probably have to over prop before i figure out what it will need. I dont think im supposed to open it up untill after 10 hours of break in. Would haveing the wrong size prop hurt anything as far as the break in period?? i believe the manual said stay above 1500 RPMs and below 3/4 throttle for the 1st 10 hours and dont hold a steady speed for to long.

No prop shops for I/O's around here that i know of.

 

[Fun Times]

Do you know what your gear ratio is???

This prop selector may help you find the prop for you also.
http://www.mercurymarine.com/propellers/prop-selector/

Here's a link so that you can check to see if you have a mercruiser dealer in your area that may be able to let you rent a prop for testing. Mercruiser calls it the Try Before You Buy program.

Demo Propeller
http://sites.mercurymarine.com/portal/page?_pageid=126,50050,126_50382&_dad=portal&_schema=PORTAL

 

[techmitch]

Here,s one in Deal MD., not to far for you. http://www.digitalpropshop.com/ and these guys, B & B Prop Shop‎
100 Radcliffe Dr, Cambridge, MD‎ - (410) 228-4725‎
are across the bridge from you in Cambridge MD. Hope this helps

 

[techmitch]

Do you know what your gear ratio is???

Good question!

 

[CAGE RATTLER]

That is a good question .... lol

I have no idea.
I'll have to try and get some number off the outdrive i guess if their still visable.
Any links to breakdowns of figuring out drive ratios from the serial numbers?

thanks for the prop shop locations.

 

[techmitch]

There is always this method if you can't find the #'s http://mercstuff.com/drivegearratio.htm

 

[CAGE RATTLER]

There is always this method if you can't find the #'s http://mercstuff.com/drivegearratio.htm

thanks ...... thats probably even easier than trying to find some serial numbers since the outdrive is off :grin:
Didnt think it would be that simple till you really think about it. Im just a backyard mechanic learning as i go :smt001

sounds like it should be a 1.5 but i'll check in the morning.

 

[No Name Sea Ray]

The Sea Ray site has Model Archives that would probably give you the drive ratio.

I found good used and new props for sale on Ebay and CraigsList. Good deals. Google for a search site to do national searches on CraigsList. Buy one, try it out, if no good sell it again.

Try Black Dog Propellers on Kent Island or Fairwinds Marina in Annapolis.

Yes, take it easy during the break in and wait to reprop. Try the boat in all conditions, empty, full loaded, tubing, etc. before making a prop change.

 

[CAGE RATTLER]

Ive verified that it is a 1.5 drive ratio

 

[AnchorsAweigh]

Hi, I've been rebuilding props of all shapes and sizes for 25 years and there is no chart that can tell you the "right" prop. You also can't be certain your drive hasn't been altered or changed in its lifetime, unless you're the original owner. If your 19p was too much before it could be just right now. As you have experience with watching your RPM's, I'd say use the 19p through the break in period, then start playing around. You can always keep it as a spare.

There are a couple typical rules of thumb. When you go up in pitch by 2" you will decrease WOT RPM by 200, and of course the opposite with going down in pitch. Also, a 18p SST prop will react the same as a 19p aluminum prop. There are some new styles of props available now in both SST and alum that have a more elongated blade, they will give you more torque out of the hole and top end speed at the same time. There are so many variables and factors such as hull design, prop design, hp, hull weight, load weight, and so on. Trying different props is the only way to find the best one. Most reputable marine dealers will let you try different props as long as you return them undamaged, and in short order. Tell them your story and don't take a week to "try" one out.