higher pitch or larger diameter?

[iron21]

hi, i already posted this but it didnt show up?! anyways, i have a 3.0 merc i/o with a 19p prop. i'm running at close to 5,000 rpm's at 3/4 throttle(38 mph)..it was screaming so i couldnt even advance to full throttle.should i try a larger diameter prop or keep the diameter and move to a 21 pitch? thanks

 

[Texasmark]

Re: higher pitch or larger diameter?

Beins I've just finished testing the answer to your question, it depends.

Want more cruising go larger dia.

Want to be the first to the fishing hole, go more pitch.

But in my tests larger dia and less pitch got me more mpg (speed at a given rpm) at the speeds where most (a lot of) boats (BB excepted) run.........But, as I said before, I significantly changed prop geometry in the process and this surely had something to do with it.
You can read my related thread about 4 down and the summary is there. If you want the point to point data, I can supply that too. Both props listed are on this site and have pics (sales catalog type pics) and features.


Like folks say. The only way you will really know it to run it. Everything else is just speculation (may be educated speculation, but still just speculation).

Mark

 

[iron21]

Re: higher pitch or larger diameter?

ok thanks for the info Mark. what kind of engine are you running? i'm looking to get a little more top end(hopefully around 45mph) as i have a some low end power i can sacrifice. i'll step up in pitch to a 21 and see what happens.

 

[Silvertip]

Re: higher pitch or larger diameter?

You are talking outboard vs I/O. If you are running that 3.0 at 5000 you are way under propped. You need more pitch to bring the RPM back down to the 4400 - 4600 range. Excluding prop geometry, pitch provides more effect than diamter. Work with your dealer. Most will provide several props to try.

 

[Almeja]

Re: higher pitch or larger diameter?

I just went through this with my Bayliner 195 classic, downpropped to a 19" from a 21".

For a typical runabout (like mine), 19" pitch on a 3.0L Alpha will be underpropped. It is great for pulling water toys and salmon trolling but will top out at about 35 mph at the 3.0L engines full rated rpm (4800). WOT at speed will over-rev the engine, so If you're using one, you have to be aware and watch the redline on the tach.

The 21" pitch prop that came on the boat (same approx 14" diameter) gives about 42-43 mph at WOT and 4800 rpm with 2 in the boat.

The 21P is for sure the best 'run-around the lake' prop on my boat, but bogs the little 3.0L down when slinging the tubes out of a corner... and it trolls a little too fast for salmon fishing.

Since I do more tube pulling and salmon fishing than just riding around, the 19" will probably be on the boat most of the time.

I had also considered getting a 14"Dx20"P Quicksilver Alpha 4 blade prop... and it might be a better compromise wheel. Maybe somebody can jump in here who is running one. For some more serious dollars, the stainless High-5 in 14X20 is available too.

Interestingly, (and I guess rationally) the 19" gives better fuel economy when pullling water toys as it's not dogging the engine as much, and the 21" give lots better economy just running around.

By the way... don't rev a 3.0L over 4800 rpm... it can turn into shrapnel.

 

[walleyehed]

Re: higher pitch or larger diameter?

The problem with a sterndrive when selecting a prop is more blades are waisted technology. We can't (easily) change the drive heigth, so the extra blades add drag which equates to about an inch less pitch per blade to keep the same WOT RPM...the "only" RPM that matters..
So, as we drop pitch on a sterndrive, we might gain hole-shot, but top-end falls short.
The best bet on a sterndrive is to go with as close as you can get to the diameter of the 19, in a 21, which will likely be slightly less diameter, but that's for a reason.
We are starting to see alot of props on the market now that of the same model and design, are offered in a "normal" diameter, and a "large" diameter. The larger diameter in a sterndrive design will give better results over-all, "most" of the time.