RPM question

[Mick2268]

2B401558

OK so re-powering later this month from 2 stroke to the 4 stroke. Running a 24 fury and getting around 5,800-5,900 wot, came from Ranger with this prop, and I love it! Dealer says I will need to go to 23 fury to get full 6,200 rpm wot. My question is why does one need to reach the top of the rpm range? I understand the concept of not wanting to lug a motor down, but isn't 5,800rpm +/- well within the top end range? It's not like I'm gonna run my motor at 6,200 rpm very dang often regardless of prop. Interested in your thoughts??

[mactlman]

if you cannot reach max rpms, you will be lugging the motor. better to get a 23p

[fuelleak]

i got a 23 on a Falcon 205 and still cant get to 6200. Ive given up and just drive it the way it is, 5800 max. Still get low 70s, enough dickin around with it, good enough.

[thommo59]

My bet is a 23 will still be 1” too tall to hit 6200 constantly

[EuropeanAM]

My bet is a 23 will still be 1” too tall to hit 6200 constantly

I would concur. You probably would need a 22P to get it up into the 6100+ RPM range.

Propping the engine to the top of it's RPM range is much like selecting the correct gear in a straight-drive vehicle for the hill you're climbing. It's just about 100 x's more important.

[Mick2268]

I would concur. You probably would need a 22P to get it up into the 6100+ RPM range.

Propping the engine to the top of it's RPM range is much like selecting the correct gear in a straight-drive vehicle for the hill you're climbing. It's just about 100 x's more important.

Ok but that's the crux of my question; why is it so important to get the last few hundred rpm? Is a 4 stroke designed to run balls out all the time like 2 strokes were? And how is running a few hundred rpm below the max going to harm the engine long term? I'm asking all of this because I don't understand ? In my mind being able to get the desired speed while staying slightly under max rpm would be better on my motor.

I mean I can raise my plate and get to 6200 easy; the rpm's I quote are when my boat is set up to get max top end.

Thanks in advance

[EuropeanAM]

Your propeller is the final "tweak" in the proper gearing of the engine. If the engine cannot reach the top of it's power band, then it is improperly "geared" (propped) from IDLE all the way to WOT. And... since peak torque and hp are made right at the top of the RPM band, you're leaving POWER on the table.

Over-propping an engine can lead to poor performance and eventual engine damage (AND damage caused by improper setup/propping is NOT covered under Warranty).

Also see: https://www.bbcboards.net/showthread.php?t=1198338

[mikesxpress]

Mick I had one of the 1st 250 V8'S and had to wade thru the propping thing on my own. Using my level one Smartcraft monitor to judge the effects of rpms, trim, and Jack plate height lead me to the following results:

Your 2 stroke props will not work on this engine.
A Fury 3 blade 23P with all PVS holes plugged and the box prop sent to the shop for true pitch work achieved the 6200 rps during summer months. Don't expect a box prop to do this!
Recording before/after Smartcraft readings showed the fuel economy on the V8 increased by 4% with the true pitched prop running at optimum cruise settings of 4200 rps/44 mph. The engine performance across all power bands also increased based on my observations. Most of us rarely run WOT for any extended periods but this engine must be propped to achieve 6200 rps at WOT.

[bassseeker]

Thanks for all of this information concerning the 4 strokes. I just blew my 2001 225 Opti and repowering with a 2023 225hp Pro XS, however Mercury told me that I can use my old 25 tempest + to brake in the new engine, will the information I get lead me to the proper prop? Or is this a trial issue with a number of props?

Thank you
Luc

[mactlman]

Thanks for all of this information concerning the 4 strokes. I just blew my 2001 225 Opti and repowering with a 2023 225hp Pro XS, however Mercury told me that I can use my old 25 tempest + to brake in the new engine, will the information I get lead me to the proper prop? Or is this a trial issue with a number of props?

Thank you
Luc

what boat would help? 25 going to be too much prop likely

[bassseeker]

what boat would help 25 going to be too much prop likely

It's a 2001 ranger 520 VX

[1BADAIR]

A few of us around here have went from 23 fury’s to 23 tempests. Little bit more rpm and much better when loaded

[EuropeanAM]

Thanks for all of this information concerning the 4 strokes. I just blew my 2001 225 Opti and repowering with a 2023 225hp Pro XS, however Mercury told me that I can use my old 25 tempest + to brake in the new engine, will the information I get lead me to the proper prop? Or is this a trial issue with a number of props?

Thank you
Luc

I would not recommend that prop for break-in, or otherwise. Get something known to be close to appropriate and run it for break-in... those first 3 hours (especially) are EVER so critical to the longevity and health of the engine!

[cmbtvet]

Mercury has a prop selector on their website: https://www.mercurymarine.com/en/us/...or/#!/step-one

When I repowered, the shop put a 24 Fury on it and right now my top speed is 68.5 with RPMs at 5620. If I had more lake I could have gotten to 70 I believe but I doubt the RPMs would've gone up another 580. The Mercury prop selector says I should have a 21 or 22 prop depending on if I want better hole shot or top speed, I just don't have $800 right now.

[bassseeker]

That prop selector stops at year 2021

[EuropeanAM]

That prop selector stops at year 2021

Same models- and really doesn't matter as Mercury doesn't actually use "years" anyway. That's there simply for the simplicity for consumers.