Should My RPMs Change This Much?

[anon1]

Hello,

Original: 3-blade 14x19 @ 4,600 rpm / 50mph

New Prop: 4-blade 14x20 @ 3,900-4,000 rpm / 54mph

I was really looking to get the boat out of the water quicker without losing too much top end. I read on these forums that a 4-blade stern lifting would lose 150-200rpm and the 1" pitch would lose about 200rpm.

Why would I lose 600-700 rpm's with this change?

This is below my engines spec of 4200-4600RPMs. I know this is bad to "lug" the engine but is this amount really bad?

Thanks.

 

[Silvertip]

Re: Should My RPMs Change This Much?

Was your original prop aluminum and the four blade stainless steel by any chance? That is another factor in prop design as they are more efficient. You possibly changed four or more factors rather than just two (blades and pitch). You likely changed manufacturers and prop material. You see, two props, even if they have the same diameter and pitch but from two different manufacturers will perform differently and not always predictably. Prop design also includes rake, blade thickness, blade shape, to name some more factors that ultimately affects how a prop performs. That's why prop selection is such a crap shoot. Research is essential and it is very important to look at more than just diameter and pitch.

 

[anon1]

Re: Should My RPMs Change This Much?

No, stainless to stainless.

Old was a Quicksilver ss 3-blade 14x19, 48 16316 A4 19P
New is Stiletto ss 4-blade 14x20, Bay Pro II

 

[volvomia]

Re: Should My RPMs Change This Much?

I have the same issue dropping 600 - 800 rpm using Stiletto Bay Pro II 4 blade 14x20. I had 3 blade aluminum 14 1/2x19. The good news is Stiletto will exchange it for me as soon as i decide which one to use. You may want to go with 14x18 on the 4 blade as this will knock at least 400rpm according to the experts on this site. I have no idea how that will perform compared to your original one but it should keep you in 4400 rpm range.

 

[Tommyfmu]

Re: Should My RPMs Change This Much?

It makes complete sense. One affect of dropping rpm's is going from 3 blade to 4 blade (a few hundred rpms). The second affect is going up on pitch by 1 notch (another few hundred rpms lower).

The general rule of thumb is when you go from a 3 blade to 4 blade, you DROP the pitch by 1 notch , not increase it. i.e. you should have gone from a 19 pitch to 18 or so.

 

[Silvertip]

Re: Should My RPMs Change This Much?

Agreed. Your engine RPM was correct before you made the change. So going four blade pulled off some revs, changing manufacturers pull off some revs due to the "design" of the prop, and going up in pitch pulled off some revs. Two props of the same diameter and pitch from two different manufacturers generally do not perform the same. Prop "design" must always be taken into consideration when making a switch.

 

[GonzagaCRO]

Re: Should My RPMs Change This Much?

Hello,

I read on this forum in many threads that 1 pitch more or less with the same diam. changes rpm`s by 200rpm, but on my boat that was around 350-400rpm`s.
I tried 4 different props.

Best regards

 

[Texasmark]

Re: Should My RPMs Change This Much?

In answering your question about engine lugging, the only engines that I know of that are designed to lug are big diesels and they have their torque curve peaks at very low rpm's (like 1600 is a reasonable number) and torque decreases as rpm's increase like occurs with most engines......some remain relatively flat however.

Normal operating range for the big diesel is well above that (2-1 or so) so that when you get into tough pulling, the load increases, the engine starts loading up causing the rpms to decrease ...but the torque increases because rpm's are dropping which backs you up the torque curve raising the torque and therefore somewhat negating the load increase.

Actually that's the way you drive an 18 wheeler in hilly terrain. You have 10-15 gears roughly with a narrow rpm operating range. As you start climbing a hill, the rpm's start dropping. When you get to your engine's lower rpm limit, you down shift a gear putting you back at the top of the rpm range, and repeat till the hill is conquered........but the rpm range is well above the peak of the torque curve.

Engines with high rpm torque peaks (most other engines) do just the opposite. If the torque peak is above the lug rpm's the more you lug it the more the engine just shuts down and it' kind of a domino effect to the point where you can actually stop the engine. BTDT accidentally of course.

Additionally, I have found that keeping the engine lightly loaded (higher rpm's) improves fuel economy and is less stressful on engine oil, bearings, pistons, rods and all.

My 2c,

Mark

 

[TilliamWe]

Re: Should My RPMs Change This Much?

4000rpm...
This is below my engines spec of 4200-4600RPMs. I know this is bad to "lug" the engine but is this amount really bad?

Yes, it's bad. Don't do it for very long, or you will be sorry. I see your original post was dated 9/23, I hope you haven't been lugging the engine for the past 2.5 months.

Lugging an engine causes increased combustion chamber temps, which causes detonation, which causes tulipped valves, which cause loss of compression, which causes...
Detonation also causes burned pistons, which causes catastrophic engien failure.

Enough reasons?

 

[hwsiii]

Re: Should My RPMs Change This Much?

Anon, you might have a better understanding of what happened if you read this.

http://forums.iboats.com/showthread.php?t=365653

If you had gotten the 18" pitch you would only have lost about 250 RPM and kept the same speed and had a much better hole shot.


H

 

[vegasphotoman]

Re: Should My RPMs Change This Much?

Im about 90% sure lugging my 3.0 mercruiser caused a headgasket to blow.....
went from 19P alum 4600 rpms (wot 43mph)

to 25P stainless 4000 rpms (wot about 49 mph)

WHOOPS! I wont be lugging my motors NO MO! hope that helps!