View Full Version : Max RPM

06-30-2014, 10:40 AM

So I finally got my 87 Donzi ZB21 running well and went for the first WOT test run. A little background: The engine (Merc 350, afb carb, edelbrock intake) was a rebuilt last year that suffered a bad intake gasket allowing water to get in. Caught it early, did about 4 oil changes and all new top end gaskets and now she's running great.

So I took her out to some smooth water with myself and my wife in the boat and put the hammer down. With the trim up to a safe position I can only achieve about 4100 rpm. I believe the prop is a 23' clever and the original prop on the boat. Any ideas on the limited rpm? I think it might be running a little rich but otherwise runs great. My old merc 3.0's could muster 4700 rpm, but is this normal for this kind of engine combo? Outdrive is an alpha.

87' ZB-21 "She Gone"

06-30-2014, 02:07 PM
I don't know about the alpha but think the motor is ok with 44-4600 rpm Glad to hear it is running ok hopefully I will see you out there one of these days

Ralph Savarese
06-30-2014, 10:52 PM
Too much pitch cut back to a 21 if you are only getting 4100 I would say 5000 WOT

07-01-2014, 12:25 AM
Maybe I missed something--what'd it do before last year's incident? The power and drivetrain were the same then, right?

07-01-2014, 10:19 AM
Last year it ran the same with the same power and drive. I'm going to double check the prop pitch. I would also think that motor should turn 4600 and maybe even top out at 5000 super trimmed. What are the advantages of a clever prop? Would a more typical Mirage prop be better? And does less pitch equal less speed or less planing power? Or Both? I've always had 3.0's with the stock Merc prop.


Morgan's Cloud
07-01-2014, 10:41 AM
If everything's good with the engine , then you've always been running over propped.

A personal example . When I had my old Merc stuff I ran a 21'' prop and got perfect factory recommended top RPM
One day I decided to invest in a 21'' Mirage to get a bit better performance. Well , I never really paid any attention to the tips that a 21'' Mirage performs more like a 23'' regular prop and voila !
I lost about 800 RPM at wot. No to mention about 20mph too !

So I went back to the standard 21'' prop . You have to get it propped for top factory RPM and an over sized prop just isn't going to do it .

07-01-2014, 10:57 AM
+1 on what the other guys have said. I feel your pain on prop selection, sometimes it seems like black magic. You really want that SBC turning around 4800 light and no less than 4600 with the boat loaded for bear.

07-01-2014, 12:20 PM
some rules of thumb
for every inch of pitch = 200 rpms
for every inch of diameter = 200 rpms
if the prop is cupped add an inch of pitch to the number
in a perfect world/vacuum a 23 pitch prop should travel 23 inches in one revolution

but the world is not perfect so each prop has slip a good prop has 10% slip or less so that means the prop really travels only 90% of the 23 inches or a little more than 20 inches

so if your motor is 100% dropping to a 21 pitch should give you an increase in rpm of about 400 bringing you to 4500

finding a prop that handles and performs is a task sometimes but choices are many for a merc

you need to fill in the blanks on this prop Calc to see how you are doing


this is just the tip of the black magic iceberg that is the world of propology
rake cup stern lift bow lift

07-01-2014, 01:26 PM
Adding to what some others have said, pitch is basically a form of gearing. Less pitch = lower gear. If the boat labors out of the hole, less pitch could be part of the answer.

Cleaver props I *think* are a fairly old technology that I don't seem to run into in the new models. Maybe they are still being made in large numbers, but I don't know of it. I *think* they are typically pretty good for top end, but slip a lot at low-mid RPMs and thus are inefficient on fuel for cruising. Perhaps terrible to useless for wakeboarding (AKA: trying to hold plane at 20 mph, where a 4-5 blade or duoprop would shine). BTW, this is speaking about the reputation of cleavers as I've heard it, not from direct experience with one.

And those are all very broad, general statements. Specific real-world results can make your head spin. As was noted, welcome to the black art of propping. :)

07-01-2014, 01:43 PM
cleavers are usually seen on setups that run near or at the surface. OBs that run high or surface type drives so you don't normally see them on a normal I/O setup now a days. the blades are very sharp and have very thin leading edges and very fat trailing edges like a meat cleaver the sharp end is thiner than the thicker back edge. making them cut into and grabbing aerated water. you may find that you are just not getting the prop high enough near the surface for it to find the type water it likes to run in. a mirage type prop may be better , bow lift is not a feature on most cleavers so that may be an issue too not getting the boat up and out of the water





07-01-2014, 03:19 PM
LOL The black art of propology. Oh boy. Well I think I'd like to look into something different just to try. Should I sat stainless or can I run aluminum? I wouldn't call my boat a big power boat at about 300hp but that might be in the stainless realm at that. With older boats we always used to like aluminum incase of a knick. Sacrifice the prop and not the outdrive, but with a stainless, the prop is the hardest thing in that equation. Perhaps I'll try going down a size and getting a cupped prop... however that sounds like it will just cancel itself out. lol

http://www.islanddesign.com/temp/JJC_8464.jpg This image looks exactly like what I have. Nasty and sharp!

07-01-2014, 04:30 PM
I'm thinking you could use a 19" with some cup maybe.

ted z
07-01-2014, 09:08 PM
Well I am interested..... I have 18 with a 350MPI,'96, with a mercury vengeance 48 16320A45 23P and barely get 4000rpm's with the alpha. Anyone got recommendations?

07-01-2014, 09:15 PM
Well I am interested..... I have 18 with a 350MPI,'96, with a mercury vengeance 48 16320A45 23P and barely get 4000rpm's with the alpha. Anyone got recommendations?i'm a thinking something else is holding you back, that combo should have no problem spinning a 23 to at least 4500-4700

ted z
07-01-2014, 09:24 PM
I don't know what...It had a 350 EFI throttle body engine in before and performed the same. The boat seems fine and gets a lot of regular use.

07-02-2014, 03:25 AM
The next thing to think about is the drive ratio there are ratios for 4-6-8 cyl motors ranging from 2:1 , 1.8:1 ,1.6:1, 1.5;1, 1.4:1, 1.3:1. A 4 cyl would have a higher ratio letting it turn a bigger pitch prop 2 revs on the motor to 1 rev of the prop. The rule of thumb is for every gear change = 200 rpm or a change of 1 inch of pitch. So going down a ratio from 2:1 to 1.8:1 is like adding and inch of pitch so with the same prop it would drop 200 rpm
I am far from a merc guru but I think i remember from what Rootsy learned me. they have a heavy duty gear set for the alpha which is in the 1.4 neighborhood the 1.4 gears are also stronger so it is likely to see them on bigger power. The merc guys can tell you how to check the ratio knowing the ratio and a good GPS speed reading you can use the calc to see how you are doing It also helps knowing your tach is fairly accurate.
Now you are prepared to join the school of It is better to spin a smaller prop faster or the school of it is better to spin a bigger prop slower

Hairy Proper
Boatwartz Academy

07-02-2014, 08:07 AM
the MPI was 300hp

the EFI throttle body was either 250hp or 260hp depending on the year

thats worth at least 1" or perhaps 2" of pitch right there..

07-02-2014, 10:43 AM
I run a 19" quicksilver on my Z21 . It has a 350 mag making 270HP. It gets to 4400-4600. That seems to be the right combo for my boat. Jamie.

07-02-2014, 11:06 AM

Are you the guy on FB with the blue Z21 you're restoring? That's interesting and probably the closest real world comparison to my boat. Maybe I'll try your setup and see. I'd like to run at that rpm. Any idea on speeds you're getting? GPS on my phone I was topping out at about 45.

So maybe I'll try the 19" cupped?