WOT rpm Question

[Beefer]

Most, if not all of us, know what the recommended rpm's at WOT is or should be on our boat, based on either the label on the motor, or the owner's manual/user's guide. Mine is 4400-4800.

My question is this; 4400-4800 based on what? What prop were they using? Right now I'm running 4100 @ wot, with a 19pitch prop. I know the prop is not the original, and I don't know what the original prop pitch was. Do I just drop down my pitch, and assume nothing else is causing the lower rpm's? As far as I and my mechanic can tell, all other parameters are correct for my engine (compression, dwell, timing, etc.).

Since different boats have different weights and characteristics, do the individual manufacturers adjust prop pitch to whatever is necessary to get the boat/motor combo operating in the proper range?

So how do we know what our recommended rpm's at wot are based on, if we aren't the original owners? Makes me wonder how many sick engines are out there running with the wrong prop to make the rpm's fall in the recommended range.

 

[Haut Medoc]

Re: WOT rpm Question

Whatever prop it takes to get you to the magical number is the prop you need, assuming the engine is tuned correctly & has no internal problems.....
The size, shape of the boat it's pushing will also have an effect....
I didn't see diameter mentioned in your post, that is just as important as pitch.....
There is a suggested prop chart in the second book of manual #1.....

 

[Bifflefan]

Re: WOT rpm Question

WOT is set by the engine manufacture.
Basiclly it is the RPM at which the engine makes the best power and lives the longest.
They also (used to) send a prop along with it, Not the case any more as I have heard.
The prop was for what the boat manufacture was making at the time.
Most, but not all were "over proped" at this time. Just you make sure you didnt (couldn't) over rev the engine.
Then its up to the owner to reprop as needed.

I know we (meaning you and I) have discussed this in another post. In my opinion, If your getting 4100 and the boat is performing well i'd leave it.
If you feel you need to drop to a 17p, then do it and see. The RPM's will come up. You will lose a bit of speed.
Sustained higher RPM's are not the motors friend in an I/O.

You dont drive your car at the red line when you have another gear.

This is just my opinion and is worth every cent you paid for it.

 

[Bondo]

Re: WOT rpm Question

Ayuh,... The 4400/ 4800rpms is where the motor is at peak Horsepower, 'n Torque...

Prop the boat to the rpm, not the other way around...

 

[HT32BSX115]

Re: WOT rpm Question

Most of the manufacturers suggest to "prop" the boat so you're turning the upper number at WOT. That way you get the best holeshot and cruising performance etc. You should not prop to turn the lowest (4400) RPM.

Also, are you certain that the drive ratio you have is correct ?

If a previous owner has replaced the drive, you may have the wrong ratio on the back. If it's close to correct, you can "get-by" with a different prop but most props have a "most efficient" RPM too.

If you get outside that RPM by too far, you can lose efficiency in the form of too much ventilation (cavitation) crummy holeshot or max speed....poor fuel consumption etc....

 

[Beefer]

Re: WOT rpm Question

I do think I'm losing on the upper end. Max GPS speed is about 27mph. It's a 22' walkaround, with a weight of about 4300lbs. If I go to a 17p, won't that increase my rpms, give me a better hole shot (I think), but not increase my upper end speed? Wouldn't a 21p give me a great top speed, but then make my rpm's even lower? Even though the 17p wouldn't get me to the 4800, I think a 15p would be too far off.

Outdrive is original. I have the original bill of sale from 1984, and it has all the serial numbers.

I knew I should have posted this in the prop section. Originally, this was a more general question, but it's definitely turned to a more specific one. :redface:

 

[a70eliminator]

Re: WOT rpm Question

My question is this; 4400-4800 based on what? What prop were they using? Right now I'm running 4100 @ wot, with a 19pitch prop


4800 RPM based on a light payload
4400 RPM fully loaded with people and gear
or anywhere in between
I have an 18p SS prop which puts me at the top of the rated RPM with just me in the boat, then with 6 people full coolers full tank of fuel and pulling a tuber the wot is lower but still in the spec. range.
I'd say either your tach is off, the engine is sick, or you have the wrong prop. in that order.

 

[QC]

Re: WOT rpm Question

In my opinion, If your getting 4100 and the boat is performing well i'd leave it.
If you feel you need to drop to a 17p, then do it and see. The RPM's will come up. You will lose a bit of speed.
Sustained higher RPM's are not the motors friend in an I/O.

You dont drive your car at the red line when you have another gear.

This is just my opinion and is worth every cent you paid for it.

Uh, this is not accurate and we need to clarify a few things.

4100 insures a deep lug condition above what is safe. It moves all of the load points along the "curve" (prop load curve is actually pretty dern straight) to a deeper lug and is actually more dangerous than the higher end of the WOT range. WOT RPM and automotive redline are NOT the same thing. Redline is typically above peak power, and we really should not discuss them together.

I disagree with Haut on diameter though. With an Alpha you're pretty much locked in.

I'll move this to the prop section and we'll get you dialed in.

 

[QC]

Re: WOT rpm Question

Wouldn't a 21p give me a great top speed, but then make my rpm's even lower?

No, and yes. And that's why it won't work. You'd be moving your max RPM even further below peak power which is probably in the middle at around 4600. You go below peak power, boat go slower.

4800 RPM based on a light payload
4400 RPM fully loaded with people and gear

a70 ^^^ has this right. I'll add: with her trimmed out as fast as she can go in either load condition.

 

[QC]

Re: WOT rpm Question

I'm getting 32% slip . . . tells me either:

1) tach's wrong (I don't really think so) or

2) Prop is shi+ or wrong for application or damaged

3) you're not trimming right or

4) weight balance sucks (too much in bow) or

5) all of the above

I would:

1) Verify tach is correct

2) Verify weight and trim are good

3) retest if there are any changes to 1 and 2

4) Report new numbers

annnnnnnndddd, CONTRADICTION ALERT!!!

5) This a big load on a small engine, horsepower compared to today's rating system (propshaft vs. flywheel) is only 165ish. You need something that will get a good bite at lower speeds, so I'd go as big a diameter as possible with that drive if you aren't already, I think that's 14 1/2 ". And I am thinking pretty much even a 15 or maybe 17 pitch to get this close . . .

 

[Beefer]

Re: WOT rpm Question

QC: Here are my thoughts on your lists, and I'll try and verify all this weekend.

1) - Tach - I'd put this as 2nd most probable. I have a new tach I can try and hook up and see if I get different numbers

2a) - Prop - Condition is like new. It's a QuickSilver Stainless, and not even a scratch on it, and no visible imperfections.

2b) - Prop - Application - This is most likely in my opinion. I think one of the previous owners (only 2, and last one had it for 1 year) put this prop on, and I think they didn't really know what they should use. I want to try a 17p aluminum and see. If that's the issue, then I'd get a 17p SS.

3) - Trim - Possibility. Haven't played too much with the trim. She gets on plane in a hurry, and that's what makes me happy. I'll definitely fiddle with it this week. I take the boat out almost daily (not this weekend tho, busy tonight, and Tampa Boat Show tomorrow ).

4) - Weight Balance - If anything, I have too much weight in the back. She sits low in the back, so much so that the outdrive is submerged when docked, and the swim platform touches the water.

5) - All of the above - See above.

So let's pretend that it is the prop. What would you all recommend as the correct prop for my application. I'm not so concerned about top end, but I'd love to get near or over 30mph as a comfortable cruising speed. Hole shot is decent enough for me the way it is. the boat is made for fishing, and that's what I use it for. No skiing, and an occasional (almost never) low-speed tube ride for the kids.

 

[QC]

Re: WOT rpm Question

This:

She gets on plane in a hurry,.

is not consistent with this:

If anything, I have too much weight in the back. She sits low in the back, so much so that the outdrive is submerged

Usually if they are stern heavy the bow goes up and they really struggle to plane. I'd move as much weight aft as possible to see if you can get the bow up a little at top speed. If she won't porpoise, or the prop blows out with any up trim, then the bow is probably heavy.

 

[Beefer]

Re: WOT rpm Question

Ok, I'll try it next run, and report back on that.

 

[45Auto]

Re: WOT rpm Question

3) - Trim - Possibility. Haven't played too much with the trim. She gets on plane in a hurry, and that's what makes me happy.

If you've got it trimmed all the way in so it gets on plane in a hurry, then you're not trimming up as it comes on plane, that could be a MAJOR cause of your low RPM and low top speed. If you're not trimming up once on plane, you're forcing the bow down and causing lots of drag like in the bottom right pic below. You should be trimming up till it looks like the top pic below once on plane.

Once it's up on plane, trim the drive up till it barely starts to porpoise then bump it back down a hair till it stops. That will be the optimum operating condition at that speed. It will vary slightly at different speeds - that's why you have adjustable trim.

From the Mercruiser Owner's Manual:

 

[Tail_Gunner]

Re: WOT rpm Question

I have thought about this for awhile...WOT When that term is being used it always starts with the Motor..THe MFG has built a motor that can withstand 4800 rpm's and for prolonged periods of time not just a 30 sec burst...and that number reflects what the {valve train} can handle without coming apart.

From there the out drive is engineered..to run effectivley at all rpm's from the top to the bottom...In that drive there is gear ratio..1.98..(3.0)....1.66..(4.3)...1.50(V8)....How they arrive at those number (gears) dunno dont care.
Now its simple as the boat mfg wants a prop on the boat that will turn the motor from 800@ idle to 4800 @ the top or what ever number they pick..
Most I/O motors are old push rods carried over and built up from the auto industry....GM primarily...And they have invested millions of dollars and thousands of man hours to reach those conclusion's....They warrant the product they build and it seems to work.. "speaking for myself its monkey see monkey do"






So when i see this:

• Manufacturer:Mercruiser
• Horsepower: 155 - 210 HP
• Years: All Years

• Model: Alpha One 4.3L Sterndrive
• Specs: Thru Hub Exhaust & 15 Tooth Spline, 4-? Gearcase
• Max RPM: 4400-4800 RPM ..............

Im thinking that is the design limit for that motor ..Valve train really...So lets put a prop on there that will get us there and all is good....:redface:...Unless you like to tinker a bit trying to outthink the mfg or push things to the limit..That is where it gets a little confusing...opininated....and subjective..

Ohh and they have given us a throttle to run at any speed or rpm our heart desires...No one is dictating terms...And yes you can change your peak band to whatever your want to...This is america and it is yours to enjoy or destroy. With one litte caveat...If you under warranty and run higher rpms the mfg suggests..your probably going to void your warranty

 

[surrender]

Re: WOT rpm Question

I had to raise the motor on my McKee to get my RPM's up. boat ran much better. Plenty of info here on prop height and rpm

 

[45Auto]

Re: WOT rpm Question

I had to raise the motor on my McKee to get my RPM's up. boat ran much better. Plenty of info here on prop height and rpm

Do you have any links to a thread on anyone raising the height of an I/O?

 

[Mariah180]

Re: WOT rpm Question

WOT specs allow for a good balance of engine longevity. Under a load and at upper rpms there is a good amount of pressure in the chamber and on the valves. Also rpms allow for a cooling effect. The air flow and even fuel spray help to cool down the combustion chamber temps. This is more noticed on the old lawnmowers that are air cooled. They are designed to run all the time at 100% throttle.

It is noted that most engines have the same rpm spec despite rated power configuration. A 160hp 4.3 has The same rpm range as the 190, 205, and 220hp. So there is room, IMO, below the lower limit in some cases. The 220hp engine turning 300 less rpms is making the near same power as xyz engine.

But if you spend alot of time wot stick with the known rpm ranges. I spend most time at 3000-3800 rpms and have a taller prop.

 

[QC]

Re: WOT rpm Question

The 220hp engine turning 300 less rpms is making the near same power as xyz engine.

But if you spend alot of time wot stick with the known rpm ranges. I spend most time at 3000-3800 rpms and have a taller prop.

Yes, the 220 can make the same power as the 160 at 300 RPM less for example, but only if torque goes up (it does) which is directly relative to peak cylinder pressure rising, soooo . . . Not a good thing. The reverse may be true though if they are mechanically identical. The 160 at WOT say 300 RPM less, could be no different than the 220 at part throttle as far as power, torque and cylinder pressure are concerned. I didn't crunch the numbers, but if anybody wants to, hp = torque x RPM/52552

Your second point, while better for fuel, can cause tuliped valves IF the prop is tall enough to put the WOT point below the recommended range. I am an advocate of slightly overpropping in some applications. The problem is that when we say it's OK here, some will take it is a blanket recommendation. The fact is, for the majority of boaters, propping in the middle towards the top of the range will offer the best compromise for performance, speed, efficiency and longevity.