Over propped – reduce pitch by 1 = +250 RPM?

[Terry Kerr]

I have a pair of 2001, 6BTA 330s, CPL 1975

When purchased, the boat hit 2800 RPM with full tank and 2 buddies on board. Would have preferred 2900, but had other issues to focus on. Stamp on prop said 20×22.

Dinged a prop and took both in the be dynamically tuned and added a partial cup (probably not a good idea, given what I have learned since).

Now boat can’t get past 2650 RPM

They worked off of what was stamped on them, but now I’m wondering if the previous owner had them adjusted down a bit. Maybe to 20×21?

I read somewhere on line (so it must be true) that lowering by 1 pitch = about an extra 250 RPM.

2560+250=2900…..

Sounds too general to me, but I need to do something.

a. Remove the slight cup?
b. Take pitch from 22 to 21?
c. all of the above?

Opinions?

Topic

[Terry Kerr]

Totally agree

It might be all in my head but she even seemed to handle better at higher speeds. I did not have to mess with the trim tabs as much.

Just filled her up with fuel. will get a couple of buddies on board. Might put the laser tach on the shaft and get the ratio first and then take her up and make the calculation from there as it is far more accessible.

If at 2900, I might leave it alone. or I might fill up the water tanks (they are usually only about 1/4 full) and see what that does.

Part of me is leaning towards taking out another 1/2 pitch (21 to 20.5) or maybe at least taking the cup from a 4-6 at the tip to a 3-5 at the tip. another $600 for the former. They will do the latter for free.

Thanx everyone for the help!!!

[Francis Valerio]

Great news on the props! It is amazing how different a boat performs when the props are dialed in correctly.

Personally my gut says you are close but need to go a bit more… I ONLY say that bc your numbers on your PORT engine are off BUT they need to be confirmed (you said the port tach has a mind of its own…)

First put a few fat guys on the boat and put some more fuel in and see where your numbers are at and then decide from there or report the numbers here and it can be discussed.

It is possible you are good where you are at. If the port engine is actually at 2900 with your other engine then I would leave well enough alone

Let us know

[Terry Kerr]

Finally got the props back (4 weeks)

Took the prop guys some time due to people out sick.
Dropped the props from 20×22 with a cup of 8 to 20×21 with a cup of 4 and just the tip going to a 6 as they recommended.

Boat went from max of 2650 (remember 2800 is spec and we seem to want 2900 just in case we get heavy or have growth)

Props are on and I sea trialed her a few minutes ago. I think I am very happy, but certainly not done yet.

Boat came on plane better than ever. Oddly I did not lose speed at 2200 RPM (maybe due to wind and current). Previously 2200 RPM gave me about 17.5 to 18.5 knots. Today it was similar up and down the river with the wind going the same direction as the current. I must have lost some speed, but it’s not noticeable.

Full throttle was much better, but not perfect. Starboard hit close to 2900 (laser corrected). Port only hit about 2800 (corrected), but that tach sometimes has a mind of it’s own. Some days it shows idle at 600 other days 800, and today 400, so not very conclusive.

Speed down river was 27.4 knots! Wow!!!! That’s the fastest the boat has ever gone. Up river against the wind was only 25 knots, but that’s all she would ever do before, out in the Gulf.

One other interesting discovery. Once I come up on plane, I seem to be able to back off and easily stay on plane at about 1800 rpm and run at 10-11 knots. Before she would start having trouble staying on plane at about 2000 rpm. Granted she does about 7 knots at about 10-12 hundred rpm, so not sure if being on plane for an extra 3-4 knots is good or bad.

All in all I am pretty happy, but think I need to take the props down to 20.5 pitch to be even safer. Changing the pitch is another $600. They will change the cup for free. Hmmmmm

Today it was just me and a half tank of fuel. Need to fill her up and get a couple of buddies on board to see what fine tuning needs to be done.

Any new suggestions are most welcome.

[Terry Kerr]

great idea!

I already have a laser tach. Never thought to use it for this purpose. Will share results when I get the props back.

[Rob Schepis]

“I also don’t know my gear ratio or type. Crawled all over the dang things and can’t find anything. Think that will be a question eventually….”

Use a laser/photo-tach to compare shaft speed to engine rpm.

[Gene Fuller]

Hypothetical pitch of 21 results in WOT under load at 2950 rpm (not overpropped). RPM at 20 knots = 2350
Hypothetical pitch of 20.5 gets results in 3100 rpm (not overpropped). RPM at 20 knots = 2450

Think about this in a slightly different manner. If the engines are in good condition the required fuel burn to maintain a given boat speed will be almost exactly the same in either RPM case. There might be some slight differences due to the prop configuration, but the great majority of the fuel burn goes toward overcoming the friction of moving the boat.

The higher RPM (2450) gives better cooling water flow, better air flow, lower combustion load per stroke, and the fuel burn is farther below the limiting fuel burn curve published by Cummins.

Yes, the crankshaft spins a bit more, but when have you heard of lower-end problems on these engines if properly maintained?

In other words, what’s not to like about the higher RPM?

Gene

[Terry Kerr]

Good question

I will have to do some research on this.

They came with the boat and I never gave them a second thought until I joined this forum.

Now I want them right or slightly under propped to be safe..

Will get with the prop guys and see what I can find out. I don’t even know the brand. Just that they are Nibral.

I also don’t know my gear ratio or type. Crawled all over the dang things and can’t find anything. Think that will be a question eventually…..

[Tony Athens]

Terry,

Since you a very concerned about getting your boat up to a vessel speed that “seems right” for the vessel, I need to ask———————–EXACLY where did the props you are trying to work with come from”..

Exact type / spec and general “vintage” is ??

[Terry Kerr]

Kind of what I was trying to confirm

Thanx for putting it more eloquently. I was not explaining my question very well.

Part two is after she is propped correctly, can I run her at 2400?, 2500?, 2600? without worrying about anything but additional fuel burn?

2200 used to be my “sweet spot” at about 20 MPH, but lowering the pitch I worry that 2400 might be my new “sweet spot” for the speed that the boat seems to like to run at.

The boat doesn’t feel like she is fully on plane until about 18-20 MPH, hence my concern about the RPMs to get there vs. possibly “plowing” at 2200……

[Francis Valerio]

2925 will give you the most speed and be 100% correct for loading.

I believe your rated RPM is 2800. Like Tony mentioned in the 2nd post that the prop shop to do what they can to get you 2925 rpms

That is giving you the best bang for you buck. Having the engine able to reach 2925rpm @WOT (as loaded for everyday use).

Anything beyond 2925rpm would be wasted (BUT CERTAINLY NOT HURTFUL) especially since you are trying to get the most speed out of your vessel without overloading it.

1 user thanked author for this post.

Tony Athens

[Terry Kerr]

Maybe I asked the question wrong…

I don’t want to be overpropped. Obviously. I get that.

Underpropped is much better. Or as you say, really can’t be underpropped…….I get that.

Rephrasing my question (removing the over-propped component).

Hypothetical pitch of 21 results in WOT under load at 2950 rpm (not overpropped). RPM at 20 knots = 2350
Hypothetical pitch of 20.5 gets results in 3100 rpm (not overpropped). RPM at 20 knots = 2450

All hypothetical, so let’s not get too caught up in whether it should be 2925 and 3125, which would be better, assuming I wanted to maintain 20 knots?

Just trying to understand the pros and cons.

[Tony Athens]

Proper Propping high HP/liter engines

“Is it better to be underpropped and run at 2500 rpm at say 20 knots or over propped and run at 2200 and get 20 knots?”

Asking this after all that has been posted on the subject of propping over the past 20 years by me personally?

Give me a break!

Tony

[Terry Kerr]

Blame it on covid

Gave them a set of 20×22 props with unknown, but low cup.
Got back 20×22 props (theoretically) with a pitch of 8 resulting in lower max RPMs.

Now just got a note that said the props were 20×21 and 20x 24???????

Not possible!

Either I got them back that way, but then I’m thinking the max rpms would have been different on the two engines as opposed to almost exactly the same……or…….They got my props mixed up at the shop last Friday and don’t know which ones are mine!!!!!

Just what I needed…. More variables!!!!!!!!!!!

[donald roth]

Propping a Cummins Mechanical Diesel Engine

[Terry Kerr]

so what if I’m “under-propped”?

I have read that my engines should hit about 3150 WOT in neutral at the dock. They do.
And they should hit something greater than 2800 (2900?, 2950?) WOT under load. They will.
Also red line is about 4000?
And Cruising RPM is 2200?

If I am “under-propped” I thought that translated into the boat coming up on plane faster, but loss of top end speed and speed at cruising RPM.

She was already kind of slow at 2200 rpm at about 17.5 knots with the props before they were cupped. after cupping she hit about 19.5 knots at 2200 rpm, but could not get past 2650 rpm.

The Gulf of Mexico is often very smooth and I would much prefer to run a bit faster than 17.5 knots.

In a different thread you mentioned to the carver owner, with these engines, to go ahead and run at 2500 rpm.

Am I mixing apples and oranges?

Is it better to be underpropped and run at 2500 rpm at say 20 knots or over propped and run at 2200 and get 20 knots?

Or do I just need to learn to like 17.5 knots?

[Tony Athens]

Not sure how many times I have said this (in so many words) , but here goes again..

It is basically impossible to underprop engines likes this (50++ HP per liter)–YOU CAN ONLY OVERPROP THEM

A graph on my own boat attached–99% of the time in local weather conditions, 18K’s in about as fast as you want to travel, although we did make a 56 mile 2-way run last Thursday at 20-22 Kts both ways. We were right at 20,000 lbs all loaded up and the numbers were basically the same.. Under propped “on paper” for sure, but does it matter–NOPE!

[Terry Kerr]

Totally agree

If I end up under-propped, at least I have good spares for when I get my new props. Unfortunately the prop guys were reluctant to go much further down than 1 pitch. Will know more in two weeks (covid delays).

In the meant time I will do more cleaning and painting in the engine room, while I wait for Brendan to get back to me on my new CCV risers. Doubt it will hurt to introduce that small variable to the equation.

[Francis Valerio]

props can only go so much when they are repitched or cupped/decupped

I would ask the prop guy how much he can go on these props and prob start around there. Worst case you will be under-propped a little more than you want. Best case you will be right where you want to be.

In BOTH cases you are SAFELY propping your engines and NOT overloading them

[Terry Kerr]

Fingers crossed

That is correct Rob. “Hoping” for an extra 300.

Went from 2800 before I added the cup to 2650. In theory putting the cup back to where it was should put me back at 2800. However I want 2900 and we are only reducing the cup partially.

As tony said, reducing the pitch much more on these props could prove to be a challenge. If I can get half my RMPS back by reducing the change in cup by half (2650 to 2725) and then get 200 RPM from reducing pitch from 22 to 21, then theoretically I am at 2925.

Not more I can do with these props. Worst case it gets me closer for when I have to buy new props.

[Tony Athens]

As I think Rob is suggesting, asking for 300 RPM will require a much more aggressive prop change. Your existing props may not be up to that challenge.

For boats that operate on plane, cup seems to be a proper part of the overall design.

[Author]

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