Reply
 
Thread Tools Display Modes
 
Old 12-01-2014, 10:40 AM   #1
Veteran Member
 
City: Sacramento Delta
Country: USA
Join Date: Feb 2014
Posts: 57
Engine RPM

I have a 44' Ocean Alexander with the Caterpillar 3208's turbocharged engines. The build sheet that came from OA show the 25x17(?) props I have came from the factory.

When I go to the Caterpillar website they indicate the maximum engine RPM's for the 3208 should be 2800, but I only get 2400 at WOT. I've been told that means I'm over propped.

I've read and heard that cruising RPM's should be 75% of WOT. So here is my question, should I cruise at 1800 RPM, 75% of 2400, my engines maximum or should I bump it up to 2100 RPM which is 75% of what the engines should turn?
__________________
Advertisement

IMG2 is offline   Reply With Quote
Old 12-01-2014, 11:02 AM   #2
Guru
 
River Cruiser's Avatar
 
City: UMR MM283
Country: US
Vessel Name: Northern Lights II
Vessel Model: Bayliner 3870
Join Date: Jan 2013
Posts: 1,228
If it was me I would change the props so the 3208s reach the specified rpm at wot. Then cruising at 80% of that rpm shouldn't be a problem. I would also install egt and manifold boost gauges if they aren't already in use.
__________________

__________________
Ron on Northern Lights II
I don't like making plans for the day because the word "premeditated" gets thrown around in the courtroom.
River Cruiser is offline   Reply With Quote
Old 12-01-2014, 11:21 AM   #3
Guru
 
Ski in NC's Avatar
 
City: Wilmington, NC
Country: USA
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 3,890
You can leave it overpropped if you only run hull speed. Running above that will be hard on the engines. There is no need to run them at 75% power, ignore that advice.
Ski in NC is offline   Reply With Quote
Old 12-01-2014, 11:25 AM   #4
Guru
 
City: gulf coast
Country: pinellas
Join Date: Jun 2014
Posts: 2,199
First make sure you are getting the data for the exact model engines you have. Look on the metal tag for engine model number and it might also show MAX rated RPM
The next step is to verify the tachometers are correct. A photo tach is cheap or a mechanic can check it for you. If both engines read the same it is unlikely the tachs are wrong.
Another quick test is with the transmission out of gear and engines warm briefly run up to max rpm. The rpm you hit will be the engines rpm setting. If you hit 2800 out of gear then the problem is probably in overpropping.


If indeed you are not able to hit MAX rated rpm then you do need to have the props repitched. At 6 knots it may not make any difference but if you are running faster they are overloaded significantly.
bayview is offline   Reply With Quote
Old 12-01-2014, 12:43 PM   #5
Guru
 
Join Date: Apr 2014
Posts: 1,291
Is the 2800 rpm the WOT no load rpm or the rated HP rpm under load. It is the under load rpm that counts for adjusting prop to match load curve of the engine. The no load rpm is usually higher than rated rpm under load. There may be a label on the motor with this information. follow Bayviews and ski's instructions and advise.
eyschulman is offline   Reply With Quote
Old 12-01-2014, 12:58 PM   #6
Guru
 
Ski in NC's Avatar
 
City: Wilmington, NC
Country: USA
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 3,890
Most 3208 tags give both rated power, as in "XXX hp at XXXX rpm". Also gives "high idle XXXX rpm"

The rated power rpm is full load rpm, usually 2800, best to prop a little above like 2850 if you are going to run planing speed.

High idle is the no load rpm, full speed in neutral. Usually around 3000, but I think some are around 3100.

Tags are usually on the front of aftercooler on TA versions, or on rocker cover if a T version.
Ski in NC is offline   Reply With Quote
Old 12-02-2014, 06:48 AM   #7
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 16,534
The worry about RPM is from 2 problems , overloading and underloading.

If you record the max RPM of each engine , and it is not pumping huge black smoke (overloaded) simply reducing by AT LEAST 10% solves any overloading problem.

If thats where you wish to cruise , your fine.I like to put a line on the tach 300RPM down from WOT., and use that as the max RPM.

Most folks want economy esp for Long Range Cruising (LRC) and that is usually about at the SQ RT of the LWL . Forget so called hull speed unless you have your own oil well.

Under loading comes from running the engine at minor power 10% or 15% of rated power for long periods of time.

A bigger cruise prop is one cure but see overloading above.

A run of 15 min at near full power will help engine life, as will oil sampling and more frequent oil changes.

For most folks its not a problem as reducing the life of a 8000 hour engine to 4000 hours is still many many years of cruising.

You will know the end is near when the engine white smokes during warm up under load until 140F -160F or more is required to clear the exhaust.
FF is offline   Reply With Quote
Old 12-03-2014, 07:23 PM   #8
Guru
 
High Wire's Avatar
 
City: Cape May, NJ and Englewood, FL
Country: USA
Vessel Name: Irish Lady
Join Date: Dec 2007
Posts: 1,602
There are many reasons a boat does not perform. Heres a few to consider:
Are the tachs true?
Are the props true?
How's the bottom paint? Smooth or fouled? It doesnt take much to drag you down in RPM.
Dirty fuel and air filters
How much extra weight added since new boat delivery?
Are the engines making rated HP?
Trim tab position? Too much or too little can have a big effect.
__________________
Archie
1984 Monk 36 Hull #46
Englewood, FL and Cape May, NJ
High Wire is offline   Reply With Quote
Old 12-03-2014, 07:52 PM   #9
Guru
 
Join Date: Aug 2012
Posts: 741
Our 44 OA has twin 250 HP engines and is propped with 24X20s. It achieves rated rpm at a speed of about 17 kts. The rule of thumb I've seen says 2 inches of pitch roughly equates to one inch of diameter. Don't know the power rating on your turbo 3208s, but intuitively, it seems they should be able to swing those props to rated rpm.
Underway is offline   Reply With Quote
Old 12-03-2014, 11:21 PM   #10
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,724
Skid,
Two " to one " dosn't seem right. That would be a 10" dia & a 20" pitch .... or the other way around a 20" dia w a 10" pitch would be OK for a bollard pull but not for a boat in motion.
From what I've read and heard there is a fairly narrow range of appropriate pitch for a given dia. For a trawler pitch should be about 75% of dia. Like a 24"dia w an 18" pitch. Some to quite a few trawlers IMO have too little pitch. Too little pitch means too much power wasted turning blades too large through the water. Get the blade big enough and all the engine's power may be required to turn up to rated rpm and w zoro pitch and absolutely no thrust will result. Extreme example of course but leaning in that direction results in power and efficiency loss .... like a 20" dia and 10" pitch. However if blade area is handled right a 24" dia w a 10" pitch can be very optimal for efficiency. It would take longer skinnier blades than usual and most all the props commonly available favor a limited prop clearance w wider "fatter" blades.
Too much pitch and too much tip loss ..... water tumbling around the blade ends but not enough moving aft creating thrust.

Skidgear I must not be reading your post correctly.
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 12-04-2014, 05:34 AM   #11
Guru
 
timjet's Avatar
 
Join Date: Apr 2009
Posts: 1,905
Quote:
Originally Posted by Ski in NC View Post
You can leave it overpropped if you only run hull speed. Running above that will be hard on the engines. There is no need to run them at 75% power, ignore that advice.
Ski is far more knowledgeable than me on this topic but I seem to recall a discussion on boatdiesel that ended with the conclusion that over propped/overloaded is still the case regardless of the RPM.
__________________
Tim
Tampa Bay
Carver 355 ACMY Twin Cummins Diesels Sold
timjet is offline   Reply With Quote
Old 12-04-2014, 10:23 AM   #12
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,724
timjet,
Not at any rpm. Most over propped boats re not overloaded at 1/2 rpm.
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 12-04-2014, 12:03 PM   #13
Guru
 
Join Date: Aug 2012
Posts: 741
Quote:
Originally Posted by manyboats View Post
Skid,
Two " to one " dosn't seem right. That would be a 10" dia & a 20" pitch .... or the other way around a 20" dia w a 10" pitch would be OK for a bollard pull but not for a boat in motion.
From what I've read and heard there is a fairly narrow range of appropriate pitch for a given dia. For a trawler pitch should be about 75% of dia. Like a 24"dia w an 18" pitch. Some to quite a few trawlers IMO have too little pitch. Too little pitch means too much power wasted turning blades too large through the water. Get the blade big enough and all the engine's power may be required to turn up to rated rpm and w zoro pitch and absolutely no thrust will result. Extreme example of course but leaning in that direction results in power and efficiency loss .... like a 20" dia and 10" pitch. However if blade area is handled right a 24" dia w a 10" pitch can be very optimal for efficiency. It would take longer skinnier blades than usual and most all the props commonly available favor a limited prop clearance w wider "fatter" blades.
Too much pitch and too much tip loss ..... water tumbling around the blade ends but not enough moving aft creating thrust.

Skidgear I must not be reading your post correctly.

Yes, I have a copy of Dave Gerr's prop handbook where the initial sizing factors are discussed in detail. Can't recall where I saw the 2:1 ratio...maybe Gerr's book, but it was in the context of a very rough thumbnail guide to estimate impact on power required when making tweaks to existing props. Nothing to do with initial sizing.

I just checked the specs on CAT marine engines and the lowest rating I could find for a turbo 3208 was 320 HP. Would be interesting to know the power rating on the engines and the transmission gear ratio....then run it through a prop calculator.
Underway is offline   Reply With Quote
Old 12-04-2014, 12:42 PM   #14
Guru
 
City: Carefree, Arizona
Country: usa
Vessel Name: sunchaser V
Vessel Model: DeFever 48
Join Date: Apr 2008
Posts: 6,376
Quote:
Originally Posted by skidgear View Post
I just checked the specs on CAT marine engines and the lowest rating I could find for a turbo 3208 was 320 HP. .
Cat offered a marine 3208TA category A rated at 235HP and 2400 RPM. The HPs go up from there with the next at 275 HP. As you correctly note, the engine plate data is needed to go any further on prop sizing.
sunchaser is offline   Reply With Quote
Old 12-04-2014, 01:21 PM   #15
Guru
 
River Cruiser's Avatar
 
City: UMR MM283
Country: US
Vessel Name: Northern Lights II
Vessel Model: Bayliner 3870
Join Date: Jan 2013
Posts: 1,228
Quote:
Originally Posted by timjet View Post
Ski is far more knowledgeable than me on this topic but I seem to recall a discussion on boatdiesel that ended with the conclusion that over propped/overloaded is still the case regardless of the RPM.

I agree with this. The govenor is going to add more fuel to maintain rpm between a correctly propped vessel and one that is over propped. Just because it's not against the pins doesn't mean it's not overloaded. And unlike a engine in a dozer or scraper that is intermittently overloaded during its life, a engine is continuously overloaded in a over propped boat. There is less harm at RPMs below WOT but the effects of overloading do not disappear. A exhaust gas temperature gauge I'am sure would show higher temps at the same rpm between the correct prop and being over propped because of the increase of fuel to maintain rpm. I doubt any of us will kill our engines by being over propped by a small amount because we're not going to put that many hours on them, but a owner who ignores being over propped is probably ignoring other items in his engines maintenance and the cumulative effect may substantial shorten engine life.
__________________
Ron on Northern Lights II
I don't like making plans for the day because the word "premeditated" gets thrown around in the courtroom.
River Cruiser is offline   Reply With Quote
Old 12-04-2014, 01:55 PM   #16
Guru
 
Join Date: Aug 2012
Posts: 741
Quote:
Originally Posted by sunchaser View Post
Cat offered a marine 3208TA category A rated at 235HP and 2400 RPM. The HPs go up from there with the next at 275 HP. As you correctly note, the engine plate data is needed to go any further on prop sizing.
Hmmmm. If the engine is the 235 @ 2400, then it's propped exactly right.
Underway is offline   Reply With Quote
Old 12-05-2014, 06:48 AM   #17
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 16,534
A exhaust gas temperature gauge I'am sure would show higher temps at the same rpm between the correct prop and being over propped because of the increase of fuel to maintain rpm.

Overloading (to not reach max rated RPM at WOT) and over proping are two different conditions.

Overloading is what most plaining boats do as they climb up on to the plane , you can see smell the black exhaust as they accelerate.

A displacement boat that has a >to large< prop to reach rated WOT would be overloaded IF it were operated at WOT all the time.

Most cruisers are far from WOT so the larger prop will show higher EGT , as it is a sign of higher efficiency.
The better heavier loading gets more HP from the fuel at that RPM , just what is needed.

So long as the cruise RPM is 10% or so less than whatever is observed WOT RPM , the engine is NOT overloaded by the larger more efficient prop.

It will have a longer service life and use less fuel when operating with the highest efficiency.

A glance at any engines prop chart will show how drastically the HP required goes down with a slight RPM decrease .
FF is offline   Reply With Quote
Old 12-05-2014, 09:36 AM   #18
Guru
 
Join Date: Aug 2012
Posts: 741
I'd overprop our 44 OA in a New York minute, and those 25x17's would be close to what I'd use... although 25x18 would be better. The only time the semi-planning boat will ever see maximum power under my ownership is for a survey. I'd slap on the old props if a buyer insisted. If the OPs boat is in fact overpropped I cut him a deal.
Underway is offline   Reply With Quote
Old 12-05-2014, 09:46 AM   #19
Guru
 
Ski in NC's Avatar
 
City: Wilmington, NC
Country: USA
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 3,890
Need to clarify this a bit:

If engine is overloaded at full power, that is, incapable of reaching rated rpm or above, then it will be overloaded also when power is reduced slightly. Say from 100% down to say 70%, or even below if boat is still trying to plane.

But if power is reduced significantly, such as dropping down to hull speed, then the overload condition will disappear.

The above applies to planing or semiplaning type boats with high output engines. Often they are overpropped to optimize hull speed running. No harm in that, but it will make it hard on the engines if they are run at high power.

A 3208TA rated at 320hp or whatever is not going to be harmed by running at say 50hp at 1200rpm, even if it can't make rated rpm at full. Just don't try to plane out.
Ski in NC is offline   Reply With Quote
Old 12-05-2014, 09:56 AM   #20
Guru
 
City: gulf coast
Country: pinellas
Join Date: Jun 2014
Posts: 2,199
Ski: True enough but I don't see the benefit of not propping to make full power use an option. Fuel use at 6 knots will probably be the same despite different RPMs.
__________________

bayview is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





All times are GMT -5. The time now is 12:15 AM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012