For smart guys and gals and problem solvers

[BandB]

Just close all the missing gaps of information and checking before starting to make major changes.

 

[ktdtx]

I'm obviously missing something but isn't 3600 RPM a really high number for a Cummins engine?

 

[smitty477]

"I'm obviously missing something but isn't 3600 RPM a really high number for a Cummins engine?"


Once again we are still not sure which engines he has 100%. If the charts I dug up are a match for his engines then the rated rpm is 3375 with no droop so that would be the high number.
Not sure of the exact engines yet = no way to really know.

 

[seasalt007]

"I'm obviously missing something but isn't 3600 RPM a really high number for a Cummins engine?"


Once again we are still not sure which engines he has 100%. If the charts I dug up are a match for his engines then the rated rpm is 3375 with no droop so that would be the high number.
Not sure of the exact engines yet = no way to really know.

Is that load or no load? My 540 hp Cummins is 2600 under load at WOT

 

[Ski in NC]

QSB 6.7 480 HO rated for 3300, want it propped to around 3350 and at 3000rpm cruise below 19gph.

But yes, not exactly sure which engine.

They sure don't mind spinning those things up. They seem to live just fine, kudos to Cummins, but that is lawnmower type rpm.

 

[Xsbank]

You use wot to get on the step? I would unload all the cr@p you are carrying and see if it still meets what it did when it was new...lead is good on a displacement boat but planing?

 

[Bigsfish]

More facts.
Yes the engines did turn 3350.
Both engines run the same.
Bottom, props and pods appear clean
Engines run cool, under 170 each.

Before doing anything rash (re prop or rerate the engines) I'll have Cummins out to check everything on the drives and engines. I'm not going to take all the stuff on the boat because I need that stuff and it would take days.

Presently wot is about 3050 and I usually will cruise about 2700 to 2800. 2700 is 15 knts and barely on a plane so I'll push it to 2750 to 2800 about 17 knts.

My own theory is the boat is over propped and if I want to get to 3300 I'll have to reduce the pitch.

My thanks to all who contributed and made me take a step back and review the problem before taking plunge perhaps in the wrong direction.

 

[NewbieFromNJ]

More facts.

Yes the engines did turn 3350.

Both engines run the same.

Bottom, props and pods appear clean

Engines run cool, under 170 each.



Before doing anything rash (re prop or rerate the engines) I'll have Cummins out to check everything on the drives and engines. I'm not going to take all the stuff on the boat because I need that stuff and it would take days.



Presently wot is about 3050 and I usually will cruise about 2700 to 2800. 2700 is 15 knts and barely on a plane so I'll push it to 2750 to 2800 about 17 knts.



My own theory is the boat is over propped and if I want to get to 3300 I'll have to reduce the pitch.



My thanks to all who contributed and made me take a step back and review the problem before taking plunge perhaps in the wrong direction.

This thread is the perfect example of why this forum is so valuable. Still 3 years away from pulling the trigger on buying a trawler. Kudos to all. You guys amaze me. Priceless for me. Hope at some point I can b helpful to others. Sorry if I interrupted the conversation.

 

[Northern Spy]

You use wot to get on the step? I would unload all the cr@p you are carrying and see if it still meets what it did when it was new...lead is good on a displacement boat but planing?

This.

 

[smitty477]

"Presently wot is about 3050 and I usually will cruise about 2700 to 2800. 2700 is 15 knts and barely on a plane so I'll push it to 2750 to 2800 about 17 knts.

My own theory is the boat is over propped and if I want to get to 3300 I'll have to reduce the pitch."


Based on the current information that you have your guess is the same as mine. Do you happen to have EGT readings on the engines at the current 3050 rpm?


Does the manufacturer have any other similar boats produced that can be compared for loads and propping data?

 

[kev_rm]

Shop around more for props or re-pitching?? My word, I would be tempted to punch a supplier in the face if they told me 9 grand for what amounts to $500 in raw materials or a dozen or so hours of labor.

 

[Bigsfish]

Smitty

No EGR but they may be available on the Cummins Vessel View, I'll have to check.

I don't trust the manufacturer, he claims the other boats get 25 knts which mine never did, taking it off the boat with half fuel, little water and practically no supplies this boat did 23 knts. I really don't believe statements from the makers.


Kev. There are four props in SS and no aftermarket supplier. Stuck with Zeus.

 

[BandB]

More facts.
Yes the engines did turn 3350.
Both engines run the same.
Bottom, props and pods appear clean
Engines run cool, under 170 each.

Before doing anything rash (re prop or rerate the engines) I'll have Cummins out to check everything on the drives and engines. I'm not going to take all the stuff on the boat because I need that stuff and it would take days.

Presently wot is about 3050 and I usually will cruise about 2700 to 2800. 2700 is 15 knts and barely on a plane so I'll push it to 2750 to 2800 about 17 knts.

My own theory is the boat is over propped and if I want to get to 3300 I'll have to reduce the pitch.

My thanks to all who contributed and made me take a step back and review the problem before taking plunge perhaps in the wrong direction.

When did the problem first surface? How long ago did you notice the decline and was it gradual or all at once? One thing that should be done is anytime engines are serviced, they should be run up to WOT to check the RPM's so you have data over time.

 

[Bigsfish]

The problem came over the first year and a half as more stuff was put on the boat and appears to have gotten slightly worse lately.

 

[BandB]

The problem came over the first year and a half as more stuff was put on the boat and appears to have gotten slightly worse lately.

Now, a good mechanic can help pin it down, but just saying that it would appear to be two issues. First, you added weight and that impacted it. However, since then your performance has further degraded which means something else going on.

Regardless, you want to be sure it's operating at it's best before you start considering changes to the engines or the props.

Also, in considering upping the horsepower, I'd talk to Cummins and anyone I could find who had done it. Often just the addition of horsepower provides a little more speed at WOT but very little change or benefit to your normal cruising. I know this to be true on many Grand Banks where they offered several options. You could upgrade 100 hp and gain 1 knot or 200 hp for 2 knots. They probably have some performance charts with various engine configurations. Just because the greater hp version is great in someone else's boat of a different model, doesn't mean it will be for you.

 

[gaston]

The problem came over the first year and a half as more stuff was put on the boat and appears to have gotten slightly worse lately.

isn't this your answer ???

 

[Bigsfish]

At your suggestion I will have Cummins do a complete service and prove that the engines are in good shape. I'll make sure the bottom is perfect and props are the same, then to try it out. Every boat I've ever had gained weight but this one lost the most RPMs and speed. It may have been over propped from the start.

Thanks to all of your suggestions I'll get to the bottom of it.

 

[Bigsfish]

Gaston

Probably, that was my premise from the start, I was going to either re prop or rerate the hp. Now I'll check the engines first before spending lots of money.

 

[smitty477]

"I don't trust the manufacturer, he claims the other boats get 25 knts which mine never did, taking it off the boat with half fuel, little water and practically no supplies this boat did 23 knts. I really don't believe statements from the makers."


I really never believe the makers claims either but I had hoped that there would be a couple of other owners out there that you could speak to about real life experiences with this set-up. I know it has been a huge help with other boats I have been involved with when owners share real information and tips.


Certainly the boat will have much less top speed and cruising speed as weight is added and 20% over a baseline on this sized hull at planning speeds will drop speed in the way you have seen.


-Do you have fuel use data from your baseline runs? Without fuel use or EGT's it is much harder to make an educated guess to your current situation.


Please consider collecting as much of this data in one organized place to assist with the Cummins techs assessment of your situation and determine your best option(s).


I believe I have collected this limited data correctly from your earlier posts:
Current use state of boat at #51,000#'s = 19.5 knts / 42 gph w-genset / 3050 rpm both engines / 98% boost both engines


Given this limited data it looks like you boat in this configuration will require somewhere around 700 hp total at 19 knots. Your engines should produce about 960 hp when they are tuned and loaded (propped) correctly.
Right now there is a likelihood that the engines are limited in their ability to pull full rpm and the ECM is limiting fuel flow to help protect the engines.
It is always safer to extract higher hp from these diesels at higher rpm that to ask them to produce the same hp at a lower rpm - the reason it is important to make full WOT rpm as a test and to monitor EGT when near the edge. Based upon the Cummins charts that I attached in an above post I believe these engines have a top WOT rating of 3375 and that is what would be best on a fully loaded boat on a hot and humid day.

FWIW - and in my humble opinion you do not need more hp to achieve your goals and to also allow the engines to see a long life. I would add EGT gages if it were my boat as they cost me less than $300 for both engines on my last 3 boats.

 

[Bigsfish]

Thanks Smitty

I sure hope you ar correct as it should be easy to get the engines to produce what they are rated for, they did when new. I will check Vessel View for the EGT readingsif there.

Certainly I have a good place to start poking.

 

[smitty477]

"I sure hope you are correct"


We know if you are burning 40 gph now on the mains that you are utilizing no more than 760 hp to reach your speed (likely less) yet you are rpm limited which leaves plenty of room for improvement.
IMHO - You say your boat performs well at above say 15 knots so this should work out pretty well for you.

If you were looking for a 22 knot cruise this discussion would be totally different.

 

[Bigsfish]

I would be very happy for a 20 knot cruise at 3000 RPMs. I would probably cruise at 17.5 to 18 knots.

Thanks.

 

[Ski in NC]

Have them check that both pods are aiming right. If angle is wrong they will fight each other and reduce speed.

 

[koliver]

In all of the above, the recurrent theme is that you are overloaded for the combination of hp and prop pitch that the boat is equipped with.

When the day comes that you want to move up, you will unload and demonstrate to prospective purchasers that the boat is again correctly propped for the hp and WOT rpm available. What could be better.

For now, get used to the slightly slower top speed and enjoy your boat.

You may find yourself changing dinghies, getting rid of the overload in the process, is a more viable solution.

 

[smitty477]

"When the day comes that you want to move up, you will unload and demonstrate to prospective purchasers that the boat is again correctly propped for the hp and WOT rpm available. What could be better."


Correctly propping the boat for longer engine life, higher efficiencies in fuel burn, and a higher flank speed just in case it is ever needed in an emergency situation.
Having a set of logs that indicate the boat was propped correctly during use and a computer that confirms those figures also helps at sales time.

 

[Bigsfish]

Kieth

So you saying I may even be able to go in skinnier water!

I know I can run the boat as is but I am running to close to the maximum attainable RPMs to feel happy.

Thanks.

 

[smitty477]

"Have them check that both pods are aiming right. If angle is wrong they will fight each other and reduce speed."


Great thought here....

 

[Bigsfish]

They were confirmed when the boat was landed in the US. I have it rechecked to be sure.

 

[Northern Spy]

Have them check that both pods are aiming right. If angle is wrong they will fight each other and reduce speed.

Interesting. Might not have come up with that one.

 

[IRENE]

Thanks Jeff

Did you go down in pitch or diameter?

Well, I don't know...

Volvo Penta sells the "propeller sets" for my drive, so I went with the next size down. It's two propellers designed to work together on a DuoProp drive. I did not measure them, but I believe the main thing different is pitch.

I agree wholeheartedly with the post regarding resale. If your boat was on my short list with 1000-2000 hours on it, then I found out at survey it was overloaded a la incorrect props, I would walk.

Conversely, if you went through the trouble shooting to address a common problem (weight gain or builder incompetence), I would be impressed with how you managed your vessel maintenance.

Good Luck

Jeff