For smart guys and gals and problem solvers

[Bigsfish]

The facts:

Cummins QSB 6.7 480 hp

When new with ½ fuel, ¼ water and minimal supplies. 3350 rpm @ 23.2 knts. WOT

Now 51000 lbs ½ fuel, ½ water normal supplies . 3050 rpm 19.5 knts. WOT

Boat has Zeus pods so new props (2 / engine) is 4 props. $9100.00 to replace.



I want to get the boat to top rpm of 3350 again. The boat has always felt under powered to me. A friend has an East Bay (12 inches longer and 10000 lbs less w/o a bridge and the same maker put the same engines in his boat but they were rated at 550 hp. (140 hp more than my boat) and his performs much better.

I can get Cummins to rerate these engines to 550 hp for $10000.00 ( includes new water pumps at $1000.00 each).



To me it's a no brainer to rerate the engines ( I know it will use more fuel and that's not a factor). The boat now cruises at 2800 and 17.8 knts but I want it slightly higher at 3000 rpm and 3350 WOT.



What am I missing? What say you?

TIA

 

[smitty477]

"What am I missing? What say you?"


Re- pitch or re-prop the boat so that it can easily hit rated WOT rpm on a fully loaded boat on a hot and humid day.
After that run the boat at cruise rpm that you require to meet your speeds.
Adding Hp that you will never use is not a best solution - and if you require that extra 12.5% of hp to cruise I suggest you will have other issues before long.

 

[smitty477]

1. "When new with ½ fuel, ¼ water and minimal supplies. 3350 rpm @ 23.2 knts. WOT

2. Now 51000 lbs ½ fuel, ½ water normal supplies . 3050 rpm 19.5 knts. WOT"


Do you happen to have EGT and boost readings for these two data points?
Do you happen to have fuel flow readings for these two data points?

 

[Bigsfish]

I do have those but would have to check to make sure, I do know at 100% throttle I am at 98% boost at 3950 WOT.

At 3000 rpm engines with gen the totalburn 42 gallons per hour.

 

[smitty477]

"

I do have those but would have to check to make sure, I do know at 100% throttle I am at 98% boost at 3950 WOT."

Is that a typo - 3950- or is that at a no load / high idle) reading?
What is the rated WOT rpm on your engines?
The more data that you have on the engines the better to work with.


"At 3000 rpm engines with gen the totalburn 42 gallons per hour.

"
Would that be before as in number 1 above or later as in number 2 above?
Do you know what your genset draws in fuel use?

 

[Ski in NC]

How much weight have you added? 300rpm and 3kt drop is a lot of change.

Some semi-planing boats are super sensitive to weight. 51000lb is heavy for a 43.

 

[Bigsfish]

Smitty

It's now but would be the same I think because of fuel burn is a function of RPM. Gen burns 1-2 gph.

 

[smitty477]

Fuel burn is a function of load that is why it is not the same when out of gear as in gear at the same rpm. Fuel burn is not a function of rpm on diesels as they have a very large range in air fuel ratio based upon load.


What about the boost at 98% at 3,950 - is that the real rpm?
Is that in neutral" what is the rated rpm of the engines?

 

[Bigsfish]

Ski. Grand Banks listed the boat at 42,061 lbs but that was without the 800 pound dinghy and the had to add 1600 pounds of lead to the bilge to balance it out plus fuel, water, parts, tools and other supplies and it now weighs 50,743 on the travelift.

I think it was under powered from the beginning.

 

[cafesport]

Big fish when was the last time you changed the secondary fuel filters. When they get fouled you won't get full rpm.

 

[Ski in NC]

Ski. Grand Banks listed the boat at 42,061 lbs but that was without the 800 pound dinghy and the had to add 1600 pounds of lead to the bilge to balance it out plus fuel, water, parts, tools and other supplies and it now weighs 50,743 on the travelift.

I think it was under powered from the beginning.

It probably was a bit underpowered even at the start. But you have added a LOT of weight to an already heavy boat. Performance numbers don't surprise me.

Why is 1600lb of lead needed to balance a 800lb dink? On a planing boat the last thing you want to do is add weight.

You need a bigger boat or a smaller dink!!!

 

[smitty477]

"think it was under powered from the beginning."


FWIW - from my perspective I do not think its under powered. I do believe it is overpropped and by getting back to correct loading you will gain both engine life, efficiency, and some speed.


Without more detailed data lets just take a "WAG" based upon this statement of yours....
"It's now but would be the same I think because of fuel burn is a function of RPM. Gen burns 1-2 gph"

So the Wag would say that you are left with 40 gallons per hour for the mains when running at about 18.5 knots.
That would mean your hull requires about 760 hp or likely less to run at 18.5 knots (40 X 19 = 760). That would be about 380 hp per engine or about 80% of the engines rating at 480 hp.
The main problem I see so far is that the engines are not able to reach near rated rpm which will cause them to run less efficiently and potentially ate very high EGT's (not good). Also if you can not get near rated rpm you will never realize the full rated hp at any of the rpm range as well as WOT.

 

[Bigsfish]

Smitty

Good info. The boat did get the proper RPMs and speed when new with little weight and little fuel.

Btw the 1600 pounds of lead were to balance the boat including the dingy (a Rib with 40 hp) that weighs 800 pounds. Not sure how the screw up happened on Grand Banks end but the dingy is not excessive in my mind.

I think what you are saying reproping is better than turning up the hp.and will increase the speed.

Thanks.

 

[seasalt007]

I am planning to take my prop down a little during the next bottom job. Right now I am short about 75-100 rpms at WOT on my QSC 8.3 540 Cummins and getting light soot on the transom at other than displacement speeds.

My 1989 45' Californian MY with 6-71 TIB's ran like a SAA after I took out some pitch. My last NT 42' with the 5.9 Cummins got much better fuel economy after the pitch was reduced. I had an old Taiwanese Albin with a 120hp Ford Watermota that ran better after I had the prop corrected.

In my recollection I have never seen a new boat that was not over propped from the factory. Don't know why this is.

 

[Bigsfish]

So I have been looking into having the pitch (not diameter) corrected on the props. Is that the consensus of this group?

I have found since these are pods with two props per engine I'm having trouble finding a prop shop that really understands the relationship of the bigger prop to the smaller prop. This may become a mission.

 

[Bigsfish]

Smitty

Sorry, I didn't answer your question. Yes it was a typo, should read 3050. So the boat has to find 200 RPMs.

Thanks.

 

[Acheron2010]

Air and fuel. Clean filters all around? Max design flow? Backpressure on exhaust?
Are you carrying a huge oversize anchor and chain?

 

[bglad]

If you reduce pitch you will loose speed for a given rpm at slow speed (hull speed). It will not effect your hull speed but your engines will turn faster to get there. However, re-pitching the props is pretty close to science with all the data you have so your result is pretty much guaranteed.

Adding horsepower may or may not get you what you need. You may spend the $10,000 to add the HP and not get the 200 rpms you are searching for. If Grand Banks has data that insures you will get the rpms with the upgraded HP I would consider that route. You need the added power when you are loaded and want to run faster. I bet most of the time you will not be tapping that resource.

Bottom line is re-pitching helps your engines but not your need for speed. Adding HP gives you another dimension to tap into but does not require you use it.

 

[Bigsfish]

A2010

Clean air filter and fuel filters.
I'll have to check the backpressure of the exhaust if it is listed on Vessel View.
I don't think the anchors are heavy, I have two 60 pound anchors on the bow platform. One with 300 feet of 5/16 ht chain and the other with 69 feet and a nylon ride. I wouldn't boat with less.

Thanks.

 

[Bigsfish]

Bglad

Thanks. I'll check with Grand Banks to see if they have an answer on the props. You are correct that I won't run at WOT other than to clean the props for a few minutes but I would sure like to see 3000 RPMs.

I wish you were correct on reworking the props being a science, I've been getting answers that seem strange as most shops don't seem to understand I have two props per engine one smaller than the other. Interesting.

 

[Ski in NC]

Adjusting pitch on those pod props may not get as clean a result as on a normal prop. More diameter at hub so some of the blade retains original pitch. Find a prop guy familiar with your props and go on their advice.

Uprating the engines may get you where you want to be with original props, but you will be running engines (same engines) harder afterward, which is close to running them as is, but on the pins.

Get that 1600lb of lead out of there first. Nuts.

 

[Bigsfish]

Ski

If I remove the lead I will list about 7 degrees to starboard, that's why the lead was added. Wasn't my choice but nothing else to correct the lean.

I'm finding it hard to find a prop shop that is use to these Zeus drive props.

Thanks.

 

[IRENE]

I switched props...

Bigsfish,

I am just a little fish, but we have some similarities:

Diesel engine
Boat gained weight over time
Proprietary propeller set on a drive
Engine not reaching full potential

We were achieving the lower end of our rated WOT, which is 3400-3700. When really loaded down, it was 3400. Upon consultation with our most excellent VP technician Ed, we decided to go down a size in the VP propeller set.

Results - The boat now reaches 3650 (or it is hitting a limiter). She just "feels" better at all speeds; she is not working as hard climbing a large sea. I do not have a pyrometer, but I am betting the numbers are better. Since we are putting our exhaust through a drive that is sensitive to heat, this can only be an improvement. No, it was not cheap but I think it will pay off in longevity.

This boating is great!

Best Wishes,

Jeff

 

[Bigsfish]

Thanks Jeff

Did you go down in pitch or diameter?

 

[Jeff F]

I'd like to understand the motivation here. Reducing pitch sounds like a good thing but is likely not going to increase max cruise speed much if any. You may be able to cruise at your desired rpm but the same speed as you go now.

Doesn't sound to me like that's the solution you seek.

 

[BandB]

The facts:

Cummins QSB 6.7 480 hp

When new with ½ fuel, ¼ water and minimal supplies. 3350 rpm @ 23.2 knts. WOT

Now 51000 lbs ½ fuel, ½ water normal supplies . 3050 rpm 19.5 knts. WOT

Boat has Zeus pods so new props (2 / engine) is 4 props. $9100.00 to replace.



I want to get the boat to top rpm of 3350 again. The boat has always felt under powered to me. A friend has an East Bay (12 inches longer and 10000 lbs less w/o a bridge and the same maker put the same engines in his boat but they were rated at 550 hp. (140 hp more than my boat) and his performs much better.

I can get Cummins to rerate these engines to 550 hp for $10000.00 ( includes new water pumps at $1000.00 each).



To me it's a no brainer to rerate the engines ( I know it will use more fuel and that's not a factor). The boat now cruises at 2800 and 17.8 knts but I want it slightly higher at 3000 rpm and 3350 WOT.



What am I missing? What say you?

TIA

Ok, everyone back up. Before you even think about repowering or changing props, you need to find out why you've lost the RPM and speed. Don't dare start changing things until.

Either engines not performing right or props damaged in some way or bottom condition or you've added that much weight, or something else about the boat.

Take it back to the condition when you first tested it. That's an easy way to see if it's all the weight. A pain but will prove it one way or another.

Check the props and the bottom.

If none of those things are revealing, then have someone check the engines. Do you have the same problem on both engines?

Evaluate anything else that has or could have changed. Once you get back to the original numbers, then you can talk about possible changes. Losing 300 RPM and 2.7 knots is major to me. In fact, to me the most likely issue is the engines need adjusting or have a problem.

I've purchased a lot of boats, loaded them, and never had a fall off like that. In fact, most every boat i've ever purchased from day one loaded to 200 hours later loaded has actually gained a bit. There's something wrong and you need to figure it out before starting down other paths. If the yard you're dealing with can't, then need to find someone who can.

 

[FF]

GlowShift | Pyrometer Exhaust Gas Temperature Gauges

These are not OK on a fly bridge in the rain but, at a little over a 1/10 of a boat buck are great indoors..

The gauge hookup is with ANY wire , not a specific proper length pyrometer probe.

Knowing how hard the engine is working will help in problem solving.

 

[smitty477]

BanB is right - assumptions were made on the original post that may be a mistake. Things that affect engine load and speeds that may have changed and need to be clarified included in his list and here are:
- Bottom cleanliness
- Props clean
- Pods clean
- Prop condition


Is this your engine listed here?....
https://cumminsengines.com/showcase-item.aspx?id=69&title=QSB6.7%2FQSB7+for+Marine#ratings


If yes you can click on the engine curve specs in the far right column and get your fuel burn and EGT specs for comparison. If this s your engine you will not right away that these engines are rated for 3,375 at WOT and at full power draw 25.4 gph each. The EGT rating guide is at 850 post turbine and you can compare that to your readings if you have it.


It is really not the best practice to continue with wild ass guessing (WAG on my part) but you will need to determine some more data points in your hunt for the problems to best reach a solution -so here are a few other guess's/thoughts to play with as you look...


FWIW - if your boat really took on 20% or more weight from baseline where you were at 3350 to begin with (less than full rpm on light load) I can see the possibility of losing that much rpm with such a large gain.
Really need to prop for a fully loaded boat on a hot and humid day and not for a super light boat in best conditions.
Do you have smoke or soot? Do you see an increased fuel consumption now at the same rpm vs at baseline? Any other symptoms?


Now your posted fuel use is at 42 gph with genset running at a max rpm well below rated WOT. Given that you will require more fuel to reach full rated power and that you need rpm to burn that fuel you are likely 'missing' at least 175 hp in the current configuration (25.4 x 2 +2 - 42 = 11 gph / /11 ghp is typically 200 hp in best conditions)


FWIW - I have seen dozens of boats overpropped from the factory , even worse than this.
And there are many times when you take pitch out of a prop but the overall max speed will increase - not as if that was the goal but it is a result of depitching a heavily overpropped boat.

 

[bayview]

Ok, everyone back up. Before you even think about repowering or changing props, you need to find out why you've lost the RPM and speed. Don't dare start changing things until.

Either engines not performing right or props damaged in some way or bottom condition or you've added that much weight, or something else about the boat.

Take it back to the condition when you first tested it. That's an easy way to see if it's all the weight. A pain but will prove it one way or another.

Check the props and the bottom.

If none of those things are revealing, then have someone check the engines. Do you have the same problem on both engines?

Evaluate anything else that has or could have changed. Once you get back to the original numbers, then you can talk about possible changes. Losing 300 RPM and 2.7 knots is major to me. In fact, to me the most likely issue is the engines need adjusting or have a problem.

I've purchased a lot of boats, loaded them, and never had a fall off like that. In fact, most every boat i've ever purchased from day one loaded to 200 hours later loaded has actually gained a bit. There's something wrong and you need to figure it out before starting down other paths. If the yard you're dealing with can't, then need to find someone who can.

This is the right answer! Are both engines hitting the same RPM?? So many missing data points>

 

[smitty477]

"Are both engines hitting the same RPM?? So many missing data points"


Yes - there are a lot of data points missing - his posts indicate 98% boost at 3050 rpm currently. Confirmation of data both on initial baseline and current condition would help a lot. Fuel use, EGT, boost with associated rpm by engine side (unless exactly the same) across a few rom ranges would help a bunch if he has it.
Confirming that the other possible rpm loss 'suspects' have been eliminated (props, bottom etc) would also help a lot.