Prop Demand Data vs. Actual Fuel Data - LONG

[Maybe A Dancer]

Several issues here, so I'm trying to learn all I can on this topic.

Running a 450DA with 3126TA Cats

Installed the FLOSCAN digital tach/flowmeter system from my 10 Meter Trojan International's Yanmar diesels to the Sundancer.

Calibrated and verified my fuel burn this summer on the 660 mile Florida Loop, and am very happy with the results, especially seeing the live data when running half fuel and water in a calm sea.

I've reduced the factory props down twice to reach 2800WOT with full fuel, water and 30% blackwater, and I am now at the max reduction for this set of propellers.

This boat runs a little heavy with fwd & rear bow thrusters, 7 batteries, cablemaster and vinyl plank flooring throughout.

I've worked with General Propeller on my last diesel boat, where on their recommendation, I upgraded to VEEM propellers.

General Propeller has recommended that I upgrade these props to 5 blade VEEM propellers, claiming there will be an additional1-2 knots and far better efficiency.

In ordinary situations given the cost, this upgrade would not make sense.

However this is (yeah, you've heard this before) my last boat. I'm 64 and the express layout combined with the remote I installed for both thrusters and the cablemaster, I should be capable of continuing to single handle her until reaching the point where my health fails me.

She's no dock queen she's been to Key West, Sarasota and just completed the Florida Loop through Marathon. I joined AGLCA this year and plan on extending these adventures to include the Bahamas and extended runs on both coasts. No immediate plans to do the loop right now, but that may be a future target depending on my situation. This boat is going to be run, go places and will be burning some fuel. Increasing cruise speed and efficiencies could even expand my between port distances.

I have no spare props, and I'd feel better carrying a set for a future Bahamas run.

Boat comes out of the water for a bottom job soon, and I'll have an accurate weight on her.

Currently running 22 x 22.5" props.

Not entirely "sold" on the VEEM 5 blade prop idea. I have a pair of same shaft 1.75" diameter Michigan wheels from my last boat, 20x20 Nibral which might be adequate spares in a pinch if I damaged something in the Bahamas.

So, where is this post going?

I've confirmed my FLOSCAN fuel burn data to be accurate within 4.5%

The 3126 CATS are rated to safely run 2400 RPM continuously. Surprised that my most efficient cruise RPM's (50% fuel and water) are 2250-2400 @ 24-26 SMPH burning .89 - .94 SMPG. Not too shabby for a 45' express likely weighing in at well over 30,000 lbs.

These efficiencies drop significantly at fully loaded fuel & water, especially in rougher seas. .79 - .82 SMPG at 20-21 SMPH

The Floscans give me the ability to have real time (now verified) instant data, combined and or separately and for both motors at any speed/rpm/load.

The data demonstrate that my actual fuel burn is significantly higher than the CAT prop curve data, which according to all known, experienced, sources, is extremely accurate.

Here's what I have:

			CAT GPH	ACTUAL	COMBINED	%
RPM	SMPH	SMPG	COMBINED	GPH	DIFF	DIFF
1200​	9.6​	1.31​	4.4​	7.3​	2.90​	66%​
1400​	10.5​	1.02​	6.4​	10.3​	3.90​	61%​
1600​	11.0​	0.73​	9​	15.1​	6.10​	68%​
1800​	16.3​	0.83​	12.4​	19.8​	7.40​	60%​
2000​	19.6​	0.90​	16.2​	21.8​	5.60​	35%​
2200​	23.6​	0.94​	20.8​	25.2​	4.40​	21%​
2400​	26.6​	0.90​	26.8​	29.7​	2.90​	11%​

Planing speed is 1800 RPM.

It's interesting to see the net difference between the CAT published Prop Demand GPH vs. actual data.

Remember, I am making 2800 WOT fully loaded.

Most owners of this exact hull/engine combination report best efficiencies at 1900-2200 RPM, with a sweet spot at 2150 RPM.

Seeking comments regarding the significant difference between the CAT table and my own data.

Also, given that I've taken as much off current props as possible, should I consider making those the "spares" and getting the VEEM 5 blades as an option, once I have an accurate weight for this vessel?

BEST !

RWS

 

[techmitch]

I have no experience what so ever but is there any reason why you skipped over 4 blades? The Acme 4 blades have always gotten good review's here on CSR.

 

[Maybe A Dancer]

I have no experience what so ever but is there any reason why you skipped over 4 blades? The Acme 4 blades have always gotten good review's here on CSR.

No reason.

Just going on the knowledge and experience of General Propeller.

Perhaps the difference in prop curve is the hull style/weight/design.

Despite making 2800 WOT, perhaps the governor is sending more fuel at a given RPM due to more load.

IDK

 

[Thornton69]

How did you confirm your floscan results? Fuel top up after testing?
I did some testing/set up several years ago on stationary generators in a remote fishing lodge on the AK border. The customer wanted to be able to monitor fuel consumption remotely (Calgary, AB).
I found the ecu (calculated) readings to be surprisingly close to the floscan results after calibration. They were Deere engines though. We had to use dual meters and measured both the fuel supply and return lines.

 

[Maybe A Dancer]

How did you confirm your floscan results? Fuel top up after testing?
I did some testing/set up several years ago on stationary generators in a remote fishing lodge on the AK border. The customer wanted to be able to monitor fuel consumption remotely (Calgary, AB).
I found the ecu (calculated) readings to be surprisingly close to the floscan results after calibration. They were Deere engines though. We had to use dual meters and measured both the fuel supply and return lines.

Calibrated fuel after my first fuel stop.

Ran the rest of the loop.

Spent a weekend on the hook, running genset exclusively on one tank, therefore able to confirm genset fuel burn @ .42 gph running all HVAC, water tank and appliances.

After 2 fuel stops confidence in Floscan totals is very good, within 4%.

Can't complain about current NMPG or performance, maybe I'm chasing a unicorn, however need to know about the significant discrepancy between CAT data and my own.

Why is she so much more efficient at 2300-2400 than at the heretofore consensus magic number on CSR of 2150 ?

BEST !

RWS

 

[Thornton69]

I guess where I was going with my last comment was, do you monitor fuel returning to the tank with the floscan as well? Only recording the supply can be misleading unless you do your fuel top ups after running each recorded rpm bracket. Was just a thought.

 

[Maybe A Dancer]

The Floscans have a flow and a return sensor/meter for both engines.

The system does the "math" and Once calibrated they are accurate

 

[Strecker25]

propping fascinates me, so I'll be following.

Not really an answer but a little more data for thought - not knowing your exact particulars (water length, weight, etc.) and assuming you're 420hp with 1.66 transmissions, the BD calculator kicks out the prop demand HP at a particular RPM. The speed is pretty close to what you reported in this table, and you can see the data backing up your theory that the engine is being loaded more than what CAT may have 'assumed' at a particular RPM. If the prop demand is pushing the engine to work a bit harder at that RPM, my understanding is the engine will do its best to deliver. That's the whole theory with overpropped boats being so bad on diesel engines, they're working past the rated load at cruise RPM, unbeknownst to the owner. I would think CAT's published consumption numbers assume a linear demand curve and were taken on a dyno?

I'm not sure the widely accepted 2150 for our 3126's is necessarily based on best efficiency but rather wear and tear on the engine. if the chart below is somewhat accurate, you're around 70% load at 2400. Nothing wrong with that, it's how CAT designed the pleasure rating on these engines, but logic would say running ~2150-2200 and closer to 50% load should result in longer engine life.

Again, just more data to confuse the conversation

 

[SeaNile]

5 blade props....now that would be a fun experiment. I'm most familiar with the 3 vs 4 blade debate and always went with 4 blade.

 

[Maybe A Dancer]

I've been told by a well respected source on another forum that the published prop demand curve data is for DISPLACEMENT VESSELS.

Now, if that holds true, it does explain a great deal, especially the data differences when climbing on plane.

The displacement vessel need not spend the energy to create the lift required in a planing vessel.

It does not explain the discrepancy between the accuracy consensus on this FORUM vs. the verified numbers I'm seeing.

This is the first question to be answered.

BEST !

RWS