Speed issues?

[Tom Jerome]

Wanted to run this by the forum, I have a 1999 Pursuit express 3000 with Cummins 4bta 250hp in it. The possible issue I am having is getting the speeds that others get with this vessel. Props were redone to 19-22 or 19-23 cant recall, aftercoolers serviced (they looked decent), new injectors and valves adjust last season. New cutlass bearings and shaft seals. No smoke out the exhaust and fuel burn seems to be on par with factory test. I cruise at around 2400 rpms and only get around 20-21knots fully loaded and WOT is 3100rpms at 26-27knots (was on a day with 3ft seas at 7 sec). I am not sure what else to check and/or if there is an issue. It’s been like this ever since I got the vessel so I am not sure if its even an issue but seems like it could be. Nothing with how the engines run is apparent that could be the issue and the running gear has all been redone. 

Factory specs: 

RPM	MPH	KNOTS	GPH	MPG	NMPG	STAT. MILE	NM	DBA
600	7.7	6.7	1.8	4.28	3.72	839	730	79
1000	9.8	8.5	2.6	3.77	3.28	740	643	80
1250	10.6	9.2	7.4	1.43	1.24	280	243	85
1500	15.3	13.3	8.2	1.87	1.62	366	318	85
1750	19.4	16.8	11.2	1.73	1.5	339	295	86
2000	24.8	21.6	14.2	1.75	1.52	343	298	87
2250	29.2	25.4	18.4	1.59	1.38	311	271	88
2500	32.3	28.1	22.2	1.45	1.27	285	248	89
2750	34.8	30.3	22.8	1.53	1.33	299	260	92
3000	37.2	32.3	23.6	1.57	1.37	309	269	92

 

Topic

[Tom Jerome]

thanks

Appreciate it, going to get one of those and also paint the balancer flat black so there is no reflection. I do have 19in 4 blade props not sure the pitch, havent had a chance to dive under the boat. It’s either 23 or 22 I do know that. I will update on monday as we are gonna test it sat and going to go under and clean the boat and check it over.  

[Rob Schepis]

Laser Tach

Mine is referenced here:  https://www.sbmar.com/community/topic/calculating-load/

[Tom Jerome]

Decent tach gun

Can you guys recommened a decwnt tachometer gun for diesels or have to verify rpms? The one I have is way of ao either the harmonic balancer is too shiny or disfunctional. Reading were a bit all over. Also if i am under on the WOT by say 100rpms do i resuce or increase pitch? My suspiscion is that my tachs are 100rpms off.

 

 

[Tom Jerome]

thanks

I can hit 3100rpms and I do run heavy, typically in the ass end….6 people and 210gal of fuel and ice an coolers. I am reading through the articles and going to keep a better log to see if I can fine tune anything. I am going to laser the rpms this weekend to ensure everything is accurate. 

[Tony Athens]

The fuel curve numbers are in a certified test cell under very controlled conditions…. You don’t operate in that environment.. Consider them a guide  at this point as what the engineers would like to see as to engine loading to get the full designed life of the engine. As long as YOU can make a solid/accurate  3100-3150 at WOT loaded as you use the boat, then you have what you have.. IMO, if you cannot make that, adjust your pitch, clean the bottom/running gear or remove weigh or windage from the boat.            

[Tom Jerome]

found

found the data sheet! if I can get those fuel burn rates id be tickled pink

[Tom Jerome]

thanks

Thanks for the reply! I absolutely understand the each vessel is different and that factory specs are on a new boat and perfect conditions. I haven’t been able to locate a performance chart on the 4bta250, maybe someone here has one? Fuel burn I am measuring based on cruise speed and time and calculating out how much a burn in a trip. Roughly 18gal/hr at 2400rpms. 

Load is full tanks 210gal, full 20gal water, full fishing outfit and 6 people, Heavy load. Running gear and boat are in good shape. I need to start keeping a chart. 

My WOT is 3100 but need to check that with a Tach gun. I have digital gauge converters and they are close to my tachs but still not as accurate as a gun. My gears is zf:hsw630a1 ratio is A=1.56 and B=1.58. I am not sure what the difference between A and B is. 

Thanks for the attachment also. 

[Steve Lewis]

Correction

I mentioned 6BTA250.  Meant 4BTA250.  Sorry about that.

[Steve Lewis]

A few questions

Each boat is going to be different.  No getting around that.  some times “It is, what it is” will be “As good as it gets”

Those Tiara factory performance tests will be the best it can be.  New boat, smooth bottom and IDEAL LOAD.  Instead of posting factory specs, post your own performance chart.  Then compare that to Cummins performance chart for the 6BTA250.

When you say that fuel burn seems to be on par with factory spec….  How are you measuring fuel burn?  Which factory spec are referring too?  Tiara or Cummins.  Should be Cummins.

What are your load specs?  Fuel level?  water level? people? stuff?  Running gear nice and clean?

I have a Sea Ray 460 Sundancer with 6CTA 480CE’s.  I do not compare to other boat tests, I compare to the 450 diamond performance curve as that is what matters.  I have smartcraft electronics so I am “Good enough” on the accuracy and I test performance with conditions as close to being the same as I can get.  My bench mark is 3/4 fuel, 1/4 water and our full cruising stuff.  I keep a chart and compare often.  I also take my performance test numbers on clam days. 

Also, I know for a fact that my props are 23×25.  I have the Class 1 scans.  That matters.  My ZF280IV ratio is 1.77:1. I take that, enter into my performance spreadsheet and compute my prop slip.  I have reduced prop pitch by 2 inches over these last 2 seasons. I have improved slip and fuel burn at the same time.  I am sure that if I asked other 460 owners what they have it would be different.

Your WOT should be 105% of rated.  So your WOT should be in the 3150 range not 3000 range.  Also you must laser/photo tach to make sure the readings are accurate.  Lots to unpack.  Read the attached and focus on the Mechanical Diesel reference.

 

 

What transmissions do you have and what are the ratios?  Figure out the exact prop spec

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