5.9 QSB Fuel burn

That came across wrong

My sentence about posting an answer twice had to do with the website.  When I was typing that last answer I had to submit it 3 times…..  Nothing to do with frustration with you or anyone else.  Just good ole technology not behaving like I thought it should.  I was trying to paste a link to the thread around optimal-cruising and I bet I was doing it wrong.  The UI Control for doing that changed recently.  Not quite sure how it works now.

Let us know how the prop tweaks go for you.  I will be sure to post ours int he spring once we are floating.

“Is it possible that these motors at 380 hp are just too small for the weight of the boat?”

The answer in most likely NO…………….They just have to propped correctly so they can run properly at their full potential………………….. On the surface without seeing a proper scan of the props you have now,  you need to remove 2.5″ of pitch or some cup or some of each so you enaging hit the governor break poiy of approx 3070 RPM and reach it easy..  Until that happens, you are not respecting what Cummins did for you..Building a world class 5.9 liter electronic engine that, IMO,  has zero competion.. 

Tony

I had these engines in a 2005 40 Sundancer which had a similar weight and once I got it dialed in it was a great running boat. It did take some discussions with the prop shop to convince them to reduce the pitch. They kept
saying “that is not what Searay says should be on there”!

A little pitch off the props?

Those QSB380’s are 3000rpm engines. you should be able to reach the governed engine rpm of 3065 – 3080 so you are almost 300rpm short of that. Ouch! Tony would say “Grossly overpropped” or something to that effect.

The important info about those props are the diameter and pitch. I am thinking that you need to modify these props so much that you might very well be looking at buying new ones instead of being able to remove enough pitch to get things into specification.

Using some linear interpolation I come up with a burn of ~11.6gph @2500. You are showing 17.1 and 15.4. That is “about” where you state that you cruise. That kind of fuel burn should be at 2800 – 2900 on this engine. So you could be in the 3inch area which could mean new props……..

I would find a very reputable prop shop and take your performance chart numbers to them and have a serious conversation. Also take a serious look at those props again and find the dia and pitch. That should be stamped somewhere. mine had 23×27 stamped right on the hub.

Need a little more info……..

What is your transmission ratio, current prop diaxpitch and do you have vessel speeds for the 2200 – 2800 GPH listings? You mention vessel speed at 2450 but we could use more data points. which HP rating do you have? is 2800rpm what you get at WOT under load?

I am in the process of working this out for my vessel and I learned something “the hard way” so maybe we can save you the same mistake I made.

I plotted my rpm, gph and speed from 2000 – 2400 rpm in 100rpm increments and then compared it to the cummins curve for the 6CTA Diamond 450. Based on my plot I took 1 inch out to move the fuel burn from 2200 up to 2300. I did not account for prop slip and should have taken out 2 inches. That is the ratio of actual vessel speed/theoretical vessel speed at rpm. So that 1 inch did not accomplish my goal of moving the fuel burn. So at winter layup we are going to have another inch taken out.

original prop spec = 23×27 (confirmed with the below mentioned scans)
current prop spec = 23×26 (I have the prop scan printouts)
“cross my fingers and hope this works” prop spec = 23×25

The goal is to be burning ~27gph @2150rpm+ and get that 30gph burn as close to 2300 as possible(and isntall new heads but that’s a story for another time). Our prop shop is telling me that I can only modify these props a total of 2 inches. So I have 1 inch left to go. If we need to reduce pitch any more, then I am looking at new props.

Here is an interesting thread from last year of an owner with QSB425’s going through the process. Give it a read and see what you think….

https://www.sbmar.com/community/topic/optimal-cruising/