optimal cruising

[JP Lazo]

For reference: 2008 SeaRay Sundancer 40, QSB 5.9 425HO

New to this boat coming from gassers.

How do you determine the optimal cruising RPM for a diesel? I was told by the seller that 2400RPM was the appropriate cruising speed. Other forums stated 80 or 85% load was the proper mechanism to determine cruising speed. What temp is too high?

Ive searched the inter webs, various forums and manuals and I can’t find the answers. I desperately want to operate my vessels as efficiently as possible.

Topic

[JP Lazo]

FWF

I don’t disagree at all. Ill take another look.

[Steve Lewis]

So one last idea about FWF. Before you watch the video in the below Tony’s Tip, think of this. With your dockside fresh water you are introducing fresh water into the existing water flow NOT necessarily replacing all of it.

Let that sink in a bit.

By using the Sea Strainer fitting(while leaving the thru hull fitting open) to introduce fresh water from the hose into the stream, you replace some of the salt/bracksih water with fresh water. That dilutes the salinity of the water that ends up sitting in your raw water side. Any lack of water pressure that “Might be an issue” is made up by the water coming in through the thru hull fitting. Watch for that and listen for his explanation of that closely. 4 minutes and 26 seconds or so along in the video.

https://www.sbmar.com/articles/the-benefits-of-fresh-water-flushing-your-marine-diesel-engine/

Ultimately the decision is yours. I personally would go for the flush as I can always get the engine stats on the Smartcraft display. Just one guy’s opinion 🙂

[JP Lazo]

flush

Absolutely.

I have looked into it. Read all of Tonys articles. Spoken to quite a few members on CSR.com as well. Between my mechanic and the Cummins mechanic that did the survey they both said it wasn’t worth it. The other issue was water at my dock. If I don’t have the proper pressure then I could cause damage so with all that I have budgeted to have the coolers pulled in 3 years and cleaned instead. Im not sure what to do.

[Steve Lewis]

This post should do ya. Based on your pictures I think you have the correct Smartcrap head unit and a newer GPS. Hopefully you have a NMEA2000 network for connecting all of your stuff. Corey Schmidt is the SBMAR guy that can help if you run into issues.

https://www.sbmar.com/community/topic/nmea-2000-gateway-to-diesel-view/

As for the engine temps, if you think about what you did it should all make sense. You made it easier for the engines to turn the props. They are not working as hard. They are not burning as much fuel. Therefore there is less system heat generated at each RPM. You are also turning more RPM so there is more cooling water running through the system so all temps should be good or even better than before. Like Exhaust Gas Temps.

Your QSB’s should be happier all the way around. New bottom paint will help, the only question is how much will it help? Not sure exactly how much as your current bottom paint condition is not known to us so we can only guess. A clean smooth bottom will always help your boat move through the water.

Might I be so bold as to suggest a fresh water flush system first instead of the Gateway? You could plumb it so that a dockside hose could be connected to a fitting that is easily accessed under that rectangular piece of the cockpit floor on the port side that comes out without having to lift the engine hatch.

[JP Lazo]

Scans

What I find interesting, as you mentioned before, is the temps remained the same at 2600rpm with the new props as they did at 2400rpm with the old. That was one of my concerns and the additional speed is fantastic.

Attached are the before/after scans. Let me know if can’t read them. I’m curious if new bottom paint in a year will make much of a difference.

On another note do you know who I can talk to about connecting my engine data to my chart plotters? Mercury said to call a local company who then said to call Cummins. Thoughts?

[Steve Lewis]

Victory is in the eye of the beholder. Still a bit over the curve but a whole lot better than being 3gph over per engine. If it were my boat I would run as is for a season or so and see how things go. Cruise a couple of trips see how things settle in.

How much pitch did you end up taking out? that would be nice to know so that we can clearly plot the before situation, indicate the prop pitch tuning and then re-plot after the tuning. We have had a couple of members provide that info and I am tweaking my understanding of prop tuning a bit. Its equal parts art and science.

Cheers and keep us posted.

[JP Lazo]

props

The props were re-installed yesterday and I just finished running the boat. Finally make WOT 3000. Fuel numbers at 2400-2600 are closer to factory but not exact. 2600 RPM yields 25+ on the speed, 79-80 on the load and 15.3/side on the fuel. Given the additional drag of a bimini top, isinglass and not perfect bottom can this be considered a victory?

[JP Lazo]

Props

First off, screw that Fleming. The 48 is my all times favorite. We seriously considered an ‘06 48 but it just wouldn’t fit in our slip. I even went as far as to inflate my paddle board and physically measure all the different depths in my slip to make sure. Between that and the 3” per side from the pilings in windy Stuart it just wouldn’t work. Perhaps some day but the 48 is a wonderful boat. The key to that 48(for us) is the open space in the back. The seating, swivel helm chairs etc. it’s a great boat to cruise and entertain outside.

Aside from my dream boat I discussed in detail our conversation and the data you requested. They also asked for specific data on their own. We went over the engine data sheet and fuel curve numbers. The prop shop is familiar with my engines and the fact the manufacturer over props these boats.

We’ll see what happens but they made me feel very comfortable because they seemed to understand what I was after.

[Steve Lewis]

When you dropped off the props did you have a conversation with them about what you were doing or did you just ask to have 1.5 inches or so taken out of your prop pitch? I am curious about the whole interaction and then I will be very interested in the outcome of the modifications. My wife and I went to Annapolis last weekend to look at a few Fleming55’s. No Joy! Nothing that was in our price range fit the bill so we either wait a few years or look at other options.

I have had my eye on a 48 Sundancer with QSC540’s. We will see what comes on the market. I have been patient but it seems like this bigger boat buying process is lasting a long time.

It’s fun to dream big……….

let us know.

[JP Lazo]

Fuel

Thanks Steve.

[Steve Lewis]

I have gas engines so I use Startron gas additive but Tony says the following:

Fuel additives

[JP Lazo]

Props

Props went into the prop shop today Steve. Anxious to see the results. While, I have you do you recommend any diesel fuel additives and, if so, how often?

[Steve Lewis]

JP,

Thank you for the extra data and let’s unpack what I am thinking. We have established that you are ~200RPM overprop’d. Meaning you are burning fuel at the 2600RPM rate at 2400RPM, 2400RPM rate at 2200RPm etc etc etc…. So my recommendation, and please take it as such as I am not a Naval Architect or Propeller specialist. I am just a guy that has learned a few things by reading different performance related posts on this site. So now that I have given you a healthy caveat/disclaimer……..

I think about 1.5 inches of pitch removal is in order. I plotted your data as seen in the pictures into a chart and see the trend of fuel burn. You should probably take your data, nicely entered into a spreadsheet, and this recommendation to a qualified prop shop and discuss with them and see what they say and ultimately make a choice about how much pitch you want removed. 1.5 inches should give you the shift of the fuel burn by 200RPM.

Recall the info about running at Reduced Power which is 300RPM below the rated RPM or lower. So again you can run your engines below 2700RPM and cruise to your hearts content as long as the fuel burn is at or below the curve.

By removing the pitch you will experience a decrease in speed at each RPM in comparison to where you are now. But that is OK. You will still be able to cruise in the 21-23mph range fully loaded and run her up to full throttle every now and again as needed. Remember this is an HO rated engine so limit WOT to 1 out of every 8 hours.

You indicated that your trans ratio is 1.64:1 and something just does not come out right in my slipage calculations but that is pretty much a side show to the more important idea of reducing fuel burn at any given RPM to make your engine last a whole lot longer than it will if you do nothing.

So in summary, remove 1.5 inches which will reduce load. Then to maintain your current cruising speed you just run the engines at a slightly higher RPM. Things should be happier.

[JP Lazo]

Last set

Last pics

[JP Lazo]

South Run

Data

[JP Lazo]

More data

More data

[JP Lazo]

Data

Ok Steve. Got all the data for you this time. North and South run. Fully loaded boat as we would use it. Fresh bottom cleaning done Monday.

[pwrobert]

Calibration baselines

When making changes like prop diameter/pitch it is required to do all these comparisons with the same baselines. Accurately calibrated tachometers and a clean bottom. Get a Chinese hand held tach From EBay for $12.00 or so and put the marker tape on the front crankshaft pulley and compare your numbers.

Noticed you said the boat was ready to be hauled in a few months. Probably should do all these tests after the new bottom job. Or at least have someone come and do an in the water scrubbing of the running gear and bottom if you want to avoid two haul outs.

Here in south Florida performance data can change fairly quickly. Depending on how much use the boat gets – the slime itself with no other growth can be a measurable factor.

Even a tiny bit of growth on the running gear from a little paint chip can throw off your numbers. Those critters will find any open metal and grow ASAP.

I get the best longevity out of Propspeed on the gear.

Fuel tank gauges for boats are notoriously inaccurate mostly due to sender issues. Consider looking into alternatives such as Maretron’s TLM100 which I have and recommend.

[JP Lazo]

Crap. Sorry Steve. We didn’t mark that info down. I’ll get on the boat this week and do it all again. Sorry about that.

[Steve Lewis]

JP,

I do need vessel speed at RPM to help make this work.

what speed are you running at 2200?
what speed are you running at 2400?
what speed are you running at 2600?

you indicated a cruising speed of 21-22mph but did not indicate at what RPM. I need that to make this work. Based on my spreadsheet I am guessing your 21-22mph is around 2200 rpm or so. with a gear ratio of 1.64 and prop pitch of 24inches that is the only this makes sense. Otherwise I need to look at the prop reports again……..

[Author]

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