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]

Transmissions: ZF V-drive, Ratio 1.64:1, SN: (p) 19417S (s) 19422S

Does that help?

[JP Lazo]

Let me check on the mode and gear ratio.

[Steve Lewis]

At what engine RPM are you doing the 21-22mph? 2300? 2400? If so no worries. You will move that boat speed up the RPM range to 2500 or 2600 RPM. That is well within the 2700RPM reduced power objective of Cummins with the ability to run at rated RPM for 1 hour out of 8. So if you need to get up and move 25mph to get to a marina or get ahead of some weather, run her up to rated power and know that you will be ok.

Which model and gear ratio ZF do you have? I can do a little math on my end and give you and idea of where you might run as an estimate. We have the same hull and diameter props so slippage should be similar. I make 21.5mph at 3500RPM. That calculates out to 75%. My boat slips at 25%. I have a trans ratio of 2.5:1 and 21.5 inches of pitch. You do the math and I should be doing 28.5mph but because the prop “slips” in the water we only get 21.5.

Now a couple of caveats. Make sure that you properly service the seawater side of the cooling circuit as SBMAR recommends. For you that means every 2-3 years you service the aftercoolers, Heat Exchangers and then keep a close eye on the fuel and gear oil coolers. The stock sherwood water pumps will go bad eventually so changing over to the SMX designed water pumps is a good idea. Keep one or both of the sherwoods in your spare parts cabinet. For me that is the cabinet that is next to the companionway door that goes down into the cabin. That cabinet is full of spares to the point that getting rags out of there is fun……

If you want to constantly run the engine at those higher RPM’s(2600-2700) you just need to make sure that the cooling circuit is operating and maintained at the optimum. Once everything is dialed in you can run all day long or until the Family says “we have had enough for one day” 🙂

Cheers!

[JP Lazo]

Thanks Steve. With a fully loaded boat heading up the inter coastal we were cruising at between 21-22mph. My only concern is taking at much pitch out and we don’t even achieve that speed anymore. Thoughts?

[Steve Lewis]

Conclusions Confirmed

So your data proves out what we have been discussing. The boat is over prop’d. Based on my read of things it looks like 2 inches of pitch need to be removed.

@2300RPM you are burning fuel at the 2600rpm rate
@2400RPM you are burning fuel around the 2650RPM rate
@2600RPM you are burning fuel at the 2800++ RPM rate

SO I conclude that 2 inches removed will get you under the curve. The other piece of info that I would like to know is vessel speed for those pictures. How fast is the boat moving? When you remove pitch the boat will slow down at any given RPM when compared to your current setup. You can run your engines at higher RPM’s to get your current cruise speed back and know that you are going to be fine since you are burning fuel at rates that are under the Cummins Prop Demand Curve.

Remove the pitch and run at RPM’s under 2700RPM and you will be fine.

Hope that helps.

[JP Lazo]

Next run

[JP Lazo]

Prop scan reports

[JP Lazo]

Here you go Steve. One run north, one south.

[JP Lazo]

Ok. Thanks Steve. Ill give it a read. Hopefully this weekend I can run the test to your specs and then go from there. I’m due for a haul out in the next few months(unless another hurricane comes first) and then I will have plenty of time to do it then.

[Steve Lewis]

You are over propped and if you are only able to make 2900ish Then taking pitch out of the props to move the burn you get at 2400 up to 2600 will also allow your engines to meet and exceed your rated RPM of 3000. Everything moves up the RPM range. The other idea is yes your boat will slow down a bit at 2400rpm after you take out the pitch. BUT since we have established that you can run your engines in a constant cruise all the way up to 2700rpm when the props are correct then you will be fine. When the pitch is removed from the props it will reduce the load at each and every RPM interval thus reducing the fuel burn at each interval. You will most likely need to run your engines at a slightly higher RPM to maintain your current cruise speed but since the pitch has been removed that is actually good for the engines. Slightly higher RPM and lighter load means the EGT’s will be less, your engine coolant temps might go down a bit, more oil and coolant flowing etc…..

Think of things like this. It takes a certain amount of thrust to move your boat at any given speed. The props have pitch which in theory is how many inches forward the prop will move through the water with each rotation. There is slipage but ignore that for this discussion. The spinning of the propeller is what generates the thrust to move the boat and the load to the engine. by decreasing the pitch you decrease the thrust and the load presented to the engine at each RPM interval. Getting the load with in spec is the goal. The way you determine that is by looking at the fuel burn at those Cruise RPMs. If you are burning too much then remove pitch. Ignore loading% as reported by your electronics. Its a distraction. Focus on the fuel burn numbers. That is the purest indication.

Tony has been doing this for many decades and he has found that “Certified Cummins Tech” does not directly translate into understanding the proper way to do things in the real world. They understand how they have been trained. Tony has amassed decades of real world experience and some of that experience has lead him to adopt processes that are more stringent than what Cummins recommends.

This whole prop loading and getting burn under the curve in the cruise RPM is a case in point plus the Aftercooler maintenance in the real world. There are lots of folks that have adopted these approaches, removed pitch, altered their maint intervals to match Tony’s recommendations which are more often and better proved than what is in the Cummins Manuals.

Hang in there and stick with us and we will give you good guidance. You will end up with a smooth and great running boat that will last a LONG time.

The attached is still a draft but give it a read. There are a few examples in there. Hopefully it helps and does not confuse.

[JP Lazo]

So I have to admit this whole situation has me a bit frustrated.

I’m going to attach some pics in order but here’s the background. The boat did not get fueled this morning but we still went out just to test if the tabs would significantly alter the fuel burn. It did not. At most it was 1 gal.

WOT with half tanks and full everything else was still 2890/2920.
2400/2390 14.5/14.4
2510/2500 15.9/15.3
2600/2610 17.3/16.8

Fully loaded boat on the way to vacation was still 2400/2400 14.8/14.6

I will do the full test tomorrow weather permitting.

I have to say I am a bit confused though. Will changing the pitch of the props alter the WOT? Will I simply achieve the lower fuel burn but still have to use higher RPM to achieve the same current speed? Could this be simply the way my boat runs?
I did the survey in late may so I have all that documentation is you need and it was completed by a certified Cummins mechanic.

[Steve Lewis]

I would fully retract those tabs and run without them. You might find that you pick up speed AND lower your fuel burn. My wife has similar visibility issues. to solve that we bring one of her exercise steps so that she can stand on that and see. I am 6′ 4″ so I see over everything………

On our trip to Mackinac we were running long days. 6-9 hours with refueling stops on the really long days. Northern Michigan has good Marinas every 50-90 miles along Lake Huron. I had Smartcraft barking at me 1 day that we were going to be out of fuel on the strbd side. Has me paranoid.

I really want a boat with Diesels so that I get longer range and less expensive fuel. Love the 40 but the gas engines are less than ideal.

[JP Lazo]

They do report and I’m tracking the numbers. My initial trip was 6 hours. Since then the longest I’ve done is 2.5 hours each way this last weekend so I saw the burn vessel view indicated and I’m getting the boat fueled today(hopefully) to verify those numbers. So that my wife can see when she’s driving we run with the tabs full down and my mechanic seems to think that could be the culprit on the fuel numbers. When I had the props done in July they were done to Searay specs based on the hull number he said.

[Steve Lewis]

A Question for you

JP do your port and starboard fuel tank senders report ~accurate fuel levels in the tanks? My Strbd side is very inaccurate in the low fuel range and the high fuel range. I am getting used to it and have not run out yet. I trust the Port side and just make sure that I fill the same amount of fuel on each side and ignore the strbd gauge……..

[Steve Lewis]

In my experience with this hull, the trim tabs will only push the bow down and cause it to plow and slow down. I do not use tabs at all when running our boat. All boats are different, even versions of the same model will have variations so I would try yours and see what happens. My intuition tells me you will experience the same.

[JP Lazo]

One more question Steve. How much affect will the trim tabs have on those numbers?

[JP Lazo]

All sounds good Steve.

We are in salt and all the coolers were serviced. I have pictures of the interiors(before and after). My mechanic always takes pictures. The first picture was a fully loaded boat heading on a 4 day vacation i.e. water, fuel, food, toys, family. I will gas up this weekend and do the requested test run and take pictures of the data points your requested.

JP.

[Steve Lewis]

Added Context

You are definitely over propped. Even with the heat and humidity factored in you should look at having pitch removed. The goal is to get your fuel burn at or below the curve in the report at all cruising RPMs. So this is 1 data point. we do kinda want a chart of your fuel burn from 2200 – 2800 so we can really dig in. Also when looking at the fuel burn the idea is to be fully loaded with fuel, water, fill your waste tank with water take your whole family and a ton of stuff. Loaded for Bear as Tony says. Then see what your fuel burn is. Chart it and then post it.

generally speaking you will increase your RPM by ~150rpm by removing 1 inch of pitch. So you should be seeing your 14.4/8 gph per engine at 2600 instead of 2400. That equates to about 1.5 inches but factor in your load and 2 inches might be called for in the end.

It all depends on how you load and run your boat. Case in point. When we are just knocking around on Lake Erie with a partial load our mercs burn about 33-34gph total. When fully loaded with fuel and stuff, like we were earlier this year cruising up to Mackinac Island we were burning 36+ gph total. 3500 RPM and 21.5mph pretty consistently. Fuel burn changes with load.

Boat load matters. Are you in Salt water? Did your aftercoolers and other coolers get serviced at the 1000 hour point? How are your raw water pumps? Post some pictures of the engine compartment. We also want to see your exhaust setup. Sea Ray used the Wet Elbows on a lot of dancers and if you are in salt and those are oriented the wrong way, well “Doomed to Fail” is how Tony describes those. There is a Tony’s Tip on that one as well.

I am working with Tony on an article around Propping a boat. It is in draft form right now so once we get if finished it will give you step by step detail on how to take the data points and then how to interpret them. What I have written above is a summary and will get you started.

3 users thanked author for this post.

Kevin Wells, fwatt77, Kevin Lee

[JP Lazo]

Thanks a ton Steve.

When I run at 2350 to 2400 my fuel burn is closer to 28GPH(see images below). The first image was taken on an extremely warm day with ambient temps in the lower 90s with a heat index of 100. The second image was taken a few weeks prior with temps in the high 80’s with a heat index close to 90. Per your response I am above the rated GPH for each RPM setting(what does this mean). Reading some of Tony’s Tips his articles indicate that an engine will see higher numbers that what the original charts indicate based on marine age and other factors. I recently had the props rebalanced and the 1000 service completed in July after purchasing the boat with 450 hours. Thoughts?

[Steve Lewis]

JP,

Welcome to the forum and you have come to the right place. Here is what you need to know. Ignore the 80% load stuff, what the seller told you might be right but here is the technical answer that will give you a nice range once you get a few things understood and taken care of.

First get familiar with the attached document. This is the performance report for the QSB 5.9 425. What you are interested in is the Rated RPM, Fuel Consumption Prop Curve and the little paragraph explaining the HO designation. Let me unpack all of that for you. Let’s do it in reverse.

The HO designation indicates that you can run your engine at rated RPM for 1 hour out of 8 but the rest of the time you need reduced power which is defined as running at or below 300RPM less than rated RPM. So that means 3000rpm – 300rpm = 2700rpm. So you can run your engines below 2700Rpm and be fine.

The caveat to the above is that your fuel burn must be AT OR BELOW the prop curve values in the Cruise RPM range. so starting at 2400rpm and then increasing in 100RPM intervals you compare your fuel burn as indicated by Smartcraft to the fuel burn in the chart. Remember Smartcraft gives you total fuel burn so divide by 2 to compare to the chart. If your fuel burn is more than the Cummins chart, which I can pretty much promise you it will be, then you need to remove pitch from your propellers.

This forum can help you determine how much pitch based on your numbers for your boat.

So in answer to your original question, you can run your boat from 2400 – 2600 all day long IF AND ONLY IF your fuel burn at those RPMs is below what cummins has published in the report.

2400RPM should be less than 23.2gph total
2500RPM should be less than 26.2gph total
2600RPM should be less than 29.2gph total

WOT RPM is important as you need to be able to meet and exceed rated RPM at WOT to make sure that things spin freely and within spec. Off plane running is not a concern since such a small portion of the rated power is being used. So run at 8kts all year long.

I have the same hull, 2006 vintage with gas engines. 9.2mph is what I have found is hull speed.

Let us know if you need some clarification on any of this.

[Author]

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