Prop load vs fuel burn vs overloaded.

[Joe Monaco]

I was rereading props move boats engines just turn them.

I understand it’s possible to load an engine rated at say 500 HP @ 2800 WOT rpm to produce 500 HP at say 2000 WOT RPM (overpropped) and the fuel burn in both cases should be the same as published on the manufacturers charts, correct? There is damage being done even if the engine is maintaining a proper temperature, I get that.

I’m trying to understand what actually is damaging the engine of an over propped boat if it’s running at less than WOT RPM but off (higher) on the fuel burn curve than that RPM is rated for. Is it a higher EGT than expected at that lower RPM that is causing the damage? Even if the EGT is not beyond what’s expected for that engine at that HP draw? Why is the same motor able to handle a given HP at a given RPM but it is stressed if that rpm is dropped but HP demanded stays the same?

It all makes sense intuitively but what really is going on that’s damaging the engine that doesn’t happen when it’s propped correctly? Any additional insights appreciated.

Topic

[Joe Monaco]

Anybody ever here of this company and it’s fuel flow options?

https://shop.interactio.co/collections/frontpage/products/4-optio-fuel-wireless-flow-sensors-twin-diesel-bundle

[Joe Monaco]

Thanks to all. Great info to review and consider. Makes more sense to me now.

So the “work done” at a specific RPM is the fuel being used at that RPM. If your burning more fuel than the manufacturer specifies @that RPM the engine is being damaged, because it’s working harder/stressed more than it’s designed to be at that RPM.

Looks like you need fuel flow to know for sure what’s happening. Without fuel flow (none on my boat) next best thing is ensuring you’re propped correctly to reach rated WOT RPM+ at your heaviest load. That should keep you safe.

To anyone with with fuel flow & the correct props, in the “real world” do you see the fuel burn #s published by that manufacturer at your cruise RPMs? Do you see the fuel burn numbers changing when going through swells without any RPM change?

Might be time to look into fuel flow gauges for my boat…

[Steve Lewis]

Engine Design and Ratings

Joe,

You answered your own question by asking it. Heat is the enemy. Fuel burn causes the heat so the rate at which you burn fuel AND the RPM at which you burn the fuel are very important. Diesel engines are designed and rated very specifically. IF you operate outside of that specification then the life of your engine is shortened. 2 very specific cases of life shortening are well documented on this site.

The 480CE and the QSM11

Both engines had Design “issues” and if you burned too much fuel at too low of an RPM then the lives of those particular engines got shortened substantially. Valve Seats, Valves, Exhaust Manifolds and Turbos were the engine parts that suffered.

QSM11 – The answers to fuel burn and flow are in Item#4
https://www.sbmar.com/articles/the-cummins-qsm11-marine-engine-story/

480CE- READ the 3rd paragraph that is below the numbers chart.
https://www.sbmar.com/articles/the-cummins-6cta-8-3-marine-engine-story/

480CE – Read the section about Dropped Valves. Title says it all
https://www.sbmar.com/articles/propping-cummins-6cta-8-3-480ce-vs-430450-diamond-engine/

So the ultimate goal for getting longer life out of your Diesel engine is to have the correct propellers on the boat so that the engine is asked to burn equal or less amounts of fuel than what is published on the curve for that particular engine. That practically means getting your boat as heavy as possible, warm up the engines and go for a cruise. Chart the fuel burn from idle to WOT at 200 rpm intervals, preferably the same intervals as the manufacturer used for easy comparison.

You then pay VERY CLOSE attention to the cruise RPM range as that is where most people spend most of their time from a recreational boating perspective. Compare the GPH observed on your boat to the GPH numbers published by the manufacturer. If you have done the work properly, your numbers will be less than the manufacturer.

I have my eye on that new Beneteau Swift 47. I can not afford it new but could afford it on the used market. Based exclusively on the boattest.com performance review, Beneteau actually propped that boat correctly from the factory. Twin QSB 6.7 425’s and they are burning ~26GPH total @2600, so 13gph per engine at 2600 RPM. That is a 3000 RPM rated engine and the fine print says that the maximum recommended reduce power operation is 300 off the top so 2700 rpm. Running at 2600 rpm and burning less than 28gph means that when I go looking for one on the used market, I might find one that has engines that will last. That all assumes that the nut behind the wheel is not a nut and the owners have performed the maintenance described on this site.

Great Loop is my retirement goal and that Swift looks like a pretty good option. Here is to hoping I find one a few years from now with engines that are in good shape so that they last longer than I do.

[Tony Athens]

LOOK UP “BMEP”……….. If you read what it means and understand what it is saying, you’ll then understand why more RPM to make the same power is way easier on the components inside the engine.. All engines are designed around a maximum BMEP as that what they need to be, and not exceed, if you want to get what you paid for as to engine life.

Tony

[Brian Katz]

Engine loading

I’m probably in the same boat as you experience wise. No pun intended.

What I pickup from the article is that if you are making (x)hp regardless of rpm you are going to need to burn a certain amount of fuel to do that.

When releasing that energy you create heat which causes EGT to rise but why?

It comes down to force x distance x time. Your asking your engine to process fuel to create a constant force aka (HP) over a shorter distance(less rpm) in the same amount of time. Stick with me.

Your engine needs to work harder to produce the power while traveling a shorter distance. The top end heats up causing excessive wear on the valve train, turbos, aftercooler, pistons etc… the force on the connecting rods and bottom end is also greater producing more wear over less distance.

Look at 500/2800 and 500/2000. At 2800 your engine is carrying much less “weight” per revolution.

I don’t talk about the prop because that’s constant in the equation.

I don’t think I’m too far off but I’ve been wrong before.

[Author]

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