Cooling water pressures

Remember that the connection point of this test is before any branches that distribute the water to the engine or exhaust. While there is a small amount of head pressure loss due to the height I am holding the gauge, I considered that it would be insignificant. The pressures turned out to be much lower than what I was surmising. What really concerns me is how an engine that has been running hard can get enough cooling at idle or even no wake speed. Apparently the engineers at Yamaha have this dialed in as an engine operating correctly does not have any problem.
So the lesson I have learned is the importance of flushing with household water. My home has pressures in the 50 PSI range. This should be more than adequate to loosen sand and dirt and move it along. I wanted to pass along that I have changed my mind about flushing and will be doing it more frequently.

Nice write up Jim. Good things to know! Thanks for taking the time out and doing this.

While flushing my boat yesterday it became apparent that my water utility pressure could only produce 8 psi (measured at the flush connection)because all the water just flows through the exhaust and engine. So I am back to believing that running your boat WOT in fresh water will flush your engine better than flushing on the hose.

I am going to guess that the volume at 25 psi is similar to the volume of your garden hose at 8?
I know that I am on a well and my pump is set about 40-50 psi I believe.

Oh, and this means I probably have one of the cleanest engines?

I really don't have a means of measuring volume. I just learned our pressure is in the 80 psi range since we are near a water tower. I just can't figure out why we need to flush our boats if we are in fresh water. Seems every time we operate it we are keeping it clean.

buckbuck said:

What really concerns me is how an engine that has been running hard can get enough cooling at idle or even no wake speed. Apparently the engineers at Yamaha have this dialed in as an engine operating correctly does not have any problem.

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Not sure how I missed this last year, but I did. Your knowledge on these boats and engines eclipses most others' here, mine included, but I thought I'd take this question on for those that may have concerns.

The answer (I suspect, without data it is really only opinion):

• it is very predictable that jet boaters will go from idle to WOT to idle repeatedly
  • while tubing, for one example
• the engine has ability to shed heat up to a certain heat transfer rate without any external cooling at all
• the external cooling (from pumped ambient cooling water from lake/river/ocean/etc. in this case) significantly increases the engine's heat shedding ability
• when engine is run at max heat generation (WOT), maximum water cooling is required to stay ahead of the heat generation
  • here, this means units of heat shed >= units of heat generated
  • this is accomplished using both inherent engine cooling + external cooling water cooling
• given the catastrophic consequences of an error in this dynamic heat transfer equation (engine meltdown), there is a big safety factor in the design
  • also, cooling water intake temp varies widely, etc., other inputs are variable
• going from WOT to idle reduces the heat shedding as the external cooling drops with cooling water flow but the inherent engine cooling capacity remains
• like a beef roast's internal temp can increase after removal from the grill, temperature of key internals can increase for a short time after reducing heat generation
• also like a roast off the grill, there is both experimental and modeling (FEA) data available to know how much latent heat remains
• the engine's ability to shed heat without external cooling is sufficient to cool both the idle/low throttle heat generation and the latent heat (residual, no longer being generated)
  • if not, there would be many, many smoked engine cases documented here and elsewhere

Is it possible to intentionally smoke an engine to prove the above wrong? Quite likely, but it would almost certainly take intentional engine abuse scenario to do so to a properly running engine because:

• the engine, its inherent cooling + its external cooling have a dynamic thermal stability in their design (per above)
• the system has engineering controls that take over in cases where it has lost its thermal stability (ECU reduces RPM to minimal when temp readings exceed programmed limits)

As was likely the case with the famous mid-2000s photo of the flying Yamaha jet boat, Yamaha's test engineers intentionally drive the boats' various systems beyond any reasonably expected real world conditions.
They do this to develop data for the extremes which help them in both system design and safety factor selection. That and engineers only have so many opportunities to have fun.

PS: I am not sure anyone has actually FEA modeled a beef roast during and after heating...

Interesting GTBRMC. Perhaps I should mention that running at WOT heats the engine and creates a condition where the cooling has a more difficult time removing heat. This according to data I have collected over the years. So even if we get greater psi and will, in my opinion flush the jackets better, it is still probably best to back off WOT for a time and run at, say, 6000 rpm to cool things a bit. What is concerning to me is the low, low pressures until you get above 7000 rpm. I know these engines are robust because I once accidently clamped off cooling water in the wrong spot and idled the engines for about 10 minutes with no cooling in the jackets.

buckbuck said:

Interesting GTBRMC. Perhaps I should mention that running at WOT heats the engine and creates a condition where the cooling has a more difficult time removing heat. This according to data I have collected over the years. So even if we get greater psi and will, in my opinion flush the jackets better, it is still probably best to back off WOT for a time and run at, say, 6000 rpm to cool things a bit. What is concerning to me is the low, low pressures until you get above 7000 rpm. I know these engines are robust because I once accidently clamped off cooling water in the wrong spot and idled the engines for about 10 minutes with no cooling in the jackets.

Click to expand...

I absolutely agree that more careful operation is better and will likely prolong the life of your engine. Yes, still have fun, of course. But warm it up a bit before hammering it and ease it down when you can.

Also, recall these (at least the MR1) engines were originally closed loop cooling designs with much higher redlines. Yes, differing duty cycle, but that's a whole diff discussion.