water pressure increasing while trimmed up

[countvlad]

i changed the water pump a few days ago and today i took the boat out. I went from 17 psi at WOT to about 20 PSI so im happy with that... unfortunately for some reason when i trim the motor up and take the RPMS down to cruise speed the water pressure increases to aprox 27 PSI... I tested the gauge at work and its about 1-2PSI off but thats not my concern, obviously....

Last yeah the gauge used to do the same thing when i would take turns but i assumed might have something to do with the pump beign few years old...

motor doesnt over heat and tee tale flow is still good... also the pressure stays at 20PSI at WOT with motor trimmed up... for some reason the pressure increases to 27 psi only when i start dropping in RPMS...

is this normal? i would think if i dont have any flow the pressure should drop not increase...

thank you

 

[countvlad]

Re: water pressure increasing while trimmed up

no one has anything to add? is it normal for the pressure to increase if there isnt enough water going through the pump???

 

[F_R]

Re: water pressure increasing while trimmed up

Why do you apologize for your English? Looks better than most.

Anyway, an increase in pressure could only happen because there is MORE water flowing through the system, not less. I can't imagine that can't be a bad thing. I assume it is because there is more water pressure available at the water inlets on the lower unit. Mind you, that is only an assumption.

EDIT: The engineers that design those things get paid to know where to place the water inlets so that water flowing past them is under pressure. That pressure-feeds the water pump so it doesn't starve for water at speed.

 

[countvlad]

Re: water pressure increasing while trimmed up

Why do you apologize for your English? Looks better than most.

Anyway, an increase in pressure could only happen because there is MORE water flowing through the system, not less. I can't imagine that can't be a bad thing. I assume it is because there is more water pressure available at the water inlets on the lower unit. Mind you, that is only an assumption.

EDIT: The engineers that design those things get paid to know where to place the water inlets so that water flowing past them is under pressure. That pressure-feeds the water pump so it doesn't starve for water at speed.

thanks for the compliment.... i don't want anybody to think i didn't finish high school

i mean sure it makes sense... i know the pump doesn't act like a compressor and if the pump was starving for water and there were air pockets i assume the gauge would jump like crazy and surge.... i guess i'll just have to keep an eye on it...

 

[metriccrescentwrench]

Re: water pressure increasing while trimmed up

I can think of a couple reasons for what you describe to happen. First, as you change engine speed you'll be operating at different points on the 'pump curve' of your water pump. Every pump of any kind has different characteristics of flow, pressure, and rpm - when put on a graph that's called a pump curve. As you change engine speed the water pump will be moving around on that pump curve, and you'll see varying pressures as a result.

Also, depending on trim angle there's a chance that the water inlets could be operating more or less efficently. Again, no big deal, as long as the engine temp stays where it belongs all is good.

 

[countvlad]

Re: water pressure increasing while trimmed up

I can think of a couple reasons for what you describe to happen. First, as you change engine speed you'll be operating at different points on the 'pump curve' of your water pump. Every pump of any kind has different characteristics of flow, pressure, and rpm - when put on a graph that's called a pump curve. As you change engine speed the water pump will be moving around on that pump curve, and you'll see varying pressures as a result.

Also, depending on trim angle there's a chance that the water inlets could be operating more or less efficently. Again, no big deal, as long as the engine temp stays where it belongs all is good.

yes makes sense i'm not disagreeing with you ... but i wouldn't think the pressure would increase by half just by trimming the motor up few inches out of the water.... but just like you've mention could be operating more or less efficiently....

but its also odd that no one has experienced this problem/issue.. i would think that i would be the only one dealing with this....

I would definitely keep an eye on it and post back if anything changes...

thank you