Water Pressure

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Mike M

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Apr 14, 2009
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Hey Guys....



I have a 2001 NX750 DC with a 90 Merc (ELPTO). It didn't come with a water pressure gauge and I installed one last Friday in the availble hole in the dash. I have never had a water pressure gauge on a boat so I am not exactly sure how it's supposed to work. So I am a little perplexed. At idle my water pressure is only around 2 PSI. And at full throttle trimmed up I was at 12 PSI. Even at full throttle not trimmed it would barely go over 10 PSI. I just don't know what to expect. I get a powerful stream out of the pee hole. I even took it home after having it on the lake and hooked up the ears and ran it in the driveway with the same results. I only idled it and reved it once and the pressure jumped up over 10 PSI. I took the hose off the back of the gauge while running and steady stream would come out the hose. So I know the hose isn't kinked. And there a lot of pressure coming from the water coming out the pee hole. If you put your finger in front of the pee hole the water would spash everywhere due to the amount of force of the water coming out.



I have had the boat since April and my Dad owned it back East for three years before me. Last fall it was winterized and had maintenance done including the impellar. There is no more than 20 hours on the new impellar. We have never had any issues with overheating. The motor has always run very well.



I have been tweaking the performace so a water pressure gauge is mandatory. This trip on Saturday was the first since I moved the motor up two holes. I don't have a jack plate. I am currently a little over 4 inches below pad running a 21p SS Ballistic prop. I was at 5 1/2 below pad with the motor on the transom. I relize that doing both things might not have been a good idea.....but I would have never expected a water pressure issue at 4 inches below pad. I read about guys running that high or higher all the time. I couldn't believe the difference in the way it handled and drove. I think I even picked up a little speed. Although I was paying more attention to the water pressure than speed. I did gain a few RPM's and trimmed out hit 5500 with that prop for the first time.



Sorry for the long post...I felt like I needed to get all information on the table.



Thanks in advance.....
 
That's where you want to be. I would not go any higher based on PSI and RPM's. If you do, bump it only a hair at a time. 12 PSI is the minimum recomended. Make sure pressure holds on sharp turns and maneuvering. Other than that your numbers sound good. Enjoy!
 
According to my '99 Merc, 90hp. factory manual, the water pressure should be 10 to 15 psi at WOT. The water pressure at idle on my motor is not very high either. Keep in mind that the pressure spec, according to the manual, is measured at WOT, not at idle. Hope this may have helped you.
 

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