water pressure tube?

[Capt. Bob]

Guys....

 

[fitz73222]

All outboards to my knowledge flow water through the exhaust side of the cooling system first; filling the block and cylinder head passages up to the thermostats and as the thermostats open the warmed water begins to flow and is regulated by the thermostats as the water warms and is maintained at operating temp while running. The tell tale stream of water you see is a way of bleeding off water pressure and giving you a visual reference that the water pump is pumping. It does not give you an indication that the thermostats are working. You can fry an engine with a steady tell tale stream and a stuck thermostat assuming no hot horn or other warning system to protect the engine from overheating. I use a water pressure gage in conjunction with a temp gage to better monitor changes in temp and pressure before the hot horn sounds. Most outboards have a connection point to use a temp gage in the cylinder head(s) and are very easy to install. Running into a load of eel grass, muck or a discarded ice bag that covers the water intakes or a damaged impeller is what a water pressure gage was meant for. Also very necessary when running with high engine heights were air may be introduced into the intakes that alter water pressure. So think about the added redundancy to monitor the entire cooling system.

[John Jones]

Wonder if the pressure changes as the thermostats open and close?

My Yamaha does.  When first cranked it runs 6-7 psi at idle and at a fast idle out the channel it goes up to  around 12-13 psi.  Before I get out of the slow zone it drops to about 6-7 psi when the thermostat opens.  At cruise it will slowly vary from 6-10 as the thermostat operates.

[RickK]

That would seem that your pressure line is ahead of your thermostats?

[John Jones]

Yep.  Otherwise it would read near zero when cold.

In fact, it is right at the t'stat.

[RickK]

To me that makes sense - shows that water is making it up to the top of the engine before the circulation down through the motor and out.
I don't have one on either of my engines - used to have one on the 170 on engine #1.  Need to put one on each - a good way to watch what is going on.

[John Jones]

I am a believer in pressure and/or temp gauges.  I had both on the CCP but only pressure on the flats boat.  It only takes one shot of those floating grass mats we get inshore around here in the spring to melt a motor down.  A friend of mine was saved by the warning horn on his Yamaha 150 HPDI. He ran through some of the floating grass and the horn went off.  He instantly shut it down and called for a tow.  The water pump impeller was melted to the housing.

[seabob4]

The water pressure gauge is the second most important gauge to have on a boat...

But the key is to scan your gauges, just as you do in your vehicle.  A lowering gauge reading is a good reason to throttle back and see what's going on...

[Circle Hooked]

What should typical water pressure be,at full throttle i get almost 30 psi.

[Capt. Bob]

What should typical water pressure be,at full throttle i get almost 30 psi.

Here's my two cents.

Boat in water/engine in water, engine running, idle speed and in neutral. The impeller on older Johnyrudes (96 back) acts as a displacement pump in that it forces the water it receives between the rubber(?) vanes and the water pump housing through the motor. Logic would dictate that pressure will be at its lowest (a couple of pounds on a gauge with somewhat speculative accuracy, if I remember correctly).

Motor in gear, boat on plane and zipping along. At this stage, the impeller vanes (which when in good shape are flexible) are compressed due to the force of the water striking the inlet ports and they now act like a circulation pump. The faster you go, the greater the water pressure on the lower unit inlets, the vanes compress more and your pressure rises on the gauge. 30 psi sounds good.

So what?
Well if the impeller fails, you could conceivably get the boat on plane and keep the engine cool enough to make it home but....
For me, the pressure gauge is the best indicator of an obstruction (weed/plastic bag/giant squid/left flush plate on) over the inlet and can warn you before the "horn" sounds.

PS... If I have the terms (displacement/circulation) backwards (probably do) please correct. Also, I've never owned a motor newer than my 96 so I can only speak for those. I'd venture a wag that most work the same way though.

[John Jones]

Different motors have different pressure specifications even though they all work about the same.  That's why they sell 15, 30, and 60 psi gauges.  My old '93 Oceanrunner 225 ran in the mid 20 range at cruise with a new water pump so a 30 psi gauge is good for that application.  You don't want a gauge with a range so high the needle barely moves off the peg.

[Circle Hooked]

You don't want a gauge with a range so high the needle barely moves off the peg.

That's what i used to have,the one thing i have to get used to since i installed my new gauge is the old gauge went to 60 and the new 30 so its almost pegged at wot.

[John Jones]

Now you can see any change that goes on with your cooling system.

[Aswaff400]

well i found that port on the bottom of the block, there is already a hose attached to the fitting that goes upto some valve... guess ill have to read a little deeper into my service manual... anyways...

installed a jack plate today. the install went pretty smooth, cant wait to try her out