Checking cooling water flow

Jun 6, 2020
Hunter 326 Hasting, Westernport Bay
The usual test for ok cooling water flow on an inboard is a quick look over the stern.

Unfortunately, the cooling water outlet on my Hunter 326 is below or at the water line and under the stern, so that there is lots of gurgling and splashing going on with the seacock open or closed! :(

The exhaust note is different I noticed, but not remarkably so - not for a newbie at least.

Tests for flow rate suggest a bucket under this outlet which is obviously not practical in this case either!

I don't have a temperature gauge, just the warning buzzer!

Any suggestions?
Jul 7, 2004
Hunter 30T Cheney, KS
Seems like you'd want a visible, more permanent indicator besides the overheat speaker. Just in case the thru hull wasn't opened or the impellor gets trashed. Some of the members may have done this.
Jan 4, 2006
Hunter 310 West Vancouver, B.C.
I have exactly the same situation as you with the underwater exhaust discharge. A number of the "gurus" around here swear they can tell the difference between "flow" and "no flow" just the same as I swear I can tell the winner of the first race tomorrow :poop:.

See #2 in the below post.

You see the lack of flow instantantly, rather than waiting for an increase in temperature.
Feb 26, 2004
Catalina 34 224 Maple Bay, BC, Canada
My engine overheated last July. My exhaust is above the water. I looked over, as I always do, to confirm flow, left my slip, and overheated less than 10 minutes later.
What I thought was the "normal" splashing was just the exhaust gas.
I traced it almost immediately to a worn raw water pump faceplate. I sanded it down, but it wasn't enough. That's when the real fun began. I bought a new one, but Oberdorfer has changed the hole spacing so the new one doesn't fit older pumps. :banghead::banghead::banghead: The change was tiny, less than half the diameter of the holes in the plate, but enough to make it not work.
I got my old one refaced at an iron works shop. Back in business.

Doesn't answer your original question about underwater exhaust, but I'm sure there are tons of folks with boats like that who have had to deal with it. If you don't get an answer here, try other boating forums and let us know what you learn.

EDIT: I read the link, finally. Yup, Ralph had the answer. He's good that way! :beer:
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Jul 7, 2004
Hunter 30T Cheney, KS
Now I'm adding an Aqualarm flow switch to my off season project list. :huh:
Thanks for the link.
Jan 7, 2011
Oday 322 East Chicago, IN
I can tell when I forget to open the cooling water thruhull, or when the impeller is not working, by the difference in the tone of the exhaust.

You might train your ear to hear the difference ( but probably harder with the under water exhaust).

But I am also lucky, and have an exhaust exit above the waterline and easy to see from the helm…

Your flow meter is a sure fire way to tell.

Jun 6, 2020
Hunter 326 Hasting, Westernport Bay
Thanks for the input guys. The flow gauge is a great idea.

After my run dry episode after forgetting to open the Kingston valve I decided to check the cooling water flow rate.
The easiest access spot on my 2GM20F was to remove the pipe at the back of the heat exchanger (before the vent and elbow), push on some 16 mm ID vinyl hose and lead it (without kinks) to a bucket. The service manual says 1600 liters/hour at 3600 rpm. This computes to 7.4 liters/min at 1000 rpm (2 gal/min). I ran it for 30 secs at 1000rpm and I got 4 liters :D

Now to fit a temp gauge in place of temp switch. The off the shelf temp gauges (Supercheap auto) come with 1/8 NPT sensors. My research indicates that the Yanmar sensor thread is 3/8 BSP :(. I can get a 3/8 BSP to 1/8 BSB adapter (Champion Parts) and will either tap the 1/8 BSP adapter to 1/8 NPT or see how well it goes with lots of teflon tape ;).

I still like the idea of an alarm so I will build an electronic detector off the sensor line to trigger the existing alarm at 90 C.

The single wire from the sensor to the control panel will now go to the gauge.

Still looking at flow gauges though!
Jan 20, 2020
Hunter H336 Milwaukee
I left my boat for a couple of weeks in August of 2021 and when I came back to it I noticed that the tone of the exhaust was different and so I started to watch the temperature. I had setup a coolant sensor temperature monitor through a Raspberry Pi previously. As the temperature increased I went down to the 3GM30F and felt the temp of the hose between the heat exchanger and the waterpump. This was warm so either the pump had failed or their was a blockage somewhere on the Raw water intake line. I went to the thru hull and this moved freely. I then closed the thru hull and took off the raw water hose - I opened the Thru hull expecting a small fountain but had a very small trickle. I inserted a screwdriver into the thru hull and cleared out all the Lake Michigan Zebra Mussels at which time the very small trickle returned to being a small fountain.