Low Flow vs No Flow Alarm

[NorthernFocus]

One important aspect of operating any type of equipment is understanding exactly what it does/is intended to do. On the forums here and most anywhere else that the topic is discussed people refer to the Aqualarm "low flow alarm". Aqualarm describes their product as a "cooling water flow detector" and claims it "monitors cooling water flow to prevent engine damage". OK, so what?

Well it depends on how one defines "low flow" and "engine damage". Last week we had a cooling water flow "experience" that may be informative to others. We were fishing in open water so left the engine idling. We had been fishing for a couple of hours when the Aqualarm began to chirp every once in a while. The alarm never latched in, just a momentary chirp now and then. Since we were in deep water and not moving I hoped it was just some weed or something that had drifted by and partially blocked the intake. Since it apparently occurred at idle maybe it didn't suck in and would clear easily by simply shutting the engine down for a bit and restarting. At least it was worth a try before getting the crew upset with more intense troubleshooting. I did so and voila, no more chirping.

Over the next few hours we continued fishing during which time we motored a bit at 1500 RPM or so without incident so it appeared the issue had resolved itself. But a while later on the way to our next anchorage I pushed the throttle up and rather than engine temp settling at 180 degrees it continued to ramp straight up. I pulled back to idle and temp dropped immediately. When I lifted the hatch to check the raw water strainer I found it empty but with a stream/trickle that I'd estimate at maybe a gallon per minute or so flowing through it.

So, moral of the story, the Aqualarm is essentially a "no flow" alarm. It might save destroying an impeller or melting down a muffler, but it won't tell you that you have inadequate flow for normal operation.

As an aside, for anyone who operates in waters where you're not keen on diving under the boat to clear the intake, it's not a bad idea to figure out in advance how to reach and clear it without getting wet. I had done that and it payed off in this situation.

 

[sheral]

Good post, thanks. We have an Aqualarm installed so will keep this in mind.

 

[knotflying]

I guess low flow warning is the temperature rise. I have often thought of carrying one of those compressed air drain clearing devices. What did you do to clear yours?

 

[HRowland]

An overheat alarm on the wet exhaust would detect a low flow condition. This would likely trigger before there was significant increase of the engine coolant temp. Ideally you would have a no-flow alarm for immediate detection of a significant flow loss and a temp alarm on the wet exhaust to detect low flow issues.

Howard

 

[Chimo]

This is a really interesting thread as I had been thinking about fitting an alarm. I have read back in the threads on the subject and there seems to be an opinion that the Aqualarm is a) of relatively low production quality relating to the barbs and the alarm on the dash and b) difficult to fit, possibly because of the barbs. Are there alternatives that anyone has tried or is it the only game in town?

Many thanks

 

[NorthernFocus]

The challenge with any affordable alarm (i.e. a single point switch)on a system that operates across a wide range is deciding on a useful set point. In the context of how Aqualarm devices fit in an overall protection scheme, the flow alarm protects the impeller and the exhaust temperature alarm protects the exhaust hose/muffler. Core engine protection still comes from an alert operator and the manufacturer's engine alarms.

 

[NorthernFocus]

This is a really interesting thread as I had been thinking about fitting an alarm. I have read back in the threads on the subject and there seems to be an opinion that the Aqualarm is a) of relatively low production quality relating to the barbs and the alarm on the dash and b) difficult to fit, possibly because of the barbs. Are there alternatives that anyone has tried or is it the only game in town?

Many thanks

The issue is that the barbs are oversize for marine water hose. You can purchase just the switch and install it in fittings purchased separately. After a cost benefit analysis my own choice was to spend a few minutes sanding down the barbs to fit.

 

[knotflying]

We are going a bit off topic, but I not only sanded the barbs a bit but placed the barbs in ice, heated the hose and applied some dish soap. They inserted into the hose quite easily after that.

 

[sheral]

Mine slid on nicely too cause I read the tricks / information used by others on this site before installing mine 🙂 I suspect as with any other alarm / sensor it’s just a second line of defence, nothing you could or should totally rely on. I also installed the muffler temp sensor and together, it helps me sleep a bit better.

 

[NorthernFocus]

...I also installed the muffler temp sensor and together, it helps me sleep a bit better.

I keep going back and forth over the exhaust temperature sensor. Due to it being a strap-on rather than a wetted probe I wonder whether the response is any faster than the engine temperature. Unfortunately it would be a difficult thing to determine other than after the fact.

 

[knotflying]

Well the exhaust temperature alarm sounds at 200 degrees and higher. So it will most likely sound off before your engine alarm would. However, my thinking is that the engine alarm sounds well before engine damage, so is there a significant difference between hearing the alarm slightly sooner? In any event you will still have to stop and figure out your issue. I purchased the flow alarm to save my impeller and avoid the dreaded situation of having to replace one while on the water and the boat is rocking away.

 

[NorthernFocus]

Well the exhaust temperature alarm sounds at 200 degrees and higher. So it will most likely sound off before your engine alarm would...

The "most likely" is the key phrase there. That depends on several variables such as where the probe is installed on the exhaust system and how much the cooling water flow is restricted. The water that's injected into the exhaust flashes off so a little bit of water can do a lot of cooling.

...However, my thinking is that the engine alarm sounds well before engine damage, so is there a significant difference between hearing the alarm slightly sooner? In any event you will still have to stop and figure out your issue. I purchased the flow alarm to save my impeller and avoid the dreaded situation of having to replace one while on the water and the boat is rocking away.

Sound logic. For those of us with the earlier boats with plastic mufflers there is a question of whether the muffler will melt down before the engine heats up enough to alarm. After my recent event I'm more comfortable that's not likely barring a complete loss of flow. Which the flow detector will catch. Regardless it's not a bad idea to always "watch the gauges" when power is applied. If one is in that habit it is quickly evident if something is amiss.

Changing an impeller on open water is not an attractive thing to contemplate. The thought of having to do so in a blow with a steep, rocky shoreline to leeward is a nightmare. Fortunately something blocking the intake is a lot less likely to occur when running at speed and particularly in turbulent conditions.