Intermittent flow is usually caused when a boat has a wet exhaust/silencer. The water comes from the manifold after the heat exchanger, then down to a wet exhaust / waterlock muffler (has chambers and weirs inside) builds up in there, then when full is pushed out by exhaust pressure, with a spurting motion over and over. There are different types of wet exhausts, but these are probably the most common. I had one in my 20ft NZ Hartley cabin boat, on a marinised MK 3 Zephyr Zodiac 6 motor and it worked well.

Things you might want to know about exhausts



and 2 working





If a boat has a tell-tail, the water volume will usually vary slightly with motor speed (like an outboard) but is self limiting , as the rubber vanes on the pump impeller will fold back at high revs keeping the flow steady (if they didn't, the pressure would probably blow all the hoses off, or blow a head gasket. High performance tunnel hulls etc with straight exhaust headers, have wet exhausts with a sleeve on the exhaust running to the outlet, (which keeps the outside cool) and the water falls out round the exhaust stream.
Some marine engines like MerCruisers have a riser as part of the manifold, with water fed into the exhaust after it . Lots of different types of wet exhausts, but most work the same way, by water cooling the exhaust externally or internally (which reduces sound) and removes gasses.

You are correct that the pump is rotary. Remember that the pump is pushing water thru relative confined/restricted passages, ie, the block in some cases, but usually just the exhaust manifolds. In some case it just pumps thru the riser. Upon exiting the riser, the H2O dumps into the exhaust pipe, huge in size compared to the H2O passage in the riser. As well, it is mixing with the exhaust gas, which, at times, has it's own pulsations. The are many types of marine exhaust, with, without, mufflers, and they themselves vary considerably. As such, I doubt that there is one straight forward explanation.

Mirangi, same here, I've always been around full sized boats too.

I would guess that the exhaust gasses passing through the system will push varying amounts of water through and out depending on the resistance caused by whatever anti flood arrangement (gooseneck, etc)is fitted to the vessel.

Hi Will
You certainly sparked a good response to a question many have pondered over. It has drawn out some useful ideas and methods used. 👍
Should you decide to delve further it would seem there are members amongst us who are prepared to offer their help and guidance.👍
Clearly not everyone will want to have such a feature but we all learn as we build our models and this site helps us all improve our skills.
Dave

JBW, thank you too for your input on this.

I like the idea of the delay between the sound module and the smoker starting, that really would give that extra bit of realism.

Will

I have been around boats all my life, cooling water is pumped by rotary pump so i wonder why flow isn't constant from the exhaust?
i assume this happens because the flow is low compared to the size of the exhaust pipe and water accumulates then flows, vessel movement and possibly non return flaps could cause this. I guess that this effect will not occur when scaled down to model size.

Probably the easiest way to simulate interrupted flow would be a piston pump or a rotary valve interrupting the flow(stopping or diverting flow), pump/valve speed could be varied with motor speed.
Just a thought!!!

Hi Maverick,

Here is a circuit diagram of the control board. It has three PIC microchips to control all the functionality. The 'smart' bit that makes it work is the software programmed into the chips. I'm happy to share that with anyone who can use it, but you need to have the development equipment and be familiar with programming PIC chips with their MPSAM assembler if it is going to be of any use to you.

Graham93

Added this system to my HSL last year when Graham and I were working on making a cheap smoker system which could be made easily at home. With this set-up experiment, I used an air pump for the smoker and a water pump which cools the in-runner brushless motors and also provides water for the exhausts. The front 'exhaust' is the constant cooling water and the rear is for water, smoke and air together to break up the water flow. Worked quite well, but required a lot of plumbing ! Graham has done a similar thing with his fireboat, but used some clever electronics to pulse the water with less plumbing.

Vid of it idling


I've fitted this system to 3 of my boats now and it works well and is cheap and reliable. If you use a sound unit, you can use timers to sync start-up with smoke/water etc. Smoke/water plumbing examples,- Pic 1 -HSL, pic 2 Hartley cabin boat, pic 3 vintage runabout.

Is there a circuit layout for the control board please

Graham, that's very interesting and realistic too, especially as the water coming from the outlets isn't just constant flow. Great out of the box thinking to achieve the desired result. 😎👍

Will

To be honest Bob, that's probably the exact set up and result that any of mine will have.
As I say, I was just curious as to whether anyone had actually managed to successfully replicate the action of a full sized boat exhaust cooling water discharge.

Will,

I added something similar to my Crash Tender. It has a water pump, an air pump and a smoke generator. Details are here:

https://model-boats.com/blogs/60819#63592

Graham93

Hi Will,
As you know, all of my power boats have brushless motors mounted on water-cooled alloy motor mounts.
The water enters the boat via a water scoop (just like when we were allowed to run our two strokes and glow engines on the UK club lakes!).
Before the water reaches the motor mount it has to pass through the water-cooled ESC, and then goes into the motor mount.
When the boat is idling, the prop only pushes a tiny amount of water through the system and so the water outlet is quite slow, but once the boat is up to speed, the outlet has a good flow.
It does "more or less" does what you want - except that it does not "pulse" when at idle, and I have no exhaust smoke to mix it with ........but that could be 'sorted".
Stay safe - Bob.

Toradog, I completely agree, a lot of faff for occasional moments when the model is actually close enough to see it happening.
Like I said, just one of the many (many many many!!) thoughts floating around in my grey matter 🤣

Someone must have done this at some point already, it can't just be me (or can it🤔) wondering how this can be achieved.

Will

Just for giggles, it should not be too hard to create the effects. A water pump controlled by an ESC that is Y linked to the throttle channel would serve to regulate the water flow. As most H2O exhausts have loop in them, pretty much the reverse of a drain loop,the surging might well be simulated with it in the exhaust line. I like the idea of somehow camming the H2O pump to simulate the surge, but what happens at speed when the surge disappears? I suppose that if one REALLY wanted to go whole hog, one could install an air pump too simulate exhaust gas as well. A lot of IF's. The last thing to consider is that once your boat is "offshore", will anyone be able to see the exhaust effect and appreciate it?
All in all, it would be a fun project to explore... but my plate is full.LOL

Maverick, something like that could work.
I suppose it would have to have some sort of sprung/over centre to it, so that didn't just get a constant pressure from the pump and rotate at a constant speed, giving a constant flow.
At lower water pressure from a pump, if it had some sort of resistance to overcome, until it went over centre and then giving a bit of surge of flow, that would look pretty realistic.
Then when if the pump ran faster as the motor did, the pressure would increase and get over centre more easily giving a faster, more constant flow. The down side to this, other the complexity, could be reduced cooling due to slower water velocity through the mount and esc.

I guess that the best bet and also the simplest (other than just a constant steady flow at all vessel speeds) would be via electronic control, stopping and starting the pump.

All ideas though and something I can think about.

Yeah, I totally get what you are saying and it's probably something that I will never attempt (who knows).

As I say, it was nothing more than curiosity as to whether anyone on here has attempted it, how they did it and how well it worked.

Will

How about trying to feed the water through something like a wankle rotary engine, a cylinder with a driven triangular rotary, that may give more of a spurting action. No idea if it would work.

It would be possible to design a very realistic system using electronics to link in with an engine sound but a simple solution often works best and is achievable for many.

Dave976, I will be doing the same also, both esc and motor.

Maverick, yes, I will be running a water cooled mount (brushless outrunner), but that connected to the outlet will give a constant discharge of water and will probably be what I will actually end up doing.

It was nothing more than simply wondering if anyone has acheived anything more accurate and realistic, than simply a constant flow.

Will

I cool both in my Sea Queen, cooling the ESC first then on to the motor and out the side.

Dave976, that's more the sort of thing that I was thinking would be fairly easily achievable a fairly realistic too.

Toradog, I agree with what you are saying and that a steady constant would be a happy compromise and much easier too.
I just wondered if anyone had attempted to simulate a more accurate and realistic effect.

Are you not using the water as a motor or ESC coolant?

When the boat is moving at speed a water scoop will give a good flow so a small pump connected via a T joint to the outlet would give the required effect when at rest. If the Pump motor was pulsed I believe the required effect could be achieved. An electronic switch connected the the ESC channel could be used to stop the pump when the motor was a rest.👍

It appears to me that there are two things that need to be simulated. First is the surge of water that appears when the engine is at idle or running quite slowly. The second is that when throttled up the exhaust water flow becomes more constant, but the increased exhaust gas and pressure makes for a a very forceful spray of water from the exhaust.
With those factors in mind, I would thing that simply simulating a fair constant stream would be the easiest and least complicated while giving a good overall effect.
I did see a video of someone's boat, I believe there across the pond from me, that had the constant stream effect and it was the first thing I noticed about the model. I was very pleasantly impressed.
When in doubt...the KISS principle will not fail you. 🙄