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

https://www.youtube.com/watch?v=L6wcLi9uc9s

and 2 working

https://www.youtube.com/watch?v=WWDZESQg66g

https://www.youtube.com/watch?v=8osu_09K7jc

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.

https://www.youtube.com/watch?v=L6wcLi9uc9s

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https://www.youtube.com/watch?v=WWDZESQg66g

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https://www.youtube.com/watch?v=8osu_09K7jc

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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
https://www.youtube.com/watch?v=mb8KFfGafiM

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.

https://www.youtube.com/watch?v=mb8KFfGafiM

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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