Are Seagull water-injected exhausts a myth ?

[SAE140]

Been reading around the subject of water-injected exhausts - not easy, as there are not too many "idjut's guides" around ...
Seems that it's a good idea on air-cooled engines, and on inboard marine engines, as in both cases it helps to keep the exhaust pipes and expansion chambers cool(ish), which in turn help to reduce the temperature of the exhaust gases.

But on a Seagull ? The exhaust expansion tube is already sitting in water, so why inject more water into the exhaust ? It can't be to cool the gases directly - that would take a high pressure spray. Directing the cooling water outlet against the exhaust tube sides higher up would certainly help, but this is not what happens on the 102: the 'injection point' is fairly well down the tube.

So - in the spirit of enterprise, I've modified a series 90 which I'm using as a test bed and general workhorse.
Nothing earth-shattering - it's all been done before. I blocked-off the exhaust tube holes with s/s nuts and bolts, and drilled one hole to take a Wade 'dissimilar-Tee' fitting at the same height as the 102's equivalent fitting. The part of the Tee which enters the tube already had a restricted lumen, and I added an additional restricter (circa 2mm dia.) so that very little water will be injected into the exhaust.
During trials I found that adding a short length of tube to the outlet of the Tee fitting helped to create a pencil-jet, and it may be that a quasi-venturi effect is occurring, to actually suck exhaust gases from, rather than inject water into, the exhaust tube.

The reduction in noise was impressive, and I conclude that this is due to the closure of the exhaust tube holes, not to any injection of water - the 'water injection' point only serves to provide a pathway for pressure relief.
To test the efficiency of the pressure relief, I then filled the dust-bin fully with water, resulting in an additional 8" of immersion. The 90 started first pull, exactly as before. Interestingly, there was an even greater reduction in noise, as the exhaust gases exit further underwater. So much so, that I then became aware of a 'clocking' sound, the source of which I eventually tracked down to the carb intake cowling. I'll have a stab at silencing that as well in the next few days ...

Attached are a couple of pics, showing normal and deep immersion.

[Oyster 49]

Interesting project, roughly how much % was the noise reduced when the water injection was introduced? I'm thinking of doing a similar thing to my FPC by connecting the outlet to a 102 connector on the exhaust. It seems that a side effect would also to improve flow at low RPMs?

[SAE140]

Interesting project, roughly how much % was the noise reduced when the water injection was introduced? I'm thinking of doing a similar thing to my FPC by connecting the outlet to a 102 connector on the exhaust. It seems that a side effect would also to improve flow at low RPMs?

That's the point of my post - that in practice there ain't actually any water injection involved ...

... so - perhaps the best way to judge the expected noise reduction on your machine would be to simply block-off the holes, (say) using jubilee clips, leaving just one open for starting purposes - then, when the engine is running, simply hold a piece of rag against that hole. That should then give you some idea of what to expect.

Forgot to mention - the tube I used was 8mm copper (used for so-called 'microbore' plumbing). Although the block tell-tale hole is also 8mm, mine needed an 8mm drill running into it, just to clear out a small amount of corrosion.

[Rex NZ]

SAE 140

Well done with the experiments

Something to consider is that the exhaust wants to vent freely with the atmosphere upon start-up. A single 5mm passage is usually enough. Once the engine is running there's no further need for atmospheric venting.

Currently you have the motor starting well, but the situation can occur where there's insufficient cylinder scavenge for the motor to start.

Might be worth considering in the design

Rex

[JoeShields]

"insufficient cylinder scavenge for the motor to start"

I was hoping to plug up those two holes in the exhaust tube to cut down on the amount of exhaust buildup in the motor well (and then in the cockpit) where I have my Seagull. Is that not a good idea?

--Joe
SJP 40-

[Horsley-Anarak]

I have made a new exhaust tube for one of my century engines from stainless.

It has no holes and engine start perfectly.

This engine is as new, older engines may need the holes possibly.

You could always just plug them up and see if it makes any difference when starting.

H-A

[Rex NZ]

Joe

Suggest setting up the exh so that it's open to the atmosphere on starting, but, becomes closed by the cooling water when pumping. No extra moving parts, very simple. The Japs use a system like this on their outboards.

If you block the holes completely, it doesn't necessarily stop the motor from starting. Rather, it lessens the cylinder scavenge at starting, & you don't want that. Not ideal design workmanship.

Ideally you'd partially evacuate the exhaust for starting, but, nobody's going to do that. Unless you found somebody insane. HMMM. Now, there's an idea. Might put that in the same box as the gas turbine outboard. LOL

Speaking of which, here's an insane South Island marathon jetboat that's been gas turbine powered.



Rex

[JoeShields]

I may try just plugging the holes and see how that works with starting and general engine performance. But I am starting to study and collect the brass parts I would need to try and build the mechanism to let cooling water flow shut the hole after motor is started. Seems pretty ingenious. I've got to do something... my wife dislikes the exhaust fumes in the cockpit and is pushing for a new motor on our boat. I like pairing a 1965 vintage motor with a 1965 vintage sailboat. My Seagull always starts on the first pull and does the job. If the exhaust all goes into the water and is pushed away from the boat by the prop, instead of seeping up through the motor well into the cockpit, that would be good. If the motor is a little quieter to boot, that would be excellent. I think my wife and my Seagull could be friends. At any rate, boat launching occurs in 10 days, so gotta get cracking...

PS: I'm still not sure what cylinder scavenging is??? It doesn't mean that it sucks water back up the exhaust into the cylinder (without the holes), does it? That would not be a good thing.

[charlesp]

British Seagull actually produced a boat with a well that attempted to answer the fumes problem; it also attempted to remedy the fact that a well full of fumes isn't feeding just air to the carburettor.

The boat was based on a Poole canoe, which is basically a flat bottomed craft with a well, suited to the very shallow waters of Poole.

The well in the B.S. was shaped to sweep the fumes away, with a slope at the rear of the well to that end.

I think they sold two. The locals immediately built their own, and the project died.

[JoeShields]

Would that be using the British Seagull called a "midi"?

If I could design the perfect motor for my sailboat, it would look like a 40- or 40+ at the top, but then have a retractable lower unit that could be cranked up or down inside the motor well. Down so the prop protrudes into the water and provides propulsion... Up so the lower unit could be stored up out of the water, kept dry and eliminating its drag when just sailing.

Hey... what's the fun of living if you can't dream.

[Rex NZ]

Joe

Here's a pic of the system I'm using
* nice & quiet
* no fumes, the exh gas discharges underwater & bubbles up behind the yacht
* most of the water dumps into the exh tube, small tell tale at the block
* auto opening of vent passage for starting, closes with water
* bonus, if the cooling water stops/blocks for any reason, then the motor suddenly becomes noisy

Water injection into exh

Tell tale in block outlet fitting

Rex

[charlesp]

Never heard of a 'midi'. The boat was designed with the Centuries & 102s in mind.

[Charles uk]

Seagull didn't make the Midi, BMB did.

[SAE140]

SAE 140
Well done with the experiments

Thanks.

Something to consider is that the exhaust wants to vent freely with the atmosphere upon start-up. A single 5mm passage is usually enough. Once the engine is running there's no further need for atmospheric venting.

As I see it - that's already happening, at least to some degree. With the engine stopped, there's a route for the pressure in the exhaust tube to equalise with the atmosphere via the open outlet tube. This remains open during starting, and for a few seconds afterwards, until the water has risen up from the pump, flooded the head, and becomes a jet of water in the outlet, thus blocking the 'equalisation route'. At the moment a 2mm hole is proving sufficient - but should this ever get clogged (or prove insufficient), then it's internal restriction can easily be removed to provide the 5mm hole you speak of.

It may be of course, that the volume of the exhaust tube itself has some bearing - and that such a modification to a 64cc Seagull (especially those with the shorter shaft), might run into problems.

I'll run the motor again this afternoon in a wheelie-bin (more depth of water), flooding the tube as high as possible, just to see what happens. Watch this space ...

[SAE140]

Well - it's just gone 4 p.m. and I've spent a frustrating afternoon out on the test tank ...

'Frustrating' because no matter what I do, I can't get any of my Seagulls NOT to start ! I must say this is a bit annoying, as Seagulls have earned themselves a reputation in some quarters as being virtually unstartable.

I took loads of pictures, at varying stages of immersion - but it made absolutely no difference whatsoever, so I'll 'cut to the chase' as our American cousins say:

This is my test-bed 90 - flooded right up to the drive-shaft clamp. Nowhere for the exhaust to go, except down and out:

Started on a gentle first pull, no problems ...

Brain-wave - maybe it's the exhaust tube volume ... so I'll try a 40 Minus featherweight.
Several levels of immersion later, with one hole blocked with gaffer tape, the other with a Jubilee clip and a patch of inner-tube rubber - again flooded up to the drive-shaft clamp - and again, nowhere for the exhaust to go, except down and out:

And - again - started first pull (and a half-hearted pull at that), no problems.

So - experiment is a failure - turned out to be more like a sales-demo for Seagulls. I'm now left wondering whether the 'hydrophobia' people speak of, through excessive immersion, is also a myth ?