Are Seagull water-injected exhausts a myth ?

[Rex NZ]

SAE 140

Very Interesting, I'm surprised at the good starting. Your experiments seem very comprehensive, the water level is way above any 'normal' operating condition. This suggests Myth-Busted

This has left me wondering at how the engine is able to scavenge, given the exhaust discharge is essentially captive in a very small volume at cranking.

I'm thinking of Possible Theory-
For the engine to start, a gas exchange must therefore be happening in the combustion chamber.
1. Perhaps if the engine starts within the first few revolutions, then maybe there's insufficient increase in exhaust back-pressure to frustrate the scavenge?
or
2. Maybe the exhaust gas has a high oxygen level (can't imagine this)
or
3. Maybe having a single small intake port cross sectional area combined with cross flow design results in a very low scavenge ratio, maybe say 20% ~ 40%. So maybe under normal conditions the engine is running with a very high proportion of stale gas.

I'm leaning towards theory #3. Here's why;
My research shows a seagull fuel-burn rate is about 1000g/hp-hr. To put this in perspective a heavy diesel might burn about 220g/hp-hr, a 4-stroke mower might burn about 300g/hp-hr, a jap 2-stroke outboard might burn about 500g/hp-hr. So the good ole seagull has a fairly high fuel-burn for it's specific output. Some of this is due to the low BMEP value, basically low compression. If the engine runs with a very high proportion of stale gas, then this would help explain both the 1000g/hp-hr & the starting with high water level.

Any other theories out there?

Rex

[SAE140]

I don't know if this is relevant - but as SCUBA divers know well enough, a depth of 30 feet of water is equal to 1 Atmosphere = 15 PSI.

The immersion in the test tank was something around 16" for the 90, and 14" for the 40 Minus, but let's say 1 foot for simplicity. So that's one thirtieth of 15 PSI, or 0.5 PSI - which ain't a lot of pressure for the exhaust gases to be pushing against.

Or is my logic screwy ... ?

[Collector Inspector]

Not that Screwy at all.

Below is a pic of the exhaust tube of an Australian Riptide Fisherman, in such you can see that the cooling water is fed back into the manifold to progress down the tube and underwater. Water cooled exhaust.

The holes (Different for every production run) were only ever touted as being "A handy cooling water tell tale". No mention of exhaust silencing at any stage.



I block these off as the smell with following breeze is eye watering.

A finger on the top of a barrel every now and again proves cooling. Even at the end of the heads unlike a Brit..........ouch!

Starts not a problem and all of the noise goes underwater.

Marlin did not have any relief at all to see and they have a cowl.

Interesting this topic.

B

[phil]

"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."

The plans for a NIS 26 have the well setup you speak of IIRC , a friend has a 4 stroke Honda 8 hp on his, its mounted on slides and moved up and down with a small winch. While sailing the hull is faired by a drop down door. The whole thing is contained under a hatch cover, very nice installation.

[JoeShields]

hhmmmmm.... thanks for the info. I think I'm going to try starting out my season with the two exhaust holes plugged with self-tapping screws smeared generously with high temp silicone. Also plan to seal top of exhaust tube with high temp silicone. I bought a cheap laser tach and bench marked the rpms before plugging. Will rerun rpm test after plugging and report back on any effect on rpms.

On another note (mainly to myself). For the past nine years, my SJM 40- has faithfully started on 1 or 2 pulls once a week (year round). This past weekend it started, sputtered out and then would not start at all. Checked tank... plenty of fuel. After way too many incredulous moments of pulling and re-pulling, I finally pulled the spark plug out and sure enough it was fouled with a hardened bit of goop. Cleaned it and re-gapped it and screwed it back in, and it started on the first pull and ran stronger than ever. Note to self: pull plug once a year, clean and check gap.

[Rex NZ]

SAE 140

Your pressure sounds about right. It'll be very low. Definitely less than 1 psi

Pressure (Pascals)= pgh
where
p = density kg/m3
g = earths gravity N/kg
h = height m
so
Pressure = 1000 kg/m3 x 9.81 N/kg x 0.4 m
Pressure = 3924 pascals
given
1x Atmosphere = 101325 pascals
then we have
0.0387 atmospheres
given
1 x atmosphere = 14.7psi
then
Max Imperial Static Pressure = 0.569 psi

So I concur. You logic is sound

Rex

[Rex NZ]

Phil

Your design sounds good

I have visions of an electric outboard running off a spiral wound battery & charged from a vertical wind turbine. Ideal for manoeuvring & not too heavy. Lovely & quiet

Rex

[SAE140]

"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."

The plans for a NIS 26 have the well setup you speak of IIRC , a friend has a 4 stroke Honda 8 hp on his, its mounted on slides and moved up and down with a small winch. While sailing the hull is faired by a drop down door. The whole thing is contained under a hatch cover, very nice installation.

The standard shaft Seagulls have a feature which I have yet to see exploited to it's full potential - namely, the ability of the drive-shaft clamp to slide all the way down the water-pump housing. Seems to me that it would be simplicity itself to arrange for a primitive gantry with block and tackle to sit above the well, and simply pull the Seagull upwards out of the water, whilst remaining attached to the clamp. With the smaller engines, even the gantry would not be necessary.

And, now that we've more-or-less established that the exhaust tube holes may not - in practice - be mandatory, it would also be possible to replace the water-pump gasket with a much larger plate, thus closing-off the bottom of the well, and ensuring that the exhaust gases cannot come back into the boat to choke both engine and human ! (shades of the 110 inboard ...)

I suppose the really clever bit would be to arrange for the well closure door of which you speak to still be operated with the elevated Seagull still hanging at the top of the well - and maybe getting in the way of the door mechanism ? Maybe pulling the Seagull up, then folding it back (exhaust tube uppermost) to lie flat on an intermediate deck is the answer.

[JoeShields]

Thanks Phil and others who have discussed the idea of raising and lowering an engine within a motor well. It's nice to know that someone has designed a boat (NIS 26) that can provide for such a feature. My boat is smaller (23') and would not be feasible to have a standard shaft motor raised and lowered. It would stick up too high in the boat once raised, and interfere with the use of the tiller. My ideal concept for my boat is a collapsible drive shaft and exhaust tube so that the shaft length could be shortened and lengthened as needed. I know... dreamer!

So here's the next best thing that I've been building, and I'm wondering if anyone has ever done, or seen anyone do this.... let's call it a "Lower Unit Catcher" (LUC). I have a hinged aluminum frame (much like a butterfly catcher, but with the net portion on a hinge so that it can be in line with the handle or raised to a 90 degree angle with the handle). Instead of a porous net, I have a waterproof bag attached to the catcher loop frame. The idea is to lower the catcher in the motor well in the vertical position until it is below the motor's lower unit. Then by pulling on a line, raise the waterproof catcher bag 90 degrees and raise it up so that it completely surrounds the lower unit with the rim of the catcher bag above the water line. I then have a hand pump attached to the handle with the input hose going to the bottom of the catcher bag, and the output hose running down the outside of the bag. The idea is to use this contraption to solve one of the problems with leaving my Seagull in the motor well - keeping the lower unit dry. When the motor is not needed (she's a sailboat.. if I've got wind, I've got propulsion), I'll deploy the LUC, pump the waterproof bag as dry as I can, and at least not worry that water seeping in is displacing and pushing out all my SAE 140 gear oil. Hopefully it should also keep hairy things from growing on the prop and other normally submerged parts of the motor. I'll sleep better environmentally too, knowing that my Seagull is not dripping petroleum into my favorite lake. The only thing it does not solve is the drag of the lower unit through the water. In fact, it may make it worse

Any way it has given me something to think about and work on during the winter months. If it works out, I'll post some pics.

[Massimo]

Greetings to all the friends of the forum, I would like to bring my small contribution with a couple of short films, filmed by mobile phone, my LLS in 1966 with the drain holes closed and the recirculation of cooling water in the muffler
.
I started closing the bottom drain hole with a bolt and how you feel, when I connected the hose to the hole above water on the cylinder valve with a tube of copper, the engine sound has been more muted, a sign that the water injected into the exhaust system helps to reduce noise
This change had no negative effect in the process of starting the engine, which is' always started with no particular difficulty

video n.1 original exhaust
http://www.youtube.com/watch?v=vA3paZRMmZA

video n.2 lower hole closed
http://www.youtube.com/watch?v=YMfIou6A1vg

video n.3 lower hole closed and water injected
http://www.youtube.com/watch?v=tqho5TauwRk

alternative run
http://www.youtube.com/watch?v=dmaf7WxSS_8