Anti Ventilation Exhaust

rosbullterier · Post by **rosbullterier** » Thu Feb 19, 2009 12:56 am

The earlier Seagull 102, in my very limited knowledge of old outboards, may have a unique advantage with its square straight out the back exhaust.

If the later downward swept round exhaust introduces forced gas to the outer ends of the propeller, considering the limited hull speed potential, there is the strong possibility of the prop churning in gas instead of clean water.

The square exhaust would direct straight back, away from and above the prop.

Why would Seagull have discontinued this exhaust - inefficiency or fashion?

Post by **charlesp** » Thu Feb 19, 2009 7:16 am

Noise. The downwards pointing exhaust was initially called a 'sub-ejector' in typical British Seagull fashion. It was supposed to shove the exhaust gases downwards so that the cooling effect of the water condensed them as much as possible before bthey rose to the surface and became audible.

There is evidence to suggest that prewar sailors attached bits of hose to 'flat out the back' exhausts to deflect the stream of gas downwards.

I tend to think it's a load of old hogwash, but a test of both at the same time wouldn't go amiss...

Collector Inspector · Thu Feb 19, 2009 11:05 am

This is very interesting!

I have many different Old Dears, not only Gulls, and have been looking at where exhaust is "Discharged" verses propeller position/relationship on them all.

It would be gleaned from this that exhaust gases are always after the thrust side of any propeller.

When the craft is moving there would be enough velocity of water to make sure that the propellers have a "Clean Bite" as forward progress is achieved.

If, however, the craft was stationary, trying to gain some momentum, there could be a difference between different exhaust outlet designs until movement was attained. Perhaps, under those conditions, an aerated prop tip could be experienced.

Very large Hp motors will cavitate props when they are spinning high revs with the hull stationary but as soon as motion is achieved this is not a problem. Aeration through motor placement and hull design in corners is a problem after that for these.

Some say that the velocity and density of the prop wash over and around the exhaust outlet, actually causes significant negative pressure within the lower exhaust volume that encourages the egress of pent up blue smoke.

Seagulls?

Just make sure the depth below the transom/hull is correct.

Cool Topic!

C.I.

Rob Ripley · Post by **Rob Ripley** » Thu Feb 19, 2009 11:27 am

These are very interesting replies -
1# I have tried different exhaust depths and found no difference in the 'noise'
2# most 'noise' seems to come from induction sources (rubber or metal attachments dont seem to make any difference!)
3# I can easlily make my C100/40+/102 cavitate - high revs-sharp turn and then there is a slight reduction on noise, it just seems to make a deeper sound.
4# has anyone tried a cavitation plate? looks like it could be fitted above the exhaust oulet. This might increase the thrust.

Collector Inspector · Thu Feb 19, 2009 11:45 am

Rob

I am awaiting delivery of an Olympic with a cav plate. Never seen an Olympic with one before so this one may well have been "Value Added" for a specific reason many moons ago.

Here.

http://cgi.ebay.com.au/ws/eBayISAPI.dll ... :IT&ih=009

I may have outbid you on this?

I have other British outboards that have cavitation plates, Ferrier, Anzani. Mallard (Sort of 1/2 attempt), Sea-Bee (Yuck), Britannia and, what I believe to be a "Fisherman" which, to me is a re-badged Ferrier with a different tank shape and a great big red oval decal plonked on for effect.

Now, next question is logically "Why did Seagull NOT introduce a CAV plate early on" when the rest of the competition obviously did!

Seagulls push a LOT of water volume/weight at low velocity/prop speeds which was what they did so well. Move a mountain with leisurely disregard etc. Surely some barge pusher having a problem would have requested a cav plate?

Regards

C.I.

Rob Ripley · Post by **Rob Ripley** » Thu Feb 19, 2009 1:12 pm

...'I may have outbid you on this?'

No mate.
1# out of my price range
2# I'll stick with Seagulls

If I can get hold of some 1/4" al plate I'll try to cut and fit one.

rosbullterier · Post by **rosbullterier** » Thu Feb 19, 2009 1:32 pm

What a handsome motor that Olympic is! (mind - you Austrafalians know how to value your outboards)

Are they a fundamental better made 102 replica? I can't quite see the antiventilation plate to gather whether it is a welded addition.

I'd be very interested in a comparison with a Seagull.

pistnbroke · Post by **pistnbroke** » Fri Feb 20, 2009 11:03 pm

The introduction of exhaust gas before the prop is common practice ..It is common for us to drill 3 x10mm holes into the prop hub in front of the blades .(exhaust thro prop hub ) The ideas is that when the boat is stationary and the throttle is opened there is a cloud of bubbles in which the prop spins making it easier to increase rpm. As the boat moves forward the prop bites bubble free water and off you go ... It increases HOLE SHOT .
Now runnig the seagull at idle in gass filled water may reduce the load at low rpm enableing it to keep running ...

Post by **Charles uk** » Sat Feb 21, 2009 1:04 am

Isn't this normally only done on cupped props on planing hulls?

This problem only seems to occur on lower units, that are not fitted with their usual props.

Hole shot on a Seagull? nice thought.