I was told that the small drilled hole in the exhaust pipe was to help in starting, especially when the exhaust was deeply immersed.
However in the FV range there is just one small hole of approx. 0.1" dia. This seems a remarkedly small hole for this purpose.
I suspect that this small hole was possibly there to stop water being sucked up into the cylinder block when the hot engine was switched off. Does this sound feasible? If not, what was its purpose?
Why were two bigger holes about 4" apart introduced later (I think for the SJMs and possibly others)? Was this for some exhaust tuning reason?
Certainly the one small hole results in a quieter engine than two bigger holes.
Any comments?
Jeremy
Small hole(s) in exhaust pipe in 40 series
Moderators: John@sos, charlesp, Charles uk, RickUK, Petergalileo
Re: Small hole(s) in exhaust pipe in 40 series
Hi Jeremy, hope your enjoying your sailing season?
That's a good one, the earlier marstons had an adjustable exhaust "vent" which allowed the exhaust to be opened up for starting. Interestingly that feature did not carry on into the ON/OP and wartime engines and the little forty had just the small hole as you say. Two strokes do need a tuned exhaust to give the best performance so it would be interesting to know what others think about this.
Is it easier to start a seagull with reduced exhaust back pressure? I've no idea!
That's a good one, the earlier marstons had an adjustable exhaust "vent" which allowed the exhaust to be opened up for starting. Interestingly that feature did not carry on into the ON/OP and wartime engines and the little forty had just the small hole as you say. Two strokes do need a tuned exhaust to give the best performance so it would be interesting to know what others think about this.
Is it easier to start a seagull with reduced exhaust back pressure? I've no idea!
Re: Small hole(s) in exhaust pipe in 40 series
Hi Adrian,
Not a great deal of sailing this year, just with separate trips to Fowey and Falmouth and sailing around the bay.
My old Seagulls have been used with FV1867T, FV2682, FV3048 and my 'big engined' (!) LM29941 all being used on the dinghy, or my Avon inflatable, with great reliability. It is really interesting how well they start from cold, (invariably first pull) even when on the Avon when they are very deeply immersed. It does seem surprising to me that the considerable mass of water in the exhaust tube can be accelerated out so quickly at start up.
It is difficult to understand how one 0.1 dia hole affects the starting performance. Perhaps I should put a hose clip over that hole and see if it makes any difference. But what do you think of my suggestion that the hole is there to stop water being sucked up into the engine when it is cooling down after being used? Of course that doesn't explain why later Forty engines had 2 bigger holes.
Any thoughts anybody?
Have the power hungry racers experimented with different size holes in different postions?
On another topic - have any other LMs appeared?
Jeremy
Not a great deal of sailing this year, just with separate trips to Fowey and Falmouth and sailing around the bay.
My old Seagulls have been used with FV1867T, FV2682, FV3048 and my 'big engined' (!) LM29941 all being used on the dinghy, or my Avon inflatable, with great reliability. It is really interesting how well they start from cold, (invariably first pull) even when on the Avon when they are very deeply immersed. It does seem surprising to me that the considerable mass of water in the exhaust tube can be accelerated out so quickly at start up.
It is difficult to understand how one 0.1 dia hole affects the starting performance. Perhaps I should put a hose clip over that hole and see if it makes any difference. But what do you think of my suggestion that the hole is there to stop water being sucked up into the engine when it is cooling down after being used? Of course that doesn't explain why later Forty engines had 2 bigger holes.
Any thoughts anybody?
Have the power hungry racers experimented with different size holes in different postions?
On another topic - have any other LMs appeared?
Jeremy
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Charles uk
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- Location: Maidenhead Berks UK
Re: Small hole(s) in exhaust pipe in 40 series
Seagull's own Dyno figures reveal that "longshafts" give circa 5% more power than their shorter brethren, so the exhaust must have some effect.
But discussing possible tuning benefits is frowned upon by the powers that be, as members that are not aware of their lack of understanding, tend to wreck Seagulls, not something we really want to encourage!
I can't see that the difference in temperature between hot & cold of the exhaust gas, would be sufficient to fill the entire exhaust tube up to the level of the exhaust port.
But worth thinking about.
But discussing possible tuning benefits is frowned upon by the powers that be, as members that are not aware of their lack of understanding, tend to wreck Seagulls, not something we really want to encourage!
I can't see that the difference in temperature between hot & cold of the exhaust gas, would be sufficient to fill the entire exhaust tube up to the level of the exhaust port.
But worth thinking about.
Make it idiot proof and someone will make a better idiot.
Re: Small hole(s) in exhaust pipe in 40 series
Charles,
I appreciate that we do not want people drilling holes in exhaust pipes on a guess basis in order to attempt performance enhancements.
My angle is a bit different, as I am wondering why this small hole in the early Forty series is there at all. My suggestion is that it is to prevent water being getting up into the cylinder when the hot engine is switched off.
If this hole wasn't there, back of an envelope calculations show:-
If the exhaust pipe temp was 250degC (523K) and the water level was 150mm below the block then when cooled down the 'air' volume would be 34cc. If the piston stopped at BTC and the block/crankcase was at 150degC with a combined volume of 80cc then after cooling down this would have an 'air' volume of 53cc. So making a total 'air' volume of 87cc.
This sounds close enough to cause concern.
I have made a huge number of assumptions:- temperatures, water level, crankcase volume, exhaust pipe sealing etc etc etc.
But maybe, just maybe this small hole is there, not specifically for starting purposes, although it will help, but to prevent water ingress, especially if the engine is tilted after it has cooled down and also being sucked up into the engine. This is just a suggestion in order to help us to understand these engines.
Any comments?
Jeremy
I appreciate that we do not want people drilling holes in exhaust pipes on a guess basis in order to attempt performance enhancements.
My angle is a bit different, as I am wondering why this small hole in the early Forty series is there at all. My suggestion is that it is to prevent water being getting up into the cylinder when the hot engine is switched off.
If this hole wasn't there, back of an envelope calculations show:-
If the exhaust pipe temp was 250degC (523K) and the water level was 150mm below the block then when cooled down the 'air' volume would be 34cc. If the piston stopped at BTC and the block/crankcase was at 150degC with a combined volume of 80cc then after cooling down this would have an 'air' volume of 53cc. So making a total 'air' volume of 87cc.
This sounds close enough to cause concern.
I have made a huge number of assumptions:- temperatures, water level, crankcase volume, exhaust pipe sealing etc etc etc.
But maybe, just maybe this small hole is there, not specifically for starting purposes, although it will help, but to prevent water ingress, especially if the engine is tilted after it has cooled down and also being sucked up into the engine. This is just a suggestion in order to help us to understand these engines.
Any comments?
Jeremy
Re: Small hole(s) in exhaust pipe in 40 series
Say for instance, the exhaust is perfectly sealed around the cylinder, putting the motor in the water wouldn't allow the exhaust to be filled to the level of the water outside, maybe its just a way of equalising the exhaust pressure, as its only a very small hole.
Re: Small hole(s) in exhaust pipe in 40 series
I am not to sure if I understand that Keith. If the engine is not running, then the water inside the exhaust tube is going to be exactly the same level as the water outside if the small hole is above the water.
Jeremy
Jeremy
Re: Small hole(s) in exhaust pipe in 40 series
Science 101.
Take a glass, turn it upside down and then put it in some water, as you will see its not at the same level as the water around the glass.
It would be exactly the same with an exhaust if it was air tight at the exhaust port and no hole in the side of the exhaust, so the small hole is for equalising the air in the exhaust.
We have all taken exhaust pipes off and we know how hard that can be, with more water in the exhaust, the quieter the motor should run, with no hole, very little water.
Or I maybe talking rubbish again.
Take a glass, turn it upside down and then put it in some water, as you will see its not at the same level as the water around the glass.
It would be exactly the same with an exhaust if it was air tight at the exhaust port and no hole in the side of the exhaust, so the small hole is for equalising the air in the exhaust.
We have all taken exhaust pipes off and we know how hard that can be, with more water in the exhaust, the quieter the motor should run, with no hole, very little water.
Or I maybe talking rubbish again.
Re: Small hole(s) in exhaust pipe in 40 series
see what you mean Keith. 
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headdownarseup
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Re: Small hole(s) in exhaust pipe in 40 series
What about the water that can travel upwards inside the exhaust tube (engine switched off) via the exhaust outlet of the pumphousing?
However good that "seal" is around the cylinder base or anywhere else along its length, there's bound to be an equal water level inside and outside the exhaust tube with the engine shut off and with the lower unit immersed. This small hole(s) must be there for a specific reason. With the motor "tilted" clear of the water, there's basically only air inside the exhaust tube.
Think about it this way. Once you're in a position to start the engine on the boat and you're the only person in the boat, the weight shifts to the rear of the boat because you are also sitting towards the rear of the boat, thus immersing the exhaust tube even further down into the water. This water level (inside the tube) is now much closer to the more sensitive parts of the engine that don't like water much. Holes in exhaust tubes, however big or small, in my opinion help to reduce some of the initial back pressure encountered when you first start the engine IN WATER. 2 or 3 bangs and most of the water has been pushed out of the bottom of the exhaust tube via the pumphousing. Same thing with the bigger seagulls.
Why do some of the very late seagulls (kingfishers,ospreys i think) NOT have any holes in their exhaust tubes? Is it performance related or is there something else going on here.
I found this which might explain things a bit better.
https://www.youtube.com/watch?v=V1rxCfsKj2c
You'll notice the canted tank too
Jon
However good that "seal" is around the cylinder base or anywhere else along its length, there's bound to be an equal water level inside and outside the exhaust tube with the engine shut off and with the lower unit immersed. This small hole(s) must be there for a specific reason. With the motor "tilted" clear of the water, there's basically only air inside the exhaust tube.
Think about it this way. Once you're in a position to start the engine on the boat and you're the only person in the boat, the weight shifts to the rear of the boat because you are also sitting towards the rear of the boat, thus immersing the exhaust tube even further down into the water. This water level (inside the tube) is now much closer to the more sensitive parts of the engine that don't like water much. Holes in exhaust tubes, however big or small, in my opinion help to reduce some of the initial back pressure encountered when you first start the engine IN WATER. 2 or 3 bangs and most of the water has been pushed out of the bottom of the exhaust tube via the pumphousing. Same thing with the bigger seagulls.
Why do some of the very late seagulls (kingfishers,ospreys i think) NOT have any holes in their exhaust tubes? Is it performance related or is there something else going on here.
I found this which might explain things a bit better.
https://www.youtube.com/watch?v=V1rxCfsKj2c
You'll notice the canted tank too
Jon
Re: Small hole(s) in exhaust pipe in 40 series
My point, ( I really feel like I'm flogging a dead horse now) last try, Take a tube any tube lets call it an exhaust and lets block one end, lets call this a cylinder shall we, tilted or dropped into the water, the air inside the tube/exhaust will be held in or will displace the water, yureka maybe not.
kingfishers,ospreys and so on have inner and outer exhaust systems, totally another jackanory.
kingfishers,ospreys and so on have inner and outer exhaust systems, totally another jackanory.
Re: Small hole(s) in exhaust pipe in 40 series
Were the holes multi purpose, at what stage of the process were the exhausts plated? Did the exhaust tube arrive in plated in full long lengths and was then cut or, did the steel arrive in bare and was then cut and plated? I don't want to derail the thread, just curious if anyone knows.
Could the holes have assisted in the plating/hooking process?
Could the holes have assisted in the plating/hooking process?
Re: Small hole(s) in exhaust pipe in 40 series
Keith has nailed it the hole on the little forty is there for pressure equalisation, to allow the level inside the exhaust to settle correctly.
Without the hole, the engine exhaust would not back fill to the right level, and the back pressure a two stroke engine needs would not be present. Google an animation of the two stroke cycle to show the importance of the back pressure. That will get lead to exhaust tuning, which is the dark side
the hole on the little forty is there for pressure equalisation, to allow the level inside the exhaust to settle correctly.Without the hole, the engine exhaust would not back fill to the right level, and the back pressure a two stroke engine needs would not be present. Google an animation of the two stroke cycle to show the importance of the back pressure. That will get lead to exhaust tuning, which is the dark side
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headdownarseup
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Re: Small hole(s) in exhaust pipe in 40 series
Pressure equalisation maybe. My way of thinking was as a means for allowing some of the "air pressure" within the tube to escape more freely when you first go to start the engine.Same thing i suppose.Once the engine has run for a few seconds there's probably not going to be any water inside the tube at all as it's now been pushed out via the pumphousing.
Why then do century's have bigger holes and more of them?
Later models for example the kingfisher don't have any holes in their OUTER exhaust tubes, or do they? My question was perhaps with these later models the reason for omitting these 2 holes in the OUTER tube was performance related, or am i thinking totally wrong again!
Why then do century's have bigger holes and more of them?
Later models for example the kingfisher don't have any holes in their OUTER exhaust tubes, or do they? My question was perhaps with these later models the reason for omitting these 2 holes in the OUTER tube was performance related, or am i thinking totally wrong again!
Re: Small hole(s) in exhaust pipe in 40 series
If the water is totally pushed out of the exhaust when the motor is running would be interesting to know, I don't know if the gas would push out all the water, I will try an experiment next bath night.
Maybe and only maybe, the more holes on the later motor was so that the motor would be more compatible with different types and sizes of boats, maybe something in the Seagull literature.
As Jeremy was saying that the hole was so small, I couldn't see it affecting the starting much if any really.
As far as the later motors are concerned, I don't really know enough about them to form an opinion, as they probably have a totally different port setup and so on, the only thing I would say is, the more water kept in the exhaust when running, the quieter the motor will be.
Maybe and only maybe, the more holes on the later motor was so that the motor would be more compatible with different types and sizes of boats, maybe something in the Seagull literature.
As Jeremy was saying that the hole was so small, I couldn't see it affecting the starting much if any really.
As far as the later motors are concerned, I don't really know enough about them to form an opinion, as they probably have a totally different port setup and so on, the only thing I would say is, the more water kept in the exhaust when running, the quieter the motor will be.