doug748
Well-Known Member
I have one such valve. If you leave it shut it busts your engine.
Use a ball valve on the exhaust outlet?
- Thread starter Ohlin Karcher
- Start date
lw395
Well-Known Member
Same as the inlet seacock then!
Ohlin Karcher
N/A
All, or nearly all HGV's can shut off their exhausts by design and generate large back pressures, to slow the vehicle down without wearing out expensive brake shoes etc.
You can put a potato in somebody's car exhaust pipe and the engine can't run, it doesn't break anything in the engine.
I think forgetting to open an exhaust seacock then trying to start the engine could possibly, on a bad day, blow the hose off a spigot, or split a hose or a water trap.
Wiki says engine braking on lorries generates a mean pressure of 60psi. So at the top of the exhaust stroke, the piston is pushing against some 60psi of burnt gases in the combustion chamber.
An interesting subject anyway.
You can put a potato in somebody's car exhaust pipe and the engine can't run, it doesn't break anything in the engine.
I think forgetting to open an exhaust seacock then trying to start the engine could possibly, on a bad day, blow the hose off a spigot, or split a hose or a water trap.
Wiki says engine braking on lorries generates a mean pressure of 60psi. So at the top of the exhaust stroke, the piston is pushing against some 60psi of burnt gases in the combustion chamber.
An interesting subject anyway.
Yngmar
Well-Known Member
You can get an exhaust skin fitting with a flapper if your exhaust installation is problematic, but normally looping the hose up just below deck level before going down to the exhaust outlet is quite sufficient to prevent water from backflooding. We've had a fountain up the heads sink from waves slamming in the hull (which does have a seacock, but one must remember to close it), but never any issue with the engine exhaust, which is just an through-hull, maybe 30cm above the waterline, but with said loop up.
I think it's safe to assume that one day you'll forget it's closed and start the engine. That's just how this stuff goes. Unless you notice very quickly, I think the engine would get flooded. I'm not really sure though. A diesel engine with a blocked exhaust won't start, but a marine diesel engine with a wet exhaust would still pump water against a blocked outlet while cranking, so the water would have nowhere else to go but up the mixer elbow and into the engine block, past the exhaust valves and into the cylinder. Someone correct me if I'm wrong here.
The Volvo brass exhaust through-hull has a sturdy but flat drip edge all around that neither takes damage nor damages fenders. I suspect it's quite pricey though, being Volvo.
Yngmar
Well-Known Member
A closed raw water intake seacock will just damage the impeller. The engine won't go bust until it overheats from lack of cooling, which you would hopefully notice well before and shut it off.
prv
Well-Known Member
What exactly does it "bust"? How many engines have you "busted" in order to discover this?
Agree, at least without some kind of interlock.
If the exhaust is blocked, will it crank? Genuine question, but I assumed this to be the reason it wouldn't start. If it does turn over against the back pressure, then why wouldn't it start? There's fuel, air, and compression in the cylinder, so it surely ought to go bang?
Right as far as it goes, but wrong in conclusion.
Most exhaust installations use the pressure of the exhaust gas to push the water in the trap up the swan-neck and out of the boat. If the engine is cranking but not running, then the trap is filling up and eventually will flood the engine. That's true in your situation (assuming it does turn over) but it's also true in every other situation where the engine refuses to start in good time. My previous boat had a placard next to the start button advising closing the intake seacock during "prolonged starting attempts or when turning over without running".
So, "if you keep cranking and cranking without starting you will eventually flood the engine" isn't a good argument against a hull valve on the exhaust, because it already applies without the valve.
Pete
elenya
Well-Known Member
We have a standard plastic ball valve in our exhaust (mercedes om636) for 20 years. A works fine and allows us to usolate exhaust when at sea in poor conditions. We simple have a small key fob we place on engine ignition when the se cock is shut. And hang it on the ball valve when we open it. No issues with Valve. No issues with blowing up the engine when i got it wrong on one occasion, simply blew the exhaust hose of the water trap. Took 5 mins to fix.
Rocksteadee
Well-Known Member
One day someone will forget to open the valve (kids, wife, new owner).
Engine may start on first turn of the key (exhaust would initially be at zero pressure)
Back pressure could build up and force cooling water back into engine.
Back pressure could blow off exhaust pipe or split it. This may not get noticed.
Engine compartment now filling up with exhaust fumes and cooling water.
Consequences are possibly now obvious
D
Deleted member 36384
Guest
Can't answer your question about temperature rating but I would not be put off fitting a valve on the exhaust outlet if you think there is a need for one. On some Rivals the exhaust does flood back and they use flap valve or gate valve to keep closed. The gate valves are bronze bodies designed for marine use before anyone has a heart attack, a slimmer fit than a ball valve.
On my own Rival 41C, with a Perkins 4236, 80HP I have a large, bronze gate valve which I believe was fitted originally. I used to close it but don't now as it has a large swan neck running right up to the roof of the aft cabin, so back flooding is neigh on impossible. I did start my engine once with the exhaust gate valve closed, started very easily, ran for a little while and popped plus split the hose at the spigot on the swan neck. No drama, just an obvious change in noise, smoke in the engine space and a minor splash of water. The hose was cut back to remove the split, refitted and off we went.
On my own Rival 41C, with a Perkins 4236, 80HP I have a large, bronze gate valve which I believe was fitted originally. I used to close it but don't now as it has a large swan neck running right up to the roof of the aft cabin, so back flooding is neigh on impossible. I did start my engine once with the exhaust gate valve closed, started very easily, ran for a little while and popped plus split the hose at the spigot on the swan neck. No drama, just an obvious change in noise, smoke in the engine space and a minor splash of water. The hose was cut back to remove the split, refitted and off we went.
doug748
Well-Known Member
The head gasket. Just one, I won't be trying again on the off chance that the same thing happens.
There are further difficulties. Gate valves tend to fail closed, so tricky to tell if it really has opened. Similar strife has been reported on these pages concerning ball valves. It's a poor idea all round but if the OP loves it - best of British.
fisherman
Well-Known Member
I would have a flap, S/S with a rubber pad, adjusted to give a pretty good seal. If the transom is raked, then a simple counterweight, studding with a small weight on the end, will keep it tight and can be adjusted. You can, of course, get a fitting with external spigot to take the drips away from the gelcoat, this can be cut to face slightly upward.
Oh, the rubber is to avoid the endless clatter when ticking over.
Oh, the rubber is to avoid the endless clatter when ticking over.
Kelpie
Well-Known Member
I was taught to always, always, always, go straight to the stern and check the water flow every time the engine was started up. No exceptions.
Is this not common practise? It has saved my bacon at least once that I can remember, when the raw water pump belt was slipping and the engine wasn't pumping water.
Maybe it's a bit like 1-2-both switches, which some people reckon are a recipe for drained batteries, whilst everyone else just gets on with life and uses them quite happily.
Is this not common practise? It has saved my bacon at least once that I can remember, when the raw water pump belt was slipping and the engine wasn't pumping water.
Maybe it's a bit like 1-2-both switches, which some people reckon are a recipe for drained batteries, whilst everyone else just gets on with life and uses them quite happily.
lw395
Well-Known Member
Exactly so.
I guess it's a bit like people with modern cars not being able to cope with operating the traditional manual handbrake.
Ohlin Karcher
N/A
More foolproofing of systems leads to enhanced foolishness of their operators..
I concur it's a reflex action to check water is coming out when the engine starts, it's the only way to be sure, and applies on all kinds of vessels not just yachts!
john_morris_uk
Well-Known Member
No it doesn't - as lots of other people have pointed out.
With the engine running I don't believe for one millisecond that water is going to be forced back into the engine against the pressure of the exhaust. It's more likely that the exhaust pipe will be forced off a fitting and the boat will start filling up with water and exhaust fumes. I also think most people would notice what was happening at this point...
Exactly. I always do it and always get the crew to practice doing it if they start the engine. We also have a tell tale water stream from a fitting midships that its easy to go and check is steaming appropriately.
I don't expect it's any surprise to anyone that I admit that we have a large good quality ball valve on our exhaust skin fitting. I fitted it because a couple of times, in the right conditions and when it's rough enough, wave slap up the counter of the boat (where the exhaust comes out on our boat) has forced seawater back up the exhaust and hydraulic locked the engine. This is despite having a larger than standard water trap, and a gooseneck as high as possible up inside the stern of the boat and a high rise water injector on the exhaust of the engine. When the engine has hydraulic locked I have had to remove a preheater element and spin the engine over to blow the water out and then start the thing as soon as possible to dry it out to prevent cylinder and piston damage. (I've rebuilt the engine once due to this problem and I don't want to have to do it again!)
doug748
Well-Known Member
People are getting worse on these pages how many are reading past posts? How many are pondering them? We have a nausea of uninformed, opinion.
These beliefs, thoughts and expectations are of great interest but -
Read post 30
So stick to your guns, blind faith is a grand thing in metaphysics, let it justify what you want to do if you wish, carry on.
However, I am telling the wider readers who are not buffoons that I have direct experience of blown head gaskets due to an exhaust valve being left closed.
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fisherman
Well-Known Member
Air, gas, is compressible, water not. Seems fairly obvious to me that forcing gas followed by water, injected close to the engine, into a dead end results in the gas compressing and forcing the water back. In certain configurations this is going in the engine.