Cruisers Forum
 


Reply
 
Thread Tools Search this Thread Rate Thread Display Modes
Old 29-07-2021, 09:12   #16
Registered User
 
Cheechako's Avatar

Join Date: Aug 2006
Location: Skagit City, WA
Posts: 24,230
Re: Exhaust expertise please

Quote:
Originally Posted by JeremyFowler View Post
Is there anyone on this forum who is a bof/fundi/guru on exhaust gas principles, and who would deign to explain to this ignoramus in a way that he can grasp?

Looking at the Vetus installation recommendations: after the water-injection elbow, a large diameter rubber pipe (75mm) is connected to the large 75mm, 10.5 liter Vetus water-lock (all good thus far, and no issues with my understanding as yet), BUT then this whacking great pipe continues (75mm dia) and it is connected to PVC piping to make a large loop to close to the deck, a lift of nearly 1.3m, before dropping down again to just above water-level. This means that the exhaust gasses have to hold a LOT of water in suspension going up that large 1.3m lift. Also, when the engine is shut down, all that water in suspension at the time falls back into the water-lock.
My intuition tells me that a smaller diameter pipe will use less engine power as less water will be held in suspension. My gut feel is that a 40mm dia pipe would be ample. This would also give greater reserve capacity in the water-lock, which is a good thing. Is there a gas/fluids engineer out there that can correct me with a reasoned explanation?

As a 2nd issue - It costs energy to get the water up the 1.3m incline: is there another good way of preventing seawater backflowing into the outlet pipe? Is so I could exit the outlet at about 0.8m height, which is still maybe 0.6m above sea-level.

Thanks in advance.
I would not go to a smaller pipe, but you can limit the rise of the loop, you dont need a whole lot of loop up, just enough to keep following seas from surging up and into the muffler etc. The exhaust is not "full" of water though, the velocity of the gas "sprays" the liquid out. On start up there may be a big "blurp" of water sent out.
From my experience though, the "push" of following seas can be substantial, and if it does it enough, it can start water over the loop top, putting a small amount in with each push... it adds up.
__________________
"I spent most of my money on Booze, Broads and Boats. The rest I wasted" - Elmore Leonard











Cheechako is offline   Reply With Quote
Old 29-07-2021, 14:59   #17
Registered User
 
Reefmagnet's Avatar

Join Date: May 2008
Location: puɐןsuǝǝnb 'ʎɐʞɔɐɯ
Boat: Nantucket Island 33
Posts: 4,827
Re: Exhaust expertise please

Quote:
Originally Posted by sailormed View Post
Reefmagnet: One final note about PVC is that it can have a low melting point and might not be suitable for exhaust use unless specifically selected for the task. Some types of PVC have maximum service temperatures as low as 60 deg C.



I installed an exhaust temp meter on my Yanmar 3JH4E just after the exhaust elbow. In the Med in summer I get a temp of 37/38 C at normal RPM of 2500. At WOT 3200 rpm the temp is 48C. I expected a much higher temperature, but these are the values observed.
The problem is if there is an issue with the water flow. Hot exhaust and low melting point plastics don't mix.
Reefmagnet is offline   Reply With Quote
Old 29-07-2021, 15:04   #18
Registered User
 
Reefmagnet's Avatar

Join Date: May 2008
Location: puɐןsuǝǝnb 'ʎɐʞɔɐɯ
Boat: Nantucket Island 33
Posts: 4,827
Re: Exhaust expertise please

Quote:
Originally Posted by Cheechako View Post
Not sure all assumptions here are correct....?

-A larger pipe offers less resistance to the exhaust as a whole, but higher velocity, (smaller pipe) ejects better. Think of it this way; if the pipe was 3 feet diameter and rising up, how much water would go out? Probably none.

-Further: the amount of water ejected does NOT equal the amount of water injected... that's why there is water left in the system to flow back into the muffler (or engine!)

Just a thought........
The water ejected has to equal the water injected otherwise the system wouldn't work. The only difference being the volume of water in the system at any instant. There is a generic mathematical formula used to calculate the volume of water left in the exhaust system after shutdown that is used to determine the required capacity of the water lock.
Reefmagnet is offline   Reply With Quote
Old 29-07-2021, 15:51   #19
Certifiable Refitter/Senior Wannbe
 
Wotname's Avatar

Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: C.L.O.D.
Posts: 12,978
Re: Exhaust expertise please

Quote:
Originally Posted by Cheechako View Post
Not sure all assumptions here are correct....?
-A larger pipe offers less resistance to the exhaust as a whole, but higher velocity, (smaller pipe) ejects better. Think of it this way; if the pipe was 3 feet diameter and rising up, how much water would go out? Probably none.
-Further: the amount of water ejected does NOT equal the amount of water injected... that's why there is water left in the system to flow back into the muffler (or engine!)
Just a thought........
Quote:
Originally Posted by Reefmagnet View Post
The water ejected has to equal the water injected otherwise the system wouldn't work. The only difference being the volume of water in the system at any instant. There is a generic mathematical formula used to calculate the volume of water left in the exhaust system after shutdown that is used to determine the required capacity of the water lock.
While Reefmagnet has provided a perfectly correct explanation, let me add this as a thought experiment.

You go to your boat and start the engine, run it for an hour and then shut it down If all the water the raw water pumped into the cooling system did not equal all the water ejected out of the exhaust, where is now? It must be somewhere in the boat if it didn't get ejected. Now repeat this a hundred times. Sooner or later you boat is going to be full of water....oh wait!

As for pipe diameters, try sucking a milkshake with a small diameter straw or a large diameter straw.

I haven't done the maths for a 3' diameter pipe yet but might later if I get bored but whether it would work of not will depend solely on the size of the engine.
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
Wotname is offline   Reply With Quote
Old 29-07-2021, 17:02   #20
Registered User

Join Date: Nov 2013
Location: Slidell, La.
Boat: Morgan Classic 33
Posts: 2,794
Re: Exhaust expertise please

Quote:
Originally Posted by JeremyFowler View Post
Is there anyone on this forum who is a bof/fundi/guru on exhaust gas principles, and who would deign to explain to this ignoramus in a way that he can grasp?

Looking at the Vetus installation recommendations: after the water-injection elbow, a large diameter rubber pipe (75mm) is connected to the large 75mm, 10.5 liter Vetus water-lock (all good thus far, and no issues with my understanding as yet), BUT then this whacking great pipe continues (75mm dia) and it is connected to PVC piping to make a large loop to close to the deck, a lift of nearly 1.3m, before dropping down again to just above water-level. This means that the exhaust gasses have to hold a LOT of water in suspension going up that large 1.3m lift. Also, when the engine is shut down, all that water in suspension at the time falls back into the water-lock.
My intuition tells me that a smaller diameter pipe will use less engine power as less water will be held in suspension. My gut feel is that a 40mm dia pipe would be ample. This would also give greater reserve capacity in the water-lock, which is a good thing. Is there a gas/fluids engineer out there that can correct me with a reasoned explanation?

As a 2nd issue - It costs energy to get the water up the 1.3m incline: is there another good way of preventing seawater backflowing into the outlet pipe? Is so I could exit the outlet at about 0.8m height, which is still maybe 0.6m above sea-level.

Thanks in advance.
First things first. We need to know, at minimum, the size of the motor, the length of the pipe/hose run and the configuration of the motor in respect to the loaded waterline. Also good to know would be if you have a turbo or non turbo, and an engine-mounted exhaust riser and its height.

Assuming you have about a 40 horse non turbo engine,and a 6-8 foot total exhaust run, your gut instinct is probably mostly OK; a 2" exhaust will probably be adequate, and will reduce (somewhat) the chances of volume-induced backflooding (or splashing, which in the long-term can cause exhaust valve issues, even if you never get enough water to hydrolock).

For instance my 200 horse Volvo turbo diesel has a 3.5" exhaust system and exits through the fairly restrictive outdrive. Of course there's no waterlift, so there's no danger of volume-related backfilling but there is no riser and the head/block interface is only about a foot about the waterline, so there is a wave-pumping concern... but I digress.

If you have some extra-ordinary design requirements, the 3" exhaust may be necessary; few sailboat installations are what would be called 'standard'. The multitudes of diagrams and pictures available should only be taken as a starting place used to design a system to fit your individual application.

Several generic points.

PVC is not a very good idea for a marine exhaust system.

A manometer reading before you start and after you finish will yield valuable information.

You only need enough water going through the exhaust system to cool the components. Usually, with a raw water pump in good working order you can divert a substantial portion, using it as a 'telltale' to keep you appraised of that subsystems operation. Done intelligently, you also can eliminate the notoriously 'dangerous' vented loops.

Redundancy (done right) is typically a good thing; on my sailboat I have an engine mounted riser, an exhaust riser where the exhaust exits the boat, and a vented bypass/telltale riser just after the raw water pump (to preclude siphoning from the r/w strainer).

We might be better able to give you some ideas with a fairly accurate schematic...
jimbunyard is online now   Reply With Quote
Old 30-07-2021, 14:39   #21
Registered User

Join Date: Apr 2017
Location: Auckland, NZ
Boat: Cavalier 39 Mk1
Posts: 37
Re: Exhaust expertise please

Thanks very much everyone for the input!
Jimbunyard, thanks, I will knock up a schematic and will be grateful for your comments.
I have spoken to an engineer that has done a lot of 2 phased piping, not exhaust gas and water specifically though. He says it is very difficult to be exact. Where there is liquid and gas that needs to rise significantly they often use 2 narrower pipes on the vertical runs so that there is not the same "slugging" action from 1 bigger pipe burping up the "lumps" of liquid. He felt my gut feel was probably about right, if one went for a smooth bore pipe.
Yes, PVC pipe seems to be bringing in an element of risk that needn't be there.
JeremyFowler is offline   Reply With Quote
Old 30-07-2021, 21:12   #22
Registered User

Join Date: Mar 2020
Location: Wherever we please, GOM
Boat: 46 ft cutter
Posts: 173
Re: Exhaust expertise please

Ultimately, what is the diameter of the engine manufacturer’s exhaust pipe?
You should never be smaller than this,
Larger diameter exhaust lines out of the water lift might make sense if there are long runs introducing large friction losses.
PirateGuy is offline   Reply With Quote
Old 31-07-2021, 18:20   #23
Registered User

Join Date: Oct 2015
Location: Anaheim, Ca. / Subic Bay, Ph.
Boat: In Anaheim: 24'Thunderbird Formula 380 hp, In Subic Bay: Polaris Flying Boat
Posts: 11
Re: Exhaust expertise please

Quote:
Originally Posted by JeremyFowler View Post
Thanks very much everyone for the input!
Jimbunyard, thanks, I will knock up a schematic and will be grateful for your comments.
I have spoken to an engineer that has done a lot of 2 phased piping, not exhaust gas and water specifically though. He says it is very difficult to be exact. Where there is liquid and gas that needs to rise significantly they often use 2 narrower pipes on the vertical runs so that there is not the same "slugging" action from 1 bigger pipe burping up the "lumps" of liquid. He felt my gut feel was probably about right, if one went for a smooth bore pipe.
Yes, PVC pipe seems to be bringing in an element of risk that needn't be there.
I think it is appropriate here to clarify a myth about PVC. It is NOT a thermoplastic material. It will soften at elevated tempertures, but will never melt. when heated high enough, it will char and decompose (like epoxy, urethane and polyurethanes), releasing copious amounts of very toxic fumes...
Harmen is offline   Reply With Quote
Reply

Tags
exhaust, lease

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Seasick Writer Seeks Your Expertise EveOkay Meets & Greets 10 31-01-2014 12:40
Here's mah boat, Lend me your expertise thompsonisland Monohull Sailboats 23 31-10-2012 07:47
Another newbie is searching for expertise! MartiJurak Meets & Greets 4 16-05-2012 12:13

Advertise Here


All times are GMT -7. The time now is 07:25.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2021, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.