wet exhaust below waterline advice please

contessaman

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Engine replacement is progressing and my thoughts turn to the exhaust system.

bearing in mind the old MD21b died from taking seawater down a bore, I'm keen not to have a repeat.

Currently in the boat is a 75mm rubber exhaust. starting at the transom it exits 6" or so above W.L.
inside the boat the hose runs in a big loop almost up to deck level ( swan neck?) before running back down below the floorboards in aft cabin. (by now well below W.L ) there is a cylindrical stainless and rubber silencer box although this hardly has enough capacity to call it a water trap, before the exhaust runs further down hill towards the engine.

hardly a surprise that water ended up where it shouldn't and surely not an original installation from a well regarded manufacturer.

New engine is 2" diameter. So, as I understand it. I need a downhill run from the engine into a suitable waterlock. then the exhaust runs uphill toward the rear of the boat. at the transom it goes way up high to form a swan neck. The water lock needs to be able to hold the capacity of all the hose uphill of it calculated by pi r squared x length of hose.

Finally I need a syphon break above the waterline in my raw water input between inlet seacock and raw water pump.

Is this all correct? also, who makes an enormous waterlock because with a centre cockpit boat with a long exhaust run , at a first glance the vetus waterlocks aint gonna have the capacity.

anyone out there made their own water lock from Grp? was thinking it could then be made to fit shape of inside of hull?

grateful for all advice, experience , opinions.

I want to get it right from the outset.
 

prv

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Is there any scope to have the high loop immediately after the engine, then a downwards run to the transom after that? Would reduce the amount of run-back water you need room for.

If you're really concerned about back-flooding (and after losing an engine I can see why you might be) then there's always the option of a big ball valve. Have a bowden cable to operate it remotely from a convenient place. I'm told that there's no risk of accidentally running the engine against a closed valve, because it simply won't start.

Also, the siphon break has to go after the pump, not before it. Otherwise the pump will be sucking in air through the valve.

Pete
 

Tranona

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Suggest you look at the schematics in the Vetus catalogue for guidance on wet exhaust systems (P 84 in the latest version). This has the watertrap calculation formulae and the indicative height dimensions. You are however, basically correct with one crucial exception. The antisyphon valve goes after the raw water pump, not before. Its exact location depends on the detail plumbing. On my Volvo for example it goes immediately after the pump and before the heat exchanger. On the Nanni in my other boat it goes in the hose between the exhaust manifold and the injection into the elbow.

Hope this helps.
 

contessaman

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Thanks guys. Of course you are both right on the anti syphon valve. There's one there but I'm going to renew it anyway.

no real way of getting the loop in early in the engine bay - the exhaust comes off the manifold then has to go immediately through a bulkhead that separates the engine room from the aft cabin. due to the geometry of the boat it achieves this in a down hill manner such that the exhaust exits the bulkhead beneath the floorboards in the rear cabin. The bilge is only about a foot deep in this location so I can see why only modest box has been fitted.

My Idea of making my own grp watertrap is appealing. Im pretty good at fibreglass fabrication.
I could make a shaped trap that fit nicely into the vee of the bilge that would maximise capacity given the limited space. excuse the child like drawing but if I made something up that worked as per the attached image.... would that be okay?

I could line the hull with cling film to. make a mould for it and I could use off the shelf GRP tube for the exit...

thoughts?

also, pete, what exhaust did your 352 have? must have been a similar arrangement to my 38 originally

cheers guys
 

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prv

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also, pete, what exhaust did your 352 have? must have been a similar arrangement to my 38 originally

You might be mixing me up with someone else - but my Maxi 34 has the exhaust coming out of the engine, into an off-the-shelf cylindrical plastic waterlock just behind it, then sideways into the cockpit locker. It runs out to the hull side, lies against it up to the gunwale, then turns inwards under the side-deck to get a bit more height when the boat's heeled well over. Then just before reaching the underneath of the cockpit seat, it turns 180º back outboard and then curves back down the hull side to a 90º skin fitting just above the waterline.

That's more or less as-fitted; I shoved it about a bit while reorganising the plumbing in the locker last winter, but I reused the same hose for the exhaust and its route is essentially the same. The previous owner wasn't very hands-on (the boat was mostly out on charter and maintained by the charter company) so I doubt he modified it from the original.

Pete
 

ianabc

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We have an exhaust system for our indboard diesel (Yanmar)

that differs from the conventional.

Often used on 1950 power boats

Moyer Marine has a diagram here


http://www.moyermarine.com/forums/showthread.php?t=7024

Read down to second post and diagram......

Modified Standpipe

With cap over pipe

Has the advantage of being fire resistant

also does not thave the disadvantage of allowing a heavily or continually cranked engine to flood.
 

contessaman

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If you have a Md21 dying of exhaust problems it's taken quite a while to develop!

got the Gippos to take it away for scrap! that or a mooring weight was without doubt the best use for that old pile of junk. ruddy thing was a peugeot painted green anyway!
 

contessaman

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You might be mixing me up with someone else - but my Maxi 34 has the exhaust coming out of the engine, into an off-the-shelf cylindrical plastic waterlock just behind it, then sideways into the cockpit locker. It runs out to the hull side, lies against it up to the gunwale, then turns inwards under the side-deck to get a bit more height when the boat's heeled well over. Then just before reaching the underneath of the cockpit seat, it turns 180º back outboard and then curves back down the hull side to a 90º skin fitting just above the waterline.

That's more or less as-fitted; I shoved it about a bit while reorganising the plumbing in the locker last winter, but I reused the same hose for the exhaust and its route is essentially the same. The previous owner wasn't very hands-on (the boat was mostly out on charter and maintained by the charter company) so I doubt he modified it from the original.

Pete

Ahh, wrong pete! there are a few on here. Yours sounds interesting. I do have a cockpit locker next to my engine bay. hadnt really thought about going sideways. so engine, downhill a bit to water trap, then sideways in locker up to gunwhale level, then a gentle run down aft to the transom?
 

prv

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so engine, downhill a bit to water trap, then sideways in locker up to gunwhale level, then a gentle run down aft to the transom?

Are you confirming my run, or musing about yours? To be clear, my exhaust doesn't exit at the transom, it comes out the side about eight feet ahead of the stern.

Pete
 

Tranona

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Ahh, wrong pete! there are a few on here. Yours sounds interesting. I do have a cockpit locker next to my engine bay. hadnt really thought about going sideways. so engine, downhill a bit to water trap, then sideways in locker up to gunwhale level, then a gentle run down aft to the transom?

You will find the latest type of exhaust hose is much more flexible than the old wire bound stuff, making runs through tight spaces easier. I used Halyard Marine hose, but you can get similar from Vetus or ASAP. Halyard also do GRP water traps in a variety of sizes and configurations. The Vetus traps are also very versatile both in shapes and the orientation of the inlet/outlets. Best to try and find a location for the trap that gives the necessary fall and work around that than follow the existing routes. The final loop is best done immediately before the outlet and you might find the vetus goosenecks useful if space is tight.
 

smithy

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Are you sure you need such a big waterlock? My understanding of it is that the waterlock only needs to take about 25% of the hose volume as much of the volume will be gas. What are you getting if you use the vetus formula?
 

contessaman

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Are you sure you need such a big waterlock? My understanding of it is that the waterlock only needs to take about 25% of the hose volume as much of the volume will be gas. What are you getting if you use the vetus formula?

well, that'd change the game for sure. what is the formula..../
 

MM5AHO

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Anoth thing to be careful about is the orientation of the water trap - when considering boat heeling.
I made this error when re-locating one of those swan neck type vetus water traps. Ended up that it back filled engine on a decent heel to port (only).
I relocated it amid ships again and used the circular type, where heel is far less trouble.
 

macd

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Are you sure you need such a big waterlock? My understanding of it is that the waterlock only needs to take about 25% of the hose volume as much of the volume will be gas. What are you getting if you use the vetus formula?

+1...and with Tranona's earlier post #3.
According to Vetus, in practice the exhaust will hold no more than 25% water, but they suggest a 100% safety factor, i.e. 50% water. Easy enough to work out arithmetically, but also no great problem to work out empirically with a measuring jug. With a below-the-waterline exhaust, an anti-siphon is a given, but the sums will determine the appropriate volume of waterlock.
 
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