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Old 10-11-2007, 09:44   #16
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Originally Posted by MidLandOne View Post
I think that you will find that you are being unduly pessimistic regarding the use of uPVC pressure pipe given that it has a service life of at least 50 years (typically 100 years is quoted for it before rehabilitation in water and waste pressure pipework). On land PVC is used in much more onerous services such as for gas pipework. You mention it as being a cheap plastic - in my experience uPVC pressure pipe and its fittings are not cheap. Perhaps you are thinking of the like of uPVC domestic waste pipe and fittings which are cheap?

In the case of a yacht main engine or generator below the waterline as long as it is protected on the raw water suction side by an anti siphon loop and the exhaust discharge is overboard then the only possibility of flooding is from the engines own raw water pumping, not from back flooding from the sea.

I would suggest that the likely risks of that are higher from failure of pipework, joints, seals on or about the engine, including the water injection elbow and riser to the water lift muffler, or from failure of exhaust rubber hose (whether used for joints or runs) or water lift mufflers such as or similar to the Vetus ones commonly fitted which have a reputation for failure (the likes of Centek excepted) than it is from a properly installed uPVC pressure pipe wet exhaust.

Also, the materials commonly used in exhaust systems all have their problems including the hose that you recommend. As an example, I recently project managed the build of two big power boats for a client and which were designed and built to Lloyds rules (SSC). The main engine risers and exhausts (exhausts approx 8 inch) were 904L ss pipe to aluminium overboard penetrations - I can guarantee the life of that high quality solution will be much less due to corrosion and fatigue than that of smaller diameter uPVC pressure pipe properly installed in a small pleasure vessel.

In the case of the generator exhaust in the original posters question, if it were my problem I would take the exhaust from the through hull (which is only 3 inches above the water line and is also subject to flooding during heeling so cannot be classed as an overboard discharge on either count) from a valve at the penetration, which it not being able to be classed as an overboard discharge it should be fitted with, to 300mm above the maximum heeled waterline in ss or grp pipe, in reality probably to the top of the anti siphon loop mentioned. I would not use a lesser material such as rubber hose (nor uPVC) for this section as back flooding is possible if it fails even though I would expect the likelihood of failure here due to raw water cooling loss would be low because of the long run from the muffler.

Personally, and only personally given my experience with it, for the rest I would use uPVC pressure pipe but grp would be preferable - however, given the long length I would not like to be the one paying for running it all in grp. I would rate both those, properly installed, as being significantly more reliable than approved rubber hose which I would personally never use in an exhaust for anything other than flexible joints.
PVC pipe is fine for homes which are static situations where the pipe is well protected, but on a boat, pipes and hoses are subject to getting stepped on, exposed to hydrocarbons such as diesel in the bilge, they vibrate and they get banged against. This is not a good thing for a material that is relatively brittle and not completely resistant to chemicals. Besides, sometimes the Coast Guard really does know what they are talking about when it comes to safety on inspected passenger vessels.

Approved rubber exhaust hose is far more reliable than PVC for the same application.


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Old 10-11-2007, 10:20   #17
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Mark, I guess I do not understand the reason to do a repair twice. It is one thing to be under way and need to just get back to somewhere. But the real answer to your question is to replace the hose since you state it is bad and be done with it.

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Old 10-11-2007, 15:18   #18
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Originally Posted by David M View Post
PVC pipe is fine for homes...
...Approved rubber exhaust hose is far more reliable than PVC for the same application.
It seems that you are talking about low pressure PVC pipe such as used for wastes, etc. as uPVC Pressure Pipe is not a simple home product as you claim. It is used widely in industry and is regarded as very reliable, often as being the most reliable (do a Google search and you will see what I mean).

I do not have a paper copy of CFR Title 46 to hand (only the online version which is difficult to search) to look for the USCG requirements prohibiting uPVC from exhaust service as you claim so would you refer me to the relevant section in that.

However, as you refer to USA requirements, I have reviewed the ABS Guide for Steel Vessels Under 90m (it probably being a relevant guide as I recall the Guide for Building and Classing Racing Yachts does not specifically cover services) and there are no specific prohibitions of use of uPVC or in fact any other plastics in there that I can see, and does not differentiate between plastics - it concentrates on design about the mechanical properties of the plastic used, required chemical resistance and fire endurance (4-4-2 Table 2) just as it does for any other material.

Furthermore, it is a common practice of classification societies that they accept the results of proof by trial (and I have used the results of such trials to allow deviation from specific requirements in their rules for vessels entering class). In my own case I have a successful 11 year trial of uPVC to report showing no degradation whatsoever when cut into and filed compared to the new section I inserted to change the layout. Others I have discussed this with have found that information of interest.

It would seem that the original poster has not found exhaust hose as reliable as you claim it to be nor is it so in my own experience with delamination and other failure very common. In fact the faults you claim for uPVC pressure pipe (brittleness, lack of universal chemical resistance, damage from stepping on, etc, etc) all exist for hose designed for exhaust service.

Your claim of insufficient chemical resistance of uPVC to substances it is likely to be exposed to on board surprised me as I can find no evidence that is so. It is certainly has a very high resistance to diesel fuel, lubricating oils and in fact most other things apart from some solvents unlikely to be exposed to in a boat. Would you point me in the direction of your information that contradicts all that?

I have suggested an alternative and have now gone to quite some lengths to respond to your simple and unsubstantiated dismissals of it (my perhaps boring some ). People can take or leave that as they like as I am not going to test people's patience with it further here. However, I have found that people in the industry have not dismissed it as an option.

Getting back to the original question, uPVC is certainly fine for the temporary splice the poster wants to make and there is a wide range of suitable materials he can use for permanent rehabilitation (ALL with their own problems, with grp probably the best if cost is not a factor).
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Old 11-11-2007, 02:41   #19
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Originally Posted by David M View Post
... but on a boat, pipes and hoses are subject to getting stepped on, exposed to hydrocarbons such as diesel in the bilge, they vibrate and they get banged against. This is not a good thing for a material that is relatively brittle and not completely resistant to chemicals...
Chemical Compatibility:

CPVC: Diesel Fuel - Excellent
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PVC: Diesel Fuel - Excellent
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Old 11-11-2007, 04:52   #20
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If water can get in, then so can carbon monoxide. Colourless, odorless and fatal. Whatever you do, that exhaust system has to be absolutely gas tight.

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