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Old 10-01-2017, 08:33 PM   #1
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How wet should a wet exhaust be

Giday all

First sea trials on new engine fpt c13 825hp testing showed excessive back pressure due to volume of water output by raw water pump. This is known issue with these engines. Supplier has advised bypassing overboard. My installer has fabricated a bypass with a gate valve to regulate flow.

Next sea trials supplier will measure back pressure at full noise and adjust gate valve to bring pressure back to acceptable readings.

Old engine was a v8 700hp that had two exchausts connected to pong box (Aussie for water lift muffler). These where 6 inch. New engine is inline six with single 8 inch outlet. Pong box has be modified blocking one outlet and enlarging other.

The back pressure issue is at max throttle as raw water pump volume increases with rpm and so does exhaust flow.

My question is how to ensure there is enough water left after bypass to provide cooling not just at full throttle but also at lower rpm settings?

Should I check exhaust temps at all rpm setting and what is acceptable? My installer says as long as it not hot to touch all is good. I would prefer a more scienetifc approach I guess. I have a one of those laser temp sensors what should I be looking for temp wise?

As a side note the pong box is made from stainless steel covered in fiber glass my installer was surprised at this. Is this common?
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Old 10-01-2017, 08:39 PM   #2
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I would talk to the engine manufacturer and be more concerned about voiding the warranty.

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Old 10-01-2017, 08:43 PM   #3
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It sounds like the lift muffler and exhaust piping is too small. That could be a valid reason for a manufacturer to deny a warranty claim.

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Old 10-01-2017, 08:59 PM   #4
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As long as back pressure is acceptable manufacture warranty is valid. Supplier is warranty agent. The have calculated the 2x 6inch is more flow then single 8 inch. They advised at the start of this conversion that the output volume of raw water pump is very high on this engine and it is routine to bypass this flow outside of exhaust.

At this point I am more concerned about exchaust temp.

If after bypass back pressure is still to high other exchaust modifications may be required.

Can anyone confirm that 2x six inch is greater flow then a single 8 inch. Is there a calculation for this?

Issue is at full rpm something that I don't expect to use for any length of time in the real world.....
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Old 10-01-2017, 09:31 PM   #5
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The cross sectional area is equal to 3.14 x r squared. Where r is the radius. Therefore, the cross sectional area of an eight-inch diameter pipe is 50.24 square inches. The cross sectional area of a six-inch pipe is 28.26 square inches. However, there are two six-inch pipes. So the total cross sectional area is 56.52 square inches. One will get more flow from two six-inch pipes than from a single eight-inch pipe.
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Old 10-01-2017, 09:55 PM   #6
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Diameter is one thing, layout, elevations, number of bends and length of pipe another. Suggest you contact Tony Athens at Seaboard and run your questions and plans by him. For starters read his articles on marine exhaust system layouts. Also, have you looked at exhaust runs on Rivieras and Maritimos with similar HP?
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Old 10-01-2017, 10:19 PM   #7
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Sunchaser I have read all of seaboard excellent infrormation unfortunately being in Australia it's a bit far to tax tony with question without being able to reward him with any work as such. That's how I feel anyways. My marine engineer has put together a very nice riser that I feel follows seaboard practices closely. The exchaust has one bend before exiting to larger boxes running down the side of the vessel. I really don't think flow restriction is going to be any issue.

Certainly better then the PO setup... old red and new white...

original question is more about temps.
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Old 10-01-2017, 10:57 PM   #8
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Y

Posting your questions on Tony's Seaboard website would be most interesting and in the realm of experts rather than guessers. I foresee no objections on Tony's part. Sounds like you've already got things pretty well figured out. I'd be uneasy using a manual gate valve controlling bypass.

Ideally after the shower head 110F or less would be in the ballpark. If the shower head is not evenly distributing water, hotspots can arise. In my case at lower RPMs one of the lift mufflers has a spot that will reach 130F. Increasing RPM lowers the temperature as flow rate increases. The other muffler has no hotspots but is at a 90 degree offset, thus moving the water around a bit more.

Does the engine builder provide a pump curve?
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Old 10-02-2017, 12:04 AM   #9
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I saw a boat with a Gardner engine, the exhaust was wrapped with an insulated wrap and hung a right and went through a truck muffler and then through the side of the hull; the water did not go through the exhaust and was vented out the stern.

Maybe a little creativity might work, how about two pumps, the current high flow engine driven water goes out of the hull through the stern, a smaller, (maybe even electric although a variable flow would be better) pump goes through the exhaust? Choose the perfect flow for the exhaust, run the diameter of pipe spec'd by the manufacturer and live happily ever after (with the appropriate temperature monitors).
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Old 10-02-2017, 03:42 AM   #10
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I'd also be a little concerned about controlling flow with a manual gate valve.
Gate valves aren't really designed for throttling or controlling flow. They would be prone to blockage and/or washing out over time. A plug valve with a stem and cage would be a much better choice.
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Old 10-02-2017, 04:08 AM   #11
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I don't know the answer to your question about temps, but I would check the rating on the exhaust hose, and the muffler, then stay well below that. I seem to recall 200 or 250F for the hose rating. My quests is that 150F is around the max desirable temp for the wet side of the exhaust, but do seek manufacturer's specs, not my swag
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Old 10-02-2017, 06:03 AM   #12
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The gate valve I am using is pvc. From what I understand gate valves are good at regulating flow. Maybe I am mistaken.
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Old 10-02-2017, 06:43 AM   #13
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Quote:
Originally Posted by yarradeen View Post
The gate valve I am using is pvc. From what I understand gate valves are good at regulating flow. Maybe I am mistaken.
Gate valves are designed to be open or closed. To throttle flow, you want a globe or butterfly valve. A 90 degree ball or plug valve will work but will wear out pre-maturely.
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Old 10-02-2017, 08:01 AM   #14
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In my opinion Sunchaser's (below) is a pretty good answer. Fiberglass components built for exhaust applications can withstand a max operating sustained temperature of 180*F. In practice what your installer told you and what Sunchaser said, 110*F or less, is closer to acceptable. If you can put your hand on it and keep it there, its cool enough. Not scientific, but there is plenty of room until you get near max operating temps with that guide.

Also as you get further from the engine, the exhaust cools very quickly. The fiberglass coming out of the lift should be significantly cooler than the fiberglass going in.

What you describe, as an engine that supplies too much water, and the suggested fix, bypassing water, is common practice in these situations. Gate valves are the recommended device because of how the water flow is interrupted, as opposed to other type valves like ball valves and the like.

Your concern:

"My question is how to ensure there is enough water left after bypass to provide cooling not just at full throttle but also at lower rpm settings?"

should be unfounded if the bypass is adjusted properly, say a little less than max allowable at WOT. As for the cooling, the engine is pumping too much water, so if you bypass a little it should have little to no impact of cooling the exhaust system. Use your temp gun on various spots to check, I like to put a small piece of tape so I always check the exact same spot to get apples to apples data. Elbows, tops of lifts (pong box,) before and after spray ring, etc.

No worries, your installer sounds like he has it under control and you are in good hands, and they are going to sign off on the warranty anyway.

One other thing we like to do when bypassing water is ty-rap the handle on the gate valve so no one fiddles with it. Some people are too OCD to let in an engine room, and seeing a valve not fully open or closed might make someone like that a little crazy. Don't ask how I know.





Quote:
Originally Posted by sunchaser View Post
Ideally after the shower head 110F or less would be in the ballpark. If the shower head is not evenly distributing water, hotspots can arise. In my case at lower RPMs one of the lift mufflers has a spot that will reach 130F. Increasing RPM lowers the temperature as flow rate increases. The other muffler has no hotspots but is at a 90 degree offset, thus moving the water around a bit more.
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Old 10-02-2017, 08:34 AM   #15
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Have you considered a smaller pump?
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Old 10-02-2017, 09:00 AM   #16
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Nothing wrong with bypassing some sw flow overboard without all of it going out the wet exhaust. It takes a lot more water to cool the engine than to cool the exhaust.

Second that about not liking the gate valve. Gate is loose in mid position and can fret and wear. Better to use a globe valve or drilled orifice once you find the right position.

Regarding wet exhaust temps: You have to monitor wet exhaust piping temps at all power settings. Often the wet exhaust gets hot spots at low power settings as the water tends to lay at the bottom of pipes and hoses. At higher power things tend to be better as the turbulence forces good water/exh mixing. Get a pilot to run the boat and set bypass valve to get good backpressure numbers at full power, then check exh piping temps, then reduce power in steps and continue to check temps.

A good wet exhaust system runs about 20-40F above sea temp, but you could go higher. FG pipes and hose probably ok at 150F, not sure of exact limits. If I see 150F, something needs fixing.

You may be forced to build a new "pong box" more appropriate for the rated power. Eight inch for 825hp sounds a bit skinny to me. May need to go ten inch.
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Old 10-02-2017, 09:36 AM   #17
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Fitted two new C18's into one of our Catamarans based in Dongara a few years back the C18's were only rated @ 875 Hp, we passed all the water out the exhaust but used 10" pipes and custom pong boxes made in Sydney from GRP from a specialist exhaust guy, your pictures of the Pong box look way to small for 800+ Hp, the back pressures in our water cooled systems were fine and the rubber sections never went over 130 degree's, most of the new exhaust temp alarms are set at at approx the 150 degree mark (give or take),

Below couple of pictures to give you an idea of how big the Pong box should be

Cheers Steve
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Old 10-02-2017, 09:47 AM   #18
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The guy in Sydney (and damn good) is Brad Gorman at info@foreshoremarineexhaust.com,

Below is the drawing of the pong box to give you an idea of what size is all about,

Cheers Steve
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Old 10-02-2017, 10:43 PM   #19
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If ur engines are dumping hot dry exhaust out at 8 inches, then u add water, which could steam, you would need at least 10 inch pipe wet. Dual 6 inch is much smaller than a 10. Need bigger pipes and bigger pong and get rid of the valve idea.u might have to go to 12 however if u really have that much water. Do it right the first time and donít cook engines, not to mention having to worry about hot spots due to low water flow, is it really with sinking or burning the boat down? I am sure you have a great installer, just tell them to do it right and give them a break if they told u it should work this way. We all make mistakes.
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Old 10-02-2017, 11:02 PM   #20
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The guy in Sydney (and damn good) is Brad Gorman at info@foreshoremarineexhaust.com,
Very well known in the exhaust field as an aftermarket supplier.
I`m thinking the gate valve may have its value as a way of adjusting flow, to establish the optimal flow. Maybe then something replaces it, or maybe there are better valves which can do the experiment and remain as well.
To the OP, Sunchaser knows his stuff (I`m sure others do too)and is certainly capable of working his way through a thorny mechanical problem.
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