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Old 01-13-2010, 09:11 PM   #1
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Installing a back pressure guage

I was told if I install a psi guage in the manafold pipe I can measure the exhaust Back pressure. I was told it could be welded in.* Could I drill and tap it instead. As I am not a welder but I do have a drill and a set of taps. Thanks*

SD*

-- Edited by skipperdude on Wednesday 13th of January 2010 10:13:40 PM
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Old 01-13-2010, 09:19 PM   #2
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RE: Installing a back pressure guage

Your exhaust back pressure should be in the range of a few of inches of water, for that you use a manometer, not a regular pressure gauge reading in psi.

As far as drilling and tapping, it all depends on how thick the material is.

Do you suspect high backpressure for some reason?
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Old 01-14-2010, 04:37 AM   #3
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RE: Installing a back pressure guage

IF the hole must be welded in , you may want to contemplate using the hole "later " for an Exhaust Gas Temp readout.

EGT is probably the best insurance for not overloading an engine.


Overloading is hardly ever a trawler problem , but EGT information is nice when you need to run hard.

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Old 01-14-2010, 08:56 AM   #4
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RE: Installing a back pressure guage

Quote:
RickB wrote:



Do you suspect high backpressure for some reason?
Doesn't want to tac up to* Wide Open Throttle . Light load clean bottom. The best I get is 2500. I am also getting black smoke at WOT. In nuteral she revs to 2800 rpm. Cat*3208 n.a.*I thought it was proped wrong. This started because I wanted to replace the 30 year old Muffler. It looks to be just a standard truck muffler that you see on any Semi Tractor.

I Am trying to understand all the specs Cat gave me on the exhaust system.* Cat sold the engine without an exhaust muffler to Halton tractor Co of Portland. Who sold it to the boat builder.**

To go with a Cat Muffler I would have to reconfigure the way the muffler is mounted. Not to mention they want about $2,000.00 for one.

I have a 24" rh prop I can't*remember the pitch I have it written down someplace. It has 1.97:1 Marine gear.

SD



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Old 01-14-2010, 11:04 AM   #5
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Installing a back pressure guage

I would check your prop size, I have an excel spreadsheet that I can send you that will calculate prop size. It seems to work pretty well, it was right on what the local prop shop recommended for my boat.


-- Edited by troy994719 on Thursday 14th of January 2010 12:04:11 PM
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Old 01-14-2010, 11:35 AM   #6
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Installing a back pressure guage

Cool!!
**Could you post it on the site. Maybe it would also be helpfull to others. *Or send a PM**Thanks**

SD

-- Edited by skipperdude on Thursday 14th of January 2010 12:35:39 PM
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Old 01-14-2010, 12:20 PM   #7
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Installing a back pressure guage

Quote:
skipperdude wrote:Doesn't want to tac up to* Wide Open Throttle . Light load clean bottom. The best I get is 2500.
99.9999 percent of the time this is due to the boat being overpropped.* Some manufacturers like Grand Banks over-propped their boats deliberately since they were intended to be cruised at slow speeds at relatively low power settings so over-propping by an inch or two yielded a bit more speed without lugging the types of engines used in these boat enough to worry about.

Our boat was set up this way.* When we thought we needed new props the prop shop wanted to know the type of engines we had (FL120), the max hp rating (120hp) the max rated rpm (2500), and the rpm we achieved at wide-open-throttle (about 2200 on one engine and 2300 on the other).* This information, plus being given the props that were on the boat, allowed them to determine the proper prop specs for our boat.

As it turned out we did not need new props, the shop could rework the ones we had which had been horribly set up in San Francisco before we bought the boat.* They repitched the props from 17" and 18" (the stock settings) to 16" on both sides.* While we never operate this way, the engines will now go to their max rated rpm and the load on the engines has been reduced throughout the rpm range.

If your engine achieves the proper max rpm with no load, I would suspect the exhaust system is not the issue.* Most likely the boat is overpropped, which depending on the type of boat and the way it's meant to be operated, may or may not be an issue.* Black smoke is simply too much fuel being fed to the engine for the power being developed.* If your boat is doing this at normal cruise power this would indicate an engine problem of some sort.* If it's doing it at WOT it is an indication that the load on the engine is too high, which--- assuming no super-high friction in the driveline--- would be the result of over-propping.

*


-- Edited by Marin on Thursday 14th of January 2010 01:21:40 PM
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Old 01-14-2010, 12:35 PM   #8
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RE: Installing a back pressure guage

I cant upload a Excel spreadsheet to the website, anyone that wants it can PM me with an email address.
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Old 01-14-2010, 12:49 PM   #9
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RE: Installing a back pressure guage

Marin,
* I was underthe asumption *that a diesel should be loaded (WOT) *for at least 5 to 10 minuts for every 4 hours of operation. Do you ever load your diesel?

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Old 01-14-2010, 01:14 PM   #10
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Installing a back pressure guage

Dude: You are misinformed about the 10 minutes at WOT for every 4 hours of normal operating time. That is a good way to have an early diesel demise, especially if you are over propped.*The most rigorous number, from a reliable source, *for a pleasure rated diesel I have seen is 70 to 80% load for 15 minutes out of every 8 hours. This issue got a thorough airing on boatdiesel about a month ago - good reading. FF is right - go with an EGT if you add anything. And for grins you can always take out your Cat muffler (and replace with a straight run) for a test to see if RPMs go up, but over prop sounds likely.

-- Edited by sunchaser on Thursday 14th of January 2010 02:34:06 PM
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Old 01-14-2010, 01:39 PM   #11
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Installing a back pressure guage

I bet you are right. She was a commercial boat to start, so full of fish or ice.* Likely set to push a load . I will check boat diesel. Thanks*

SD*

-- Edited by skipperdude on Thursday 14th of January 2010 02:41:23 PM
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Old 01-14-2010, 01:40 PM   #12
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RE: Installing a back pressure guage

"This started because I wanted to replace the 30 year old Muffler."

Does this mean that at some point since you got it the boat ran fine with*no smoke and reached full rpm or did the smoking start recently and you believe it is due to the muffler?

If the boat ran fine until recently, it is quite possible that the internal baffles have failed in the muffler. It is also possible you have an air inlet issue, dirty filter?

Do like Sunchaser suggested, remove the muffler and try it before jumping in the deep end.

I would think that unless you just changed props or tweaked yours enough to create an overload, this problem would have been around for a while and you or someone else would have tried to chase it down.*
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Old 01-14-2010, 01:45 PM   #13
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RE: Installing a back pressure guage

Rick

when I got her I new nothing about diesel engines so I never pushed the throttle at all. Only after*I learned that WOT would not cause it to explode, that I found this issue with the RPM's. and the black smoke.

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Old 01-14-2010, 01:51 PM   #14
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RE: Installing a back pressure guage

"Likely set to push a load hard ..."

In which case running light should keep you far away from smoking due to overload.
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Old 01-14-2010, 01:57 PM   #15
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RE: Installing a back pressure guage

"Only after*I learned that WOT would not cause it to explode, that I found this issue with the RPM's. and the black smoke."

In that case, take the muffler off and run it to see what happens. Did the previous owner change the prop just before you got it or is it the same prop as was always installed?

What engine is it?

The point I am trying to make about the prop is that unless*it has been replaced recently, it would not overload the boat when running free. And if it overloads while running free (empty) then the boat would have had real problems with any kind of load onboard much less a full fishhold.
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Old 01-14-2010, 02:12 PM   #16
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Installing a back pressure guage

Wow to much information./ Old fishboat guy hit it on the head she does plow down when I open her up. Any thing past hull speed she dig down into the water and I push a bow wave like a snow plow. Prop is the same as was on her. It's a cat 3208 n.a. 210 hp

-- Edited by skipperdude on Thursday 14th of January 2010 03:13:45 PM
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Old 01-14-2010, 02:26 PM   #17
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RE: Installing a back pressure guage

"Any thing past hull speed she dig down into the water and I push a bow wave like a snow plow."

And that is*one definition of hull speed ... when you don't have a boat big enough to carry an engine big enough to go any faster.
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Old 01-14-2010, 02:44 PM   #18
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RE: Installing a back pressure guage

Quote:
oldfishboat wrote:


RickB wrote:

"Likely set to push a load hard ..."

In which case running light should keep you far away from smoking due to overload.
*

The*Skipper dudes Vessel looks more like a semi displacment that could add into this ?*

> ?*

*So is this a semi displacement hull?



*
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Old 01-14-2010, 02:49 PM   #19
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Installing a back pressure guage

"My understanding is this creats issues when you start to come over hull speed."

For sure but hull speed is what it is and unless he has a few thousand horsepower that boat isn't going to go much beyond hull speed. Trying to push it faster is a waste of fuel and really hard on the engine. It might get a tiny bit faster when trimmed better but slightly bowdown is still the better trim.

Those trollers run empty most of their lives and they run them fast to get out of town so if the current prop is the original and it hasn't been bent ( a small tweak can cause big load problems) that boat shouldn't overload running light.

With a full fishhold and a stern trim it takes a lot of power to move that boat and I doubt if it ran with the engine overloaded all its life and I really doubt if it spent 30 years with a driver who never put the throttle all the way up.

Ask the previous owner how he ran the boat. But after 30 years maybe it's worth pulling the muffler off and seeing how it runs. A failed baffle can really cause some grief. Is the engine running hot as well?

"So is this a semi displacement hull?"

It looks like a displacement hull to me, but with hard chines and aftermarket bilge keels. It would take a Westlawn grad to officiate a discussion of that hull type I think.*


-- Edited by RickB on Thursday 14th of January 2010 03:59:57 PM
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Old 01-14-2010, 03:22 PM   #20
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RE: Installing a back pressure guage

The P.O said he never ran it over 2150. He had her for 10 years. Befor that Who knows.
No issue with overheating. Had an issue with the water heater and the red dot type engine heat being higher than the engine but a back flow preventer? (one way inline valve) fixed that.

I will try pulling the muffler this spring. Thanks for the info.

SD
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