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 Author  Topic: PT Exhaust and Muffler Systems
Michael Vorrasi

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of Michael Vorrasi  Posted on: Sep 26, 2013 - 11:26am
Questions for the Motor Mac experts.

I understand how Elco boats handled engine exhaust. Flaps closed, exhaust goes though the mufflers and exits under water. Can only be used at low power. High power requires un-muffling by opening the flaps and having the exhaust exit straight back out of what is now basically open headers in drag racing parlance. Open the throttles with closed exit flaps and you will likely blow the mufflers off the back of the boat due to excess back pressure, or kill the engines altogether. At least that is what I have read.
Am I right so far?
Next question. How did Higgins boats, with the same three Packards, do it? They have exhausts and mufflers that exit underwater. Was there a way to un-muffle a Higgins? Was the added back pressure from exiting underwater all the time an issue for Higgins boats when throttles were opened up? And if not, why was it an issue for Elcos?


Mike

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bubbletop409

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of bubbletop409  Posted on: Sep 26, 2013 - 10:11pm
I'll venture a guess and say as the Higgins speed was increased and the hull rose up on a plane, due to the way the Higgins exhaust outlets were cast they allowed the free flow of the exhaust.

The Higgins outlets were shaped so that they directed the flow down and to the rear. There were also deflection plates mounted to the hull directly in front of the exhaust to divert water away. As I see the system, it appears to me as speed was increased, so was the amount of water directed away from the exhaust outlets, in effect reducing the back pressure in the system, and allowing the engines to perform up to their potential.

But then again I could be all wrong, but I don't believe the Higgins had mufflers like the Elco's, at an idle the water was the mufflers, and at speed it didn't matter.

Larry
62 Bel-Air
260 Eagle EXP
79 Cole TR-2

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  Jerry Gilmartin

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of Jerry Gilmartin   Send Email To Jerry Gilmartin Posted on: Sep 27, 2013 - 1:36am
Hello Michael and Larry,
I think both of you may not have a totally clear picture of what is going on here. First and foremost is the Higgins Mufflers can exhaust either below or slightly above the water level depending on the position of the internal butterfly valve damper. Also instead of saying "high power" it should be more precisely termed as "1200 rpm" for both types of boats. For comparison, the top engine rpm is 2900 at full throttle. The Higgins Mufflers did have wooden wedges "deflection plates" just forward of them on the hull, but they were actually 'spray reducers" to reduce the amount of water spray coming up onto the deck and not to help lower exhaust backpressure as you surmise. I myself have personally installed these wedges onto our mufflers on PT658 and they really do help a lot in keeping the passengers dry when we are underway. I have copied a blueprint showing a cutaway of the Higgins Muffler here. It also has a 5 inch Damper Butterfly valve inside of the casting that serves to direct exhaust flow down into the water vice out along to top of the water. (the butterfly is within a few inches of the aft outlet of the casting), The butterfly is operated from within the engine room (Pull to Close) with a ball handled "choke cable" type control linkage. The butterfly is normally left open except when the order to close them is given from the helm. Whenever starting the engines you must ensure all butterfly valves are open or you will damage the butterfly valve, as well as the engine, the exhaust pipes (stacks) and valves as the engine hits higher rpm's. I also included some explanations of how the muffler system works from the Melville Motor Mac Training class textbook so you can get a better idea. Operating instruction #3 labeled Paragraph E "Maintenance of Higgins Mufflers" explicitly states the Higgins Mufflers exhaust "aft just above the water" when the butterfly is open. I hope this was helpful.

Jerry PT658 Portland

Cutaway of Higgins Muffler casting shows butterfly within



Muffler operation part1

Muffler Operation part 2


Muffler ops part 3



Muffler operation part 4 (last one!)



Jerry Gilmartin

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Michael Vorrasi

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of Michael Vorrasi  Posted on: Sep 27, 2013 - 8:15am
Jerry, Outstanding! I had not realized that the Higgins mufflers had flaps internally, similar to Elco external flap valves, that directed open un-muffled exhaust out and back but above the water, or down through the mufflers and into the water. So in reality, the operation was fairly similar to Elco, just a different layout
Thanks,

Mike

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Jeff D

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of Jeff D   Send Email To Jeff D Posted on: Sep 27, 2013 - 8:36am
Like Mike said Jerry, thanks from me too for taking the time to give us a detailed, illustrated answer.



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Higgins Fan

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of Higgins Fan   Send Email To Higgins Fan Posted on: Sep 27, 2013 - 8:57am
Good stuff! BTW
PT-305 is missing one exhaust muffler. So does anyone know where I might find a Higgins exhaust muffler?

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National WWII Museum
PT-305 Restoration
Phone: 504-528-1944 Extension 375
http://www.nationalww2museum.org/see-hear/collections/artifacts/pt-305.html
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bubbletop409

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of bubbletop409  Posted on: Sep 27, 2013 - 10:04am
Thanks for the correction Jerry, that certainly clears up the picture in my mind of how the Higgins system operates. I had seen pictures of the castings before, but was unaware they had a butterfly and two separate outlets.
Now I have an Elco question, were the Elco mufflers true mufflers in the respect they had sound deadening capabilities and packing or were they just a sheet metal version of the Higgins casting that only served to divert the exhaust depending on the rpm and the Captains orders?
Thanks for the help.



Larry
62 Bel-Air
260 Eagle EXP
79 Cole TR-2

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Will Day

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of Will Day   Send Email To Will Day Posted on: Sep 27, 2013 - 11:14am
Once again, Jerry - you are the man!

Will

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Roy Forbes

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of Roy Forbes  Posted on: Sep 27, 2013 - 7:15pm
I never could figure out how they worked on a Higgins. You ROCK Jerry !!


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  Jerry Gilmartin

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of Jerry Gilmartin   Send Email To Jerry Gilmartin Posted on: Sep 27, 2013 - 8:55pm
Thanks you guys!
Yeah I was recently looking at our mufflers on the PT658 before we put her back in the water and stuck my arm up inside the muffler to check the bolts holding on the zinc. We removed our butterfly valves from our mufflers because we don't want to reduce the awesome sound of those Packards, and besides we don't have to worry about sneaking up on any enemy barge traffic! Our crew actually prefers the loud rumble of those un-muffled engines!

I don't know where to find any more of those muffler castings Higgins Fan but have you looked in the Dagsboro Bay Delaware anchorage where the PT308 was demolished? I bet if you used that strong ground penetrating radar or that "sidescan sonar" used by Dr Ballard in finding PT109 torpedo tube it may be able to find what a metal detector cant? In heard that Jerry Strahan already had some divers use a metal detector with no luck.


Here is a photo showing the Muffler butterfly valve control linkage operator balls. Do you see them just above the aux generator control box on the bulkhead?



Here is the other 3 just above the other generator control box (look for 3 black balls)





Jerry Gilmartin

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