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Topic: Tropical jets fitted to carburetors? |
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Allan
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Posted on: Jul 27, 2013 - 8:31pm
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Guys:
Can anyone help me understand the following referene to "tropical jets"?
"Those first boats [in New Guinea waters] were not able to reach speeds that they had done in cold waters, but after tropical jets had been fitted their performance was much improvd."
Thanks for the help.
Allan
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bubbletop409
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Posted on: Jul 27, 2013 - 9:23pm
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For a gasoline burning engine to operate at maximum efficiency the air fuel ratio must be correct. An engine that was tuned to run in cool Atlantic air would be rich when run in the heat and humidity of the South Pacific, meaning smaller carburetor jets would have to be installed to correct the air fuel ratio and allow the engines to perform at maximum efficiency.
Larry
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Allan
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Posted on: Jul 28, 2013 - 4:33pm
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Larrry:
Thank you so much. This clears up something that has been at issue for a while. Very helpful.
Allan
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bubbletop409
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Posted on: Jul 28, 2013 - 9:39pm
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Allan
Your quite welcome, a jet is simply a replaceable orifice in a carburetor that meters the fuel. When the ambient air has a large amount of oxygen you can burn more fuel, as the quality of the air decreases, you must also decrease the amount of fuel you are inducing in the engine.
The use of the term "tropical jet" loosely translates to a jet size suitable for use in a tropical area, and is used in a generic sense as jets comes in a multitude of sizes.
Larry
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Stuart Hurley
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Posted on: Jul 30, 2013 - 12:07am
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Hi,
I don't think that would be the whole story.
Just replacing the jets alone would not completely cure the problem of performance loss in hot ambient conditions.
Correcting the rich fuel air ratio would make the engine run properly, but there would be a problem of reduced fuel and (hot) air density causing another power loss. Just as with aircraft at high altitude, it would be necessary to increase the fuel/air charge into the engine to regain performance. There was continual evolution throughout the war to increase the performance of the PTs by adding bigger superchargers and inter/aftercoolers to claw back losses due to temperature, hull fouling and increased weight.
Best Regards,
Stu.
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Jeff D
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 Posted on: Jul 30, 2013 - 12:51am
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That's why they couldn't fix the problem simply by using the jetting installed on the high altitude boats.
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David Buck
TOP BOSS
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Posted on: Jul 30, 2013 - 6:19am
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Is that the one that the Japs were afraid of?
Flap its wings ,roars etc. etc.
(next section to be added to your website ? Can't wait to see how that turns out)
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bubbletop409
MASTER
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Posted on: Jul 30, 2013 - 9:17am
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Jeff D
Could you please explain a high altitude boat. I thought all the oceans of the world were at sea level.
Changing jets alone will not allow an engine to operate at it's maximum efficiently regardless of ambient air conditions, but it will allow an engine to perform at it's maximum for the conditions present.
The quality of the air being in jested by the engine is measured by a term called corrected altitude. It is a combination of several factors including base altitude, temperature, and humidity or grains of water in the air. The closer to sea level the corrected altitude is, the greater the power potential of the engine or engines. It is actually possible to have a condition where the corrected altitude is below sea level, and when present the power potential is at it's greatest.
Some race tracks around the country located close to an ocean or bay will at certain times of the year have a corrected altitude the racers refer to as "mine shaft" conditions, meaning the corrected altitude is far below and better than sea level.
Larry
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Jeff D
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Posted on: Jul 31, 2013 - 2:02am
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I said it in jest Larry, Good explanation to Allan's question though. I didn't know they had the performance issue, I wonder if they tweaked the carbs for winter / summer use in areas with a wide variation of temperatures.
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Stuart Hurley
MASTER
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Posted on: Jul 31, 2013 - 2:31am
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Jeff,
That's what the smiley faces are for. The high altitude boats were always a handful.
I would be wary of being on being on a boat with fueling corrected for below sea level. 
Best Regards,
Stu.
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