The PT Boat Forum
http://www.ptboatforum.com/cgi-bin/MB2/netboard.cgi

ť Forum Category: PT Boats of WWII
http://www.ptboatforum.com/cgi-bin/MB2/netboard.cgi?cid=101&fct=showf

ť Forum Name: PT Boats - General
http://www.ptboatforum.com/cgi-bin/MB2/netboard.cgi?fct=gotoforum&cid=101&fid=102

ť Topic: Tropical jets fitted to carburetors?
http://www.ptboatforum.com/cgi-bin/MB2/netboardr.cgi?cid=101&fid=102&tid=3284

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

Posted By: Allan | Posted on: Jul 27, 2013 - 8:31pm
Total Posts: 161 | Joined: Sep 18, 2007 - 7:07pm

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

Posted By: bubbletop409 | Posted on: Jul 27, 2013 - 9:23pm
Total Posts: 164 | Joined: Apr 22, 2013 - 11:48pm

Larrry:

Thank you so much. This clears up something that has been at issue for a while. Very helpful.

Allan

Posted By: Allan | Posted on: Jul 28, 2013 - 4:33pm
Total Posts: 161 | Joined: Sep 18, 2007 - 7:07pm

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

Posted By: bubbletop409 | Posted on: Jul 28, 2013 - 9:39pm
Total Posts: 164 | Joined: Apr 22, 2013 - 11:48pm

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.

Posted By: Stuart Hurley | Posted on: Jul 30, 2013 - 12:07am
Total Posts: 255 | Joined: Mar 19, 2013 - 3:32am

That's why they couldn't fix the problem simply by using the jetting installed on the high altitude boats.

Posted By: Jeff D | Posted on: Jul 30, 2013 - 12:51am
Total Posts: 2200 | Joined: Dec 21, 2006 - 1:30am

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)

D.buck

Posted By: David Buck | Posted on: Jul 30, 2013 - 6:19am
Total Posts: 332 | Joined: May 4, 2008 - 2:59am

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

Posted By: bubbletop409 | Posted on: Jul 30, 2013 - 9:17am
Total Posts: 164 | Joined: Apr 22, 2013 - 11:48pm

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.

Posted By: Jeff D | Posted on: Jul 31, 2013 - 2:02am
Total Posts: 2200 | Joined: Dec 21, 2006 - 1:30am

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. [:-happy2-:]

Best Regards,
Stu.

Posted By: Stuart Hurley | Posted on: Jul 31, 2013 - 2:31am
Total Posts: 255 | Joined: Mar 19, 2013 - 3:32am

OK all jokes aside (at least for a minute or two) there are a number of Lakes and Inland seas that are either a certain amount above sea level or below sea level this I know from my friendly Dive Instructor.

Why does this matter the main reason is that when diving at high altitude you have to be very careful with the decompression tables, IE; rather fatal if you get it wrong, same for the other way round however this does lead into the area of feeding an engine on your friendly PT Boat, if a boat ever ended up on any of these bodies of water there would have to be various mods. needed to keep the Packards running to their peak output, but then I really would like to see someone drag a 50 ton PT Boat up to a 10,000 ' lake just to prove a point.

Told you it would only be a minute or two!!!

And I still think it would look better with Wings!

D.buck

Posted By: David Buck | Posted on: Jul 31, 2013 - 5:55am
Total Posts: 332 | Joined: May 4, 2008 - 2:59am

I remember the 41 boat was destined to be transported to an inland lake to be used as a gunboat, but she never made it. Don't know if the lake was at sea level or not but if she had made it, her engines would have been in such bad shape by then that speed parameters would have probably been a mute point. . .

Will

Posted By: Will Day | Posted on: Jul 31, 2013 - 11:43am
Total Posts: 1955 | Joined: Oct 8, 2006 - 4:19pm

Lake Lanao in central Mindanao is 2300' above sea level. I've read that while the 41 boat had to be destroyed in route, her engines actually completed the trip.

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

Posted By: bubbletop409 | Posted on: Jul 31, 2013 - 5:07pm
Total Posts: 164 | Joined: Apr 22, 2013 - 11:48pm