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 Author  Topic: Question for PT Boat Experts
TGConnelly

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Post a Reply To This Topic    Reply With Quotes     Edit Message   Posted on: Dec 27, 2008 - 9:53am
Thank you for setting me straight ...

Garth


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Ray Wilbur

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Post a Reply To This Topic    Reply With Quotes     Edit Message   Posted on: Dec 27, 2008 - 1:32pm
I think they thought it would get the boat up and plane quicker.

Ray

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victorkchun

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Post a Reply To This Topic    Reply With Quotes     Edit Message   Posted on: Dec 30, 2008 - 10:40am
Hi Ted,
This is the first time I post on message board. I hope you get it ok.
Thank you Ted for your answer, especially how you indendified the boats.
The steps as mentioned in your answer were called louvres according tto Elco's note on the back of the original photo. I think the louvres were
part of the Elcoplane system. The steps were fitted to the bottom of the
boat. Six of them can barely be seen below the louvres in the second picture of PT 487. The purpose of these steps was to help lift the hull
further out ofthe water thus reducing drag and increase speed. However,
can you explain how the louvres help the system reduce drag. Jeff D
mentioned in his answer about introducing air behind the steps under
the hull to reduce drag. Do you think the louvres have anything to do
with that? If so, how does it work?
Happy New Year and smooth sailing.
Victor

Victor K Chun

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David Buck

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of David Buck   Send Email To David Buck Posted on: Jan 6, 2009 - 3:22am
Hi, Victor the idea behind the Elcoplane was to take advantage of the stepped hull design. This has the effect of creating an area under the hull that rides on a layer of air bubbles, these bubbles lower the boats surface tension or drag thereby allowing the boat to achive higher speeds with less power/effot. The idea of bolting the steps/louves on to the hull was so that a new hull design did not have to be made during wartime production.
For further info and to see drawings of Vospers approch to the same idea see page 58 and 59 of Allied Coastal Forces of World war 11 by John Lambert and Al Ross.
By the way the Vosper idea did not make it to a fighting boat either the trial boat ending up target towing after trials, but it is interesting to note that only two engines were fitted and she was quite fast with those two.

Cheers David.

D.buck

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victorkchun

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Post a Reply To This Topic    Reply With Quotes     Edit Message   Posted on: Jan 6, 2009 - 10:14pm
Hi David,
Based on your reply I concluded that the 6 louvres on the sides of the boat are sir intakes that provide air to the bottom of the boat as a layer
of air bubbles to lower the boat's surface tension or drag. If that's the
case, how did the air get from the louvres to bottom of the boat? Was
a fan or compressor uaed? I don't think the speed of the boat can
push the air down, for the openings of the louvres seem to be facing aft. Any idea?
Thank you for all the help.
Victor



Victor K Chun

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Dick

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of Dick   Send Email To Dick Posted on: Jan 6, 2009 - 11:00pm
Hi guys, just adding my two cents into the discussion,

Its my hunch, and hunch only, the items referred to as louvers are merely foils used to smoothly divert the water from the flat face of each of the haul brackets used to attach each plane step, hence reducing drag from structural components. If I recall correctly, somewhere in At Close Quarters there was a mention of the ELCO Planes as being five or six separate steps attached o the boom and to the sides of the boats with brackets. If my assumption and recall is correct, Photo 2 shows the unprotected metal brackets running down the side wall of the boat to the edge of plane step which is flush with the Chine. With photo 1 showing the louvers (Foils) in place. Also at any high speed the ELCO 80 footer haul would naturally begin to plane and lift itself out of water, where at least the first few louvers would be out of the water and have no effect whatsoever.

OR

I could be completely wrong!

All the best,
Dick . . . .




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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: Jan 6, 2009 - 11:43pm
I'd imagine the weight of the water would draw the air Victor. As the speed increased, the water would have to fill the gap behind the steps if air couldn't get in. That would mean lifting a considerable weight not to mention the water having to make a sharp turn up. So the vent holes would supply the much easier flowing air to fill the gap. At speed, most of the air was probably supplied from the side of the steps as the hull lifted up and brought them out of the water.

Modern design, as seen in the link I posted, seems to embrace a deeper step with a curved forward surface and flatter bottom on them. I don't think they need air vents, the deeper step cuts come up high enough on the sides to supply the air. I used to race an R/C catamaran hydro hull that had a stepped bottom, the cuts behind the steps were deep and rose up past the chine quite a bit.

Hydrodynamic experts can stop sniggering now. I'm done propounding theories on subjects I know little about.



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David Buck

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of David Buck   Send Email To David Buck Posted on: Jan 7, 2009 - 4:15am
Hi Victor, both Dick and Jeff are close to the mark. The steps under the hull would have some form of connection to the louves on the side of the hull, this now explaines why the chine is so wide.This connection could be as simple as a section chiseled through the original chine.

So what makes it work .

When the boat starts to move forward the water is sucked out of the steps this creates a vacumn within the step which is filled by the air coming through the conection from the louver.With vents at the rear so they do not fill up with water(dosen't work to well like that).The air then exits the step under the hull (this will continue as long as the vacumn i.e. forward moition is maintained) on exiting the step the air and water meet creating a certian amount of turbulance.This turbulance breakes the boats surface tension by a certian amount this then allows the boat to move forward with less drag acting on its hull.

Thus accounting for the extra speed achived

This also explains why the boat was hard to control at slow speeds without enough turbulance being created the steps and louves would become just extra drag creaters on an other wise clean hull

Surface Tension."Those forces acting on a boats hull in water and equal in all directions."Thats why they put those massive great Packards in them , damm good engines too.

As i said Dick and Jeff were close, hope this helps you Victor .

Cheers from an Aussie, David.

D.buck

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Steve Tuhy

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of Steve Tuhy   Send Email To Steve Tuhy Posted on: Jan 7, 2009 - 8:52am
I DON'T KNOW WHAT ITS WORTH, BUT INTRODUCING AIR INTO THESE STEPS SEEMS UNNECESSARY. THE HOLES IN THE SHEETMETAL SIDESTEPS SEEM TO BE FOR DRAINAGE. AT SPEED THE FREEBOARD DOESN'T CONTACT MUCH WATER. THE STEPS ON THE BOTTOM OF THE HULL WOULD BE EFFECTIVE THROUGH SIMPLE CAVITATION. THE AMOUNT OF AIR FLOW NEEDED WOULD BE TREMENDOUS. SINCE THIS WAS EXPERIMENTAL, I DON'T THINK THAT ELCO WOULD CUT LARGE HOLES INTO AN UNDELIVERED PT BOAT AND GET THEIR MONEYS WORTH. SLAP-ON STEPS MAKE MORE SENSE

Steve Tuhy

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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: Jan 7, 2009 - 12:31pm
Dick, I've been wrong so many times that almost any decision I come to about anything, I'll do the opposite.Except when fashion models are asking me to come to their place of course.

I still don't think that the 2nd image has the louver skins missing... The sun is low and forward in the 1st one casting deep shadows backwards. In the 2nd image the sun is high straight off the port making the shadows weak. And I'd think some structures would be visible if the skins were gone. The waterlines look like they're going over a curved surface too. Do you see that at all or is my past catching up to me?

One interesting bit is their height, almost snorkel-like. It would seem to point to low speed / high waterline use at least in part, maybe to help get on plane.

One of the profile plans I bought from the ptboats.org has the stepped hull, I'll try to dig it up and see if it can help explain anything.




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