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 Author  Topic: ELCO and HIggins Planing Speed and Trim Angle
Andy Small

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of Andy Small  Posted on: Jan 5, 2014 - 9:20am
Been looking, but have not located the following info posted or mentioned:

1) I'm trying to find out at what speed the war loaded ELCO 80' and Higgins 78' started planing in relatively calm water?

2) At what trim angle were the boats prior to planing, and then throughout their planing speeds?

3) Also, could planing speed be maintained using just a single prop? two props? at what engine rpm?

4) What configuration (1, 2 or 3 props and rpm) and speed gave the maximum combat radius for the boats?

Thanks for the help!
Andy


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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: Jan 5, 2014 - 11:13am
While I have no direct answer for your question regarding PT's, I can tell you from actual experience in my 28' offshore style boat that it takes more speed to get up on a plane, than it takes to maintain a planning attitude' once on top of the water.

I can also tell you overall weight and balance play a major part in how the boat reacts. A full crew with personal gear in my boat will require different trim and speed to plane, than it does with just my wife and I.

I have also read that in combat it was better to fire the stern torpedo's first as the boat would be in better trim.

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

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  Jerry N Gilmartin

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of Jerry N Gilmartin  Posted on: Jan 5, 2014 - 11:31am
Hi Andy,
In operating the PT658 for the last 10 years, I have determined that it takes a speed of about 23 knots before our Higgins 78 footer will get up on the step. I am relatively certain the 77 ft Elco and the 80ft Elco could get up on the step at a couple of knots less than that, say 19 or 20 knots. I am not too sure what you mean by "what trim angle" We do not have any trim angle adjustment capability, like that on a modern day Outdrive. I suppose on a small Inboard/Outboard motor boat, you can adjust the angle of the outboard drive, and that is the "trim angle" that you are referring to? I want to assure you there is no such trim adjustment on either the 80 foot Elco or the 78 foot Higgins PT Boats. The propeller shafts always maintains the same angle as they penetrate through the bottom of the boat. If you are talking about what angle does the boat rise up out of the water as a function of speed, I dont even know how to measure that.

As far as how many engines needed to get up on the step, you must have at least 2 engines running to get it up on the step. One simply cant push it fast enough. You can look at the chart to determine how many rpm it takes to get to 20-23 knots which would get the boat up on the step. It is different for 2 vs 3 engines. The cruising radius vs engines running is determined by a chart, but common sense tells us that with 1 engine running you can go much further. See the chart below taken from the "Motor Torpedo Boats Tactical Orders and Doctrine" book available on the internet at the HNSA website.

I hope that answers some of your questions. Jerry

TABLE 4.-Gasoline consumption with Admiral (29-30) wheels -77' Elco boat


ONE-ENGINE OPERATION
R. P. M. Knots Gallons/hr Cruise radius
700 8.3 17.5 1,420
800 9.5 22.5 1,270
900 10.3 28.8 1,070
1,000 11.1 31.7 1,050
1,100 11.8 39.2 903

TWO-ENGINE OPERATION

800 11.8 45.8 772
900 13.2 62. 5 633
1, 000 14. 7 70. 8 623
1, 100 17. 0 75. 8 672
1, 200 19. 3 80.8 718
1, 300 21. 5 87. 5 740
1, 400 23.5 100.8 704
1, 500 25. 1 120. 8 625

THREE-ENGINE OPERATION

700 10.4 46.6 669
800 12.8 72.5 531
900 15.2 81.7 558
1, 000 17.2 100.0 516
1, 100 19.4 107.5 542
1, 200 21.8 129.2 506
1, 300 25.0 140.0 536
1, 400 27.2 145.8 560
1, 500 28.4 162.5 524
1, 600 29.5 175.0 506
1, 700 32.4 183.3 530
1, 800 33.2 200.0 498
1, 900 34. 0 234.2 436
2,000 34.9 292.5 358
2, 100 37.2 314.2 355
2, 200 39.4 350.9 336
2, 300 40.2 426 6 282
2, 400 40.9 474.2 259

Sea-Moderate.
Displacement 94,500.
Draft aft-5 feet 3 inches.
Draft forward-2 feet 5 inches.

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Andy Small

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of Andy Small  Posted on: Jan 5, 2014 - 3:10pm
Thanks all, great info!

The Trim Angle I'm referring to is the angle between the surface of the water and the boat's angle of attack ( how high the bow raises up). It would be measured by a pitch/roll gauge (if you have one).





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Andy Small

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of Andy Small  Posted on: Jan 14, 2014 - 11:11am
I looked over the ELCO 77 graphs in theTactical Orders and Doctrine and in some cases it was hard to recognize when the planing step took place. When looking at a speed vs rpm graph of a planing hull, you normally see a notch where suddenly the speed increase much faster in relation to the rpm due to the lower resistance of the hullcommencing to plane. This would also normally be a point where the trim angle would drop (the nose of the boat lowers). Under two wing props, the 77' has a nice obvious point (15-17 knots and 1000-1100 rpm) where it looks to be planing. With the 3 props all going it looks like it almost has three different points where the hull starts to do some sort of planing (approx 21 knots, 30 knots and 35 knots) but I am not very sure of this hypothesis.

For the 80' ELCO, I did find a comment in the PT 105 book by Keresey (page 52) that the 80 ft ELCO had a critical speed of 27 knots where the boat had a high trim angle with stern squat, and once it got over that speed, it was planing the rest of the way to over 40 knots. In the discussion they had troubles getting over this speed (once the boats aged a bit) until they reduced the size of the props. This to me indicates the the ELCO 80 had a pile of resistance to overcome to get up on plane at 27 knots. I assume since the ELCO 50 cal turrets where at quite an angle to the waterline, that these boats tended to ride at a rather high trim angle most of the time.

Does anyone have any supporting or differing info than what I just presented? I'm sure there was/is a vast pile of test data recorded somewhere, just a matter of finding out where it resides. Did the ELCO 80' have a lower speed where it also planed, or was it more like a Deep-V displacement hull all the way up to 27 knots?

Thanks for your assistance.

Cheers,
Andy






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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: Jan 14, 2014 - 11:21am
Hi Andy,
Unfortunately, We never had an inclinometer mounted on the PT Boat. I am not sure who to ask for this information that you are seeking. My best guess would be Dr Al Ross, who sometimes monitors this board, or maybe Jeff Davidsons PT103 website may have some technical information like that as well. Also possibly the Elco Book "Specifications for building the 80 foot Elco Motor Torpedo Boat" may have some of that type of information as well. Sorry I could not be of more assistance. Jerry

Jerry Gilmartin
PT658 Crewman
Portland OR

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alross2

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of alross2   Send Email To alross2 Posted on: Jan 14, 2014 - 2:17pm
Quote:

Al Ross, who sometimes monitors this board



Actually, Jerry, I lurk here several times a day, every day... Just don't have anything to contribute most of the time.

Al


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Stuart Hurley

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of Stuart Hurley  Posted on: Jan 15, 2014 - 2:37am
Quote:

I assume since the ELCO 50 cal turrets where at quite an angle to the waterline, that these boats tended to ride at a rather high trim angle most of the time.





Andy,
The .50 cal turrets were at right angles to the waterline on the 103 class boats, so it could be assumed they planed at a similar angle to when at rest as built. I would think the addition of heavier armament such as a 40mm Bofors would affect the trim of the boat considerably so they probably varied.
Best Regards,
Stu.





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Andy Small

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of Andy Small  Posted on: Jan 15, 2014 - 6:54am
Good Point Stu,

I was looking at the later series which had a turret angle of just about 3 degrees. I assumed they did this based on feedback from the fleet. Now was that the normal trim angle, or a median compromise between a higher trim angle and normal waterline? I believe the squat that was mentioned in the book, would have been just before the boat wanted to plane, where the turbulence and suction forces are the greatest. This would have probably been alleviated by increasing the flat surface area (my personal observations from line drawings show that the Higgins looked to have a bit more surface area).

Since ELCO was working on external add-on hull mods (such as the ELCO slipper to help it plane more easily), the 27 knots might have been the actual planing speed. Also, the LCDR Alan Montgomery notes on the Nov 43 runoff record an acceleration from idle to 2000 rpm (which I'm guessing is about 35 knots), of 24 sec for PT296 and 42 sec for PT553. That's quite a difference. Since planing reduces resistance going through the water and should therefore increase acceleration, it seems that the Higgins went up on step much earlier than the ELCO. Again, these are just my thoughts on the subject.




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Andy Small

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Post a Reply To This Topic    Reply With Quotes     Edit Message     View Profile of Andy Small  Posted on: Jan 15, 2014 - 9:43am
Jerry,

Do you have a speed to rpm chart (or info) for PT658? This would really help in this discussion. Thanks!

Andy


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