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Forum Category: PT Boats of WWII
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Forum Name: PT Boats - General
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Topic: ELCO 103 + turn characteristics
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My question concerns the turning characteristics of the ELCO 80 footers. As a model builder, I have found, and heard of many others finding the same characteristics. The 80's during a hard turn have a strong tendency to nose dive inboard during a turn. A lot of this can be attributed to the amount of rudder angle applied to the model.Once the boat heels into the turn, the rudder, being semi-balanced, assumes a degree of elevator control, pitching the bow down. On my 5 ft model, when I reduced the angle of rudder available, the issue disappeared. Of course she did not turn on a dime.
I suspect that the Higgins did not share this attribute largely because of a different shaped rudder and hence effect.
My question is: did this happen with the full size boats or was the rudder angle limited to prohibit it?
I am open to ideas.




Posted By: JEno | Posted on: Nov 6, 2023 - 5:47pm
Total Posts: 75 | Joined: Oct 13, 2019 - 9:52am



The ELCO 80 foot boats had a rather small rudder which limited the turning performance of the boat. The Higgins had two, much larger runners than the three rudders on the ELCO. Providing a bit more turning performance. I have a document related to relative boat performance based on tests done in 1944. I will share this later.

I do not recall any discussions of boats nosing down in a turn. I would suggest the turning radius of the actual boat was not tight enough to create that effect. Nor was there enough power to push the bow down against the V created by the hard chine. Most likely the power you apply in a turn with your model, combined with an over scale rudder, is causing the effect. Not to mention the performance of the scale hull does not represent the performance of the full sized boat in a linear way.

Bill

Bill Smallshaw

Posted By: smallwi | Posted on: Nov 8, 2023 - 4:28pm
Total Posts: 140 | Joined: Jun 21, 2007 - 3:02pm



Pretty complex issue related to the major differences in hull design, which then affected the rudder design. Elco and Higgins had rudder stops at 35 degrees. Huckins had stops at 30 degrees due to the use of outboard rudders and the issues with ventilation. The initial 80' Elco's turned relatively well, but as weight was added (weapons, electrical suite, people), the boat quickly outgrew the hull's capability and by 1945 the hull was plowing into seas during high-speed turns. This is the primary reason why the 613 series had all the ordnance shifted back which sort of improved the turning. Improving the performance was also the reason that the last four operational Elco had the slipper installed on the transom, a relatively simple method to update the out of date hull design, and improving turning at high speed knocking off between 20-40 seconds off 360 degree high speed turn.

I greatly simplified things, but that's my take from my research of primary source material.

pcbT5.jpg



Posted By: Andy Small | Posted on: Nov 10, 2023 - 5:32am
Total Posts: 265 | Joined: Nov 20, 2013 - 9:04pm



Thank you Gents, for your replies.
Just to clear up one point. My model is 1/16th scale and I am using 1/20th scale rudders, So they are certainly not oversize. Also, I limited my rudder throw, initially to 30 degrees, but have since reduced that further. I installed and ballasted my weight according to the "plan" water line, but I have no way to measure if she retains the proper water. while underway.
My suspicion, purely speculative and with no foundation, is that the weights of the prototype were proportionally even further aft than I mine set.
Anyhow, Thanks again for the input.



Posted By: JEno | Posted on: Nov 10, 2023 - 2:08pm
Total Posts: 75 | Joined: Oct 13, 2019 - 9:52am