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 Boat Design Forums Round stern vs box stern??

#16
02-09-2009, 12:30 PM
 robherc Designer/Hobbyist Join Date: Dec 2008 Rep: 102 Posts: 432 Location: US/TX
Mostly guessing...at slow enough speeds I see having similar gains by a rounded stern to what is gained by a double-ender (though the DE would get higher efficiency, and at higher speeds). At planing speeds, I see the rounded bow as increasing the overall wetted area aft with very little added lift, thus loss of efficiency & more of an "X" shaped stern wave until a high enough speed is reached to completely disconnect the flow from the curved sides of the stern (which would be above the stated 25kts max speed).

While I will not work up fluid-dynamc/hydrodynamic formulas to post here to prove my assumptions, if you depart from your calculator for a minute & try to look at things from the viewpoint of logic & physics intuition, you'll see what I mean very quickly.
Yes, I CAN do calculus in my head, but I very quickly learned the worth of "throwing away the calculator," so to speak, for some things after High School.
#17
02-09-2009, 12:32 PM
 robherc Designer/Hobbyist Join Date: Dec 2008 Rep: 102 Posts: 432 Location: US/TX
Oh yeah...almost forgot:
All of the above information is based on the assumption that the rounded portion(s) of the stern ARE wetted.
Also, by rounding the stern, you're adding a couple feet to the LWL...thus raising hull speed, but I still don't see where that's a benefit for a planing-speed craft.
#18
02-09-2009, 12:38 PM
 Leo Lazauskas Senior Member Join Date: Jan 2002 Rep: 1934 Posts: 1,849 Location: Adelaide, South Australia
Quote:
 Originally Posted by robherc Mostly guessing...at slow enough speeds I see having similar gains by a rounded stern to what is gained by a double-ender (though the DE would get higher efficiency, and at higher speeds). At planing speeds, I see the rounded bow as increasing the overall wetted area aft with very little added lift, thus loss of efficiency & more of an "X" shaped stern wave until a high enough speed is reached to completely disconnect the flow from the curved sides of the stern (which would be above the stated 25kts max speed). While I will not work up fluid-dynamc/hydrodynamic formulas to post here to prove my assumptions, if you depart from your calculator for a minute & try to look at things from the viewpoint of logic & physics intuition, you'll see what I mean very quickly. Yes, I CAN do calculus in my head, but I very quickly learned the worth of "throwing away the calculator," so to speak, for some things after High School.
Sorry, but I can't accept your faith-based assertions.

And as for "I have seen similar gains...", where is the data to support this? It would have some credibility if you could supply real figures, but at the moment it is little more than hand-waving.
#19
02-09-2009, 12:39 PM
 Deadrise13 Junior Member Join Date: Feb 2009 Rep: 10 Posts: 19 Location: Sebastion inlet fl
From what I have read they started using round sterns on work boats for one reason. When thay would pull there big gill nets in the rear deck of the boat they would not get cought on the back corners of the boat.
#20
02-09-2009, 12:46 PM
 robherc Designer/Hobbyist Join Date: Dec 2008 Rep: 102 Posts: 432 Location: US/TX
OK, then is was guess #4: Something work/fishing related

Leo:
If you meant "I see having similar gains..." then it's self explanatory...don't mis-quote me and you won't confuse yourself.
I have already explained my position perfectly, and have at no point asked anyone to accept my theorems as fact. That is where you went wrong, you're wanting fact & mathematical support for everything that I've already directly stated I wasn't going to give you. And if you re-read my post #14
Quote:
 I think it's a safe bet that every poster in this thread is guessing.
I don't think I can put it any more clearly than that. If you don't want to hear theories based on sound logic, but NOT on math, then simply ignore most of my postings, and I'll not be too offended. There are plenty of us in here that can accept something on a logical level without needing a mathematical equation to explain it to us...if you need the equation, run it for yourself, or ignore posts from anyone who doesn't offer their equation up-front.
#21
02-09-2009, 12:52 PM
 Leo Lazauskas Senior Member Join Date: Jan 2002 Rep: 1934 Posts: 1,849 Location: Adelaide, South Australia
Quote:
 Originally Posted by robherc Deadrise: If you don't want to hear theories based on sound logic, but NOT on math, then simply ignore most of my postings, and I'll not be too offended.
But it's not sound logic. It's gibberish.
#22
02-09-2009, 12:55 PM
 Deadrise13 Junior Member Join Date: Feb 2009 Rep: 10 Posts: 19 Location: Sebastion inlet fl
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#23
02-09-2009, 12:57 PM
 DMacPherson Senior Member Join Date: Mar 2005 Rep: 195 Posts: 106 Location: Durham, NH USA
I am loathe to step into the fray here, but the tone has taken a turn and I'm compelled to reply. I think you are missing Leo's very reasonable statement - that the assertions are anecdotal, at best, without the benefit of test data. I don't read any of his posts that says he wants the equations or formula, just some real in-the-water performance results to compare. That's just good engineering, and has nothing to do with crunching any numbers.

Regards,

Don MacPherson
HydroComp
#24
02-09-2009, 12:59 PM
 Deadrise13 Junior Member Join Date: Feb 2009 Rep: 10 Posts: 19 Location: Sebastion inlet fl
More pix of the Albatross III of the Foster fleet Hatteras NC. Thanks for all the replys
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#25
02-09-2009, 12:59 PM
 robherc Designer/Hobbyist Join Date: Dec 2008 Rep: 102 Posts: 432 Location: US/TX
@Leo
I think you need to re-research your basic physics & hydrodynamics buddy...I've shown you no disrespect, but you're being spiteful (not to mention WRONG)...please do try to clean it up a bit...you're quickly backing yourself into an indefensible position here.
#26
02-09-2009, 01:02 PM
 robherc Designer/Hobbyist Join Date: Dec 2008 Rep: 102 Posts: 432 Location: US/TX
@Don,
Sorry, but I don't have \$10,000's to invest in test results on every idea that is expressed in this forum. I am a hobbyist, not an engineer with a company/college and unlimited testing resources at my disposal. Deadrise asked for opinions, and I gave mine....so far yourself & Leo are the only ones who can't seem to accept that.
#27
02-09-2009, 01:03 PM
 Deadrise13 Junior Member Join Date: Feb 2009 Rep: 10 Posts: 19 Location: Sebastion inlet fl
Thanks for all the thoughts guys, I guess if it has worked all these years for the Albatross fleet it must do ok. They have fished these same three boats since the therties.
#28
02-09-2009, 01:06 PM
 robherc Designer/Hobbyist Join Date: Dec 2008 Rep: 102 Posts: 432 Location: US/TX
Quote:
 Originally Posted by Leo Lazauskas Do you have any support for this guess. The air cavity behind a transom is vented to atmospheric pressure, and so it cannot sustain a pressure. ...
I present exhibit #1...there is no "air cavity" in the (wetted) rounded-stern I was illustrating, therefore THIS post (by Leo) is the one that's filled with "jibberish." ...cut by your own knife...

Anywise, this arguing is becoming pointless now, as Deadrise posted another picture, and the new, profile-view, picture clearly shows that the rounded-portion of the stern is NOT wetted.
#29
02-09-2009, 01:09 PM
 Deadrise13 Junior Member Join Date: Feb 2009 Rep: 10 Posts: 19 Location: Sebastion inlet fl
Sorry for all the pix I am just happy to know how to upload. When you guys figure out what the right answer is could some one tell me in plain english. Thanks for all the input.----
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#30
02-09-2009, 01:11 PM
 Leo Lazauskas Senior Member Join Date: Jan 2002 Rep: 1934 Posts: 1,849 Location: Adelaide, South Australia
Quote:
 Originally Posted by robherc @Leo I think you need to re-research your basic physics & hydrodynamics buddy...I've shown you no disrespect, but you're being spiteful (not to mention WRONG)...please do try to clean it up a bit...you're quickly backing yourself into an indefensible position here.
Physics and engineering are about measurement and calculation. You were trying to promote a brand of snake-oil and I called you on it.