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Old 02-06-2019, 12:17 PM   #101
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"Semi Displacement" is as much a marketing term as anything. All boats are semi displacement in that they all create some lift and they all displace some water. Arguing about whether certain hulls are semi displacement or planing has no resolution. The best marketing definition would be a hull intended and powered to operate a significant amount of time in both displacement and planing regimes.

Of course, a larger anchor will result in more displacement. There, I got anchors into the thread.
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Old 02-06-2019, 12:22 PM   #102
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"Semi Displacement" is as much a marketing term as anything.
That is what it looks like to me as well. Since SD boats have flat after sections, if you put enough hp in those hulls, they'll plane, so at that point, aren't they planing hulls? If so, it makes the distinction between SD and Planing kind of meaningless.
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Old 02-06-2019, 12:31 PM   #103
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Perhaps my comprehension has been hampered by your abstruseness?
When offering an insult, it is always better to choose the appropriate pejorative so you don't look like a dope. If I am abstruse, then I am hard to understand, or obscure. Since my comments are easily understandable if read, I think what you wanted to say was that I am obtuse, which would mean I am stupid, not inscrutable. I guess you could mean I am abtuse, which means I am simultaneously abstract and obtuse, which is kind of difficult to achieve since being abstract would seem to imply complexity beyond the reach of the stupid. I'd stick with obtuse. More direct.
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Old 02-06-2019, 12:37 PM   #104
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I'm going to take a shot at clarification. According to Dave Gerr a well known naval architect and author of numerous books on the subject a semidisplacement hull is one that is "designed to operate above SL ratios of 1.3 or 1.4 but below SL ratios of 2.5 to 3.0". Please note the word "designed" rather than actual speed produced by excessive horsepower to exceed an SL of 3.0 or insufficient horsepower to attain SL of 1.3. Hence regardless of performance it is the designers desire for a particular operating speed range that determines a SD hull and is determined by the hulls designed buttock angle. This reference can be found in the "Propeller Handbook" on page 12 along with a more detailed discussion on this subject.
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Old 02-06-2019, 12:45 PM   #105
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So based on the piece that Delphin posted, if the formulaic hull speed is 10kts, and it cruises (with the 450s) at 13, what the hell does that 3 knots do - certainly not taking it over the bow wave - maybe pushing it at extreme cost? No?
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Old 02-06-2019, 01:00 PM   #106
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Grand Alaskan, not Grand Banks.
Menzies definitely SD.

I used GB as I know they are but most importantly few here would know what a GA was. This in not a brand specific forum.
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Old 02-06-2019, 01:04 PM   #107
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Does everyone think all hull shapes clearly fall into one of 3 categories?


All those that know better should not let those trying to "fit" absolute definitions into the discussion sway them and read technical white papers on the subject....and with those often in disagreement....form your own opinion.


There are hulls that clearly fit the 3 descriptions because of hull designs that clearly fit the loose description of FD, SDP, and planing... but a huge amount of hulls fall somewhere in between those loose definitions.


And many professional discussions discuss just that.
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Old 02-06-2019, 01:23 PM   #108
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I disagree with just about everything in that post (#56) but I'll just say this: we must have common terms or we are sunk . Whether lift exceeds drag or not has nothing to do with speed. Lift and drag are orthogonal vectors on a boat. Lift is up, drag is back (thrust forward, weight down). Lift definitely reduces the displacement of a planing boat (planing is here a verb not a noun), as Archimedes would agree. Planing is not required to exceed "hull speed", it just reduces the power required to do so.

You'd have to agree that a nearly submerged sphere is a "displacement" shape? And a submerged sphere is nearly unique in that it produces essentially no lift. Do you doubt that I can tow one at many times faster than 1.34 x diameter ^ 0.5? All that will happen is the stern wave will be well behind the sphere, and it will require a lot of power. This is an existence proof of a displacement shape exceeding hull speed that you can do yourself.

I don't think 10,000 hp is going to do it for Delphin - that's why I suggested 100,000 .
Bit hard to visualize how towing a sphere relates to a displacement hull under its own power. I suppose if I hooked Delfin up to the USS Ronald Reagan while under full power, she'd exceed hull speed, although as has happened to sailboats under tow, likely she would be pulled under water. She'd also exceed hull speed if she was mounted on a flat bed and driven down the road, but I think we're talking about a vessel under its own power in these discussions.

Be that as it may, I agree with NAs like Gerr that full displacement hulls are not designed to provide hydrodynamic lift. They may still generate some, but if full displacement, not enough to offset drag. Absent lift, you have a maximum hull speed where drag simply increases as you power the vessel faster. You disagree, which is fine, but I don't think the physics supports your position.
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Old 02-06-2019, 01:37 PM   #109
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So based on the piece that Delphin posted, if the formulaic hull speed is 10kts, and it cruises (with the 450s) at 13, what the hell does that 3 knots do - certainly not taking it over the bow wave - maybe pushing it at extreme cost? No?
The flat sections in the stern generate increasing lift the faster you go, so the 3 knots translates to sufficient lift to keep the stern afloat and get the vessel on top of its bow wave (a bit), further reducing drag (a bit). At the cost of a hellacious amount of fuel.

Comparing your underbody to Delfin's, you can see that my boat has basically no surface under the water that can generate vertical lift. At the bow, the hull is effectively vertical, so no lift can be generated there. In the stern, a bit might be provided at the turn of the bilge, but not much. As Psneed suggests, the lines between hull type classification are fuzzy, but fuzzy or not, some hulls are definitively full displacement, and some are definitively planing. In between are shades of grey where some underbodies have flatter sections and some less. With sufficient area of flat sections to provide lift to offset drag, whether the boat is SD or planing seems to be a matter of how much hp you apply to propulsion. Enough propulsion and the lift generated by flat sections increases, decreasing the power required to keep the vessel on plane.
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Old 02-06-2019, 02:22 PM   #110
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That may be more obvious on a canoe stern, but look at these KK photos.
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Old 02-06-2019, 02:40 PM   #111
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Here’s an article by Kady Krogen: “Spelling-out the Full Displacement Hull”

Spelling-out the Full DisplacementÂ*Hull FormÂ*
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Old 02-06-2019, 03:09 PM   #112
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One question. Can't we all agree that this is a full displacement motor cycle?
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Old 02-06-2019, 03:17 PM   #113
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The Krogen in 110 shows how the percent of submerged transom definition I mentioned some time ago is imperfect. But the Quarter Bean Buttock Line saves the day. There are not many FD boats that have submerged transoms but this is one.

In the photos it’s easy to see the difference between between menzies V bottomed hull and the KK. Lots of rocker over most all the KK’s length.
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Old 02-06-2019, 04:21 PM   #114
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. There are not many FD boats that have submerged transoms but this is one..
Over here I would say most FD boats have submerged transoms
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Old 02-06-2019, 04:49 PM   #115
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Greetings,
A bumble bee isn't supposed to fly based on "calculations", yet it does. There are a plethora of flying objects that also fly regardless of their shape. I suspect it is similar to a "full displacement vessel" not being able to plane.










and, of course...





Even the term "full displacement" is vague IMO.
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Old 02-06-2019, 04:55 PM   #116
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Hey guys, a boat is a boat. Everyone loves their own boat. Lets let this thread pass into the night - lets talk about something benign like anchors
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Old 02-06-2019, 05:04 PM   #117
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Greetings,
Mr. FC. I tried to derail the thread in my post #84 but...


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Old 02-06-2019, 05:59 PM   #118
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Hey guys, a boat is a boat. Everyone loves their own boat. Lets let this thread pass into the night - lets talk about something benign like anchors
I don't think we were talking about particular member's boats, just what makes one over the other. I am sure there are plenty on here who wouldn't want a FD and others the same for SD or Planing.

So it in't a case of "everyone loving their own boat," or even preferring it to others. Just what the heck do "people" mean when they describe one. And who are these "people" anyway!
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Old 02-06-2019, 06:06 PM   #119
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Foxy Charlie who gave you the authority to end threads?

RT,
The boats and the birds eh

And Fox re anchors I still haven’t tested several anchors but it seems nobody is very interested. But I’ll prolly dump it on ya anyway but no promises when.
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Old 02-06-2019, 06:28 PM   #120
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Over here I would say most FD boats have submerged transoms
Fery few have submerged transoms IMO.
What boats would that be? An old GB? They are SD to be sure. Some of the finest FD boats on the planet are represented in Voyaging Under Power. Steven Seaton and Chuck Neville offer wonderful FD designs w more than a tad bit of transom submerged. I love their designs (especially those by Neville) and nobody would mistake them for SD. They are deep and heavy boats and have a fairly steep QBBL ... the bottom aft.

But perhaps there’s some interesting designs down under that most of us are not privy to.
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