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Old 02-07-2014, 09:25 AM   #21
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PS, perhaps you're right in the context of 30- 50 ft pleasure boats but I think overall in the world of water craft it has been fairly well defined.
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Old 02-07-2014, 09:55 AM   #22
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Nope...applied to all size boats the way I see it....full displacement dingies versus planning ones run in the "displacement mode"...same principles apply.
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Old 02-07-2014, 11:48 AM   #23
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I hate to say it but I'm convinced there is something that falls in neither camp. Do we need to add "nearly full displacement"?
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Old 02-07-2014, 11:57 AM   #24
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I hate to say it but I'm convinced there is something that falls in neither camp. Do we need to add "nearly full displacement"?

Sure...exactly what I have been saying for years...none of the formulas or constants are as absolute as many would have you believe....

By constantly changing just a few measurements on any brick shaped object...at some point you could keep reshaping that hull in tiny increments from full displacement right up to planning...I can't figure out why people thing there's some magical wall that you hit and bots jump from one design to the next....

Lets face it we all know that GBs all evolved with the times ...so why not keep tweaking the hull to get a few more knots as the public wants to go faster at times (a few years at 6 knots will sometimes make you think) and fuel is cheap?

At some point you add another engine and really tweak the hull to get all the way into the middle range of the "next design"...so the design change can be radical or minor...but there's no clear cut anything that defines the "transition hull shapes" as I think MANY trawlers are.
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Old 02-07-2014, 01:05 PM   #25
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I hate to say it but I'm convinced there is something that falls in neither camp. Do we need to add "nearly full displacement"?
Dave Gerr seems to take care of it by contending that displacement encompasses a fairly wide range of designs. If you have a hull that won't semi-plane (significant hull lift), regardless of installed power, but it is capable of exceeding the 1.34 number (up to a point), then he would place it in the displacement grouping.

For those of you who prefer calculations and data, I once again provide Naval Architect Dave Gerr's excellent paper (pages 12-17 of the linked document). The article provides mathematical methods for calculating potential speed and power required, which is what it's really all about for recreational boats. He explains the contribution of hull design on the "bottom line". In those explanations he contends that displacement encompasses a fairly large range of S/L and D/L that go well beyond the traditional 1.34 factor. Virtually all of what has been said thus far in this string is discussed in engineering terms if anyone cares to look.

I found it particularly interesting that on page 16 in the link he used his personal "displacement" boat as an example for a speed calculation using both the Crouch formula and the Gerr formula. He explains that the hull has "rounded bilges and full keel....although the buttocks have been flattened aft for the purpose of reducing squatting at modestly higher speed". Now, a purist/traditionalist would contend that flattened buttocks for adding lift would automatically place the boat in a semi-planing category. Gerr says his hull would have to be modified to semi-plane or plane.

Boat calculations Dave Gerr.pdf
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Old 02-07-2014, 01:44 PM   #26
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I hate to say it but I'm convinced there is something that falls in neither camp. Do we need to add "nearly full displacement"?
Absolutely correct. Some of DeFever's earlier FD designs were vessels with canoe style sterns much like Eric's. He was not pleased with the canoe stern vessels' hobby horsing, stability at anchor and lessened carrying capacity aft. So in the late 60s and into the 70s he started designing very large FD yachts with no canoe stern and gasp, active stabilizers! I was fortunate this past summer to be on one of his earlier canoe stern designs, his first yacht type vessel. A DeFever FD design all the way

In a few brief years the DeFever canoe stern had morphed into Daddyo's fuller stern design with a very large stern curvature angle going well below the water line, an improvement in Art's mind over other FD vessels built to that point. The FD yachting world agreed with copies springing up numbering into the thousands and owned by most on this Forum.

Of course the purists still say if not a canoe stern, it isn't FD. As PSneeld says, "many compromises" which is absolutely correct. Compromises to improve upon the 1.34 number, lessen rolling, provide more fuel space, better cabin arrangements and contain costs.

The FD vs SD discussion dramatically changed years ago with the advent of tank testing, computer simulation, CAD programs and construction techniques to get the shape just right in the NA's mind.

Some of the discussions on this Forum regarding Jay Benford's designs fits into the current design of FD vs hard chines vs space and economy. Tad Roberts too, with his designs yielding fast, roomy and economical vessels.
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Old 02-07-2014, 02:35 PM   #27
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All boats at rest are full displacement.
Harumph, harumph, harumph!! (I didn't get a harumph outta that guy!)
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Old 02-07-2014, 03:03 PM   #28
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once you accept a hull as being full displacement, you feel comfortable with lower horsepower and often a single engine. This then means lower fuel and maintenance costs. While true that a semi-displacement hull will operate at displacement speeds, it will generally have more horsepower so will not do it as inexpensively.
I think this factor is too often overlooked in place of sea keeping attributes.

When I look at cars, mpg is a driving consideration - not because I can't afford the gas, but because there are other things I'd rather spend my money on.

When it's time for me to hang it up and start living the dream, a liveaboard with double digit horsepower is going to better allow me to cover the grounds I'd like to cover.

Now if I were a Friday night run away from the San Juan's like many of you are, I'd probably want something that would get me there quick to maximize the weekend.

Now that is just the opinion of a frozen Midwesterner who is merely a dreamer here at TF!
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Old 02-07-2014, 04:43 PM   #29
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Dave Gerr seems to take care of it by contending that displacement encompasses a fairly wide range of designs. If you have a hull that won't semi-plane (significant hull lift), regardless of installed power, but it is capable of exceeding the 1.34 number (up to a point), then he would place it in the displacement grouping.

For those of you who prefer calculations and data, I once again provide Naval Architect Dave Gerr's excellent paper (pages 12-17 of the linked document). The article provides mathematical methods for calculating potential speed and power required, which is what it's really all about for recreational boats. He explains the contribution of hull design on the "bottom line". In those explanations he contends that displacement encompasses a fairly large range of S/L and D/L that go well beyond the traditional 1.34 factor. Virtually all of what has been said thus far in this string is discussed in engineering terms if anyone cares to look.

I found it particularly interesting that on page 16 in the link he used his personal "displacement" boat as an example for a speed calculation using both the Crouch formula and the Gerr formula. He explains that the hull has "rounded bilges and full keel....although the buttocks have been flattened aft for the purpose of reducing squatting at modestly higher speed". Now, a purist/traditionalist would contend that flattened buttocks for adding lift would automatically place the boat in a semi-planing category. Gerr says his hull would have to be modified to semi-plane or plane.

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Heresy!!! Say it ain't so......lies like that have NO PLACE hear on TF!!!

Before you know it...we won't be able to tell what's what from pictures anymore!!!!!!
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Old 02-07-2014, 04:45 PM   #30
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Absolutely correct. Some of DeFever's earlier FD designs were vessels with canoe style sterns much like Eric's. He was not pleased with the canoe stern vessels' hobby horsing, stability at anchor and lessened carrying capacity aft. So in the late 60s and into the 70s he started designing very large FD yachts with no canoe stern and gasp, active stabilizers! I was fortunate this past summer to be on one of his earlier canoe stern designs, his first yacht type vessel. A DeFever FD design all the way

In a few brief years the DeFever canoe stern had morphed into Daddyo's fuller stern design with a very large stern curvature angle going well below the water line, an improvement in Art's mind over other FD vessels built to that point. The FD yachting world agreed with copies springing up numbering into the thousands and owned by most on this Forum.

Of course the purists still say if not a canoe stern, it isn't FD. As PSneeld says, "many compromises" which is absolutely correct. Compromises to improve upon the 1.34 number, lessen rolling, provide more fuel space, better cabin arrangements and contain costs.

The FD vs SD discussion dramatically changed years ago with the advent of tank testing, computer simulation, CAD programs and construction techniques to get the shape just right in the NA's mind.

Some of the discussions on this Forum regarding Jay Benford's designs fits into the current design of FD vs hard chines vs space and economy. Tad Roberts too, with his designs yielding fast, roomy and economical vessels.
.........................

had to repost the whole thing in case anyone missed it the first time...
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Old 02-07-2014, 05:46 PM   #31
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DAMN! Am I ever convinced after reading all the posts in this thread!

D, FD, SD, SP FP, P: All acronyms that either carry a D or a P as their base... which means to me... the hull is either D or P. It either is or it ain't one or the other! If it can't plane ever it is a D. If it can plane ever it is a P.

I/we simply LOVE our very stable and pretty quick (whenever desired) Tollycraft P boat!

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Old 02-07-2014, 06:41 PM   #32
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[QUOTE=Art;211692] If it can't plane ever it is a D. If it can plane ever it is a P. /QUOTE]

I'd revise that to say if it can't plane ever it's a D. If with a large application of power it can skim along the surface with the majority of the hull exposed, then it is a P. If the hull can clear the bow wave, but cannot skim along the surface, even with application of an inordinate amount of power, then it is probably an SP. I have doubts as to whether many of these very deep forefoot (non-displacement) hulls could be lifted free and clear of the surface even with a pair of high powered, light weight turbine engines. But it does depend on how you wish to define "plane".
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Old 02-07-2014, 06:49 PM   #33
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Great stuff here and I was a bit afraid the thread wouldn't get a single post as so many don't like beating the dead horse. But I always say as long as posts keep coming a thread isn't dead.

Spy I agree. Semiplaning. Semi means "partly" to me and a SP/SD hull that we are most familiar w is "partly" planing (to a small degree). Tried to float that about 4 or 5 years ago. SD for hulls closer to planing and SP for boats closer to FD. Made sense to me but I got negative feedback so let it slide.

Skidgear re your post .. sure you can build a canoe w a perfectly flat bottom but the hull is 100% FD. And the QBBL shows a Full blown planing hull. Like psneeld says definitions and even formulas won't hold water at times. But if we had the QBBL angle and a good cross sectional section area ratio (transom/midsection) we could nail it down for typical trawlers such as most here have.

Tom hard Chines have nothing to do w FD/SD. And basically pointy stern or perfectly right angled stern don't either. But the transom being in or out of the water is very definitive. If the whole transom is out of the water it's FD. Look at the older sailboats w their transforms fully out of the water. FD.
And yes even most trawler people don't like tippy so the typical trawler has a wide fairly flat bottom aft for a good purchase on the water for good inital stability. Almost never do we need secondary stability. But the notion of a big wide flat stern being more seaworthy is rubbish to these ears. May make a better yacht though.

My computer's in the shop and I'm on my i-pad (no pics and awkward). But I'll be reading these posts for several days as there is so much to absorb.
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Old 02-07-2014, 07:11 PM   #34
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Some of DeFever's earlier FD designs were vessels with canoe style sterns much like Eric's. He was not pleased with the canoe stern vessels' hobby horsing, stability at anchor and lessened carrying capacity aft. So in the late 60s and into the 70s he started designing very large FD yachts with no canoe stern and gasp, active stabilizers! I was fortunate this past summer to be on one of his earlier canoe stern designs, his first yacht type vessel. A DeFever FD design all the way
I'll be a bit nit-picky...... It's (another) fine point but I wouldn't say DeFever ever designed a canoe stern. If you google "canoe stern" you'll get a lot of pictures of double-enders and boats (as the Willard) with narrow, tight-radius round sterns. They used to be called "Cruiser" sterns in the UK. These could be called canoe sterns. The early DeFever stern was that of Marda, pictured below. This is a wide, rounded shape, with large transverse flat, usually with a knuckle well above waterline where the hull bottom ends. Here in BC we call this a Horseshoe stern, it's common on trollers and other fishboats.

The Horseshoe is very definitely a displacement hull feature, designed to ease the return of water from under the hull to the "at rest" level somewhere astern. At the correct speed this stern leaves no wake at all. The deeply immersed transom of DeFever's later designs is far more "draggy", but does allow greater speed without squatting. It costs money to drag that around though......of course I know you toy boat owners don't care about such things

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Old 02-07-2014, 07:35 PM   #35
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I'll be a bit nit-picky...... It's (another) fine point but I wouldn't say DeFever ever designed a canoe stern. If you google "canoe stern" you'll get a lot of pictures of double-enders and boats (as the Willard) with narrow, tight-radius round sterns. They used to be called "Cruiser" sterns in the UK. These could be called canoe sterns. The early DeFever stern was that of Marda, pictured below. This is a wide, rounded shape, with large transverse flat, usually with a knuckle well above waterline where the hull bottom ends. Here in BC we call this a Horseshoe stern, it's common on trollers and other fishboats.

The Horseshoe is very definitely a displacement hull feature, designed to ease the return of water from under the hull to the "at rest" level somewhere astern. At the correct speed this stern leaves no wake at all. The deeply immersed transom of DeFever's later designs is far more "draggy", but does allow greater speed without squatting. It costs money to drag that around though......of course I know you toy boat owners don't care about such things

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Right you are Tad, it is a horseshoe in the early DFs. But using 4.5 gph at 7.7 knots and 1/2 that at 6 knots in our DF 48, even with our draggy buttocks, is not too shabby.

How would describe your PL designs in light of this discussion?
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Old 02-07-2014, 08:28 PM   #36
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How would describe your PL designs in light of this discussion?
Oh boy...are you sure you want to incite this? I'm not at all interested in making any sales pitch but I'm happy to try and shed some light on questions of naval architecture.

The PL (Passagemaker Lite) design, like Dashew's FPB, the Vripack Ned, and Nigel Irens LDL designs, is a "High-speed" displacement hull. It's ratios, shape, and performance are similar to a naval Frigate. Somewhere between a Frigate and a high-speed displacement catamaran. A lot faster than typical heavy "Full" displacement hulls, these hulls achieve speed by being small (in cross section), relatively high-powered, and light weight for their length. Unlike a semi-displacement hull, they are not intended to lift with speed at all. Rather the idea is to reduce wetted surface and wave-making to allow the hull to slip along at higher speed. They lack the bottom area to get any useful dynamic lift as big wide (and usually heavy) semi-displacement boats do.

You might find some reference to this in the Dave Gerr link above. He couples it to displacement/length ratio. The lighter (displacement) a hull is for a given length, the higher it's "Hull Speed" will be.
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Old 02-07-2014, 08:30 PM   #37
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Old 02-07-2014, 08:31 PM   #38
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Oh boy...are you sure you want to incite this? I'm not at all interested in making any sales pitch but I'm happy to try and shed some light on questions of naval architecture.

The PL (Passagemaker Lite) design, like Dashew's FPB, the Vripack Ned, and Nigel Irens LDL designs, is a "High-speed" displacement hull. It's ratios, shape, and performance are similar to a naval Frigate. Somewhere between a Frigate and a high-speed displacement catamaran. A lot faster than typical heavy "Full" displacement hulls, these hulls achieve speed by being small (in cross section), relatively high-powered, and light weight for their length. Unlike a semi-displacement hull, they are not intended to lift with speed at all. Rather the idea is to reduce wetted surface and wave-making to allow the hull to slip along at higher speed. They lack the bottom area to get any useful dynamic lift as big wide (and usually heavy) semi-displacement boats do.

You might find some reference to this in the Dave Gerr link above. He couples it to displacement/length ratio. The lighter (displacement) a hull is for a given length, the higher it's "Hull Speed" will be.
Thank you...one more intelligent explanation that the world does not revolve around 3.14 and QBBL.
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Old 02-07-2014, 08:34 PM   #39
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I prefer the motion of going through waves rather than bouncing over.
ever ride a honking Hobie Cat or a 40 something, trip engine go fast from wave top to wave top?

you may not change your opinion but you'll see another way to travel comfortably though surprisingly large chop.
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Old 02-07-2014, 08:35 PM   #40
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The PL (Passagemaker Lite) design, like Dashew's FPB, the Vripack Ned, and Nigel Irens LDL designs, is a "High-speed" displacement hull. semi-displacement boats do. .
So given this frigate design, how does the stern look and behave in the design?
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