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Old 02-13-2019, 12:23 PM   #281
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Every hull in the world is exactly the same and only fits into 3 categories...you have seen it right here on the internet as proven by a few.
Besides a vessel that cannot get ahead of its bow wave, a vessel that can, but cannot plane, and a vessel that can plane, what would you suggest as a fourth alternative?
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Old 02-13-2019, 12:37 PM   #282
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Mid 70's to mid 80's in winter I ran my two snow plow trucks at 5K to 6K feet elevation - in Sierra Nevada mts. Being a masonry, concrete and tile contractor... had to keep my two best installers busy during winter! Man, did we ever have some fun snow plowing... The Stories!!!...
It wouldn't be so bad but I have to also feed and water one horse, two cats, three dogs, forty chickens and three hundred hogs, and I'm retired.
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Old 02-13-2019, 12:46 PM   #283
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Besides a vessel that cannot get ahead of its bow wave, a vessel that can, but cannot plane, and a vessel that can plane, what would you suggest as a fourth alternative?
That's just the point, there is no other alternative a boat is either designed for operation as full displacement, semi-displacement or planing. Certainly there exists some ambiguity as to the precise SL ratio where the change takes place but ultimately if a vessel is designed to have a capability of exceeding an SL ratio of 2.5 or 3.0 it ceases to be either full displacement or semi-displacement regardless of a capability to operate at those speeds. Please note the word "designed" as it plays an important role in these definitions.
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Old 02-13-2019, 01:08 PM   #284
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Fish53,
How-a-bout a boat w an S/L ratio of 2.46?
Or 3.1 ?
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Old 02-13-2019, 01:12 PM   #285
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Well, I guess if you feel that catamarans have full displacement hulls, which is a hull form that has a definite speed wall, and is unlike a catamaran that will readily plane, then I suppose there isn't much more to say on this subject.
There are power cats that are designed to plane, and many that don't. Most sailing cats do not plane, if the definition of planing is "a significant reduction in running displacement". They displace the same amount of water at 3x hull speed as they do at rest. There have been attempts to design hulls that promote planing, these experiments were not thought to be particularly fast.
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So, if the argument is that a hull that is a displacement hull can plane if you provide the power, that would be true since all hulls are displacement hulls at displacement sppeds. That is, until they get on top of the bow wave because they are semi displacement, or ahead of it because they are planing.
A boat never gets ahead of its bow wave. Its bow is creating the wave. Planing or displacement makes no difference. If the bow wave height is large compared to the length of the boat, yes it will appear to be climbing its bow wave. It will never get ahead of it though. It will get ahead of the second crest of the bow wave, which will be (speed x 1.34) ^2 behind. It will never get ahead of the stern wave either, though that is less obvious to observe.
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By the way Baker, dues this Hobie really look like a FD hull?

https://www.google.com/imgres?imgurl...&iact=c&ictx=1
The Hobie is pure displacement hull. In fact, when flying one hull the other must carry twice the displacement. It isn't an anti-gravitation machine. If a boat is planing, it will be seen to rise out of the water somewhat. That due to lift reducing the running displacement. A Hobie will do 3 or 4 x hull speed and be just as sunk, or more so, than at rest. Most sailing cats will do the same. Also power cat trawlers, which do not create significant lift. Here are the lines and sections of the Hobie 16. Can you point out the flat sections that promote planing? (BTW, cats sail fastest when kept nearly flat, with the windward hull just off the surface to reduce wetted area.)



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And yes, a full displacement hull can "start to climb their wave". Problem is, it can't ever get to the top. If it can, it isn't a full displacement hull because it is no longer travelling at displacement speed.
That is a circular argument: A displacement boat traveling at more than displacement speeds isn't a displacement boat. You may define it that way, but it is more a definition of speed than hull type. And again, you will never get to the top of your bow wave, even on a high powered runabout. You are creating the bow wave and will always be just behind the crest, no matter how fast you go.
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Old 02-13-2019, 01:22 PM   #286
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As I said ...... pure FD
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Old 02-13-2019, 01:29 PM   #287
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Fish53,
How-a-bout a boat w an S/L ratio of 2.46?
Or 3.1 ?
SL ratios of 0 to 1.34 full displacement, 1.34 to 3.0 semi-displacement, 3.0 to I assume infinity are planing. Obviously these are not precise numbers but represent a general range applied to the majority of boats.
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Old 02-13-2019, 01:29 PM   #288
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It wouldn't be so bad but I have to also feed and water one horse, two cats, three dogs, forty chickens and three hundred hogs, and I'm retired.
300 Hogs! Bacon, Bacon, Bacon!! Do they plane... Bet they all are FD!!
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Old 02-13-2019, 01:41 PM   #289
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3) Re: "sailboats...and on the face of a wave will do a pretty good job of planing."

Also false, and yet another ridiculous analogy. Think. If the force of gravity is no longer perpendicular to the water on which you are 'planing' (as it will be on the face of a wave), then the upward forces on the hull (hydrodynamic and bouyancy) will be less than the downward force of gravity on the boat. In this state the boat will be 'falling' down the surface of the water. On a hull that is capable of planing, this is known as surfing. Now...as the face of the wave approaches vertical, the weight of the boat relative to the wave's surface approaches zero. So...falling is not planing, and we don't need a definition to know that. It occurs to me that if you had actually experienced this, you'd already know that you were falling because you can feel it in your stomach, just like you can anytime you lose altitude rapidly.
So much in that post, so little time. But on this in particular.

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then the upward forces on the hull (hydrodynamic and bouyancy) will be less than the downward force of gravity on the boat. In this state the boat will be 'falling' down the surface of the water.
A boat will do this in steady state, that is, for many minutes at a time. The upward forces on the hull and downward force of gravity are in balance, otherwise the boat would quickly accelerate towards the bottom of the sea. That is high school physics. It is a steady state, unaccelerated condition and the only thing you will feel it a slightly bow down trim. "Falling" is an accelerated condition, not what we are talking about here.

A non breaking wave in deep water and no current is never even close to vertical, about 1:7 height/length is as high as they will be so the bow down trim, and the slope of the water surface really isn't that great. The boat surfs because there is a component of gravity parallel to the water's surface and opposite drag which adds to drive, making the (normally) displacement hull plane or perhaps semi-plane. In other words, it is adding power enough to plane.

Yes, I've experienced it many thousands of times. On my pure displacement sailboat, with 11,000 lb lead keel, I've averaged 1.7 x WL ^ 0.5 for many hours on end, and somewhat more than that for a few minutes. In flat water with lots of wind, it will do 1.5 x, for as long as the wind and water last.
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Old 02-13-2019, 01:43 PM   #290
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"Obviously this explanation is highly simplified, and there are plenty of other design nuances that I haven’t covered. So it’s helpful to think of the three types of hull forms—planing, displacement, and semi-displacement—as pure types. Most hulls you come across will be variations on them. So here’s a final observation that might be worth keeping in mind: Overused though it may be, the maxim holds that there is no free lunch. In hull design, as in so much of life, the addition of something—speed, say—presupposes the subtraction of something else—perhaps stability or a smooth ride. Everything has its price, and in the end it’s up to you to decide what you must have and what you are willing to give up to get it."
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Old 02-13-2019, 01:53 PM   #291
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A boat never gets ahead of its bow wave. Its bow is creating the wave. Planing or displacement makes no difference. If the bow wave height is large compared to the length of the boat, yes it will appear to be climbing its bow wave. It will never get ahead of it though.
Got it. This vessel is behind its bow wave because as you say, a boat never gets ahead of it.
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Old 02-13-2019, 01:54 PM   #292
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So much in that post, so little time. But on this in particular.



A boat will do this in steady state, that is, for many minutes at a time. The upward forces on the hull and downward force of gravity are in balance, otherwise the boat would quickly accelerate towards the bottom of the sea. That is high school physics. It is a steady state, unaccelerated condition and the only thing you will feel it a slightly bow down trim. "Falling" is an accelerated condition, not what we are talking about here.

A non breaking wave in deep water and no current is never even close to vertical, about 1:7 height/length is as high as they will be so the bow down trim, and the slope of the water surface really isn't that great. The boat surfs because there is a component of gravity parallel to the water's surface and opposite drag which adds to drive, making the (normally) displacement hull plane or perhaps semi-plane. In other words, it is adding power enough to plane.

Yes, I've experienced it many thousands of times. On my pure displacement sailboat, with 11,000 lb lead keel, I've averaged 1.7 x WL ^ 0.5 for many hours on end, and somewhat more than that for a few minutes. In flat water with lots of wind, it will do 1.5 x, for as long as the wind and water last.
"A non breaking wave in deep water and no current is never even close to vertical" As water molecules travel in a vertically circular pattern in non-breaking waves I find it difficult to imagine that at some times they aren't moving vertically at two points in the circle, say maybe 3 and 9 o'clock?
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Old 02-13-2019, 02:01 PM   #293
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https://www.powerandmotoryacht.com/b...ng-hull-design



"Obviously this explanation is highly simplified, and there are plenty of other design nuances that I haven’t covered. So it’s helpful to think of the three types of hull forms—planing, displacement, and semi-displacement—as pure types. Most hulls you come across will be variations on them. So here’s a final observation that might be worth keeping in mind: Overused though it may be, the maxim holds that there is no free lunch. In hull design, as in so much of life, the addition of something—speed, say—presupposes the subtraction of something else—perhaps stability or a smooth ride. Everything has its price, and in the end it’s up to you to decide what you must have and what you are willing to give up to get it."
Don't feel we gave up much of anything and gained plenty to have full planing hull shape. That is, for the very recreational in-side-waters use we undertake with our Tolly!

For long costal cruises... semi displacement is my choice.

To cross oceans - FD or SD

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Old 02-14-2019, 06:32 AM   #294
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"To cross oceans - FD or SD"


Plan B could be to put the boat in a shipping container and relax in a 787 or 777 to cross oceans.
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Old 02-14-2019, 07:39 AM   #295
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Don't feel we gave up much of anything and gained plenty to have full planing hull shape. That is, for the very recreational in-side-waters use we undertake with our Tolly!

For long costal cruises... semi displacement is my choice.

To cross oceans - FD or SD

Happy Hull Shape Daze! - Art
I gave up sails and a lot of elbow room for an 18hp diesel and .25 gallons an hour@5kts.. It's mostly me going it alone the majority of time and all I need is hot coffee, a good seaworthy vessel and a plastic bucket.
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Old 02-14-2019, 08:24 AM   #296
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Amen, brother....

What makes internet forums so much fun is that any proposition, no matter how mundane or grounded in physics, will be challenged for pages and pages.
Or dangerous.
It's why measles are now a problem again.
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Old 02-14-2019, 08:28 AM   #297
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I think some of the discussion is about definition/labels/terms and absolutes as much as pure physics....



One post clears things up and puts people in agreement, then another with a slight twist starts it back up again.
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Old 02-14-2019, 09:09 AM   #298
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By the way Baker, dues this Hobie really look like a FD hull?
Yes it does. There is no flat sections to provide lift. Just because a boat goes planing speeds does not make it a planing hull.
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Old 02-14-2019, 10:00 AM   #299
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Yes it does. There is no flat sections to provide lift. Just because a boat goes planing speeds does not make it a planing hull.
There's thousands of ships and supply boats with flat bottoms for almost their whole length that were never designed to be planing or SD hulls. Just because there's something flat doesn't mean it was intended to plane.
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Old 02-14-2019, 10:43 AM   #300
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There's thousands of ships and supply boats with flat bottoms for almost their whole length that were never designed to be planing or SD hulls. Just because there's something flat doesn't mean it was intended to plane.
I never said that. I just said that a planing hull needs flat sections to provide lift...and that Hobie Cat did not have them.
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