Definition Of Full Displacement

[Delfin]

Delfin,
Don’t think bending is a problem on boats like this. I could be wrong as I’m no expert on steel building but I’ve seen a few come together w not very sophisticated equipment that had bends about the same as the bottom on these boats. It’s a bigger boat and takes more material but that shouldn’t be that much either. Bigger that a flatter boat bottom w/o such a big “underbelly”.

Which stern do you think would be easier and cheaper to build - the above or the below?

 

[markpierce]

The Coot has a similar crescent-shaped bottom.

 

[Nomad Willy]

Delfin,
My opinion would be the above by far. Lots of compound cirves on the “below” hull. The “above” hull is much more like a simple plywood rowing skiff but lots and lots of rocker. The panels or plates curve in only one direction like flat panels tend/want to do. The “below” boat has much more developed and complicated curved in many planes and in more directions. Rocker being the inverted curve of the bottom running fore and aft. Like the “rocker” on a rocking chair. Kinda like Marks boat on steroids. Marks boat has the same kind of rocker but to a far less degree. The “above” fishing boats are huge and very heavy. Marks QBBL would be much more shallow or put another way a lower angle than the big fishing boats. But the same type of hull basically. Most trawlers have fairly shallow QBB Lines. The IG’s for example have such a shallow angle it’s more like a planing hull. But none the less a SD hull due to her fairly heavy weight, big keel, and prop and gears suited more to heavy craft. Other trawlers like the GB are inbetween. More like a FD hull but still a SD.
All of this that I say is an opinion. And if you heard it from a NA it would likely be far more complicated.

 

[Nomad Willy]

Mark 99.9 % of this question pretains to the stern, not the bow.
And your Coot has the simple plywood shapes of the “above” boats.
But your Coot has the “bone in her teeth” bow wave of a heavy boat having a fairly wide angle of entry. Unlike a “slicer” as you say.
But your hull is very much related to the “above” very heavy fishing boat hulls.

 

[caltexflanc]

 

[sunchaser]

George

Hull extensions or running gear forward can prove interesting.

Often with the rudder, keel and prop well forward from the stern, a vessel wanders a lot. The after market vertical wavelets on the stern sides are possibly an attempt to force the vessel to stay in a straight line.

 

[Nomad Willy]

caltex I assume you’re looking for comment.

Looks like someone extended a FD hull.
This is a “head swim” example. Has a shallow buttock line angle suggesting a SD hull. She has more than most FD hulls have in submerged transom. Knee jerk reaction is “wow she looks really fast”. But that would assume a FD. Another thing I can say is that she looks like a boat designed to break the rules.

OK,
I’ll pigeon hole this boat as SD.
Why?
Buttock line is very long and almost flat. Transom submerged is significant for a FD and not much for a SD ... not a decisive element w this boat.
This boat is not at all like a FD ... well it is in several ways. She’s oviously built for speed and FD boats are not. She’s SD

But something I should bring into the discussion .... she’s VERY round and would be almost (if not dangerously so) scary in roll and perhaps lacks directional stability. See those little finns at the corners of the stern? I’m think’in they are add on’s because she may not come back out of a turn ... or at least be ruluctant to. Could even be a “bad design” but I may be over thinking it.

Can honestly say I’ve never seen a boat even closly resembling this one. I wanted badly to call her on the fence and so close to uncallable but the QBBL dosn’t lie. She’s SD

sunhaser I’m glad you agree to even some extent.

 

[caltexflanc]

The last pictured boat is a Nordhavn 62.

 

[Fish53]

 

[caltexflanc]

The little aft outboard fins are aftermarket, but otherwise it is a stock hull.

https://photos.app.goo.gl/zBX7pigkggSEHBr78

And to Sunchaser's comment yes, typically boats with the prop and rudders well forward have issues in following seas. You see that on some old Hatteras motoryachts where an aftermarket cockpit was put on and the props and rudders left in their original position.

 

[DDW]

Another new term. A bowsprit is now a hull, and not a, well, bowsprit. But on the chance that hull still means what it did before this thread, and the bow of the hull still means the same thing, then yes, in the picture the bow is ahead of any wave associated with the bow.

Hydrostatics and hydrodynamics are ruled strictly by immersed volume as defined by wetted surface. To say otherwise presumes the water is somehow intelligent and prescient, sensing what is above it and out of its reach. By your definition, every boat with a bow overhang is perennially ahead of it bow wave, no matter what its speed.

A planing boat reduces its immersed volume due to lift, but the volume is still there, and still creates a bow wave from which it cannot escape.

 

[Nomad Willy]

The last pictured boat is a Nordhavn 62.

Cal,
I knew it was a bobby trap. HaHa
Except the 46 I never have liked Nordhavn. I wonder if it was an in house extension? But whatever it’s faults I’ll bet that big Nordhavn is fast given enough power. But her faults couldn’t have been too bad as they are selling them right?

But in my mind this excercise I felt compelled to do led me to think a fourth hull designation.
Planing
Semi-planing
Displacement
Full Displacement

Too many of these on the fence designs exist. Haterass .. the typical big Hatt boats of about 65’ we usta call houseboats. Very hard to classify by most normal methods. From a hull shape standpoint they arn’t FD and the are by hull shape actually much like a planing hull. A lot like the Chris-Craft of the early 50’s. Yet I think they are called FD by most all owners that would be offended if I called them SD. But I could be convinced to call them “Displacement” boats. They are (as designed) and powered, not capable of more than a tad over calculated hull speed. But I really don’t know much about the boat.

Only supportable classification would put them in the underpowered SD classification.
But the option of calling them just displacement boats has merrit. It’s not fully supportable from a hull shape based classification standpoint ... as I have alwaysinsisted upon ... but it may be better than calling them clearly what they are not.

I only mentioned the big Hatt’s but there are many others. Lots or most of the DeFevers are similar.
If I see another post to this end I’ll start another thread. OK I will.

 

[sunchaser]

Cal, I knew it was a bobby trap. HaHa
.

Eric

Not a booby trap at all. Easily recognizable. Nordhavn has expended much effort on getting their rudder forward design to perform well. Especially on the extended hulls resulting in the 52, 60, 63 and a few others.

BTW, without the running surface extension, the flatter immersed under water aft section on the N62 is common to many Selenes. Somewhere I have a picture of a S58 we were considering.

 

[Nomad Willy]

Tom,
HaHa .... I don’t follow Nordhavns and was thinking it may be a Selene.
So what does Nordhavn call it? FD I suppose.
And I was refering to caltex re the bobby trap. Of course he probably just thought “holy cow ... I wonder what he’ll think of this” but since he didn’t mention me I was only assuming and yea I know about the assuming saying. But I thought who else could he be refering to?

 

[caltexflanc]

You guys would all do well to toss all your ill-conceived presumptions and, as I recommended at the very beginning of this thread, read Dave Gerr's The Nature of Boats. It's winter, no better time to do it and eminently readable. After that, spend some time in big boat yards and look at various boats on the hard.

 

[Fish53]

You guys would all do well to toss all your ill-conceived presumptions and, as I recommended at the very beginning of this thread, read Dave Gerr's The Nature of Boats. It's winter, no better time to do it and eminently readable. After that, spend some time in big boat yards and look at various boats on the hard.

I agree however looking at boats isn't necessary because as I keep repeating it's simply the SL ratio the hull was designed for that determines one of all too often repeated designations. Dave Gerr's information is also contained in the Propeller Handbook amongst many other books on the subject.

 

[Delfin]

View attachment 85527

Planing hull, as long as it has a Saturn V rocket for propulsion. With that arrangement, you could put a warhead on the bow and declare war on Costa Rica.

 

[Delfin]

Hydrostatics and hydrodynamics are ruled strictly by immersed volume as defined by wetted surface. To say otherwise presumes the water is somehow intelligent and prescient, sensing what is above it and out of its reach. By your definition, every boat with a bow overhang is perennially ahead of it bow wave, no matter what its speed.

A planing boat reduces its immersed volume due to lift, but the volume is still there, and still creates a bow wave from which it cannot escape.

Could you point out on where the bow wave is on this boat? I mean the one the bow is not in front of.

 

[Fish53]

This whole thread could have been ended with the sharing of one page from numerous books.

 

[caltexflanc]

I agree however looking at boats isn't necessary because as I keep repeating it's simply the SL ratio the hull was designed for that determines one of all too often repeated designations. Dave Gerr's information is also contained in the Propeller Handbook amongst many other books on the subject.

Well yes, but that means you have to know what the maximum speed is. SL is that speed divided by the square root of the water line (a displacement speed boat having one of about 1.5 or less), and thus comes by default of the actual boat design... which includes weight, powertrain, and hullform.
A

One way to get at it is to measure the quarter beam buttock angle, typically about 7 degrees or more of a "full displacement" boat. Another way to get at it is the prismatic coeffecient, as another poster discussed., which is displacement divided by 64, in turn divided by the area below the waterline at midship times the waterline length. That'll be a little over .50 and under .6 for a displacement boat, over .7 for a planing boat. You'll have to read up to understand how those measurements are taken and what they mean. So can arrive at various types of displacements (full, semi, planing) via a number of hull designs, as illustrated. Then you get into the powertrain calculations and can derive SL from there.

 

[sunchaser]

This whole thread could have been ended with the sharing of one page from numerous books.

Somebody had that opportunity on post #9 but elected instead to keep us muddling along. But, it is all good fun, the ultimate objective of TF.

 

[Giggitoni]

Could you point out on where the bow wave is on this boat? I mean the one the bow is not in front of.

This photo is a boat doing about 235 kts. What bow wave?

 

[Fish53]

Well yes, but that means you have to know what the maximum speed is. SL is that speed divided by the square root of the water line (a displacement speed boat having one of about 1.5 or less), and thus comes by default of the actual boat design... which includes weight, powertrain, and hullform.
A

One way to get at it is to measure the quarter beam buttock angle, typically about 7 degrees or more of a "full displacement" boat. Another way to get at it is the prismatic coeffecient, as another poster discussed., which is displacement divided by 64, in turn divided by the area below the waterline at midship times the waterline length. That'll be a little over .50 and under .6 for a displacement boat, over .7 for a planing boat. You'll have to read up to understand how those measurements are taken and what they mean. So can arrive at various types of displacements (full, semi, planing) via a number of hull designs, as illustrated. Then you get into the powertrain calculations and can derive SL from there.

See that's the mistake that keeps jerking this around, none of that matters, not horsepower, not reduction and not buttock angle. A competent naval architect designs a hull for certain performance and depending on the SL ratio it's designed to operate at defines it, period. How it's designed to operate at that SL ratio doesn't matter, a planing boat can have a flat bottom or a deep vee with lifting strakes yet either way it's designed to operate at above approximately an SL ratio of 2.5. A full displacement boat can also have a wide variety of hull configurations yet it's still defined as full displacement by virtue of being designed to operate below an SL ratio of 1.34. All the rest is just distraction and mental masturbation.

 

[Fish53]

Somebody had that opportunity on post #9 but elected instead to keep us muddling along. But, it is all good fun, the ultimate objective of TF.

I'm not entirely sure how to read that?

 

[Nomad Willy]

well yes, but that means you have to know what the maximum speed is. Sl is that speed divided by the square root of the water line (a displacement speed boat having one of about 1.5 or less), and thus comes by default of the actual boat design... Which includes weight, powertrain, and hullform.
A

one way to get at it is to measure the quarter beam buttock angle, typically about 7 degrees or more of a "full displacement" boat. Another way to get at it is the prismatic coeffecient, as another poster discussed., which is displacement divided by 64, in turn divided by the area below the waterline at midship times the waterline length. That'll be a little over .50 and under .6 for a displacement boat, over .7 for a planing boat. You'll have to read up to understand how those measurements are taken and what they mean. So can arrive at various types of displacements (full, semi, planing) via a number of hull designs, as illustrated. Then you get into the powertrain calculations and can derive sl from there.

yes

 

[caltexflanc]

See that's the mistake that keeps jerking this around, none of that matters, not horsepower, not reduction and not buttock angle. A competent naval architect designs a hull for certain performance and depending on the SL ratio it's designed to operate at defines it, period. How it's designed to operate at that SL ratio doesn't matter, a planing boat can have a flat bottom or a deep vee with lifting strakes yet either way it's designed to operate at above approximately an SL ratio of 2.5. A full displacement boat can also have a wide variety of hull configurations yet it's still defined as full displacement by virtue of being designed to operate below an SL ratio of 1.34. All the rest is just distraction and mental masturbation.

You've got it completely backwards, think about it a little: exactly HOW is it designed to have a lower SL?

 

[Nomad Willy]

I'm not entirely sure how to read that?

A good one by Fish53.
But at what point is the hull not fully “in” the water? IMO as soon as the bow begins to rise it is not.
How many FD hulls would qualify to that? I can’t remember when my FD hull starts to rise but it may be before her usual cruising speed.

 

[Nomad Willy]

You've got it completely backwards, think about it a little: exactly HOW is it designed to have a lower SL?

That’s easy or obvious.
Just give the hull a buttock angle and WLL that will play ball w whatever kind of boat you want.

 

[Fish53]

A good one by Fish53.
But at what point is the hull not fully “in” the water? IMO as soon as the bow begins to rise it is not.
How many FD hulls would qualify to that? I can’t remember when my FD hull starts to rise but it may be before her usual cruising speed.

As is evident by the name "full displacement" that type of hull does not change the volume of water it displaces when at rest or underway. Apparently I need to fix my scanner or get a new one so I can just post pages from the books previously cited.

 

[Plane Nauti]

This photo is a boat doing about 235 kts. What bow wave?

235 kts?