Definition Of Full Displacement

Pulling or pushing a D hull fast enough over water trying to get it on "plane", in semblance, is not too unlike pulling or pushing a person fast enough over ground trying to make them "fly".


End result is neither form actually accomplished "planning" nor "flying". Just forced alterations to their physical forms' intended stance-results. And, both items are likely wrecked at close of the grossly overpowered pulling/pushing endeavor.


Why o' Why - would any adult for any reason want to pull or push a D hull fast enough to try to get it on plane???? Makes no sense!!


This thread kinda reminds me of playing with model boats in bathtub when very young... Little intention for actuality nor care for reality!

Art said:

Pulling or pushing a D hull fast enough over water trying to get it on "plane", in semblance, is not too unlike pulling or pushing a person fast enough over ground trying to make them "fly".


End result is neither form actually accomplished "planning" nor "flying". Just forced alterations to their physical forms' intended stance-results. And, both items are likely wrecked at close of the grossly overpowered pulling/pushing endeavor.


Why o' Why - would any adult for any reason want to pull or push a D hull fast enough to try to get it on plane???? Makes no sense!!


This thread kinda reminds me of playing with model boats in bathtub when very young... Little intention for actuality nor care for reality!

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Well said, that's why design is the key, at least in the bathtub there's bubbles.

Baker said:

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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The flat sections are quite visible in the photo. At speed, it doesn't take much to get a very light rig up on plane. For older Hobie designs that don't have the "at rest" flat sections you can see in the picture I posted, the planning surface becomes the hull side when the Cat gets up on one sponson and you have to hang way out to windward to keep from capsizing.

But as I said, if one wishes to call a hull that planes a non planing hull, it's fine by me. To paraphrase the Red Queen, hull type is whatever she says it is, regardless of what it actually is.

In that spirit, since all hull designs of whatever shape displace water at rest, and at displacement speeds push the water out of the way rather than riding on top of it, I declare that the answer to the OP's question is that anything that touches the water is a full displacement hull, whether wineglass shaped blue water cruiser, a rock, a Hobie Cat skimming the warer at 30 knots, an inner tube being pulled by a hydroplane, the hydroplane itself - these are all full displacement hulls.

I personally have always wanted to own a hydrofoil, so the one exception to the above definition is going to be Delfin, which henceforth will be referred to as a hydrofoil. The fact that it lacks foils is no deterrent anymore than the ability to plane indicates a planing hull, which no longer describes anything anyway.

Delfin said:

I personally have always wanted to own a hydrofoil, so the one exception to the above definition is going to be Delfin, which henceforth will be referred to as a hydrofoil..

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In the two latest America's Cups the arguments over wings vs hydrofoil language and definitions is prodigious. Even the top designers argue over the minutiae of the definition and guard their curve linear differential equations carefully.

Flat vs round bottoms vs exposed aft sections seems more, well, gentlemanly.

Delfin said:

Got it. This vessel is behind its bow wave because as you say, a boat never gets ahead of it.

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The vessel in that picture is certainly not ahead of its bow wave. The only parts of the hull that have any effect on hydrodynamics are the parts that get wet. Everything else can be cut away without effect. In that photo the effective bow of the boat is where the water and fiberglass meet. You might as well say a heavy tub with a long bowsprit is "ahead of its bow wave" since the end of the bowsprit is - which would be ridiculous.

Fish53 said:

"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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The surface is not vertical, and 3 and 9 o'clock positions are not on the surface. In any case the water motion is insignificant compare to the boat speed - do the calculations.

Delfin said:

The flat sections are quite visible in the photo. At speed, it doesn't take much to get a very light rig up on plane. For older Hobie designs that don't have the "at rest" flat sections you can see in the picture I posted, the planning surface becomes the hull side when the Cat gets up on one sponson and you have to hang way out to windward to keep from capsizing.

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A Hobie (and any cat) sails fastest with very little heel. Sure you can have fun with one like the picture shows, but that will not win any races. Even with both hulls in the water, a Hobie 16 will do 16 knots easily - that is 4 x WL ^ 0.5. It is not planing as anyone would define it. Even the fastest cats in the world ( non-hydrofoil) have no flat planing sections, they are semi-circular bow to stern. (Hydrofoils are not bound be wavemaking drag and are a whole different subject.) The most recent ocean racing cats have wave piercing bows as it is quicker to go through waves, rather than over them.

Baker wrote;
“Originally Posted by Nomad Willy View Post
There are boats that have no flat planing hull surfaces that exceed hull speed considerably. Old sub chaser’s do come to mind.
But they have some lengthly straight lines aft and a well submerged transom. And lots of power.

DDW said:

The vessel in that picture is certainly not ahead of its bow wave. The only parts of the hull that have any effect on hydrodynamics are the parts that get wet. Everything else can be cut away without effect. In that photo the effective bow of the boat is where the water and fiberglass meet. You might as well say a heavy tub with a long bowsprit is "ahead of its bow wave" since the end of the bowsprit is - which would be ridiculous.


The surface is not vertical, and 3 and 9 o'clock positions are not on the surface. In any case the water motion is insignificant compare to the boat speed - do the calculations.


A Hobie (and any cat) sails fastest with very little heel. Sure you can have fun with one like the picture shows, but that will not win any races. Even with both hulls in the water, a Hobie 16 will do 16 knots easily - that is 4 x WL ^ 0.5. It is not planing as anyone would define it. Even the fastest cats in the world ( non-hydrofoil) have no flat planing sections, they are semi-circular bow to stern. (Hydrofoils are not bound be wavemaking drag and are a whole different subject.) The most recent ocean racing cats have wave piercing bows as it is quicker to go through waves, rather than over them.

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So if it's "insignificant" then why even say it much less bother to correct others? I apparently didn't understand your original comment correctly, for that I'm well, apathetic.

Fish53 said:

So if it's "insignificant" then why even say it much less bother to correct others? I apparently didn't understand your original comment correctly, for that I'm well, apathetic.

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53 - There must be a 12 step program for addicted hull shape persons!! :lol:

Art said:

53 - There must be a 12 step program for addicted hull shape persons!! :lol:

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12 steps will take too long. As there is apparently no end in sight for this thread I'll declare my boat with it's 5000 pounds and 18hp a wicked slow boat and let it go at that. I did enjoy your posts however.

DDW said:

The vessel in that picture is certainly not ahead of its bow wave. The only parts of the hull that have any effect on hydrodynamics are the parts that get wet. Everything else can be cut away without effect. In that photo the effective bow of the boat is where the water and fiberglass meet. You might as well say a heavy tub with a long bowsprit is "ahead of its bow wave" since the end of the bowsprit is - which would be ridiculous.

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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.

DDW said:

A Hobie (and any cat) sails fastest with very little heel. Sure you can have fun with one like the picture shows, but that will not win any races. Even with both hulls in the water, a Hobie 16 will do 16 knots easily - that is 4 x WL ^ 0.5. It is not planing as anyone would define it. Even the fastest cats in the world ( non-hydrofoil) have no flat planing sections, they are semi-circular bow to stern. (Hydrofoils are not bound be wavemaking drag and are a whole different subject.) The most recent ocean racing cats have wave piercing bows as it is quicker to go through waves, rather than over them.

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Of course a semi-circular hull generates lift, and with a long narrow hull, the Hobie under sail generates more lift than drag, so planes. Which is something a full displacement hull cannot do, except with physics that don't seem to apply to this space/time envelope. Again, you can redefine the language to your heart's content to make your point, and as I said, I am a ok with that.

Delfin I think I'll pass on a counter post.
We've got enough balance on this thread now.

Or... imbalance!

simple no flat sections aft. P and SD have flat sections aft.

At the risk of putting another log on this fire, I found this article comparing KKs FDs to Fleming's SDs (on KK's site) offered a practical explanation for those of us not interested in being towed by aircraft carriers.

http://www.kadeykrogen.com/articles/pages/Krogen54designprobe.htm

bgilroy,
Simple eh?

314 posts and there’s only some agreement at this point.

But probably 49 out of 50 you’re right. But then it gets complicated. Very very few boats are perfectly flat. Flat fore and aft or athartships or flat at an angle. Or several combinations of flat.

Airstream345 said:

At the risk of putting another log on this fire, I found this article comparing KKs FDs to Fleming's SDs (on KK's site) offered a practical explanation for those of us not interested in being towed by aircraft carriers.

http://www.kadeykrogen.com/articles/pages/Krogen54designprobe.htm

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Interesting article, although out of date based on the new physics discovered on this thread, since we've learned that the reference to a speed limit for a full displacement hull is obsolete. KK should have consulted with Hobie. Perhaps then they could have built a full displacement hull that planes.

If I saw an M30 sitting at the dock, looking at its stern would suggest an FD vessel. 43 knots out of this beauty.
http://www.mooresyachts.com

Here you go, have fun discussing this one and all its brethren:

caltexflanc said:

Here you go, have fun discussing this one and all its brethren:

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Almost every dragger I ever worked on had an immersed transom, including my own but it's important to remember that these vessels are designed to take on a considerable amount of load, far more than a pleasure boat of similar size. In practice minus ice and fish that may or may not be immersed. Oh boy this thread isn't dead after all.

sunchaser said:

If I saw an M30 sitting at the dock, looking at its stern would suggest an FD vessel. 43 knots out of this beauty.
http://www.mooresyachts.com

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Cool looking boat.

sunchaser said:

If I saw an M30 sitting at the dock, looking at its stern would suggest an FD vessel. 43 knots out of this beauty.
http://www.mooresyachts.com

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Chaser,
It probably has a stern very much like the Bartender.
Both planing hulls.

Fish53 said:

Almost every dragger I ever worked on had an immersed transom, including my own but it's important to remember that these vessels are designed to take on a considerable amount of load, far more than a pleasure boat of similar size. In practice minus ice and fish that may or may not be immersed. Oh boy this thread isn't dead after all.

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Not dead HaHa ....
you mean this FD hard chine vessel?
Nothing to talk about. FD
But perhaps the partially submersed transom. I can’t think of a boat w it’s transom fully above the water as being anything but FD. But there are some, quite a few actually, that have partially submerged transoms. The design element that is decisive on these boats is the angle (fore and aft) that comprises most of the bottom well aft of amidships. These boats have a quite steep angle aft that identifies them 100% as being FD. If the bottom was straight and about horizontal they could be even planing hulls.

This angle that is called Quarter Beam Buttock Line by naval architects has in the past been assigned numbers such that a shallower # than “X” indicates a SD hull and steeper than “X” = FD.
But whenever one assigns a specific number to something that number fails to hold up under all circumstances. But the QBBL is probably the best decisive road to separting FD from SD.

One other way to separate FD/SD is to evaluate the area and it’s percentage of boat submerged to the deepest point and how much area is submerged of the transom. But I know of no hard numbers to attach to that.

bgillroy said:

simple no flat sections aft. P and SD have flat sections aft.

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I guess the FD prawn trawler guys never got your memo

I thought I tried to be a bit ambiguous with that reply by making the loading observation.

Simi 60 said:

I guess the FD prawn trawler guys never got your memo

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A SD hull that lacks the power to get on top of its bow wave of still a SD hull. With enough power, this boat could plane, because the flat sections aft will provide the lift to keep the stern from going under. It's a lot cheaper to build this shape hull, even if you never expect or want to go faster than displacement speeds.

Why is it cheaper Delfin?
To build this shape of hull?

Delfin said:

A SD hull that lacks the power to get on top of its bow wave of still a SD hull. With enough power, this boat could plane, because the flat sections aft will provide the lift to keep the stern from going under. It's a lot cheaper to build this shape hull, even if you never expect or want to go faster than displacement speeds.

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yeah

Nomad Willy said:

Why is it cheaper Delfin?
To build this shape of hull?

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Flat steel not requiring shaping, less steel.

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”.

Nomad Willy said:

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”.

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All you need is an English wheel and a couple of guys that know how to use it. The vessel in question may have been intended for a significant load in shallow water such as carrying a load of pots, but curved steel would be stronger for the weight if the design would allow it. A large "underbelly" gives an offshore boat better carrying and seakeeping properties.