Definition Of Full Displacement

Any hull can be made to exceed "hull speed" with enough power, however if the height of the bow or stern wave exceeds freeboard first, then it will sink first. The 1.34 x WL ^ .5 formula is just speed of the wave with a length = WL. Assumption is once the wave is as long as the waterline it swallows the boat.

The lift/drag ratio of the hull form is really the determining factor. Planing boats develop enough lift to ride on top of their bow wave, leaving the stern wave behind. Many racing sailboats plane readily, even though they are soft bilged and do not have an immersed stern. Very narrow hulls like catamarans have U shaped or even completely round sections, and readily exceed "hull speed". These narrow hulls also have better lift/drag ratios than traditional displacement hull shapes.

The distinction is really not amenable to a simple definition. For ordinary trawler type yachts, hard chine vs. soft bilge or different sterns are characteristic, but not determining.

markpierce said:

If the boat makes wakes like these at cruising speed, it has a displacement hull:

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I reckon if the boat makes wake like that you are pushing past cruising speed.

We make almost no wash at 7.5 knots, our towed dinghy puts out more.

DDW said:

Any hull can be made to exceed "hull speed" with enough power, however if the height of the bow or stern wave exceeds freeboard first, then it will sink first. The 1.34 x WL ^ .5 formula is just speed of the wave with a length = WL. Assumption is once the wave is as long as the waterline it swallows the boat.

The lift/drag ratio of the hull form is really the determining factor. Planing boats develop enough lift to ride on top of their bow wave, leaving the stern wave behind. Many racing sailboats plane readily, even though they are soft bilged and do not have an immersed stern. Very narrow hulls like catamarans have U shaped or even completely round sections, and readily exceed "hull speed". These narrow hulls also have better lift/drag ratios than traditional displacement hull shapes.

The distinction is really not amenable to a simple definition. For ordinary trawler type yachts, hard chine vs. soft bilge or different sterns are characteristic, but not determining.

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Some hull forms can and routinely are powered past hull speeds, but a displacement hull cannot. The reason is what you identify - the lift generated by the hull vs. the drag as the vessel moves through the water. Displacement hulls generate very little lift, which is why the Navy has, for a very long time, built higher speed boats like destroyers with flat aft sections that generate enough life to get anywhere from 25% to 80% improvement over calculated hull speed. Without that lift, and without the ability to "leave the stern wave behind", a displacement hull simply steams itself under water, stern first, due to drag.

In the link above provided by Menzies, the author notes:

"On the other hand, there would be no benefit to powering the Kadey-Krogen used in our example with a 400 or 500-HP engine. Unlike semi-displacement hulls, long range trawlers with true displacement design cannot, in any practical sense, be pushed over the bow wave to get “on plane.” An over-sized engine would merely add to the weight and purchase cost of the vessel and occupy more space in the engine room than required."

I guess the key is what constitutes a "true displacement hull", and I believe that unless one is working to score Internet Forum points, the accepted definition is a boat that cannot get on top of its bow wave regardless how it is powered, and therefore cannot benefit from increasing horsepower beyond the roughly 1 hp per 500 pounds of displacement all displacement trawler builders generally provide in their vessels. A hull like a Fleming isn't a full displacement hull, and Fleming doesn't call it that for the simple reason that it can exceed hull speed due to its underbody configuration, which like Navy destroyers is fairly flat with minimal keel.

Simi 60 said:

I reckon if the boat makes wake like that you are pushing past cruising speed.

We make almost no wash at 7.5 knots, our towed dinghy puts out more.

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AFAIK, the most efficient speed for a displacement hull is around 1.1 x sq root of LWL because that is where speed to drag is most favorable. For your boat, that will be around 7.5 knots. Mine is slightly less.

Delfin said:

AFAIK, the most efficient speed for a displacement hull is around 1.1 x sq root of LWL because that is where speed to drag is most favorable. For your boat, that will be around 7.5 knots. Mine is slightly less.

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What do you mean by "most efficient speed"?

The most fuel efficient speed (in gallons/nm) is much slower than 1.1 x sq root of LWL; Drag is much more favourable at idle.

The most time efficient is maximum speed.

Is there another way to measure efficiency?

Two excellent and authoritative books I can recommend on the subject:

"The Nature of Boats" by David Gerr, a naval architect
"Voyaging Under Power" originally by Robert Beebe and updated by Nordhavn / Jee Leishman.

AusCan said:

What do you mean by "most efficient speed"?

The most fuel efficient speed (in gallons/nm) is much slower than 1.1 x sq root of LWL; Drag is much more favourable at idle.

The most time efficient is maximum speed.

Is there another way to measure efficiency?

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Good point. Perhaps a working definition of "most efficient speed" would be that speed where maximum distance is travelled in the minimum amount of time without drag ruining fuel economy, is maximized, but I will leave you to make up your own.

Fish53 said:

More correctly any hull being operated at displacement speed.View attachment 84941This boat is being operated at the transition from full displacement to planning or semi-displacement. Note the characteristic depression almost to the transom and the building wave in front and behind. In full displacement mode the depression is midway as in your photo, at planning speed the hull has climbed over the wave forward and is riding n the surface.

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See the picture in post #22
This boat in the picture is exceeding hull speed.
The stern wave is behind the boat. On a FD hull the water would be higher at the stern than amidships. Water would be high as a bow wave fwd and the water level would be lower than at rest amidship (where you can see bottom paint that is normally below the surface. Then at the stern the water humps up, actually before the stern ... mostly under the stern. So the boat is supported more so at the ends than at rest.
On a FD boat the stern wave would apear to be lifting the boat up right under the stern. To a degree it does, and this is one of the reasons the FD hull is more efficient. You could say the stern is surfing on the wave that is under her buttock. And to some degree she’s being pushed ahead recovering some of the energy expended fwd pushing the water down and to the sides. Kinda like a regenerative system on some electric cars whereas the motor drives the car up the hill and that motor turns into a generator on the way down the hill partially recharging the batteries. Also like when you drop a ball into the water it makes a wave traveling out all around from the center where it sliped into the water. Part of that wave bounces back to the center and pops up vertically. Energy lost but that energy makes the water in the center bounce up and down until all the energy is lost.

So looking at FD boats underway you’ll see a significant hump of water at and under the stern. If the wave (like the above pic) is aft of the transom it’s exceeding hs. FD, SD or planing. The wave exists for all three hull forms. It’s a matterof where the stern wave is that shows it’s speed.

Efficiency has nothing to do w the OP’s question.
Our discourse is interesting though and relavant or highly related.

Nomad Willy said:

See the picture in post #22
This boat in the picture is exceeding hull speed.
The stern wave is behind the boat. On a FD hull the water would be higher at the stern than amidships. Water would be high as a bow wave fwd and the water level would be lower than at rest amidship (where you can see bottom paint that is normally below the surface. Then at the stern the water humps up, actually before the stern ... mostly under the stern. So the boat is supported more so at the ends than at rest.
On a FD boat the stern wave would apear to be lifting the boat up right under the stern. To a degree it does, and this is one of the reasons the FD hull is more efficient. You could say the stern is surfing on the wave that is under her buttock. And to some degree she’s being pushed ahead recovering some of the energy expended fwd pushing the water down and to the sides. Kinda like a regenerative system on some electric cars whereas the motor drives the car up the hill and that motor turns into a generator on the way down the hill partially recharging the batteries. Also like when you drop a ball into the water it makes a wave traveling out all around from the center where it sliped into the water. Part of that wave bounces back to the center and pops up vertically. Energy lost but that energy makes the water in the center bounce up and down until all the energy is lost.

So looking at FD boats underway you’ll see a significant hump of water at and under the stern. If the wave (like the above pic) is aft of the transom it’s exceeding hs. FD, SD or planing. The wave exists for all three hull forms. It’s a matterof where the stern wave is that shows it’s speed.

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Eric, I think you can see what you are describing in the photo below. This was taken at hull speed. While I had power in reserve, the only effect of asking for more speed would have been to increase the size of the stern wave as drag increased, negating the extra power except in terms of generating a bigger wave. The Fleming semi-displacement wave pattern is very different.

Delfin said:

In the link above provided by Menzies, the author notes:

"On the other hand, there would be no benefit to powering the Kadey-Krogen used in our example with a 400 or 500-HP engine. Unlike semi-displacement hulls, long range trawlers with true displacement design cannot, in any practical sense, be pushed over the bow wave to get “on plane.” An over-sized engine would merely add to the weight and purchase cost of the vessel and occupy more space in the engine room than required."

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The key is "in any practical sense". There is no magic in displacement hull forms, they simply have low lift/drag, and the power required to climb the bow wave impractical. Very much full displacement hull forms in both power and sail will do this surfing down a steep enough wave. The extra power provided by the wave and gravity. You can make a KK plane. Might take a gas turbine and have a range of 50 miles, making it impractical.

A tire tube doesn't have the look of a planing hull, yet pulled by a ski boat they plane routinely. A human foot doesn't either, yet people barefoot ski. It's pretty hard to define a characteristic or set of characteristics that confines a shape to displacement speeds. Designers tend to pick forms that are efficient at the chosen operating conditions which we have come to recognize as typical planing or displacement types, but these definitions have very grey edges.

DDW said:

The key is "in any practical sense". There is no magic in displacement hull forms, they simply have low lift/drag, and the power required to climb the bow wave impractical. Very much full displacement hull forms in both power and sail will do this surfing down a steep enough wave. The extra power provided by the wave and gravity. You can make a KK plane. Might take a gas turbine and have a range of 50 miles, making it impractical.

A tire tube doesn't have the look of a planing hull, yet pulled by a ski boat they plane routinely. A human foot doesn't either, yet people barefoot ski. It's pretty hard to define a characteristic or set of characteristics that confines a shape to displacement speeds. Designers tend to pick forms that are efficient at the chosen operating conditions which we have come to recognize as typical planing or displacement types, but these definitions have very grey edges.

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I don't think that falling down a wave due to gravity quite demonstrates that displacement hulls can exceed hull speed. The basis for asserting that full displacement hulls cannot exceed hull speed is a consequence of the effect of increased drag on those hulls when speeds approach hull speed. Apply more power and drag increases. As drag increases, wetted surface increases as the stern of the vessel is literally dragged into the water, further increasing drag until you reach the hull speed limit. It probably is an asymptotic function, but there is a speed limit that can't be exceeded.

On my boat, if I apply WOT, I go no faster than I do at WOT minus 20% of rated rpm, but my stern is sucked under enough to start to bury it and slop water through the hawseholes. Delfin is, by any definition, a full displacement hull and changes to hull form that provide compensating lift I don't have allow for speeds in excess of hull speed, but for me, it simply isn't possible no matter how much power I employed.

And yes, a person can plane on their feet, and an inner tube can also skim along the surface of the water. That is because those flatish "hull" forms generate lift, something a displacement hull is designed not to provide. Those hull forms are intended to reduce drag, not produce lift which increases drag as lower angles of attack, as in less than planning speeds.

I think of FD hull shape as that of a tear drop or wine glass.

I have some more pics that show the FD dynamics well. Had to wait till I got home to my computer.

1. This pic shows a sailboat at WOT (or very close to) bucking a lot of current. You should be able to see the crest of the stern wave about 2' aft of the transom.

2. This shows the stern of my rowboat. The up-turn of the hull bottom aft is very clearly shown. Look closely at her keel and you can see her WL at rest presumably with no people aboard. The transom is well out of the water. With a full load the water level would be close to the transom or a smudge below. Despite her small size she's a good example of a FD hull.

3. Here is Willy at WOT just days after we bought her. It's easy to see she's just starting to outrun her stern wave as it's about 3' aft of the stern. While cruising it would be ideally about a foot fwd of the stern.

SD and Planing boats have a very different following wake. The water boiling out under the bottom at the bottom to transom edge is a frothing mass of extremely disturbed water. Much like the water in a washing machine. Water is literally jumping up and down. FD wakes aft are very different whereas the water could be described as "flowing" w only a small fraction of the turbulence from SD boats.

Delfin said:

The basis for asserting that full displacement hulls cannot exceed hull speed is a consequence of the effect of increased drag on those hulls when speeds approach hull speed. Apply more power and drag increases. As drag increases, wetted surface increases as the stern of the vessel is literally dragged into the water, further increasing drag until you reach the hull speed limit. It probably is an asymptotic function, but there is a speed limit that can't be exceeded.

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Every form produces lift, some not as efficiently as others. Drag increases but not without limit. Wetted area has a limit and is in any case a small component of drag at those speeds. And as I said, if the hole it digs exceeds freeboard and the hatches are open it may sink first. Nevertheless, every shape can be made to plane if enough power is applied. For Delphin, that would be a lot of power . Imagine a dinghy with the same hull form as Delphin. If I tow it at 15 knots it's going to plane if the decks are sealed. It's like skipping stones on a creek: the flat ones work better but a round one will skip if you throw them hard enough.

DDW said:

Every form produces lift, some not as efficiently as others. Drag increases but not without limit. Wetted area has a limit and is in any case a small component of drag at those speeds. And as I said, if the hole it digs exceeds freeboard and the hatches are open it may sink first. Nevertheless, every shape can be made to plane if enough power is applied. For Delphin, that would be a lot of power . Imagine a dinghy with the same hull form as Delphin. If I tow it at 15 knots it's going to plane if the decks are sealed. It's like skipping stones on a creek: the flat ones work better but a round one will skip if you throw them hard enough.

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I guess we'll have to agree to disagree. If Delfin's 65 tons were hauled through the water at 15 knots, I think she'll get sucked under by the stern. You think she'll put on her dancing shoes and skip across the waves like a paddle board. I think physics are on my side....

For a vessel to plane, lift must exceed drag. For a displacement hull, that never happens, drag just increases, not infinitely, but enough to pull the boat under. If this weren't the case, there would be no such thing as hull speed anymore than there is something called "car speed". Just an endless increase in speed as you add propulsion power.

DDW,
I don’t think any shape will plane w enough power.
They go out of control and loose their directional stability and achieve un-godly high angles of attack .... but not plane. Just my opinion though.I’ve never seen a boat like Delfin w 30,000hp.

Nomad Willy said:

DDW,
I don’t think any shape will plane w enough power.
They go out of control and loose their directional stability and achieve un-godly high angles of attack .... but not plane. Just my opinion though.I’ve never seen a boat like Delfin w 30,000hp.

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I believe you're correct as the sinking sailboat analogy indicates, but really we've gone a bit too far afield from what is practical or common. The picture of your boat is an excellent example of a displacement hulls wave forming. I've played with ideas to make my hull a bit more efficient primarily by sharpening the entrance. I'd like to try a bulbous bow but the technical details are beyond me.

https://en.wikipedia.org/wiki/Hull_speed


Hull speed or displacement speed is the speed at which the wavelength of a vessel's bow wave is equal to the waterline length of the vessel. As boat speed increases from rest, the wavelength of the bow wave increases, and usually its crest-to-trough dimension (height) increases as well. When hull speed is exceeded, a vessel in displacement mode will appear to be climbing up the back of its bow wave.
From a technical perspective, at hull speed the bow and stern waves interfere constructively, relatively large waves, and thus a relatively large value of wave drag. Though the term "hull speed" seems to suggest that it is some sort of "speed limit" for a boat, in fact drag for a displacement hull increases smoothly and at an increasing rate with speed as hull speed is approached and exceeded, often with no noticeable inflection at hull speed.
The concept of hull speed is not used in modern naval architecture, where considerations of speed-length ratio or Froude number are considered more helpful.





https://en.wikipedia.org/wiki/Hull_speed

Hull design implications

Wave making resistance depends dramatically on the general proportions and shape of the hull: many modern displacement designs can easily exceed their 'hull speed' without planing.
These include hulls with very fine ends, long hulls with relatively narrow beam and wave-piercing designs. Such hull forms are commonly realised by some canoes, competitive rowing boats, catamarans, fast ferries and other commercial, fishing and military vessels.
Vessel weight is also a critical consideration: it affects wave amplitude, and therefore the energy transferred to the wave for a given hull length.
Heavy boats with hulls designed for planing generally cannot exceed hull speed without planing.
Light, narrow boats with hulls not designed for planing can easily exceed hull speed without planing; indeed, once above hull speed, the unfavorable amplification of wave height due to constructive interference diminishes as speed increases. For example, world-class racing kayaks can exceed hull speed by more than 100%,[1] even though they do not plane. Semi-displacement hulls are usually intermediate between these two extremes.
Ultra light displacement boats are designed to plane and thereby circumvent the limitations of hull speed.



https://www.boatinternational.com/y...e-efficiency-by-reducing-hull-resistance--635



Plus, of course, the longer a hull is, the greater its top speed in the displacement mode according to Froude’s law which states that displacement speed is directly proportional to the square root of the waterline length, or (1.34 x vLWL). Because of overhangs, this number if often expanded to 1.5 x vLWL.

"I'd like to try a bulbous bow but the technical details are beyond me."

The big hassle with a bulbous bow is they are optimized for one speed.

The big boys spent big bucks having their shape modified in the last economic slowdown , when slower delivery times became the norm.

FF said:

"I'd like to try a bulbous bow but the technical details are beyond me."

The big hassle with a bulbous bow is they are optimized for one speed.

The big boys spent big bucks having their shape modified in the last economic slowdown , when slower delivery times became the norm.

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Precisely, I of course don't intend to operate a one speed so why bother, although it could be said I have one speed, slow.

FF wrote;
“The big hassle with a bulbous bow is they are optimized for one speed”

So is a FD hull. Optimized for

ha, Maybe this goes beyond the laws of physics, but the guy here in Finland with the old FD Hull open Trawler installed a lot of hp (500) earlier about 20 hp. In addition, the hull reinforced steel beams withstand the stress. Previous speed about 7kn now +40kn.


I do not just question the shape of the body as these "trawlers" have a tapered stern and a round base with full long keel and now it is both FD, SD and additionally planing Hull?

Litle trip this video

https://youtu.be/fTHYD_hVAK0




Typical hull profile this series trawler, this boat have litle cabin.

NBs

North Baltic sea said:

ha, Maybe this goes beyond the laws of physics, but the guy here in Finland with the old FD Hull open Trawler installed a lot of hp (500) earlier about 20 hp. In addition, the hull reinforced steel beams withstand the stress. Previous speed about 7kn now +40kn.


I do not just question the shape of the body as these "trawlers" have a tapered stern and a round base with full long keel and now it is both FD, SD and additionally planing Hull?

Litle trip this video

https://youtu.be/fTHYD_hVAK0

Typical hull profile this series trawler, this boat have litle cabin.

NBs

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While not the same it sort of makes me think of Bartender boats, look like FD but plane. Of course they have a sizable flat section though. I have a 28' seine dory that's flat for most of it's bottom but I'm happy with the 9hp it has now, something to be said for being able to fuel for a days cruising with a five gallon can.

Delfin said:

For a vessel to plane, lift must exceed drag.

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No, lift must reduce displacement enough so that power can exceed drag. A planing boat displaces less than it's weight. Even with NO lift, a boat can be made to exceed its hull speed by pushing it hard enough. It simply creates a wave that is longer than it's length. It's inefficient, but quite possible.

Read PSneed's post #49. This is correct, most NA's do not talk about "hull speed" which is pretty much a meaningless number, though ok to compare similar heavy boats. In particular:

in fact drag for a displacement hull increases smoothly and at an increasing rate with speed as hull speed is approached and exceeded, often with no noticeable inflection at hull speed.

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If you look at drag vs. Froude numbers for hulls with a lot of rocker (what we think of as "displacement") the curve bends upward with speed but does not go vertical - for any shape. If lift is sufficient to reduce displacement significantly, the curve flattens out again.

Yes, it is impractical to think about 100,000 hp in Delphi. A similar hull form of lighter displacement could more easily be made to exceed "hull speed", but there is little practical application. The canoe stern design shown is a poor candidate for planing, as there is likely insufficient floatation in the stern to keep it from sinking before sufficient lift is developed - but there are plenty of planing canoe sterned boats.

A practical definition of a displacement hull is a hull that is designed to operate most efficiently at speeds of less than "hull speed". Typical characteristics are deadrise, rocker, and angled sections aft. Planing hulls are designed to run efficiently on plane, typical characteristics are low or no deadrise aft, low rocker, a flat run aft. However you will see characteristics crossing these lines frequently. The bastard child is the SD hull, not as efficient at either mode, typically has characteristics of planing hull but is over weight and underpowered for that mission. Nevertheless, it is very popular for other reasons.

Delfin said:

For a vessel to plane, lift must exceed drag.

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DDW said:

No, lift must reduce displacement enough so that power can exceed drag.

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That is just another way of saying that for a vessel to plane or get ahead of its own bow wave, lift must exceed drag. You're simply identifying that propulsive power is what causes lift to exceed drag, but that doesn't negate that physical reality.

DDW said:

A planing boat displaces less than it's weight.

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Well, not according to Archimedes. All vessels displace their weight. I gather you mean that a planing hull on plane displaces less than its weight, and that is certainly true. It is able to pull this trick off because the lift provide by the hull (flat sections) provide more lift than drag getting the boat up and out of the water it would normally displace at rest or at less than planing speeds.

DDW said:

Even with NO lift, a boat can be made to exceed its hull speed by pushing it hard enough. It simply creates a wave that is longer than it's length. It's inefficient, but quite possible.

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Well no, it's not. Without lift, any applied propulsion simply increases drag. With a displacement hull, there is some lift, as you mentioned earlier. However, because it is full displacement, drag increases greater than lift resulting in the existence of the formula mentioned by PSneed in his reference. Speaking of which, why do you think that the "overhangs" change that formula a bit? Answer: Because the faster a displacement hull goes, the lower it sinks in the water and the greater it LWL increases resulting in a higher hull speed. Keep applying power and you will hit the hull speed wall where any additional power simply increases enough drag to prevent the boat getting in front of its bow wave.

DDW said:

Read PSneed's post #49. This is correct, most NA's do not talk about "hull speed" which is pretty much a meaningless number, though ok to compare similar heavy boats.

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NAs use Froude numbers because they provide apples to apples comparisons between hulls, not because the hull speed formula is wrong. It just varies enough between hull forms that Froude numbers are preferred for accuracy.

DDW said:

Yes, it is impractical to think about 100,000 hp in Delphi. A similar hull form of lighter displacement could more easily be made to exceed "hull speed", but there is little practical application.

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Essentially what you are saying is that the physics that apply to Delfin do not apply at smaller scales, even with all other variables remaining constant. Physics doesn't work like that. Which is why the Froude number calculation doesn't include displacement as a component of the formula.

DDW said:

The canoe stern design shown is a poor candidate for planing, as there is likely insufficient floatation in the stern to keep it from sinking before sufficient lift is developed - but there are plenty of planing canoe sterned boats.

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Sure there are. And all of them that plane are not displacement hulls, which cannot be made to plane. Incidentally, my former boat, a 36' Cape George did not have a canoe stern, but it certainly could not plane or ever get ahead of its bow wave. The faster I went, the deeper into the drink the stern went.

DDW said:

A practical definition of a displacement hull is a hull that is designed to operate most efficiently at speeds of less than "hull speed".

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I'm afraid that gets us into the problem of defining efficiency, as AusCan pointed out is a spongey term. Perhaps a better definition is that a displacement hull is one where drag increases faster than lift as you apply power due to hull design. Either such vessels exist, or they do not. But if you would acknowledge that adding 10,000 hp to Delfin isn't going to get her 7'6" draft and 130,000# up out of the water on plane, then maybe we found one.

The theories mostly pretain to normal aspect ratio boats.
My 18’ freight canoe will go about 13 knots w an old 6hp Johnson.

Here's another thought.

Planing hulls - do they use fin stabilizers (for when they chose to go slower) and, if so, what is the impact of those on stability at planing speeds?

There are several sentences that I take issue with but some may be me not getting your point exactly. But this one "The bastard child is the SD hull, not as efficient at either mode, typically has characteristics of planing hull but is over weight and underpowered for that mission. Nevertheless, it is very popular for other reasons.". Up here in Maine there's tons of SD hulled lobsterboats that do quite well in the planning department, I have one friend with a 42' lobster boat that does over 40kts and race boats do up to 70kts with an SD hull. I 've had a couple one was a BHM 32 with a 215hp Isuzu that did 18kts WOT and the other a Sisu 26 with a 200hp TAMD41a Volvo that did 25kts.

menzies said:

Here's another thought.

Planing hulls - do they use fin stabilizers (for when they chose to go slower) and, if so, what is the impact of those on stability at planing speeds?

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They do, but the movement of the fins is smaller as speed increases. If I want my system to be less aggressive in deflection in response to roll, I just tell it I'm doing 20 knots rather than the 8 I am doing.