Definition Of Full Displacement

[Delfin]

When a wave speeds up it gets longer, not necessarily higher.

So, as a full displacement vessel approaches hull speed, the bow wave speeds up as it lengthens, but you are apparently saying that the bow and stern wave don't also increase in height. Do I understand you correctly? if we can agree that they do increase in height, then you are agreeing with my point, not disagreeing with it.

 

[Riverguy]

Never, never...

Never say "never". You can apply a tremendous amount of power to a boat, most normal hull shapes will eventually plane, though some will sink due to lack of freeboard first.

Huh? I was sure we were talking about "FULL DISPLACEMENT" hulls.

Never is the right word. The question is not about '...most normal hull shapes...' (whatever that means). The question is about Full Displacement hulls.


A true 'Full Displacement' hull will never plane out, no matter how much power you put into it, which (if I read you correctly) you seem to acknowledge, like this...

Re: "...though some will sink due to lack of freeboard first."

In the real world of hulls that someone has actually built (or would actually build), a hull that is up on plane cannot 'sink due to lack of freeboard'. If a hull on plane lacks enough freeboard to prevent it sinking, then it would sink immediately as soon as it came down off plane. This would not be called a hull, it would be called an artificial reef.

While we're at it, maybe we need to make sure we're all talking about the same thing when we say 'planing'?

https://www.boatdesign.net/threads/definition-of-planing.45248/

So, when you said that a full displacementy hull 'can be forced to plane, but will sink due to lack of freeboard first' -- how is that different from saying it can never plane?

Re: "You can apply a tremendous amount of power to a boat..."

Tremendous is not enough. How about this. Show the attached photo to any marine architect, and ask them "how much power will I need to get this boat on plane"?

 

[psneeld]

Actually, I beieve there have been reports of racing sailboats that have gone completely under and popped right back up. They were being driven hard and surfing big swells.


Cant say they were displacement , but they were well betyond hull speed, most likely sleds that develoed too much low pressure below for their shape/bouyancy.

 

[Fish53]

Actually, I beieve there have been reports of racing sailboats that have gone completely under and popped right back up. They were being driven hard and surfing big swells.

There's also reports of sailboats being given a tow by ships and being towed under and not popping up.

 

[SteveZ]

Full Displacement Speed

I find it more helpful to think of full displacement as an operating speed rather than a hull design. A fast boat can run at "full displacement speed." That speed is typically 1.2 x the square root of the waterline (1.34 is the technical number, but not the ideal one).

I worked for a naval architect who was found of saying that if you tied a sailboat (full displacement for sure) to an aircraft carrier, it would plane. As someone said earlier, the accident report would be interesting.

Some boats are designed to operate solely in full displacement mode. Some boats are designed to be semi-displacement, but due to engine choice or poor selection, then can only run in full displacement mode.

As for rolling, I would argue that all boats roll badly at full displacement speeds. Is a hard chine boat any better? Marginally perhaps but they still roll like mad at low speeds in moderate sea conditions.

Steve Zimmerman

 

[Chris_K]

A full displacement trawler is one that displaces it's own weigh to remain afloat and it must push that same weight of water out of it's way to move. Any boat can be over-engined and made to go faster than it's hull speed. Hull speed is the velocity at which power and drag lines cross on a graph, it is where speed and fuel economy are optimized. Planing hulls are typically faster and the "get on plane" meaning they are skimming on top of the water to an extent so that it is not displacing it's weight but rather a combination of buoyancy and the physical action of planing keeps it above water.

 

[seabum]

I can make a square rock plane with my slingshot. Enough power and anything will plane, but likely not in an elegant manner.

 

[Art]

I find it more helpful to think of full displacement as an operating speed rather than a hull design. A fast boat can run at "full displacement speed." That speed is typically 1.2 x the square root of the waterline (1.34 is the technical number, but not the ideal one).

I worked for a naval architect who was found of saying that if you tied a sailboat (full displacement for sure) to an aircraft carrier, it would plane. As someone said earlier, the accident report would be interesting.

Some boats are designed to operate solely in full displacement mode. Some boats are designed to be semi-displacement, but due to engine choice or poor selection, then can only run in full displacement mode.

As for rolling, I would argue that all boats roll badly at full displacement speeds. Is a hard chine boat any better? Marginally perhaps but they still roll like mad at low speeds in moderate sea conditions.

Steve Zimmerman
ZIMMERMAN MARINE

"As for rolling, I would argue that all boats roll badly at full displacement speeds. Is a hard chine boat any better? Marginally perhaps but they still roll like mad at low speeds in moderate sea conditions."

You're talking explicitly about beam seas... I take it. And, with a full plane hull, having ample power, there are speed increase levels that can reduce even beam sea rolls. Although adding some time, fuel use and actual boat-through-water/over-land distance... for any shape hull at any speed... angles of tacking through beam sea conditions can help reduce "roll" circumstances.

 

[Delfin]

I can make a square rock plane with my slingshot. Enough power and anything will plane, but likely not in an elegant manner.

You can make a rock, or an inner tube, or a canoe plane because it has surfaces that are more or less horizontal to the water, so generates lift to keep it above the water. A full displacement hull has completely inadequate such surfaces and drag will overcome whatever lift the hull does generate, resulting in a swamped boat if pushed. This is an observable phenomenon that has been mentioned by others - FD hulls have been pulled under the water when you overpower them during a tow. According to the theory that any FD hull can plane with enough power is simply incompatible with this observable fact.

Now, if you are talking about strapping a couple of Saturn V rocket engines on each side of a FD boat, then I guess you could turn it into a rocket, Wiley Coyote style. But the idea that a FD boat, under its own power, can power through the bow wave created as you approach hull speed, is simply false.

 

[Chris_K]

I can't unsee that!

 

[Fighterpilot]

Our 33 Trojan International express with the Delta Conic hull
handles the roll issue the best of any of the 5 boats we have owned.

 

[Delfin]

And speaking of how you make a full displacement hull plane, this gentleman figured it out. Just add the lifting surfaces not present on a FD hull.

https://dory-man.blogspot.com/2012/06/how-to-make-displacement-hull-plane.html

 

[mikecambrai]

True displacement hulls

The traditional true displacement hull is a fishing boat. The form is that of a 'double ender', pointed at each end at the water line. They are also fat bellied which reduces to some extent the rolling, but then a displacement fishing boat gives with the sea and does not fight it.
The HP requirement is low. My 50 ft MFV had an 88hp Kelvin diesel. Which drove her happily at 7 knots with frugal use of fuel.

 

[Delfin]

The traditional true displacement hull is a fishing boat. The form is that of a 'double ender', pointed at each end at the water line. They are also fat bellied which reduces to some extent the rolling, but then a displacement fishing boat gives with the sea and does not fight it.
The HP requirement is low. My 50 ft MFV had an 88hp Kelvin diesel. Which drove her happily at 7 knots with frugal use of fuel.

Envious of your Kelvin.....

 

[garycasey]

I agree with Eric. A full displacement hull will have its stern out or nearly out of the water. A semi-displacement hull will have a squared-off stern that will likely be significantly underwater at rest. An SD hull will generally have hard chines, but I don't think it's a firm criterion. Either will have substantial "belly". A planing boat will have an essentially straight bottom. My SD hull also has a slight concavity at the stern, keeping the stern from descending too much above hull speed. By the way, any hull, even a bowling ball, will plane given enough horsepower.

In action, a FD boat will generally have a power vs. speed curve that is concave upward. A SD boat will have an inflection point in the curve where the power required won't, at some speed, go up as fast as at other speed. A planing hull has a power curve that actually drops above planing speed. So a SD hull has a curve between the two extremes. At speeds below hull speed a FD hull will be more efficient, but driven above HS the SD hull will be more efficient. Hope this helps.

 

[Art]

Now, if you are talking about strapping a couple of Saturn V rocket engines on each side of a FD boat, then I guess you could turn it into a rocket, Wiley Coyote style. But the idea that a FD boat, under its own power, can power through the bow wave created as you approach hull speed, is simply false.

And... That... settles it! Thanks!! Delfin

 

[Baker]

As for rolling, I would argue that all boats roll badly at full displacement speeds. Is a hard chine boat any better? Marginally perhaps but they still roll like mad at low speeds in moderate sea conditions.

Steve Zimmerman

There are terms in aviation realted to this. Static stability and dynamic stability. Stati stability is the INITIAL tendency for the vehicle to resist being upset. Dynamic is the tendency for the vehicle to return to its previous state. Static stability is really what we are talking about here.

SD hulls have a hard BUOYANT chine the resists roll initially. FD hulls do not. So they are not the same in this regard. When you step aboard an FD hull it will "give" more than and SD boat would. That is what people are talking about reference roll. Now once the roll starts, as in a beam sea, it has a tendency to snap back for the same reason described above. That is what most folks are talking about. A mild wave will roll an FD hull whereas an SD hull may not roll at all due to the hard chines.

As you were...

 

[Fish53]

The traditional true displacement hull is a fishing boat. The form is that of a 'double ender', pointed at each end at the water line. They are also fat bellied which reduces to some extent the rolling, but then a displacement fishing boat gives with the sea and does not fight it.
The HP requirement is low. My 50 ft MFV had an 88hp Kelvin diesel. Which drove her happily at 7 knots with frugal use of fuel.

Don't tease me share a picture, I have 26' double ender with a Lister in it. I'd love to see the boat and the Kelvin, some of the guys from Australia have Gardeners which also makes me jealous, Gardeners and Kelvins are almost unheard of in this country, sadly as they're fine pieces of machinery.

 

[menzies]

I can make a square rock plane with my slingshot. Enough power and anything will plane, but likely not in an elegant manner.

What happens to it when it runs out of power?

 

[clyde]

as to the 1.34 x the sq root, the dividing # is dependent on the hull configuration, ie, round deep hull, hard chines, etc. i was told many many years ago, by a boating friend, when there is a small wave on top of the stern wake, depending on the size, is diesel being thrown away! then i read the same in an older boating mag!...clyde

 

[koliver]

Years ago my dad, in his C&C 30 sailboat (FD) broke down and was rescued by the CG boat, the Ready, in English Bay, Vancouver. Ready offered him a tow, which he accepted, not knowing that their idea of low speed was about 12 knots. For his hull speed of 5.5 knots, that would be as close to planing as that hull could go.
The reality of getting that hull to 12 knots was to create a wave that took away so much support in the middle of the boat, leaving it only at the ends, that the rolling became extreme, comfort and steering difficult.

 

[Delfin]

Years ago my dad, in his C&C 30 sailboat (FD) broke down and was rescued by the CG boat, the Ready, in English Bay, Vancouver. Ready offered him a tow, which he accepted, not knowing that their idea of low speed was about 12 knots. For his hull speed of 5.5 knots, that would be as close to planing as that hull could go.
The reality of getting that hull to 12 knots was to create a wave that took away so much support in the middle of the boat, leaving it only at the ends, that the rolling became extreme, comfort and steering difficult.

Keith, that particular hull isn't really full displacement. The fin keel, spade rudder and flat after sections would allow it to go over hull speed without being sucked under, as hairy a ride as that no doubt was at 12 knots.

 

[DDW]

So, as a full displacement vessel approaches hull speed, the bow wave speeds up as it lengthens, but you are apparently saying that the bow and stern wave don't also increase in height. Do I understand you correctly? if we can agree that they do increase in height, then you are agreeing with my point, not disagreeing with it.

I said "won't necessarily" increase in height. The speed of the wave is related only to its length. The height is the height, and in the case of a boat is related to the displacement. A light planing boat will not raise much wave height because it is not displacing much water (it's running displacement is its weight - lift generated). A heavy planing boat displaces a lot more, and raises a higher wave. If they are both running the same speed, the length of the wave will be the same. If there is very little lift, and the boat very heavy, the wave will be higher still, but still the same length.

Now let's do a Gedänken experiment. Take a displacement type hull, perhaps double ended and with rocker and angled buttocks. Make it very light. Pull it through the water with high power. It will exceed hull speed by however fast you want to pull it. Now begin adding weight. The wave will get higher, and the power required more. Add enough weight and the crest of the bow wave will come over the rail. You can add freeboard to stop this, or change the bow shape to better cast the bow wave aside. The angled buttocks will tend to suck the stern down, and there may be some shapes having fine enough sterns that they would get sucked underwater at the stern and flood. Again more freeboard would prevent this, and few displacement shapes have these types of sections.

Catamaran hulls are classic displacement shapes: semi-circular sections, rocker, angled buttocks, non-immersed transoms. Yet they are expected to exceed hull speed by large multiples regularly. The reason is the narrow shape and relatively light displacement create a low wave. They have no flat planing sections that promote planing at all. They look like a stretched Delfin underwater. This is the practical application of non-planing hulls exceeding 'hull speed'. Delfin is a less practical application, for sure.

 

[DDW]

Tremendous is not enough. How about this. Show the attached photo to any marine architect, and ask them "how much power will I need to get this boat on plane"?

We are throwing about terms like "displacement hull" and "planing" without much of a definition. I said a displacement hull can be made to exceed "hull speed" by applying enough power. Pretty much every NA will agree with me on that.

What do you mean when you say "planing"? I'd call it a significant reduction in running displacement. That hull looks very similar to many sailboats which will exceed hull speed on occasion, and on the face of a wave will do a pretty good job of planing.

Yes that boat in the picture can be made to do that. It isn't practical, but it is possible. I've already referenced the power source that will do it. 418,000 lbs of thrust, weighs only 7700 lbs. The fuel pumps alone generate nearly 100,000 hp. Mileage will be poor though.

 

[Fish53]

Too many experts with less than expert "opinions", great examples and explanation DDW.

 

[Delfin]

I said "won't necessarily" increase in height. The speed of the wave is related only to its length. The height is the height, and in the case of a boat is related to the displacement. A light planing boat will not raise much wave height because it is not displacing much water (it's running displacement is its weight - lift generated). A heavy planing boat displaces a lot more, and raises a higher wave. If they are both running the same speed, the length of the wave will be the same. If there is very little lift, and the boat very heavy, the wave will be higher still, but still the same length.

Now let's do a Gedänken experiment. Take a displacement type hull, perhaps double ended and with rocker and angled buttocks. Make it very light. Pull it through the water with high power. It will exceed hull speed by however fast you want to pull it. Now begin adding weight. The wave will get higher, and the power required more. Add enough weight and the crest of the bow wave will come over the rail. You can add freeboard to stop this, or change the bow shape to better cast the bow wave aside. The angled buttocks will tend to suck the stern down, and there may be some shapes having fine enough sterns that they would get sucked underwater at the stern and flood. Again more freeboard would prevent this, and few displacement shapes have these types of sections.

Catamaran hulls are classic displacement shapes: semi-circular sections, rocker, angled buttocks, non-immersed transoms. Yet they are expected to exceed hull speed by large multiples regularly. The reason is the narrow shape and relatively light displacement create a low wave. They have no flat planing sections that promote planing at all. They look like a stretched Delfin underwater. This is the practical application of non-planing hulls exceeding 'hull speed'. Delfin is a less practical application, for sure.

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.

 

[Baker]

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.

Be careful there Delfin. There are full displacement hulls on cats that will do planing speeds....a Hobie Cat is a perfect example. ANd there are Cats that have planing hulls. This is an extremely complicated subject that is beyond me. I am just here to state what I stated above.

 

[Fish53]

Be careful there Delfin. There are full displacement hulls on cats that will do planing speeds....a Hobie Cat is a perfect example. ANd there are Cats that have planing hulls. This is an extremely complicated subject that is beyond me. I am just here to state what I stated above.

Oh no!!

 

[RT Firefly]

Astounding. 12 pages, 238 posts about...and no, it isn't dead yet.

 

[Delfin]

Be careful there Delfin. There are full displacement hulls on cats that will do planing speeds....a Hobie Cat is a perfect example. ANd there are Cats that have planing hulls. This is an extremely complicated subject that is beyond me. I am just here to state what I stated above.

Baker, does it not seen to you that a "displacement hull" that planes might be better described as a planing hull?

Any hull is a displacement hull when traveling at displacement hull speeds, which is defined as the speed where the wave generated by the vessel underway is equal to the length of the vessel. The type of hull that can't exceed hull speed is being referred to in this thread as a full displacement hull that can't generate enough lift to overcome the drag of water past the hull plus the resistance of the view wave. These hull types can, and have been pulled under water if you try to climb that bow wave with excess power.

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. However, if the hull is full displacement, it will have a maximum speed very close to its hull speed, unless gravity lends a hand.

From the Wiki already posted by someone:

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.


Because the height of the stern wave increases as boat speed increases, increases in power applied to a full displacement hull simply generates a deeper hole in the stern, which explains how such boats get pulled under the water if towed too fast.