Semi displacement hull [Archive]

dskira

11-26-2009, 09:26 AM

Semi-displacement is a bizarre term who means nothing.
I will contradict Mr Blount, I think the term is wrong.
It was invented by boatbuilder to sell their short and beamy shoebox with thousand of horse power and didn't went to planing, but push enough water to go a little faster that a fishing boat.
In other term Semi-displacement do not exist.
You can go very fast without planing if you keep the waterline narrow and the displacement/lenght (Imperial) at 50 to 60 like a escort destroyer of the 1950. Beautiful ship, fast and seaworthy.
The best will be to have a length waterline beam waterline ratio around 10.
Difficult in small boat but possible in multihull.
But the condominium accommodation will be not allowed, so the builder will go under :D
Anyway, journalist invented a lot of terms in their glossy magazine, as the marketing people from some boatbuilder. Boy they are inventive, a country music song writer from the South can keep up with them :P
Cheers and happy ThanksGiving
Daniel

gonzo

11-26-2009, 10:10 AM

If you overpower a displacement hull enough it will sink. Early speedboats had that problem. They found that at a certain speed, the hull would start going down. Planing hulls are designed to produce dynamic lift. I think that climbing over the bow wave is a good definition of planing. However, planing is like running, you are either doing it or not.

Tad

11-26-2009, 02:17 PM

Blount's view......
Displacement and planing hulls operate in very different manners, as the planing hull lifts it's waterline length changes, making any coefficient based on it's length meaningless. Naval Architects use the Volume Froude number as a dimensionless coefficient for comparing boats, using speed in relation to at rest displacement.

Equation for Volume Froude Number.
FNV= v/(g x dis^1/3)^1/2

v= velocity in feet per second (knots * 1.6889)
g= acceleration due to gravity (32.2)
dis= volume displacement in cubic feet

DISPLACEMENT HULLS operate at a FNV of 1.3 or less. SEMI-PLANING HULLS operate at a FNV of 1.o to 3.0. PLANING HULLS operate at a FNV of 2.3 or more.

As can be seen in the above table there is some overlap between groups because there is no precise point of differentiation. Where exactly does planing start? There are many different opinions; when water breaks cleanly from the transom, when water breaks cleanly from the chines, or when the boat's CG lifts above its static position.

Faltinsen states.....

"The pressure carrying the vessel can be divided into hydrostatic and hydrodynamic pressure. The hydrostatic pressure gives the buoyancy force, which is proportional to the submerged volume (displacement) of the ship. The hydrodynamic pressure depends on the flow around the hull and is approximately proportional to the square of the ship speed. Roughly speaking, the buoyancy force dominates relative to the hydrodynamic force effect when Fn is less than approximately 0.4. Submerged hull-supported vessels with maximum operating speed in this Froude number range are called displacement vessels. When Fn > 1.0-1.2, the hydrodynamic force mainly carries the weight, and we call this a planing vessel. Vessels operating with maximum speed in the range 0.4-0.5 < Fn < 1.0-1.2 are called semi-displacement vessels."

From Hydrodynamics of High-Speed Marine Vehicles.

Willallison

11-26-2009, 11:41 PM

Yes - Daniel, I'd say you're contradicting some fairly noble designers there!
A displacement vessel that is overpowered is still a displacement vessel, just as un underpowered planing hull is still a planing hull. But a vessel optomised to travel in that middle ground of Fnv 1 to 3... what are they if they are not semi-displacement boats?

fcfc

11-27-2009, 05:20 AM

But a vessel optomised to travel in that middle ground of Fnv 1 to 3... what are they if they are not semi-displacement boats?

Who would optimize a boat intended to be operated in middle of the worst resistance hump he can have ... :P Would not be better to change target length or speed to get out of that zone.

I am joking, but it is sadly the truth ...

BTW, a bunch of paper do not speak of semi displacement, but of high speed displacement crafts. And in fact , it is this : displacement boats trying to get the most speed they can.

Crag Cay

11-27-2009, 06:02 AM

Semi-displacement is a bizarre term who means nothing.
I will contradict Mr Blount, I think the term is wrong.
It was invented by boatbuilder to sell their short and beamy shoebox with thousand of horse power and didn't went to planing, but push enough water to go a little faster that a fishing boat.
In other term Semi-displacement do not exist.

It's odd then that many of the seminal design textbooks of the 40's, 50's and 60's had whole chapters on 'semi displacement' power boats that were usually characterised by being long, slim and underpowered.

Laurent Giles' Woodpecker and Arthur Mursell of TT Designs and their Nelson range of pilot boats are all quoted as being the definitive examples.

Whilst some builders might have used the moniker to excuse their under performing planing designs, you can not dismiss the amount of effort that went into searching for a design that wasn't a compromise, but combined the best attributes of both displacement and planing hulls. The continuing market for work boats of this type around the world would suggest that some people still believe the search was successful to some degree. To say they 'don't exist' would suggest the healthy order books of yards like Halmatic and the buoyant second hand market is illusionary.

dskira

11-27-2009, 06:19 AM

This what I said:
Semi-displacement is a bizarre term who means nothing.
I will contradict Mr Blount, I think the term is wrong.

The term is wrong
I was talking the terminology, the name, not the naval architecture aspect which I also worked with.
The term is for me not accurate and bizarre.
It is needed in my view a better term
I don't why some people need to put all the finding by all these naval architect. I know these finding, it is not the point and as nothing to do with my thought in terminology.
But thanks anyway
Cheers
Daniel

Crag Cay

11-27-2009, 06:25 AM

It was invented by boatbuilder to sell their short and beamy shoebox with thousand of horse power and didn't went to planing, but push enough water to go a little faster that a fishing boat.
Why embellish a simple concern about terminology with 'facts' that are completely wrong?

dskira

11-27-2009, 06:35 AM

It's odd then that many of the seminal design textbooks of the 40's, 50's and 60's had whole chapters on 'semi displacement' power boats that were usually characterised by being long, slim and underpowered.

Laurent Giles' Woodpecker and Arthur Mursell of TT Designs and their Nelson range of pilot boats are all quoted as being the definitive examples.

Whilst some builders might have used the moniker to excuse their under performing planing designs, you can not dismiss the amount of effort that went into searching for a design that wasn't a compromise, but combined the best attributes of both displacement and planing hulls. The continuing market for work boats of this type around the world would suggest that some people still believe the search was successful to some degree. To say they 'don't exist' would suggest the healthy order books of yards like Halmatic and the buoyant second hand market is illusionary.

Again you are confonding the term and the design.
But it seams this term appear quite late and it's used exensively.
The buoyant market as nothing to do with the term, as for the work boat, which kind.
Perhaps I go to far, but it seams a wrong term. If it used by very knoweldgeble naval architect, fine, do not means it make the term is right. Like trimaran and catamaran, so "cata" means two hull and "maran" means hull? Perhaps but I am not sure.
I don't deviate, I try to explain that some terms are off the shelf, perhaps for practicality, and one started the other followed since it's easier.
I don't know, will see.
Healthy conversation by the way, thanks all.
Cheers
Daniel

apex1

11-27-2009, 06:50 AM

Why embellish a simple concern about terminology with 'facts' that are completely wrong?

Well Crag, so completely wrong it was´nt! The industry misused the term quite often enough.

dskira

11-27-2009, 06:52 AM

Why embellish a simple concern about terminology with 'facts' that are completely wrong?

crag I realy don't understand what you said. You seams contradicting your former post.
Embelish a simple concept? I don't get it. Since when these boat are wrong in the water. I am not talking naval architecture again, I designed (looks at my gallery the 50' motor boat) it is a semi displacement she goes to 20 knots and do not plane. I designed and built a lot of work boats, they are not automatically SD thanks God, the fuel bill will be a killer! so I know what I am talking about. The term is in my view wrong. I repeat, the term.
Cheers
Daniel

Crag Cay

11-27-2009, 07:19 AM

I have quoted the part I said was factually incorrect. Twice!

The term was not invented by boat builders to excuse their poor designs. To suggest that is wrong. It may have been used on numerous occasions for this purpose but to say it was invented solely for this purpose is to dismiss all the real work that went into refining and exploring the semi-displacement hull form.

If you read Douglas Phillips-Birt's Motor Yacht and Boat Design from the early 1950's, you will see his characterisation of 'fast power boats' only stretches from a speed/length ratio of 1.1 to 1.8. His dismisses faster boats as only being suitable for the calm waters of the USA and the Riviera. This association of true planing vessels with being un-seaworthy for UK waters persisted for years (still persists?) and lead to thousands of hours being spent refining the idea of semi-displacement hulls, predominantly for the work boat market. (This idea didn't only have supporters in the UK - West Palm Beach and Fernandina Beach pilots were both good clients).

The RNLI are also locked into the search of this holy grail, unlike the Dutch who have adopted full planning for their boats.

I myself am not convinced that the difference in rough weather performance is clear cut between the semi and full planing hulls. There are many other factors that seem to be more important and can be incorporated into hulls of both types. However there are thousands of Pilot boat skippers and Lifeboat coxswains running their semi displacement hulls in all weathers, that are convinced that their boats are the way to go, and are not using these boats because they were fooled by the marketing strategy by some poor boat builder. Fifty years of daily service in ports around the world would have rumbled these boats if they were nothing but an illusion.

Ad Hoc

11-27-2009, 08:23 AM

Daniel

The word "catamaran" is French Polynesian. It comes from katta-maram, which means tied wood.

dskira

11-27-2009, 09:03 AM

Daniel

The word "catamaran" is French Polynesian. It comes from katta-maram, which means tied wood.

Every day worth living since I learn something!
Thank you, I didn't know the meaning of catamaran.
Thanks again
Cheers
Daniel

Tad

11-27-2009, 01:00 PM

KC Barnaby's Basic Naval Architecture of 1954 did not mention semi-displacement or semi-planing. He did however refer to a "double wedge form" of high speed round-bottom hull.

In 1934 George Crouch refers to "Round Bilge Wide Transom" hulls. In 1965 GN Hatch titles a paper "Fast Displacement Craft and the Transition Zone". Nither Du Cane (1964) or Lord (1963) refer to semi-anything. But about the same time Edwin Monk Sr. was calling some hulls "Semi-Planing Round Bottom".

In the late 1970's Tom Fexas called them "Penetrating Hulls", these were of round bilge form but incorporating a number of lifting strakes. Tom also wrote that no hull was truly planing until water broke cleanly from the chine full length and the transom was completely dry, often a S/L of 4!

My personal view is that a Semi-Planing hull is one designed to lift substantially, and it's probably hard chined aft for maximum lift. A Semi-Displacement hull is one designed to Penetrate....ie slide through the water and (at least) not sink as a full displacement hull does. Semi-displacement includes (In my mind) the Maine Lobster Boat form for example.

A Semi-Planing Hull
37763

A Semi-Displacement Hull
37764

baeckmo

11-27-2009, 05:14 PM

dskira

11-27-2009, 05:24 PM

Hey, gentlemen try to focus on the subject of the thread! The guy asked for knowledge and info on "semidisplacement hulls". The term is widely known, and technically correct. To be precise, the prefix "semi" per definition says that the boat is operated in such a way that its weight is carried 50 % by buoyancy, and 50 % by other means (normally, but not necessarily, by hydrodynamic lift), end of discussion!

Most of us can associate to the type of vessel he is looking for, so why are you flipping his nose with a dozen of irrelevant notes on linguistics? The towing tank archives are full of reports on these hulls, and there are certainly numerical attempts galore, so come on with the real stuff and skip the "semi********"!

Thank you for your order my captain.
And thank you for your sense of open discussion.
Semicheers
Daniel

Willallison

11-27-2009, 10:41 PM

Hey, gentlemen try to focus on the subject of the thread! The guy asked for knowledge and info on "semidisplacement hulls". The term is widely known, and technically correct. To be precise, the prefix "semi" per definition says that the boat is operated in such a way that its weight is carried 50 % by buoyancy, and 50 % by other means (normally, but not necessarily, by hydrodynamic lift), end of discussion!

Most of us can associate to the type of vessel he is looking for, so why are you flipping his nose with a dozen of irrelevant notes on linguistics? The towing tank archives are full of reports on these hulls, and there are certainly numerical attempts galore, so come on with the real stuff and skip the "semi********"!

You just pipped me to the post in regards to the appropriateness, or otherwise of the terminology. A displacement boat is held afloat entirely due to buoyancy. A planing boat as a result of dynamic lift. A semi-displacement, or semi-planing, boat is supported by a significant proportion of each.

Now, as to the question itself. I fear I can be of little assistance beyond that which has already been given... sorry

gonzo

11-28-2009, 11:45 AM

baeckmo

11-28-2009, 01:05 PM

Any hull that moves through the water produces waves. That is caused by the pressure of the hull against the water. It is impossible to move a hull through the water without some component of dynamic lift. Also, it is not possible to have a hull plane and have no component of displacement.

....yes, and.......?

That is common knowledge here, Gonzo! Still, I (and the thread creator?) am awaiting info on the vessel type that is operating with about half of its "lift" (or at least a substantial part thereof) from displacement buoyancy, and the remainder from hydrodynamic forces, ie semidisplacing. What are the important features of such hulls, where can we find knowledge? That is what this thread is about!

BTW, it is completely possible to design a hull that is producing zero or negative hydrodynamic lift in the vertical plane, even if there will be dynamic forces acting on the hull as a result of movement. And what do you call a hull that has 50 % aerostatic lift assistance from a "lighter-than-air" vessel?

gonzo

11-28-2009, 02:40 PM

This is the first time you define what semi-displacement is for you. Some people define it as more than 1.8 hull speed but less than three. By your definition, a hydrofoil qualifies.

baeckmo

11-28-2009, 04:05 PM

By your definition, a hydrofoil qualifies.

Formally yes, as long as the basic requirement is met; roughly half lift from buoyancy, the rest from a combination planing lift plus foil lift, which occurs at some speed before coming onto the foils! That is not to say that it is a good semidisplacement configuration as such, though.

My point here is that a definition relating to wave system and Froude number is adding a restraint that may be misleading, because you can have a pure displacement hull working far into "planing territory", provided it is slender enough, just as you can have extra hydrodynamic lift assistance at lower speeds.

But if you use the definition in the original sense of the phrase, we can analyse the two "lifting systems" regarding their individual optimum shapes, in terms of some performance parameter, and what possible trade-offs that may result in a combined optimum, that is better than the "flat" sum of the individuals. The problem with this analysis is that we are depending on different tools for the two (at least for the moment).

The regression methods mostly used for displacement speeds fail when it comes to speeds where dynamic lift is coming into play, and the empirical algorithms for planing are of little use in the intermediate speed range. I have some hope, that numerical methods for resistance prediction (see Almeter f.i.), may be developed to a "working tool" level, substituting empirical algorithms like Savitsky's, but until then we only have towing experiments to lean on for the semidisplacement range. Unless Leo L or Ad Hoc or ??? has something up the sleeve right now, huh..?

gonzo

11-28-2009, 06:17 PM

Which comes to the original discussion, what is semi-displacement? Baeckmo got kind of pissy about it, but unless it is first defined, we don't even know what we are discussing.

Ad Hoc

11-28-2009, 06:47 PM

Not sure why stating basic facts is seen as a pissing contest by others?

One of my very good friends and I often debate this simple questions, what is a the definition of a semi-displacement hull. In simplistic terms:

To be precise, the prefix "semi" per definition says that the boat is operated in such a way that its weight is carried 50 % by buoyancy, and 50 % by other means (normally, but not necessarily, by hydrodynamic lift), end of discussion!

But beyond that is speculation, since other factors come into play which cannot be defined by a simple root x1.8 or whatever simplistic term one has been told to us or learnt from a book.

My point here is that a definition relating to wave system and Froude number is adding a restraint that may be misleading, because you can have a pure displacement hull working far into "planing territory", provided it is slender enough, just as you can have extra hydrodynamic lift assistance at lower speeds.

Exactly, fully concur.

Take the lines of the boat posted by Tad above:
http://www.boatdesign.net/forums/powerboats/semi-displacement-hull-30169-2.html#post318948

If i were to have seen these lines and asked for my opinion, i would have suggested that it may be, but depending upon several other factors may be not too. Since the waterlines, in plan view, are not conducive to what one would normally design to be "semi-planning"...but that is the point.

Pushing a boat through the water or riding a bit on top, both can be achieved in totally different ways, with both operating at the same speed. Both shall not be compliant with any "formula" either.

Generally looking at the boat and seeing how she runs too, one can say, ahhh yes....that is semi-planning, but as soon as you attempt to make it prescriptive, you've missed the point.

The person i referred to above, having this endless debate, simply because it amuses us, has been referred to here already. My good friend Arthur.

Laurent Giles' Woodpecker and Arthur Mursell of TT Designs and their Nelson range of pilot boats are all quoted as being the definitive examples.

Finally, it was a long time ago, but i was given a "reason" for the evolution of a semi-displacement hull. Can't recall by whom now too...so long ago.

I was told that Thornycroft was getting fed up slow boating across the Solent from the IoW to Portsmouth/Southampton commuting to work every day. The solent can be rough, short sharp chop at times, so a fully planning boat is out especially in the winter times. So he discounted this type of design. So he decided to make a "half and half", semi-displacement. Whether this is true, i have no idea....

dskira

11-28-2009, 09:19 PM

Not sure why stating basic facts is seen as a pissing contest by others?

One of my very good friends and I often debate this simple questions, what is a the definition of a semi-displacement hull. In simplistic terms:

But beyond that is speculation, since other factors come into play which cannot be defined by a simple root x1.8 or whatever simplistic term one has been told to us or learnt from a book.

Exactly, fully concur.

Take the lines of the boat posted by Tad above:
http://www.boatdesign.net/forums/powerboats/semi-displacement-hull-30169-2.html#post318948

If i were to have seen these lines and asked for my opinion, i would have suggested that it may be, but depending upon several other factors may be not too. Since the waterlines, in plan view, are not conducive to what one would normally design to be "semi-planning"...but that is the point.

Pushing a boat through the water or riding a bit on top, both can be achieved in totally different ways, with both operating at the same speed. Both shall not be compliant with any "formula" either.

Generally looking at the boat and seeing how she runs too, one can say, ahhh yes....that is semi-planning, but as soon as you attempt to make it prescriptive, you've missed the point.

The person i referred to above, having this endless debate, simply because it amuses us, has been referred to here already. My good friend Arthur.

Finally, it was a long time ago, but i was given a "reason" for the evolution of a semi-displacement hull. Can't recall by whom now too...so long ago.

I was told that Thornycroft was getting fed up slow boating across the Solent from the IoW to Portsmouth/Southampton commuting to work every day. The solent can be rough, short sharp chop at times, so a fully planning boat is out especially in the winter times. So he discounted this type of design. So he decided to make a "half and half", semi-displacement. Whether this is true, i have no idea....

I think Peter Thornycroft knew very well the work of Cooper, which was essentialy in high speed planning hull in hard chine and round bilge, but he was also very well award of the quality of the round bilge hull, designing a lot of small speedy military craft for recovery and other task.
And of course every body at the time knew the precursor, Tamahine, from Laurent Giles, also they knew the work of Saunder, Du Cane, Uffa Fox with Black Maria and other. It was a very numerous bunch of great designer who was interrested in different state of speed, depending the use of the craft. All with beefy contrct from the Royal Navy. So yes it is possible that Peter Thornycroft designed Nigella in 1959 mixing all these knoweldges. She became the Nelson Class.
I don't think a design come alone by itself, I think it is the result of a knoweldge of predecessors, and with this knoweldge a personal design is created depending of the circonstances in which the boat as to sail (motor)
Personaly reding this thread I mellow, and I start to understand better why the appelation semi-displacement. Still bizarre, but I read great explanations.
Just before Uffa Fox death, I went to Paris were he resided half the year, been maried with a French woman and then to Cowes and spend a lot of time with him. He talk at length of planning hull (Avenger of course) and other speed boat saail and motor. His great passion (beside singing!).
I was with my fiancee. She feel in love with the great man! He told us all the story of the other designer, his great involvement with Saunder Row, the design of is own high speed motor boat, and also his joy of rowing!!
Coweslip was hanging on her crane in front of his home.
And the splendide enormous all brass mechanical integrator.
The story of the thickness of the fin of the FlyFifteen, and coutless other stories. It was the Summer of my life.
Cheers
Daniel

HJS

11-29-2009, 03:54 AM

The attached illustrations may help to understand the relationship between displacement and planing hulls. The graph is created by Marine Director Börje Huss. Together with Hadlers graph from series 62, 65A, and 65B you may get a good picture of a boat's properties to begin with.

Then of course there are a lot of other parameters to get an accurate picture of the boat's characteristics, such as center of gravity, bottom width, deadrise, entry angle, prismatic coefficient,………..

Good luck.

hjs

www.sassdesign.net

HJS

11-29-2009, 04:33 AM

Daniel

The word "catamaran" is French Polynesian. It comes from katta-maram, which means tied wood.

The word "Cattumaran" is tamil. It is a common "raft" in Tamil Nadu, India.

hjs

Ad Hoc

11-29-2009, 06:31 AM

HJS

Those are great pic's..they show the "technology" very well.

I do find it strange that, as you correctly say, the word is Tamil (I personally thought from Sri Lanka, not India), yet always associated with the pacific Polynesian islands.

Since good old Capt. Cook et al when arrived in that part of the pacific was shocked at the speed of the proa's and the other similar two hulls boats, the generic term used for those boats with two hulls in the Polynesian islands, was catamaran. Did Cook use the Tamil word, or were the Polynesians using the same word, or even give the word to the Tamils (they did travel all over the pacific, why not west?).

Was it just that those in India/Sri Lanka were just not fast and so no one really took notice until Cook arrived in the Pacific?

Yet, strange that the origins being in "India" for the word (the method of construction -tied wood) but the commonly known reference of two hulls being used, is always Polynesian. (Well has been that I know).

I've never been able to fully establish which came first and where and why, you??

Sorry slight digression from semi-planning hulls.

baeckmo

11-29-2009, 07:38 AM

Yup, let's deviate for a while; interesting how the term has migrated. Friend of mine, NA who worked for UN in Bangladesh and India also said the "Katu maram" was a standard phrase for the vessels Jürgen refers to. But what is the connection to Polynesia, and how did it occur??

Willallison

11-29-2009, 05:06 PM

I think Peter Thornycroft knew very well the work of Cooper, which was essentialy in high speed planning hull in hard chine and round bilge, but he was also very well award of the quality of the round bilge hull, designing a lot of small speedy military craft for recovery and other task.
And of course every body at the time knew the precursor, Tamahine, from Laurent Giles, also they knew the work of Saunder, Du Cane, Uffa Fox with Black Maria and other. It was a very numerous bunch of great designer who was interrested in different state of speed, depending the use of the craft. All with beefy contrct from the Royal Navy. So yes it is possible that Peter Thornycroft designed Nigella in 1959 mixing all these knoweldges. She became the Nelson Class.
I don't think a design come alone by itself, I think it is the result of a knoweldge of predecessors, and with this knoweldge a personal design is created depending of the circonstances in which the boat as to sail (motor)
Personaly reding this thread I mellow, and I start to understand better why the appelation semi-displacement. Still bizarre, but I read great explanations.
Just before Uffa Fox death, I went to Paris were he resided half the year, been maried with a French woman and then to Cowes and spend a lot of time with him. He talk at length of planning hull (Avenger of course) and other speed boat saail and motor. His great passion (beside singing!).
I was with my fiancee. She feel in love with the great man! He told us all the story of the other designer, his great involvement with Saunder Row, the design of is own high speed motor boat, and also his joy of rowing!!
Coweslip was hanging on her crane in front of his home.
And the splendide enormous all brass mechanical integrator.
The story of the thickness of the fin of the FlyFifteen, and coutless other stories. It was the Summer of my life.
Cheers
Daniel

You are a lucky man, Daniel... a lucky man...

dskira

11-29-2009, 06:37 PM

You are a lucky man, Daniel... a lucky man...

Yes I know, and I never forget it. :)

tom28571

11-29-2009, 07:55 PM

If the focus of this thread is to promote discussion and the spread of knowledge about hull characteristics and their effect on performance, then it is proving very useful. If it is to generate definitive numerical formulas to describe the action of hulls in the transition range, it must fail, because no one has ever satisfactorily defined when a boat is considered planing in the first place. There are lots of individually preferred ratios, etc., but not everyone or even the "experts" will agree with them.

Tad offered lines of two boats and called one semi-planing and the other semi-displacement. He must have been smiling when he did that since it added yet another layer of confusion. If these are not one and the same, all bets are off.

If we take a hull designed for full planing with completely parallel aft buttock lines and start to drive it forward, it will immediately experience some small dynamic force. By the time the speed reaches "hull speed", significant lift is exerted on the hull. As it moves faster, more and more dynamic lift appears. Unless it goes fast enough to become airborne, some of the hull will always be immersed and buoyancy will be part of the force holding the boat up. I believe the first hulls that can be called semi-planing were developed by William Hand. The work of a continuum of other designers and builders resulted in the range of hulls we have today.

It is quite possible to have a hull that develops a high level of dynamic lift and is considered to be planing before it reaches what would be normally called hull speed. Long and light hulls do that with ease. Then there is the slender multis which throw another wrench in the works.

My point is that these terms mean nothing without reference to hulls within a narrow range of characteristics, Therefore general definitions are impossible. They do lead to much argument from varying viewpoints though.

Squidly-Diddly

11-29-2009, 08:11 PM

About 130' long, 30' wide and able to hit 40 knots....but said to be 'displacement' and leaving far less white foam trail than a PT boat, and able to operate at fairly high speed in seas that a planing hull like a PT boat couldn't?

Ad Hoc

11-30-2009, 04:04 AM

Yup, let's deviate for a while....... But what is the connection to Polynesia, and how did it occur??

Precisely!...since there is plenty of documented evidence of "multihulls" being used for centuries by Polynesian islands, long before Cook. Some even suggest also long ago as 2000 years. Whether millennium or centuries, how come the "word" used for each is the same, yet the "boat's" origins are many thousands of miles/kilometers apart...and many attribute the usage of the boat, not the word, to the Polynesians.

I've not come across any evidence that pins this down...it is intriguing.

Willallison

11-30-2009, 05:24 AM

You know the Polynesians invented the wave-piercer too....;)

Wayne Grabow

12-01-2009, 07:58 PM

Good question and, by now, you have received a variety of expert answers. I (an amateur) have been interested in the same question. Some of the best answers I have found come from looking at plans, descriptions, and museum exhibits of turn-of-the-century "high speed" powerboats, say about 1900 to 1925. Back then, the power-to-weight ratio of most engines was not favorable enough to permit true planing. By necessity, designers were optimizing hulls to perform in the semi-displacement or semi-planing performance range. Some characteristics I noticed in such hulls were slight rocker in the keel, variable deadrise continuously decreasing aft, a transom which was only slightly immersed at rest, a pronounced immersed forefoot (usually a plumb bow), and a B/L ratio significantly higher than today. Even within this group, characteristics varied depending on whether the displacement or planing part of the performance spectrum was favored. The listed characteristics may only illustrate the limitations of naval architecture research then or fashion trends of that period, but I am sure that much practical knowledge was also included. Some of the stated speeds attained with moderate power are remarkable, if true.

Ad Hoc

12-01-2009, 10:07 PM

Will

Yes, i am particular impressed with the way the heave and particularly the vertical accelerations are dramatically reduced by the amazing thin slender revolutionary wave piercing hull. Well isn't that what everyone "thinks" and "feels" so, it is surely true :P

(this should really be on your thread...sorry)

Tad

12-02-2009, 03:24 PM

I believe the first hulls that can be called semi-planing were developed by William Hand.

I disagree Tom.......While Hand is generally acknowledged as inventor of the vee bottom, that's also (IMO) a bit of hyperbole. He did take a form that already existed (the Chesapeake deadrise hull), lifted the chine forward and dropped the fairbody, to create a better handling boat.

As to developing semi-planing hulls.......that's a bit like claiming Columbus "discovered" North America. Long, lean, round-bottom powerboats were running at above "displacement" speeds before Hand came along.

What I was trying to get across in various posts above was that we're dealing with physical constants, it matters not what we call them.....which is all marketing. I've heard hulls called Penetrating, High-speed Displacement, Double Wedge, Semi-Planing, Semi-Displacement...etc. Unless one adopts and defines the precise criteria, it's all meaningless.

tom28571

12-02-2009, 07:27 PM

I disagree Tom.......While Hand is generally acknowledged as inventor of the vee bottom, that's also (IMO) a bit of hyperbole. He did take a form that already existed (the Chesapeake deadrise hull), lifted the chine forward and dropped the fairbody, to create a better handling boat.

As to developing semi-planing hulls.......that's a bit like claiming Columbus "discovered" North America. Long, lean, round-bottom powerboats were running at above "displacement" speeds before Hand came along.

What I was trying to get across in various posts above was that we're dealing with physical constants, it matters not what we call them.....which is all marketing. I've heard hulls called Penetrating, High-speed Displacement, Double Wedge, Semi-Planing, Semi-Displacement...etc. Unless one adopts and defines the precise criteria, it's all meaningless.

Tad,

You may well be right about that. I don't know enough of history to argue that. I'm not sure the long lean powerboats should be part of the same discussion though. I was thinking more of semi-planing as referring to boats of more "normal" L/B of 3.

fcfc

12-03-2009, 10:24 AM

You can find this : http://mnvdet.com/Resistance%20&%20Powering.htm

And spredsheet available here http://mnvdet.com/Other%20Section%20Header%20Sheet.htm , bottom of page.

Easy Rider

12-23-2009, 07:09 PM

I once thought a boat achieved hull speed as soon as it's bow started to rise. I once thought a boat With any emersed transom at rest was not a full displacement hull. I once thought a boat was not planing if her transom was not dry. I once thought any boat that was not FD or FP was a SP or SD hull the SP being slower than the SP. Semi being "partly" would seem to me to mean less than 50%. Then I was told the quarter beam buttock line expressed as a number made everything black and white. These four concepts D, SD, SP and P are clearly very elusive and subject to much discussion/argument as long as there are boats. What I would like to learn is what in-specific elements of boat design produce a SD boat that is graceful, efficient, well mannered in rough seas and whose basic dimensions are not extreme in any way. Boats that perform really well in the lower end of the SD range are rare. I think some designs by Hand and Atkin are excellent and perform well at SD speeds. What are the design features that make them shine.

Easy Rider

TollyWally

12-23-2009, 08:11 PM

I've pondered in great detail over some of Atkin's boats. I inherited an old 1st edition book of his from my old man who bought it when he was a kid. After reading it dozens of times I realized some of what my father taught me came from that book.

I believe Atkin was very good at optimising his boats. He seemed to be always tweaking the underbody to get the prop in the best water possible. He would splay the shaft and stick the prop farther back away from the underbody in less turbulent water. He was very particular about matching motor, gear, and prop. In a way much the same as optimising a racing machine except his "high performance target" was smooth cruising efficiency not top speed.

FAST FRED

12-25-2009, 03:34 PM

I have always thought many of the old wooden boat designers had it GREAT!

The same design could be tweaked (mostly every wooden boat is a one of) so the designer could experiment and widen the buttocks an inch and see what it does.

Our 90/90 is a Maurice Griffins hull design .
During WWII he sold his boat and worked for the Navy.

Lone Gull II was the result of years of doodeling FOR HIMSELF , and remains one of his finer vessels.

The ride is sweet , the decks stay dry in most blows , and she cruises well.

FF

pasty63

01-19-2010, 05:37 PM

Well, I don't know much, but I currently have a boat that has a hull form labeled "semi displacement". My experiences with semi displacement have led me to a keen interest in displacement and planing hulls :)

TollyWally

01-19-2010, 06:04 PM

Wisdom from perceptive observation.

breschau

12-28-2010, 06:44 PM

Why is it called semi-planing and not semi-displacement if its a middle ground, compromise solution?

daiquiri

12-29-2010, 05:33 AM

Why is it called semi-planing and not semi-displacement
It's the same stuff, imho, just a matter of personal choice.
"Semi" means "half" - so if 50% of the weight is sustained by the hydrostatic buoyancy force (which is due to water displacement), the other 50% has to be sustained by some other force (hydrodynamic planing, in this case). Of course, the vice-versa is also true. ;)
Some folks use the two terms to distinguish the situation when one of the two forces predominate over the other, but in my opinion it is an unnecessary linguistic complication. Even what we call "planing hull" is sustained partly by a hydrostatic force, for example. And, on the other side, a hull moving at "displacement" speeds is always subject to more or less small hydrodynamic forces.
Cheers

Mr Efficiency

12-29-2010, 06:34 AM

Semi-Displacement or Semi-Planing are technically incorrect terms if " semi " is strictly meaning half. In practice, any attempt at measurement of the split between hydrostatic and hydrodynamic would be migraine-inducing.

Easy Rider

01-02-2011, 04:33 PM

Daiquiri,
I see you are right the D says "half". I thought semi meant partly .....thinking maybe 10-30%. So I had my own personal concept that semi-planing was "partly planing" like a GB36 going 11 knots. And I thought semi-displacement was a boat that did 15-18 knots gracefully but would be bow high and lacking control at 35 knots. But now that you've posted I must put all boats not full planing or full disp into an extremely wide range of
"semi displacement". Oh well.

Easy Rider

Mr Efficiency

01-02-2011, 04:50 PM

Makes the term "semi-conscious" a bit doubtful doesn't it, we are presumably 100, 50, or zero % conscious, and nothing in between ? Sounds like rubbish to me, words mean what people more or less collectively understand them to mean, I think the term "semi-planing" would be generally accepted by boaters to allow a bit of percentage leeway either side of half.

gonzo

01-02-2011, 05:44 PM

The term is used loosely and means different things among different groups. Dinghy sailors call planing as soon as the bow lifts and start getting clear water off the stern. Speed boaters may call that semidisplacement. "Generally accepted"(vox populi) is not a definition. Even if we agree to a bit of percentage leeway either side of half, first someone has to define what half is. Boat operate on a changing boundary between mediums. It is not as if you pass the speed limit while driving.

tom28571

01-02-2011, 08:53 PM

All discussion of definition of displacement speed, semi displacement and planing speed ranges is not ever going to produce a definition. It's a transition range which, by definition is not specifically defined at any one point and never will be. As a boat starts to move forward, the vertical force always divides between dynamic lift and buoyancy, near zero lift and nearly all buoyancy at first and dominated by dynamic lift when fully planing. The beginning and end of the transition range as well as the width of the transition speed range vary from one boat to another.

A semi displacement hull form can be looked at as a full displacement hull that has some of the underbody shape flattened out toward the stern. A full planing hull can also be modified by adding rocker and lifting aft buttock lines toward the water surface. Both wind up with the same shape and both are called semi displacement or semi planing, whichever you desire.

Whether or not a semi hull of either designation is suitable for you depends largely on whether you opt for a light weight boat with its limitations or a heavy one with its own set of restrictions. If you can be satisfied with the limitations imposed on accommodation by light weight, then a low speed planing hull may be the best for you. If you demand to include the kitchen sink and all that it implies and still want to run at speeds beyond hull speed, then the modified displacement hullform is likely what you need. Power requirements and fuel consumption also figures importantly into the choice.

By lightweight planing hull, I don't mean actual weight but rather lightweight in the sense that the waterplane is loaded lightly per square unit of lifting area. Such a boat, with proper design, can easily run in what is considered the transition speed range for most boats and achieve power requirement and fuel use numbers equal to the full displacement hull. This boat will require much less power and fuel than the semi displacement hull at the same speed.

If you demand a heavy boat, it makes no difference which direction you start from, the result will be the same.

Demands for definition will never result in the best designs. Talent and experience of the designer is more important than the tools of formulas or spreadsheets that the designer may make use of.

Landlubber

01-03-2011, 01:29 AM

...my understanding of a planing vessel is when the water breaks free of the transom, at that speed the vessel is planing...is it possible to be planing without this free surface, i do not think so, it is still in displacement mode (maybe semi displacement as it reaches the required speed.

...from my use of so called, semi displacement hulls, every one of them can make the water break free of the transom at a particular speed, maybe it is this phenomenon that describes the semi as it is being referred to...it is a vessel that can run comfortable at either mode.

Mr Efficiency

01-03-2011, 05:14 AM

Semi-displacement boats go well in semi-(there's that word again ! ) sheltered harbour or river situations, where wave action can't work up to the point where you are chasing the backs of sizeable crests, when the boats become slugs, unfortunately.

Easy Rider

01-03-2011, 01:39 PM

LL,
"when the water breaks free of the transom"
I thought that for a long time. At another time I thought that planing commenced when the bow started to rise. That's WAY less than hull speed though. Can't remember what's wrong w the "breaks free of the transom" theory but at one point I decided that it was so. Tom says "It's a transition range" and it seems it cannot be pinned down to a specific point in the series of evolving events that spread out over the entire range of boat speeds. So many different shapes to act on water in different ways. Tom's right too in that there never will be a black and white ..now you are and now you aren't.

Landlubber

01-03-2011, 04:31 PM

tom28571

01-04-2011, 08:01 AM

ER..yeah, it does seem that is the case, watching the transom carefully and havintg the speed read out (from the GPS), it also depends on water flow under the boat, as in current moving the boat, funny to watch it.

Make a hole in the water and the natural tendency is for the water to either fill the hole or push up on what is making the hole. In the case of a boat moving through the surface, the water wants to fill the void left as the transom passes.

Landlubber talked about the flow of water under the boat. A better understanding takes place when we realize that the water is not actually flowing under the boat. It may be mathematically accurate, but it impedes understanding. The water is reacting to the boat passage by being forced down. If the boat is slow and/or the transom immersion is deep, much or all of the water has time to fill the void behind the transom. Make the boat go fast enough to be fully planing and the downward impulse given to the water by the hull is still taking place, the water separates at the transom and even continues its downward path after the boat has passed.

It is just Newton's laws at work. In between displacement and full planing, the picture is somewhere between these extremes. Giving a displacement or semi hull some rocker does not impart as much down impulse to the water and allows the water to make an easier path back toward the surface. That takes less energy than driving the deep immersed transom of a heavy planing boat at slow speed. Try to run a rockered hull fast and the forward part of the bottom imparts a large down impulse to the water and it just doesn't want to come back to the surface as soon as the after part of the hull demands. This creates suction or negative lift and sinks the hull further into the water. this is where the "hull speed" limitation comes in.

Flattening out some of that rocker will cause a decrease in that suction and the boat can go a bit faster. Flatten the aft buttocks into straight parallel lines and the hull looses most or all of that negative lift and becomes a fully planing boat. Of course there is a lot more to the story but this may help in the basic understanding of what is happening under the boat.