Semi displacement hull

Discussion in 'Powerboats' started by Steph357, Nov 15, 2009.

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Steph357Junior Member

Hello everyone,

I am getting interesting in semi displacement hull and I would like to be able to go deeper in the subject. I did basics research online but barely find something satisfying, does anyone knows a book where I can learn more about then ? I am especially interested in the power prediction of such a hull and the equilibrium calculation while running.

Cheers

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gonzoSenior Member

Semi-displacement or semi-planing is the vague area in-between displacement and planing. It is not accurately defined or agreed upon.

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fcfcSenior Member

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fcfcSenior Member

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Steph357Junior Member

Thank you for you link, I will keep searching and let you know if I find anything on my side too.

Cheers

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robbie2161Junior Member

Using Fn (speed in kts*0.515)/Sqrt(9.81*length of waterline in meters) you can categorize motor yachts in a different way. There are three distinct types of motor craft

Displacement

Transitional (or semi displacement)

Planeing

There are different opinions as to what the boundaries between the three types are.
Generally speaking if the speed to length ratio is 0.4 or less, it is classed as a displacement craft.
0.4 to 0.82 is the semi displacement category and 0.82 and above is classed as planeing. There are exceptions for example if you over power a displacement vessel it is possible to get it to go beyond the 0.4 boundary but at the expense of fuel efficiency. (Norman L Skene. ‘Elements of yacht Design’ Seafarer books)

7. dskiraPrevious Member

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
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
Cheers and happy ThanksGiving
Daniel

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gonzoSenior Member

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.

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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.

oataru likes this.
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WillallisonSenior Member

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?

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fcfcSenior Member

Who would optimize a boat intended to be operated in middle of the worst resistance hump he can have ... 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.

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Crag CaySenior Member

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.

13. dskiraPrevious Member

This what I said:
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

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Crag CaySenior Member

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

15. dskiraPrevious Member

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

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