# 19m semi displacement / planing hull "Port service boat"

Discussion in 'Boat Design' started by Devu De Goa, Oct 29, 2016.

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### bitStudent

Hi
...some existing data (145 motoryacht)

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### RabahSenior Member

Hi Devu,
But I shall try once again:
1. If you have ultimately decided that the top speed it is necessary to limit up to 25kts then it be no point from these composite outlines of the bottom.
Once again I advise - select one of two versions / see the file - the size of radius not necessarily to be 100mm/.
At any alternative version for you the lift force will be less because the breadth gliding plates will be less!
2. After relation of speed and power I am sure that your 550 kW not will sufficient that the vessel was maintained in a mode of gliding. I think that for you will be only semi-displacement mode, because Fnv = 2,192 < 3
V = Displacement /density = 44,42/1,025 = 43,336 m^3
At v = 25 kts = 25 x 0,5148 m/sec
3. For my calculation with Orca3D must to know that
Power total = Power effective / efficiency
where the efficiency is total for the propellant installations.
It is received = 0,50 - 0,65
Thus 0,50 it is received in initial stage of the project when engines are still unknown.
And 0,65 it is received at more precise calculations for correspondence of the propeller from the engine and the hull already in consequent blueprint stages.
I have accepted efficiency = 0,50 to have reserve of a power.
Continuation follows...
_______________________
NA Razmik Baharyan

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• ###### Recomendation for Boat design.dxf
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### Devu De GoaJunior Member

@Rabah

Thank you Sir for your comments. Please do not misunderstand me. I am very much open to all guidance from everyone. But as a student of a University I can not copy something as it is and say that it is my work. That is called plagiarism which is punishable. Instead of ready made work I would like help in the form of references and ideas which help to take some critical decisions.

As you have pointed out, the power will now go up from that previously estimated because the displacement has gone up from the earlier estimated about 25 t to the new of about 48 t. I would like to reduce the weight a little bit to stay within the data distribution provided by @bit in the graph.

The top speed of 25 knots is the requirement of the project. I must meet that.

When you say "At any alternative version for you the lift force will be less because the breadth gliding plates will be less!", can you please explain me why we can not have a flatter deadrise for the shapes you suggested.

Why does it have to be deep V? Since the boat will operate only in ports, it doesn't need a cushioning effect against the waves, which is the purpose of the deep V. So with flatter deadrise we should be able to get lesser resistance right?

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### RabahSenior Member

Hi Devu,
1. I have not said to you direct to use my file! You could use only the form which I offer.
I want to straighten out what for a plagiarism it is not. Look the file - it is special help generator for making hulls by different characteristics but with one and too form under name Orca3D Hull Assistant for planing hull. Students and designers from all world use it and it does not mean that all of them plagiary.
2. Usually under Rules of Classification organizations the bow and the stern is projected on a transversal framing system.
But if Bureau VERITAS allows a longitudinal framing system in the stern then that's all right and it is necessary to change nothing.
3. Your calculations of the weight justify my forecasts - I wrote to you that I expect Total Displacement> 25 t, so that 44,42 t is quite possible and actual value.
My forecast for VCG = 1,7m too was acknowledged from a minor variance.
4. You will be sure that your draft at Total Displacement is 0,94m?
Do not forget that is necessary to increase molded displacement not only for the sake of saltiness of water, but also from additional coefficient for appendages.
Continuation follows...
_________________________
NA Razmik Baharyan

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### RabahSenior Member

Hi Devu,
I respond on the problem which troubles you - what to make if calculated LCG = 8,08 m differs from LCB = 6,92 m from the transom received for Displacement = 44,42 t at an even keel by Maxsurf Modeler.
1. If you yet have not accepted the ultimate form of the hull then it is necessary to act on classical version:
1.1 To construct the curve of the sunk squares of frames.
The square encased by this curve is molded displacement, and the centroid of square of this curve X = LCB m.
First we plot a trapezoid which on square and the centroid responds on above specified conditions.
Then the trapezoid it is changed by an isometric curvilinear figure on square.
Ordinates of this figure are squares of the sunken frames.
1.2 We plot each frame separately so to receive the relevant square.
1.3 After already received curves of frames we plot computer model of the hull on which it is made dxf file of the Lines plan. For it will be LCB = LCG = 8,08 m, i.e. the vessel will be on an even keel.
2. If you have ultimately taken the form of the hull /for example on the close prototype/ and it will not change, then the solution will be the following:
2.1 Moving some frame spaces to the stern of the tank of the fresh water and the fuel for decrease LCG from the transom.
2.2 Moving engines some frame spaces to the stern.
2.3 General moving ER with transverse bulkheads.
2.4 Moving the pilothouse some frame spaces to the stern.
It is necessary to receive LCG = LCB = 6,92 m.

And at two versions you should receive LCG = LCB that the vessel was on an even keel at inoperative engines.
Continuation follows...
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### Devu De GoaJunior Member

Thank you Sir.

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### RabahSenior Member

Hi Devu,
Let now we shall see what there will be the variance if shall make calculation of speed - power for model which I was offered, at new displacement = 44,42t.
We receive that LCG = LCB, i.e. some masses are already moved to the stern to satisfy condition a vessel to be on an even keel when is not move.
From Design Hydrostatics in Orca3D it is received for 44,444 t:
Draft = 1,174 m; LCB = 12,131 m from FP / LCB = 6,869 m from AP/; Cb = 0,477; Cwp = 0,805; Cx = 0,686; Cp = 0,695; GMt = 0,915m when VCG = 1,82m
Usually the ratio of effective to engine power figures of 50 % to 65 %. From - for it the calculation of the Planing Analysis is make in two variants - with Propulsive Efficiency = 50 % and with 60 %.
The outcomes of calculation / see the files /:
Will be necessary 2 х 850 kW = 1700 kW for achievement 25 kts at Propulsive Efficiency = 60 %.
The vessel will be in semi-displacement mode at Fnv = 2,191 <3 and dynamic trim = 6,792 deg.
Continuation follows...
______________________
NA Razmik Baharyan

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• ###### Motor Yacht 19m-Planing Analysis Report,efficiency60.pdf
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### RabahSenior Member

Hi Devu,
The program Orca3D have a possibility at ready computer model to make calculation what power is necessary to reach the speed specified in the task of the project in initial blueprint stage.
It is from a prime significance for the exact choice of engines.
In Orca3D Help it is precisely specified what surfaces enter in the structure of the gliding plate /see post No 52/-it is the bottom up to the lowermost spray chine plus the horizontal segment of the same spray chine. Remaining surfaces do not enter in the structure of the planing surface.
It is necessary to remember as a rule - the motorboat glides from that part of the bottom which reaches up to the first chine /with or without a spray chine/ or before the first knuckle. For example if we shall take that the bilge will consist of two knuckles then the gliding plate on breadth reaches to the first knuckle.
If between CL and spray chine are arranged a longitudinal redans, they are not actuated in calculation from for small breadth - usually the redans have a horizontal segment 60-75 mm.
The longitudinal redans increase the stability of the planing ship also moderate on-board and pitching motion.
The breadth of the spray chine always it is more than for the longitudinal redans. At deadrise of the bottom > 10 deg the breadth of the spray chine = 4-5 % B.
For our model planing breadth = 3,87m.
4 % x 3870 = 155 mm
Whether but how much signs the designer it depends on that is close prototype or from model tests.
At calculation with Orca3D only half of gliding plate always is selected.
Except for that it is necessary to indicate location of the propeller on X and Z after relation the beginning of the adopted coordinate system and an angle of dip of the shafting.
What will be the conclusion from that I explain?
If in your cross section in the bottom area there is a spray chine /not longitudinal redan/ then the breadth of the planing surface undertakes up to the extremity of the horizontal segment of this spray chine but not of the above arranged spray chine where starts the side plate.
And if the breadth follows less as the lift force will be less.
After relation the horizontal segment in CL - it would have sense only if was more broad and with knuckle instead of with radius of rounding to the deadrise bottom. Then the breadth of the gliding plate actuates only this horizontal segment in CL. It is possible only at small boats for which the smaller lift force is necessary. That is for gliding we hope only on the horizontal segment of the bottom in the CL.
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### Devu De GoaJunior Member

@Rabah

Thank you Sir. This looks interesting. I have understood the concepts which you explained.

I need to ask some things:

(1) I discussed the project with my Professor. The total weight will now come down to about 30t. Should help us to reduce the power.

(2) Based on the attached screenshot of the report given by you, does the software say that we can further reduce drag considerably by changing those values like deadrise? Will it give much benefit?

(3) If the current design will operate in semi planing range, under what conditions do you expect to clear the resistance hump? Is the planing analysis reliable to compute pre-planing performance?

(4) Could you please provide the angle of entrance value.

(5) Also, how do we consider the vertical component of the propulsive force in this? Does the software take care of that? The horizontal component will push the boat froward, but the vertical component will have a rotating effect which will try to bring the aft of the the boat up.

Can you please provide the latest hull shape which you are working on so that I can pick some ideas?

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### Devu De GoaJunior Member

Also, for other students who may follow this thread in future, here are some interesting articles I found on internet:

(1) The influence of aft buttock shape on high speed hull performance by Sten Hellstrom and D.L. Blount.

(2) Semi displacement boats : an alluring compromise by Charles Neville

Best Regards,
Devu

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• ###### p21 The influence of aft buttock shape on HS hull performance.pdf
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### RabahSenior Member

Hi Devu,
I write you last time. I think that helped you to understand the problems of gliding enough. Already it is time to you to cope with originating problems itself. My answers:
1. I publish for you Planing Analysis by Orca3D for displacement 30 t.
At draft T = 0,94 m D = 30,031t in salt water
LCB = 12,497 m from FP /6,503m from AP/
Cb = 0,428; at VCG = 1,82m GMt = 1,218 m
Beginning of planing - at 33 kts Fnv = 3,088; dynamic trim = 4,925 deg; PP total = 1254 kW
With operating speed 35 kts Fnv = 3,275; dynamic trim = 4,692 deg; PP total = 1321,2 kW
At velocity 35 kts the required power for engines is 2 x 660 kW = 1320 kW
At v < 40 kts motor yacht has semi-stable longitudinal stability.
At v = or > 40 kts - there is not porpoising, motor yacht has stable planing mode.
2. My model is type "Monogedron" - with stationary values of deadrise angle of the bottom from the midship up to the transom, equal 10 - 17 deg.
Deadrise in Middle = 14,567 deg < 30 deg
If the velocity it is necessary to be higher then also the frames should be projected with a major deadrise angle of the bottom /more than 20 deg/. Such hull a type „deep V” ensures a most comfortable course from the minimal loss of speed on heaving. But the considerable deadrise of the bottom has also a lack – low initial stability of the motorboat both at rest and on the move. The stability of the monohull motorboat hardly falls with increase of the deadrise angle.
3. The calculation with Orca3D concerns for a range of velocity from 20 - 40 kts, i.e. actuates two modes of floating - pregliding and gliding.
The maximum water drag is received 46343 N at v = 23 kts.
4. The angle of bow cusp it is possible to receive in Maxsurf Modeler on DWL of model at T = 0,94 m.
5. From a horizontal component lift force it is possible is afraid at catamaran vessels when aerodynamic lift force can the vessel through the transom is overturned or at sporting racing gliders.
At Monohull more dangerous is the development of the longitudinal instability /porpoising/ when the vessel starts to spring on water.
6. I publish too for you my model in Maxsurf Modeler and dxf file of Linesplan with shm zip file by Shape Maker. The beginning of the coordinate system is in the stern. Model is without change except for new displacement.
_______________________
NA Razmik Baharyan

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• ###### Motor Yacht 19 m,24112016.zip
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Last edited: Dec 21, 2016
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### RabahSenior Member

Hi Devu,
I have understood in what your error. Title of your thread <19m semi displacement / planing hull "Port service boat”> is wrong. When I was see your new thread “Hybrid method for semi displacement hull forms (B. Taravella, 2009)” has understood that yours title is necessary to look so: 19m semi displacement / semi planing hull " Port service boat" From that all misunderstanding also were received. Already you are clear to me what for want velocity 20-25 kts, i.e. it is necessary to you a mode of floating at volumetric Froude number < 3. It is not necessary to you planing mode.
Me the following now interests:
Method of Taravella concerns for vessels with weight above 500 t. How it you will adjust for yours 30t?
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### Devu De GoaJunior Member

Dear Sir,

Sorry for all the misunderstanding. I haven't designed a boat like this before which works in semi displacement range. So when I started the thread I really did not know whether it will plane or pre-plane. But by now I have reached some understanding and agree with you that we have to stay within the semi-planing range. The confusion arises because of Maxsurf results which do not distinguish based on Volumetric Froude Number. If you see the resistance results I have posted before, it show that the vessel can begin planing in the range of Fv less than 3.0.

Key to this is the understanding as we have from the reference I have provided in comment #70. Quoting from "Going Half Fast" by Charles Neville:

"So here's the ugly truth. There are three primary ways of achieving semi-displacement speeds on water. The first technique is to load up a displacement hull with excessive power. This technique usually is obvious because of huge wakes and radical trims these boats generally produce.

The second, and most difficult technique is to design a boat specific to the semi-displacement task. Often this means developing a relatively narrow, light hull with a fine bow. Such geometry results in a boat with less interior volume than others of a similar size, making it harder to sell.

The third method, and unfortunately the most common, is to take a typical planing hull shape, not worry too much if it gets overweight, and pack enough power to push it to whatever speed one believes customers will demand."

Secondly there is no clear boundary where pre planing stops and planing starts. The same article says that the fully planing range is between a Froude number of 2.5 or 3.0 and above because of variations in the performance of different hulls.

I'll continue this in the next comment.

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### Devu De GoaJunior Member

To continue the previous comment, suppose we assume a rectangular plank of plywood of the following dimensions: 1m x 1m x 0.050 m

Consider this is placed on still water. If you tie one edge of the plank and drag this plank with sufficient power it will behave like a planing craft.

Now assume that the same plank is made of a heavier material. You will be able to create the same planing effect, but with a very high power.

This works in practical situations. So I think every vessel will have some planing ability if the hull shape is suitable for planing, irrespective of the Froude numbers, but in a range of power which is either impracticable or inefficient.

I feel, apart from the two of us, nobody else is participating in this discussion or sharing their knowledge. I do not know why.

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### Devu De GoaJunior Member

To your second question regarding Taravella method, I have tried to contact Mr. Brandon Taravella of the University of New Orleans himself but he has not responded till now.

The matter presented in the paper is very mathematical and with my exams due very soon I do not have a lot of time to go into all the details of it. But as far as the mention of 500t is concerned, the author has expressed the need for extending the semi displacement analysis between the range of (500t to 3000t). That does not limit the use of the method to that range.

To quote from the paper: "The second misrepresentation in the Michell’s near field free-surface boundary condition is eliminated by reformulating the near field in terms of “slender body” theory, versus “thin body theory.” This was originally completed by Ogilvie (1972). However, Ogilvie limited his results to wedge shaped bodies. Vorus and Taravella (2008) have developed a general solution based on Ogilvie’s (1972) formulation which yields very accurate results for length Froude numbers greater than 0.38."

I have created a model which displaces about 30t. To my surprise the Holtrop method was able to show better results as compared to the previous results posted earlier for another hull form. We have a effective power requirement at full load of 30 t and speed 25 knots of about 350 kW (700kW bhp at eff=0.5) by Holtrop method. The Savitsky preplaning method somehow stops performing beyond 21 knots. But both methods are in near agreement upto 21 knots.

You will notice the reduction in wave making resistance upto about 25knots, after which Holtrop method shows a sharp rise in resistance. Doesn't it mean that the bow wave effect has been shifted from 18 knots to 25+ knots. After that the wavemaking component will not decrease any further.

It will be interesting to test this in the model basin.

Attached are the results. You can point out if you find any discrepancy.

Best Regards,
Devu

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