Written discussion on: Paper No.2 of the SNAME Annual Meeting 1991. Resistance and Seakeeping Characteristics of Fast Transom Stern Hulls with Systematically Varied Form by E. Lahtiharju, T. Karpinnen, M. Hellevaara and T. Aitta

Written discussion on:

Paper No 2 of the SNAME Annual Meeting 1991.

Resistance and Seakeeping charac-teristics of Fast Transom Stern Hulls _{with Systematically. Varied} Form by E _{Lahtiharju e.a.}

J.M.:J. Jöurnée Report No. .905-P

Deift Unlvoi'elty of Technology Ship Hydromechanics Laboratory Mekelweg 2

2628 CD DeIft The Netherlands Phone 015 -7868 82

J.MJ. Journée.

Ship Hydroinec.hanics Laboratory,. Deift University _{óf Technology,} Mekelwe:g 2,, 2628 CD Delft, The Netherlands.

Tel: + 31 15 783a8l Fax: + 31 15 7al:836.

Written discussIon on:

Paper No 2 of the SNAME Annual Meeting 1991.

Resistance and Seakeeping Characteristics of _{Fast Transom} Stern Hulls with Systematically Vried Form.

by: E. Lahtiharju, T. Karppinen,, M. _{HelÏevaara and T. Aitta.}

The authors present in their _{paper very valuable information on} the vertical motions of high-speed vessels.

The differences between the two versions of the strip theory _are attributed to an inclusion in the original _{strip theory (OST) and} an exclusion in the modified strip theoy _{(MST) of the added mass}

"end- terms" . in fact, an inclusion or an èxciusion of

these terms

i,n the left hand side of the

equations of, motion is defined by the integration boundaries of the _derivatives:

L-e L+e

inclusion: int jf1(x).d = Jf'x).dx-(f(0)-f(L))

O-I-e O-e

E

« L

L+E . L-e

Exclusion: mt _{if'(x).dx jf'(x)idx+(fO)-f(L)) O}

O- e O+.e

However, when using numerical _{integrations from O+e until L-e,} the :expressions "inclusion" and "exclusion" are confusing: in

case of an exclusion of the "end-terms" in _{the left hand side of} the equations of mot:ion, _{extra "end- terms" have to be introduced} at the wave loads in the right hand _{side of the equations.}

To verify the explanation for the _{differences between the strip}

theories, _{the vert:ical motions of the bow of the NOVA-II vessel} have been calculated _{at Fn=O.864 with the Delft University}

of

Technology six degrees of freedom ship motions program SEAWAY for the two strip theorie:s without _{and with these "end-terms". To}

ob-tain the huilform, 'a _{Lewis approximation has been}

used. The re-sults are presented n figure 1, _{below. Comparable results have} been obtained with the lO-parameter _{conformal mapping}

J.M.J. Journée _/ DU.T. Discussion on Pap.er No. 2

SNAME Annual, Meeting Ï991

and Franks puisati.n.g source method for _{the NOVAI vessel, of} which a body pian is given in the paper. Figure _{1 below, taken} from figure 28 of the paper, does not confirm the explanation fully. But certainly, _{it is one of the reasons for differences.}

In _{thi.s particular case, the experimental results are situated}

between the OST without "end-terms" and the _{MST with, "end-terms',} so close to strip theorie.s with inclusions -and _exclusions o.ppo-site as used in the paper.

It is mentioned _{n the paper that different -expressions for}

t'he

heave exciting force and the pitch exciting moment should have a relatively small effect _{on the computed heave and pitch. In} lite-rature a lot of attention. i,s _{paid to the left hand side of the} equations of motion, but generally less _{attention is paid to the} definition of the exciting wave loads in the right hand side f the equations, which _{are of equal importance.. These loads}

con-sists of _{a Froude-Krjlo.v part and a diff;raction part.}

For the calculation of the diffraction part,, _{often a pressure level in} the fluid will b.e chosen. _{However, this, can be done in different} manners., The definition of the _{wave loads and the presence or}

absence of "end-terms" in the _{paper is not fully clear.}

In SEAWAY, _{the Froude-Krilov loads are expanded in series. Then} the dominating term i.n the relevant part of these series

deli-vers _{equivalent directional components of the orbitai}

a'ccele-ration-s of the water particles. From this, _{equivalent orbital} velocities are found. These orbital _{motions and the hydródynamic} coefficients are used to caiculate _{the diffraction part of the} wave loads. _{t high forward speeds the contribution}

of this part i.ñt6the tótïl wave loads _{can become significant, which holds}

that it;s definition can be irnpo;rtant.

3..

3.

2

2.0

H1.5

o

1.o

0.5

o

0.5

_1.0

_1.5

_2...0

_'2.5

WAVE/SHIP LENGTH Figure 1. The Vertical Motions _{at PP of NOVA-II}

in Head Waves at Fn 0. 864

J.M.J _{Journée / D..U.T.} Discussion on Paper No. 2

S NAME .Annia i M eet:ing 1991

3.0 3.5

OST Original Strip Theory _{(+) with "end-terms"} MST Modified' Strip Theor ..

.. (-) without end-terms" ) NOVA

-

Fn =

II

MST(-) L .MsT(+) 0.864 L

1.

F

-A,

-. I

-,/

'\\_

::;

fr

/

EXPERIMENTS . CALCULATIONS: i'

t

: Lahtiharju, Program SEAWAY

et.ai..

I

iii

II_

I

3.5

z

o

H.

E-1 1.0 0;5 O 0.5 WAVE/SHIP LENGTH

OST Original Strip Theory .(+) with "end-termS" MST Modified Strip .Theor (-) withoüt "énd-terma"

NOVA Ii

Fn=0.864

IMST(-)

_MST(+)

-A,

i __

-/

OST()

\_OST+)

H EXPERIMENTS I CALCULATIONS: : Lahtiharju, Program. I SEAWAY et.al.: I

/

1.0 1.5 2.0 2.5 3.0 3.5

3. 5

.à

2.5 2.0 : 0':

OST Original-Strip Theòry

MST Mòdified Strip Theor.

/

A

"NOVA II-'Fn 0.864'

I

/

0.5 1.0 1.5 2.0 WAVE/SnIP LENcTH (+) with "end-terms" (-) without "end-termá" et.al. 3.0 3.5