Ship Hydromechanice Laboratory Deift University of Technology
Longer ¡s better?
The seakeeping of a monohull in head seas can be
improved by an increase in length without altering
other parameters, giving improved operational
characteristics at an affordable cost
J.A. Keuning and J. Pinkster
Report 1070-P
October 1995
Ship & Boat International - Issue 95/8
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Ship & Boat International October 1995 1
Contents
News Review 2
Longer is befter?
s-
zu
-The seakeeping of a monohull in head seas can be improved by an increase in length
without altering other parameters, giving improved operational characteristics at an
affordable cost.
A research programme at Deift University of Technology in the Netherlands has been
test-ing the validity of the above assertion, and finding it basically true. The results to date were presented in a paper by Jakob Pinkster
and JA Keuning at the Fast 95 conference. The vessel used as the yardstick in the study is the Damen Stan Patrol 2600, a 26 metre, 25 knot patrol boat which has proved an effective
package, with examples in service with the
Royal Hong Kong Police.
It combines a steel hull with aluminium 'wperstructure and has a moderate deep vee illform. The main particulars are given in
Table I.
For the purposes of the study the boat was considered to be lengthened, without in prin-ciple changing other parameters, to give 33m
and 40m long craft. To do this the stations
were spaced 25% and 50% further apart than
the basic design, and the lines faired. For
weight calculation purposes the frame spacing of 1m was retained. Apart from this the boat remains the same. The wheelhouse is identi-cal. as is the machinery, and all are located at the same distance from the transom as in the
standard design. The payload remains the
same, and internally the extra space generated by the radical length increase is considered to be unused void space.
Damen Shipyards provided design
infor-mation on huilform,
stability and trim,weights and building costs and the analysis
97.0 1039 IfO.0
460 9.91 7.03
5.61 679
'iii
ç 2X
U
,3 2? a? a?
Fig 2. Resistance plotted against speed for the various boat lengths.
was carried out using Delft Ship-hydromechanics Laboratory SEAWAY and FASTSHIP software. SEAWAY ship motions
program uses a linear strip theory approach
and a North Sea operation area was assumed,
using the JONS WAP sea spectrum.
Hydrostatic particulars were computed for
the new (thus lengthened) body plans. The increase in structural weights of these two
alternatives was also computed via the
origi-nal weight data which was augmented with extra frames and hull plating while, at the same time, taking into account the relevant
positions of the centres of gravity of all
corn-length o/a m
lengthw.I. m
Beam mid. m
Depth mid.(1/2) L) m
Draught midships m
Draught aft. approx m Displacement kN Deadweight kN
GM m
Tot. Engine Power kW
ponents of the designs. The resistance and propulsion calculations were also made for
each alternative and the position of the system centre of gravity of each of the two
alterna-ives were optimised with respect to
minimis-ing of the required installed horsepower for
the given speed of 25.00 knots.
Since the idea behind the enlarged ship con-cept is equal payload and speed for all possi-ble alternatives it stands to reason that the ves-sel configuration (i.e. also position of accom-modations etc.), remains unchanged to that of
the basic design for each design alternative concerned. Both enlarged alternatives are
Table 1 Main vessel design particulars for basic ship and alternatives
Fig 1. The Damen Stan Patrol 2600 Qn which the study is based is shown at the top, with the 33 and 40 metre theoretical versions below. Table I gives the particulars.
St. Patrol 3300 4000 26.70 33.70 40.70 24.85 31.85 38.85 5.80 5.80 5.80 3.35 3.35 3.35 1.60 1.47 1.38 1.95 1.82 1.16 970 1040 1110 170 170 170 1.62 1.93 2.19 2000 1300 1200
f5
i.
05
LO
(rad/J)
shown in Fig. I along with the basic vessel
configuration. The main design particulars for
the alternative designs are shown in Table I
a1on with the basic vessel configuration.
Most interesting are the conclusions one
may make from the results shown in Table 1, namely that the larger the design the relatively
lighter the ship becomes, and to a lesser
degree the lower the engine power becomes to
propel the vessel at a constant speed of 25 knots. It should be noted, however, that the
basic design is rather over-dimensioned with
regard to scantlings in view of the working
at philosophy of the designing yard.
One of the
interesting effectsof the
enlarged ship concept turns out to be that all Fig 4. Heave behaviour for the three hulls.
VERrICIL MOTYON W WHffLI4SE V5 25 X*pp Is m 15 1.5 LO HEAVE RAO
V .25 ¿(n
05 1.0j_ ¿rd/)
le
Fig 3. Vertical motions in the wheelhouse at25knots in head seas.
the parameters which are important for the
determination of the ship's resistance are
improved considerably yielding a far lower specific resistance, i.e. resistance per ton ofdisplacement, of the enlarged concepts. The usual design trend, i.e. to cramp all the func-tions of the ship into the shortest overall
pos-sible length (driven by the supposed direct
relationship between building cost and length), yields a relative low length to beam
and high loading of the planing hull. The resistance calculations of the three designs
have been carried out using the code FAST-SHIP of the DeIft Shiphydromechanics Laboratory (Keuning 1994). The results of the
resistance calculations for the three designs
Fig 5. Pitch behaviour for the three hulls.
i-
:s
-are presented in Figure 2.
The enlarged designs show a considerable decrease in actual total resistance, largely due to the high L/B ratio and in particular higher loading factor API(DISP**213), when com-pared with the original design.
In order to be able to assess the operability of the three designs in a realistic environment, which is the basis for conclusion with regard to possible improvement in economics of the advanced ship, use has been made of the cal-culation method (Beukelman 1988).
In the framework of this present study only the behaviour of three ships in head seas was considered, since it is known from the litera-ture and from real world experience that this condition generally imposes the largest restrictions on the safe (and comfortable) use of the ship. A comparison of the three designs in this condition will yield a clear insight in any possible improvement.
As operational area of the ships, the south-ern part of the North Sea has been chosen. To limit the amount of work, the all year statistics were used, and only one forward speed of the ships, i.e. 25 knots, has been considered. No attempt has been made to incorporate different mission profiles of the ships into the calcula-tions.
The large number of ship motion calcula-tions necessary to perform the operability cal-culation makes use of a linear superposition
approach quite attractive. Therefore, as a first approach, the necessary ship motion calcula-tions have been carried out using the software
SEAWAY. Although the behaviour of fast
planing boats in head waves may be consider-ably nonlinear, in particular when the vertical accelerations are concerned, the use of such a finear theory for high deadrise boats, as long as only significant values are being used, i.e.
significant motion and acceleration
ampli-tudes, may be justified for the sake of compar-ison (Keuning, 1994).
The limiting criteria with respect to the safe
operation of the ships in waves are derived from the regulâtions of the Dutch National
Authority. For patrol boats (and similar craft)
to
9-Z! t7-4 3-z LO W (,ud/3) PiTCH RAO V1. 25 ¿(n.5
on the North Sea these regulations state that
the maximum significant vertical acceleration in the wheelhouse must be less than 0.35 times the acceleration of gravity.
The results of the calculations are presented in Figs 4 and 5 presenting the heave and pitch
response functions for the three craft at a
speed of 25 knots in head waves. The plots are presented on a basis of wave frequency for the
sake of direct comparison. This is possible
since the forward speed and the waves
encountered are the same for all three craft
considered.
From this figure the improvement in sea-keeping behaviour with increasing length is
obvious. This of course is a well known phe-nomena in ship motion analysis.
Since the accommodation (payload) area is
identical for all three designs yet another
improvement with respect
to seakeeping behaviour may be introduced in the enlargeddesigns. The accommodation can be moved
aft into the area of minimum vertical motion. rhis relative movement aft of the wheelhouse is clearly visible from the side elevation plans presented in Figure 1. The effect of this on the vertical accelerations in the wheelhouse there-fore yields a further improvement of the
sea-keeping behaviour of the enlarged concept.
This is clearly shown in Fig 3 showing the
ver-tical accelerations in the wheelhouse as a
function of the wave frequency. From this graph the dramatic reduction in the level of vertical acceleration in the wheelhouse overthe entire frequency range is
obvious.
Economic case
-In order to make an economical evaluation the building costs of the different design alter-natives have been estimated. These were esti mated using the original building costs of the
Stan Patrol 2600 (of which all costs compo-nents were known) and correcting this for changes in steel weight of the hull and extra
painting costs (i.e. cleaning, preparation and
painting). This main engine installation has
been left unchanged as
far as costs and
weights are concerned as the authors seek the reduction in engine power via derating of the engines in this stage of the design exercise; in
doing so, however, the outcome of building
costs is more pessimistic. The differences in building costs are indexed with regard to the Stan Patrol 2600 in the bar chart. Note the low
increase in building costs of approx. 3% per
25% increase in vessel length.
The operational costs of all the design alter-natives are considered for a scenario of a ten year economic life, sailing 6 hours per day at
full speed, 7 days a week for 48 weeks per
year and crewed by 5 persons (3 shifts per 24
hours). The differences in operational costs are indexed with regard to the Stan Patrol
2600 in the bar charts. Note the relatively high
decrease in operational costs of
approxi-mately 6% for design alternative 3300. This decrease is less dramatic in the case of the4000 design alternative (i.e. 7%).
The transport efficiency (TE) - defined as
(payload(kN) x service speed(mls))/installed
power (kW) - has been calculated for all three
designs.
The differences in TE are indexed with
regard to the Stan Patrol 2600 in the bar chart.
Note the relatively high increase in TE of
approx 54% for 3300 design alternative.
Again this increase is less dramatic in the case of the 4000 design alternative (i.e. 67%).
Conclusions
Given the three designs presented in this
paper, along with the wave scatter environ-ment, seakeeping criteria and the patrol boat mission profile, the authors drew the follow-ing conclusions to the enlarged ship concept, also shown visually in the charts.
The enlarged ship concept is, for the
case presented here, very attractive indeed An increase in vessel length of 50%, i.e. alternative 4000. results in the best vessel in
terms of operational costs, operability and
transport efficiency (i.e. vessel resistance)
With regard to vessel resistance, an
increase in length of 25% or 50% does notlead to large differences. The sign of these
dif-Fig 6. intermediate design result - weight indices.
e -J, e
a:
I I S,Fig Z Intermediate design results with displacement, lightship, deadweight and speed indices.
WORKBOATS
ferences vary with speed domain. In absolute sense these differences remain below approxi-mately 10%
With regard to operability, an increase in
vessel length of 50% leads to a much larger increase (i.e. approximately 4 times as much)
than that compared to an increase in vessel
length of 25%
The building costs of the vessel, provid-ed vessel speprovid-ed and payload capacity remain unchanged, increases remarkably little with length (i.e. approximately 3% for every 25% increase in length)
The operational costs of the vessel,
pro-vided vessel speed and payload capacity
remain unchanged, initially shows a large decrease of 6% for 25% increase in vessellength, remaining fmally however, almost at
the same level for a 50% increase in vessel
length.
* Optimisation of the seakeeping behaviour of a fast monohuil. By JA Keuning and Jakob
Pinkster, Delft University of Technology. Fast
95,Tra vemünde, Germany.
-:
Fig 8. Final design results; the overall performance indices.
