HUNGARIAN JOURNAL OF
INDUSTRY AND CHEMISTRY
Vol. 49(2) pp. 35–38 (2021)
hjic.mk.uni-pannon.hu
DOI: 10.33927/hjic-2021-19

INVESTIGATION OF THE RESISTANCE A SAILBOAT IS SUBJECTED TO
IN THE CASE OF DRAFT CHANGES CAUSED BY MODIFYING THE POW-
ERTRAIN

EMESE LÉVAI*1 AND PÉTER FICZERE1

1Department of Railway Vehicles and Vehicle System Analysis, Budapest University of Technology and
Economics, Műegyetem rkp. 3, Budapest, 1111, HUNGARY

In our research, calculations were performed using the example of a specific ship when a modification was made to the
powertrain of a vehicle. These results were used to observe whether the ship is expected to decelerate or the change
in the draft is likely to cause stability problems. In recent years, the rules concerning the use of inland waterways by
recreational crafts have been tightened for environmental reasons. In many cases, these restrictions affect the drive
chain, making the workflow of the conversion involved frequent. In this case, it is extremely important for shipowners to
know how the draft will change and at what speed the ship will operate after the conversion. This study on a numerical
flow simulation provides an excellent opportunity to find out.

Keywords: sailing, computational fluid dynamics, simulation, modifying the powertrain

1. Introduction

With the boom in lake navigation, the technical equip-
ment of small watercraft has also expanded to include
important details. Since the number of sailing vessels,
which were originally only wind-powered, has increased
in ports, in order to prevent subsequent disruptions and
facilitate emergency maneuvering, these vessels carry an
engine as a secondary source of propulsion. In addition,
small boats that are purely motor-powered have also en-
tered the market for private individuals. Generally speak-
ing, this size of engine has had a detrimental effect on life
in shallow lakes, such as Lake Balaton, in several ways
and emissions have noticeably increased. As a result, a
regulation [1] came into force whereby recreational (i.e.,
not emergency, port maneuvering, etc.) trips can only be
powered by electric motors on Lake Balaton. For this
reason, shipowners have had to convert their propulsion
systems from internal combustion engines to electric ver-
sions.

Although the effect of this on the draft and speed
varies from boat to boat, this data is important for
shipowners. Our article presents a method for calculating
such data for a specific ship.

*Correspondence: levai.emese@edu.bme.hu

2. Methods

2.1 Weight and dive calculation

The examined vessel is a 50 m2 cruising, capital-
weighted sailboat designed for tours that is 13.2 m long
and weighs 6.8 t (Figs. 1 and 2). The differences between
the results of the weight calculations on the blueprints
and the actual ship, as well as the knowledge of the mass
of the elements of the electric and diesel powertrain, were
sufficient to determine the change in the draft [2]. The
draft varies from 1.68 m to 1.7 m (Fig. 3). The calcu-
lation was performed using the MAXSURF Hydromax
program.

2.2 Fluid dynamics simulation

The purpose of the calculation in this case is twofold: to
determine the drag (resistance) force acting on the hull
while in motion (‘x’ component) and the buoyancy (‘z’
component). The practical goal of the tests is to calculate
the resistance force acting on the boat during two dif-
ferent dives using the same engine power, as this has a
major impact on the speed of travel (because if the boat
moves in the ‘x’ direction, the resistance force will al-
ways be in the ‘x’ direction). The tests were performed
using the FloEFD extension of the Siemens Solid Edge
program. The initial values were the dive (1.68 m and 1.7
m according to the weight calculation) (Fig. 3), flow char-
acteristics - test, free surface, and the boundary between

https://doi.org/10.33927/hjic-2021-19
mailto:levai.emese@edu.bme.hu


36 LÉVAI AND FICZERE

Figure 1: A piece of the original item and weight chart
(1950)

different media, as well as their required properties (e.g.,
density, method, etc.). As an external force, the magni-
tude and direction of gravity had to be taken into account.
The flow rate of the tested medium was also adjusted to
the medium in the appropriate direction, that is, opposite
to the direction of travel. The mesh was compressed lo-
cally by 3-stage compression around the surfaces delim-
iting the body (Figs. 4 and 5).

This is partly due to the fact that unnecessarily ac-
curate calculations of values at points unrelated to the
hull would lengthen the duration of the test and partly the
result of the calculation being as accurate as possible at
critical locations (where plating and water intersect). The
load cases were chosen according to the dives and the se-
lected speed points at a Froude number of up to 0.45 can
be assigned to the characteristic operating conditions of
the ship (while displacing water) [3]. The values at the
selected speed points were recorded within this range.

3. Results

After running the analysis, the series of measurements
was tested separately for each load case at each of the
six speed points. Convergence was observed under all cir-
cumstances. The results, which are shown in Figs. 6 and
7, were plotted on graphs and presented in tables. It can
be seen that in both the ‘x’ and ‘z’ direction, the higher
the speed at which the vessel travels, the greater both the
force acting on the hull and the greater the vertical dis-
tances measured between the points of the curve on the
graphs are. As can be seen in Fig. 4, the resistance (verti-
cal axis) resulting from lighter load cases (denoted by the
orange line) was less than for heavier load cases when the
electric drive chain was in use (denoted by the gray line).
The expected results were also recorded in the evaluation
of the buoyancy forces. Regarding the load case belong-
ing to the original drive chain (denoted by the gray line),
the buoyancy forces are lower than in terms of the load

Figure 2: A portion of the new batch and weight chart
(2020, field survey)

case belonging to the new drive chain (denoted by the
blue line). The parabolic nature of the curves was also
in line with the preliminary expectations as the resistance
and velocity are square proportional. On the curve depict-
ing the resistance, the wavy nature (at a Froude number
of approximately 0.5) is due to the effect of the wave re-
sistance on the total resistance.

This also means that even as a result of small changes
in dive, the increase in resistance becomes more signifi-
cant as the vessel accelerates. Therefore, even on a larger
ship, it might be beneficial to select lighter components
for the powertrain.

Not only does dive cropping change the size of the
wetted surface and thus the resistance force, the shape of
the wetted surface and the waterline section is also mod-
ified. In the present case, it geometrically cuts a wider
shape out of the water surface as a result of the hull, so

Figure 3: Characteristic curves calculated from vessel data

Hungarian Journal of Industry and Chemistry



INVESTIGATION OF THE RESISTANCE A SAILBOAT 37

Figure 4: Locally compressed mesh around the stern of
the hull

Figure 5: Locally compressed mesh around the stern of
the hull (side view)

the current image must also be examined (Fig. 7). If the
immersion shape changes leading to the flow rate gener-
ating early-breaking vortices around the hull or acceler-
ating too quickly around the maximum width, a drastic
reduction in speed results.

4. Analysis

By examining the results, several findings are made. At
higher speeds, as was expected, the resistance force on
the ship differs greatly between the two dives, increasing
the draft from 1.68 m to 1.7 m by 4606 N. Therefore,
the effect of changing the draft in the order of a few cen-
timeters is also significant. By plotting the velocity dis-
tribution around the vessel from the current image, it was
found that behind the point where the width of the vessel
is greatest (towards the stern) next to the side plate, the
flow rate accelerates locally and then decelerates again
back to the velocity observed around the front of the ves-
sel. Around the ship, in addition to the surface that is in
the shadow of the overflow (i.e., the accelerated flowlines
next to the ship do not–or only partially–affect), the ve-
locity of the medium decreases significantly. The velocity
of the medium at and around the intersection of the wa-
terline area and the axis of the steering bearing is close to
0 m/s. It can be seen that neither the magnitude (13.7 m/s)
nor the location of the maximum flow velocity (the nar-
rowing arc behind the main rib) causes a large decrease
in velocity (Fig. 8).

5. Conclusion

It can be said that by replacing the internal combustion
engine and its associated drive chain, this ship will be
subjected to an excessive resistance force which will re-
duce its forward speed. The test vessel is mass-produced

Figure 6: Comparison of the magnitude of the resistance
a ship is subjected to under two load cases

Figure 7: Comparison of the magnitude of the buoyancy
acting on a ship under two load cases

Figure 8: Flow velocity around the ship (higher: red,
lower: orange)

and, in the case of vessels operating with possible mi-
nor modifications to its class, the calculation is expected
to yield the same result with similar engines. For other
types of vessels, by following the testing methodology,
accurate answers to the questions raised in Section 2.2 are
provided, which are vital, for example, before a sailing
race (in which case, the engine is merely excess ballast).
Since the calculations were performed on a horizontally
floating sailboat, the subject of a further study could be
the examination of a tilted vessel or of a hull protruding
whilst accelerating using the same methodology. Ques-
tions may also be raised about an excessive number of
additional batteries that may be inserted to increase the
range of the electric motor and their possible placement
in the light of swimmers as this presupposes additional
dive options.

REFERENCES

[1] XLII/2000,Water Transport Act
[2] Lévai, E.: Elektromos segédhajtás beépítése egy

acél építési anyagú, 50 m2-es tengeri cirkáló típusú,

49(2) pp. 35–38 (2021)



38 LÉVAI AND FICZERE

kedvtelési célú vitorlás kishajóba, 2021, 2–21, 30–
35

[3] Simongáti, Gy.; Hargitai, L.: Kishajók. Budapest,

2012, 14–15 ISBN: 978-963-279-643-7

Hungarian Journal of Industry and Chemistry


	Introduction
	Methods
	Weight and dive calculation
	Fluid dynamics simulation

	Results
	Analysis
	Conclusion