Roll stabilisation of motor yachts: Use of fin stabilisers in anchored conditions

ROLL STABILISATION OF MOTOR YACHTS: USE OF FIN STABILIZERS

IN ANCHORED CONDITIONS

By: Ir. R.P. Dallinga, MARIN, The Netherlands

1. INTRODUCTION

Early 1998 Amels Shipyards, Makkun, The Netherlands, was looking for solutions

improve the comfort of

rolling motion.

After the evaluation of various alternatives in terms of performance, _{weight and impact on the}

interior design the yard and owner agreed to test the idea to use the planned two pairs of

KOOP NATJTIC fin stabiisers

way. The present paper describes

captive and free-floating model tests. BASICS

The basic idea behind the development is that lack of roll damping _{(a moment coufiteracting} the roll velocity) is the main reason for rolling problems If the _{water surrounding the hull}

does not provide sufficient resistance one might tzy to obtain a larger reaction force by _using

the fins actively This idea

located quite far forward (chordwise) on the present type of_{non-retractable fins. The original}

idea was that this arrangement would yield relatively large inertia force when kicking" the

fin up or downwards.

VERIFICATION

To check the concept and to obtain quantitative information on the fin loads model tests _were performed with a 1:20 scale model of the 62.5 m L, vessel. During these tests the transverse

and longitudinal force and the _{shaft moment were recorded fri the four fins. Figure 1 shows}

the used model.

3.1. Character of the fin reaction

To obtain insight in the reaction forces ftom the fins, tests were performed with the model held captive in calm water. _{Three tests were performed in which the fin was moved from}

one

extreme to the other (and back) at high, iriedium and low angular velocity.

Considering the character of the recorded fin forces it showed that the fin loads show four

distinct 'phases' indicated in Figure2.

In phase 1 the fin accelerates from zero angular velocity to the required value. This_{phase lasts} relatively short.. A surprising result was that the forces and arcelerations do _{not match exactly in} time. The build-up of a starting vortex may explain this result.

in the second phase the fin _{moves stationary at a constant angular rate. The duration depends}

(obviously) on the fin angular rate., at high velocity its duration _{was around 1 s, at the lowest} velocity it lasted around 3 s.

Deift University of Technology

Ship Hydromechanics Laboratory

Library

Mekelweg 2, 2628 CD Deift

The Netherlands

In the third phase the fin decelerates to zero velocity, similar to the first phase the duration is very short.

In the very last phase the wash in the water creates a slowly decaying force on the fin in the rest position.

As expected the magnitude of the force due to the acceleration was substantial. Not anticipated

on forehand, but in retrospect obvious, the forces in the deceleration phase showed a very

similar trend Because the forces are oppositein this phase the deceleration forces cancel part of the benefits from the acceleration loads.

This result may be quite sensitive to the control characteristics of th acuator. The model

actuators seem quite fast in their reactiOn. The forces in the second, stationary moving, phase increase more or less proportional to the angular velocity squared.

3.2. Roll damping from active fins

The roll damping is related to the energy dissipated by the forces or the integral of the product of the roll velocity and the opposing damping forces In this process the roll velocity may not be treated as a constant factor, in particular at the lowest fin velocity where the fin transition may take as much as one-third of the natural period of roll.

To obtain the effective damping the forces measured during the captive test were convoluted with a harmonic roll motion. Figure 3 indicates the integral of the product of roll velocity and the force per fin for unit roll velocity amplitude The conti-ibution of the foregoing four phases in the fin forces is indicated as well.

The results of the analysis showed that the initial inertia force during the initial accelerationand the drag forces m the transition phase yield the largest damping contributions

The highest fi±i velocity yields the highest dsmping.

The effect of roll velocity is not as dramatic as might be expected from a quadratic

drag-velocity relation; at the medium speed the damping is still two-thirds of the value at the highest

speed.

An explanation of the above follows when considering the timing of the events, in particular the negative contribution of the deceleration.

The fin transition is triggered at the moment the velocity is highest. When thefins move fast the

negative force from the deceleration occurs at a moment when the roll velocity still has the

same direction (within one quarter of the roll period). This yields a negative damping

contribution.

in case the fins move slower the negative force comes at a time when the roll velocity is much

reduced, this reduces the negative damping conthbution. At the lowest fin velocity the roll

velocity has changed sign at the moment the deceleration force matenahses Because of this the damping contribution becomes even positive.

A systematic variation of the timing shows that the optimum moment to start the fins varies from a small lead (starting before the maximum roll velocity is obtained) at the highest fin

33. Relative magnitude of all roll damping contributions

Roll decay tests in calm water are often used to establish an estimate of the roll damping. Figure

4 shows typical results that indicate a drastic effect of fin activity However, because of the differences in the flow characteristics in calm water and waves and because of the expected

non-lineari ties in the forces it is far from certain if the derived damping estimate has a predictive value in realistic irregular wave conditions.

To overcome this problem afd to obtain explicit insight in the relative magnitude of the

derived from tests in irregular waves by measuring the fm and bilge keel forces and evaluating the dissipated energy This approach neglects the eddy damping of the hull and the increase of the eddy damping due to the presence of the bilge keels.

Figure 5 shows the contributions of the appendages derived from three tests in resonant

conditions with passive fins.

A first observation is that the various contributions are more or less proportional to the roll angle A second observation is that, despite their relatively small area, the bilge keel _sections provide just as much damping as the passive fins Dividing the damping contributions by the

projected area the bilge keels yield approximately three times as thuch damping as passive fins.

Comparing the sum of the contributions with the results of the decay tests in calm watershowed

that the decay tests yield a relatively high damping estimate (roughly twice the foregoing

results). The eddy damping of the hull and the contributions of skeg and rudders and differences in the damping mechanisms between calm water and waves may explain the ifférences.

Repeating the tests with active fins and re-analysing the results showed that the fins yield a dramatic increase of the roll damping. The character of the results is indicated in Figure 6,

which shows the total equivalent linear damping (from the fin and bilge keel force

measurements) as a function of roll amplitude for passive and active fins.

A linear damping contribution is independent of the roll amplitude. The quadratic nature of drag forces (like experienced by passive fins and bilge keels) implies a damping that is proportional with the roll amplitude. Tn the case of active fins there is hardly any relation between the fin

forces and the roll velocity at the moment the frns are "kicking". For this reason the related

damping contribution is more or less proportional with the inverse of the roll angle a character that makes it particularly suitable in the low amplitude 1-ange.

3.4. Roll response in anchored coiiditions

The fmal "proof of the pudding" was of course the roll response during the previous tests in

irregular waves.

The increasing roll damping was expected to have the most effect in Wave conditions with a dominant wave period around the natural period of roll of the vessel For lower wave heights

these conditions are met when the vessel is exposed to remnants of waves from the open sea, for

instance by refraction of waves around a protecting barrier or the swell from a distant wind

field. Because of the non-linear characteristics of the roll response, tests were performed in two different wave heights. In addition a test was performed in a typical, shorter, wind sea condition.

The results of a test with a high fin rate in a 0.5 m beam swell yielded a more than 10-fold

A repeat test in a I m swell shows that also in higher waves fin activity is quite beneficial, _a reduction of 66% Was obtäin&L

As discussed in the previous section a reduction of the maximum fin angular velocity affected

both the height of the inertia loads at starting and stopping (by means of the dynamic

characteristics of the electro-mechanical model fin actuator) and the timing of the deceleration peak In practice the result was that the roil response was not very sensitive to the adoptedfin

rate In a 05 in swell the roil reduction was 79% at the medium rate, 63% with the lowest rate

When comparing the transfer functions obtained from the tests with various fin activitiea it

shows that the effect is concentrated around the peak response See Figure 8 This confirms that the fin activity primarily affects the roll damping.

4. CONCLUSIONS

An idea for an innovative way of using fin stabilisers at zero speed to reduce rolling of a motor yacht was tested by means of model tests Considering the results it seems justified concluding

that:

also at zero speed fin stabilisers can b used to obtain a dranatic increase in roll

damping;

a medium fin activity seems adequate to obtain very good results,

in modest wave conditions the increase in roll damping yields roll reductions above 75%.

ACKNOWLEDGEMENTS

The author want to thank the fin manufacturer Koop-Nautic and Amels Shipyards for their

innovative attitude and their kind perrmssion to publish the basic idea and the character of the

Fig22:l:20 Model in Bowq.Seas Total Decay Deceleration Transition Acceleration a a. E a

Fig.3: Roll Damping Contributions

afifl, AO rndbk

Ofdfi,,

mis Roll '

Fig. 5: Roll Damping Contributions

Wave Frequency

Fig.7: Roll ResponseFunctions

Fig.2:Character Fin Reaction Forcess

Fig.4: Roll Decay Tests

Active Fins

Passive Fins

rms Roll

Fig. 6: Roll Damping Comparison

Fig. 8: Roll Response in Irregular Waves / / Passive Fims Active Fins a. E a a

_a