Measurement provides yacht design data
Stresses o n sailmg yachts at sea
by Jiirgen Hausen, Jukka Talvia, Richard Wiefelspiitt and Uirich HeinemannI n order to achieve the optimum design of the hull structure and the rigging components on modern sailing yachts, information is required about the loads occurring on the yachts during operation. Decisions regarding the dimensions of a yacht's constructional elements are normally made manually based on load statements which are obtained using empirical guidelines or which are determined according to unreliable rules from the Classification Societies. Measurements are needed if reliable load statements are to be made. These measurements should provide data on the external loads affecting yachts during various weather conditions and maneuvers. This article describes load measurements taken on a number of yachts with lengths between 8.6 m and 35 m. H u l l pressures, rudder torques, forces in the mast, stays, shrouds and keel bolts together with accelerations at various points were measured.
Background
The design and construction of sailing yachts are com-prehensively described in the specialist literature, e.g. [1, 2, 3]. The frequent international specialist seminars together with the boat shows which run i n parallel to them reflect the continuing progress in yacht construc-tion, giving information on the latest developments. Also, particular subject sectors appear in special publi-cations such as the arrangement and design of the keel and rudder [4, 5] and the aerodynamic properties of yachts [6, 7, 8].
Until now there have been no generally applicable state-ments regarding the loads which have to be considered
BOAT
Type Main dimensions Measurements ANNE Duetta86 Loa = 8.60 m B = 2.95 m Disp. = 2.93 m^ Preliminary tests S M I L Y dbl Loa = 10.10m B = 3.40m Disp. = 3.22 m^
Hull pressures, keel-bolt forces, rudder torques, accelerations, forces in whole rigging V A G U S
Longkeeler
Loa = 11.40 m B = 2.80m Disp. = 9.76 m^
Forces in shrouds, rearstays, forestay, jib headstay, mast
UWA
12m
Loa = 19.14 m B = 3.66m Disp.= 25.82 m^
Forces in mast and shrouds, accelerations, longitudinal bending of the hull F R A T Z Z Z
12 m
Loa = 19.14 m B = 3.66m Disp. = 25.82 m^
Forces in mast, shrouds, backstay, accelerations ONYX Special Loa = 35.00 m B = 8.70m Disp. = 126.00 m^
Forces in mast, shrouds, rearstays, forestay, genoa-stay
Loa = Overall length; B = Breadth; Disp. = Displacement
Table 1: Summary of the investigations showing the yachts and test programs
during the design of the yacht's hull or of the individual components. Reliance is placed upon empirically de-rived guidelines or upon the regulations issued from the Classification Societies. These Classification Societies normally dispense with an accurate analysis of the effec-tive loads and instead base their design equations on "design stresses". The six most popular equations for the specification of frames, external plates and rudders on yachts i n wood, FRP and metal construction have been compared in [9], including that from Lloyd's Regis-ter and the American Bureau of Shipping. The result of this comparison has shown that the "design stresses" on which the equations are based can vary up to 1000 % de-pending on the size of ship or component.
It is only by conducting and evaluating large-scale tests that measurement data can be obtained, enabling the correction of these "design stresses".
Large-scale trials with sailing yachts
With the support of the Deutsche Forschungsgemein-schaf t (German Research Association), it has been possi-ble, starting in the autumn of 1983, to investigate a number of yachts of various classes of size and construc-tion [10 to 15]. Table 1 shows a summary of the dimen-sions of the yachts in the investigation and the scope of the relevant program of trials. After preliminary tests aboard the ANNE, which was kindly made available for the tests by Dehler Yachtbau GmbH, a very comprehen-sive program was carried out with a " d b l " boat, a three-quarter tonner according to lOR. This boat was provided by the same manufacturer for a duration of two years and was named SMILY (Strength Measurement andFig. 1: Photograph of the test yacht SMILY (Strength Measurement and Investigation of Loads of Yachts)
vestigation of Loads on Yachts). Figure 1 shows a photo-graph of the SMILY at its berth. The pressures occurring on the hull of the SMILY could be measured by pressure transducers recessed into the boat's hull. The objective of these pressure measurements was to measure loads and pressure peaks caused by the slamming action of the boat moving in the sea under realistic conditions. Then followed test programs aboard the UWA which was originally built as the SVERIGE for a Swedish partici-pant in the America's Cup elimination round in 1980. Measurements were also taken during participation i n regattas on the North Sea, at Kiel and at Flensburg in the summer and autumn.
Fig. 2: Test yacht VAGUS during the Atlantic crossing
Loads occurring during a Baltic crossing from Flensburg in northern Germany to Copenhagen in Denmark were measured aboard the FRATZZZ, another 12 meter (39 f t ) boat.
Load measurements formed part of the daily routine for the crew participating in an Atlantic crossing with the VAGUS from St. George (Bermuda) to Bremen in Ger-many. The 35 m long ONYX was also the subject of mea-surement programs during its transfer from Bremen to Alicante in Spain. Figure 2 shows the VAGUS during its Atlantic crossing.
Measurement techniques
Arrangement of the measuring points
During all the measurement trips the results of which are presented here, the mast force, tensile strains in the shrouds and stays and the acceleration levels on the hull at the side, amidships, i n the bow and at the stern were all measured. The measurement program for the SMILY also included recordings of the tensile stresses i n the mainsheet and the compression i n the main boom. I n ad-dition, the stresses in the keel bolts, the bending and tor-sion stresses i n the rudder shaft and the comprestor-sion load on the hull were measured. Figure 3 shows the ar-rangement of the measuring points aboard the SMILY.
A 1 ... A 6 B12/200 Acceleralion Transducer P 1 ... P 13 P11/1 bar Pressure Transducer M 1 ... M11 strain measuring points on mast.
LY11 6/120 Strain Gage
R 1 ... R 3 strain measuring points on the njdder shaft, LY11 6/120 Strain Gage
F l ...F4 force measurement on the keel bolts (specially made) S 1 ... S 8 force measuring elements on shrouds and stays (specially made) WD wind direction sensor
WS wind speed sensor
Fig. 3: Arrangement of the measuring points on board the SMILY
The direction and speed of the wind were recorded simultaneously, as were the wave height and frequency using a wave measurement buoy provided by the Laboratorium voor Scheepshydromechanica at the Tech-nical University in Delft, Netherlands.
The measured signals were acquired using 18 channels on an HBM KWS 3073 amplifier with a carrier frequency of 5 kHz and the signals were then recorded on magnetic tape through a PC modulator for evaluation later. For onboard operation the measurement equipment was modified for supply on 12 V DC provided by the boat's batteries. On the SMILY a generator supported in gim-bals in the forecastle below deck supplied the complete measurement system with 220 V AC. This method of mounting was necessary, because the motor could not tolerate the heeling of the boat on account of the lubrica-tion system used.
Preparation of the measuring points
The tensile stresses i n the fixed and moving parts of the rigging were measured with strain gages which were applied either directly to the shrouds and stays (on the UWA, FRATZZZ and ONYX) or, where this was not possi-ble, to intermediate measuring elements. These measur-ing elements, developed by the authors, remained on board the boats after the tests had terminated. Figure 4 shows the basic construction of the measuring elements used for determining the forces i n the rigging.
Strain gages and solder terminals were bonded to the roughened and cleaned surfaces which had been de-greased with acetone, The adhesive used was the rapid, cold-curing Z 70 Adhesive. The connecting strips from the measuring grids were passed through isolating com-pound to the solder terminals and then soldered to them. Two cable clamps held the connecting lead f i r m to
shroud / slay
solder terminal
measuring element
longitudinal strain gage LY11 6/120 ABM 75 Protective Putty transverse strain gage LY11 6/120 screw socket
lead to measuring amplilier protective lube
shroud tightener
Fig. 4: Basic construction of the measuring elements fit-ted with strain gages for the measurement of forces in the rigging
Fig. S: Two measuring elements f i t t e d to the starboard shrouds as shown in Fig. 4 for measurement of the shroud forces
Fig. 6: Installation of the strain gage measuring points for the measurement of the longitudinal force i n the mast
prevent the transfer of tensile forces from the lead to the strain gage. The complete measuring point was care-fully covered to protect i t from mechanical damage and from humidity. The covering agent used was the well-proven ABM 75 which consists of aluminum foil and putty. A plastic tube was then pushed over the measur-ing point to provide additional protection against mechanical damage. Figure 5 illustrates two measuring points mounted using this method.
Strain gages were fitted to the mast in a similar manner where they were protected against mechanical influ-ences, but above a l l against the effects of splashing water and humidity. Figures 6 and 7 show various stages in the application of the measuring points to the mast.
Fig. 7: Strain gage application at the foot of the mast
When installing the measuring points, special care had to be taken to ensure that neither the measuring point it-self nor the cables leading to i t could become damaged during sailing maneuvers and that they were not a hin-drance to the operation of the boat. I t was essential that the boat's routine was not impaired. This was a particu-larly important requirement during measurements taken on the UWA during regattas. Therefore, leads from a number of measuring points were combined to-gether and passed as a cable loom to the acquisition equipment below deck. Figure 8 shows a photograph, taken during signal recording, of a 6-channel KWS 673.A2 amplifier installed below deck.
Fig. 8; The KWS 673.A2 six-channel carrier frequency amplifier installed below deck
In order to measure the pressure loads on the hull, the shell of the SMILY was bored at 13 points. These mea-surement points were located only in the forebody, be-cause the greatest loads were expected in the region of wave entry. When selecting the points for boring, consid-eration had to be given to the position of the boat's inter-nal equipment and to the repair of the hull after the termination of the test program.
Mounting flanges were set into the bored holes and silicone was applied as a sealant between the hull and the flange. Pressure transducers of type P l l with a nom-inal pressure of 1 or 2 bar could be inserted as required in the flanges. Figure 9 shows one of the 13 installed P l l Pressure Transducers on the inside of the bottom of the yacht.
The movement i n the vertical direction (heaving and pitching), the rolling about the longitudinal axis of the boat and the yawing motion about the vertical axis were monitored using acceleration transducers. These were mounted i n the bow and stern pulpits and on the railing stanchions at the point of the boat's greatest breadth.
Fig. 9: One ot the 13 P l l Pressure Transducers viewed from the internal side of the yacht bottom
Adhesive textile tape was used for fixing the transduc-ers and for protection against splashing water. This method was found to be very satisfactory in practice. The measurement signals were recorded satisfactorily even during regatta runs.
The stresses resulting from the longitudinal bending mo-ment acting on the boat were measured with strain gages mounted in pairs on the deck and bottom of the boat. The analysis of these recordings gave the maximum stresses and the maximum bending moments which occurred.
Also, the deflection of the hull was measured directly on the SMILY and its relationship to the compression at the foot of the mast was found. To achieve this, a steel wire was stretched in the region of its yield point between the ends of the boat. The deformation of the boat over half of its length was then measured with an inductive displace-ment transducer as the backstay was tightened.
Measurement results
The recorded measurements were evaluated with the ASYST analysis software package. The conversion of the electrical measuring signal voltages into the relevant mechanical quantities was carried out taking into account the measurement parameters, such as k-factor, strain gage circuit, measurement range, output voltage and component geometry.
At this point, just a few examples of the measurement results w i l l be extracted from the multitude of those taken. Figure 10 shows the variation of the normal stres-ses in the deck and bottom i n the boat's longitudinal direction during a tacking maneuver. I t can be clearly seen that the boat's hull is subject to a bending load be-fore the tack due to the tensile forces from the be-forestay and backstays and the compression force from the mast; tensile stresses are present i n the bottom and compres-sion stresses in the deck. During the turn, the sails flut-ter, the boat returns to the upright position and the
stres-r
V
bottc m/
r
U
\
_t1
/
deck r ) 1 2 2 -t 3 6 4 3 60 lime, sFig. 10: Example of measurement results: Variation with time of the normal stresses in the longitudinal direction i n the deck and bottom of the boat during a tacking maneuver
SMILY VAGUS UWA & FRATZZZ ONYX Mast force 45.5 kN not
measured 170 kN 1000 kN Tension in shrouds: lower intermediate upper 20 kN 6kN 14 kN 7.5 kN 4.5kN 4.5 kN 45 kN 45 kN* *Intermediatea shrouds toge 120 kN 170kN* id upper ther
Table 2: Maximum forces measured in the rigging (in kN)
150
\ r • <
t r—
/
Ort bow starboard bov; port bow
Fig. 11: Example of measurement results: Variation with time of the stresses i n the starboard shrouds and in the starboard stay on the FRATZZZ for t^vo consecutive tacking maneuvers
ses decrease sharply. Now on the other bow, the sails f i l l , the boat heels over, the stresses increase again, putting a bending load on the hull.
The variation of the stresses in the starboard shrouds and i n the starboard backstay is shown in Fig. 11 for two consecutive tacking maneuvers with the FRATZZZ. Finally, the maximum forces measured in the rigging for all of the boats investigated are listed in Table 2. For the two 12 m boats, UWA and FRATZZZ, different maximum forces were measured. The higher values are given in Table 2.
Conclusions
At the Lehrstuhl fiir Schiffbau, Konstruktion und Statik at the Aachen Technical University (RWTH) a data base for touring and racing yachts was formed using the data obtained on the test runs that had been conducted so far. New data w i l l be added as i t becomes available. A t
sent, no definite conclusions regarding the validity of the approximations given in the literature can be drawn from the limited number of yachts already investigated. For example, the exact computation of the mast com-pression is extremely difficult, because, apart from the pressure that the mast trimmers transfer to the mast via the backstays, the position and size of the sails, particu-larly the foresail, affect the mast compression. The equa-tion presented in [9] for the computaequa-tion of the mast force gives a force of 30 k N (6744 Ibf) for the SMILY, whereas a force of 45 k N (10,116 Ibf) was in fact mea-sured. The computation for the UWA produced a force of 270 k N (60,696 Ibf) compared to a measured value of only 170 k N (38,216 Ibf). The design of the mast accord-ing to this equation would have led to an undersized mast in the case of the SMILY and to a substantially over-sized mast for the UWA.
On the basis of the measured values, a new relationship was derived, expressing the mast force only as a func-tion of the wind speed. However, although the depen-dence on the wind speed was the same for all the yachts investigated, the resulting curve had to be multiphed by different factors depending on the shape of the yacht. The narrow bandwidth of test parameters obtained to date indicates that further tests are urgently required. A significant part of the loads on the hull and rigging of modern sailing yachts results from the wind forces act-ing on the sails. I t follows that for the optimum design of the rigging components and the hull structure informa-tion is required about these wind forces which is as accu-rate as possible. The loads could be determined in ad-vance for future designs using a finite element computa-tional model currently under development. The data ob-tained from the trials serves to validate the finite ele-ment prediction. However, reliable predictions can be achieved only if the size of the boat and the configura-tion of the sails conforms or is very similar to one of the boats investigated. The development of universally applicable computational techniques for the determina-tion of the structural loads on sailing yachts is only possi-ble with a significant expansion of the data base. Above all, i t would be interesting to take measurements on large sea-going yachts, multi-hull boats, future conten-ders for the America's Cup and participants in the Whit-bread Round-the-World Race.
It is to be hoped that the German Research Association is pi-epared to give further financial support to relevant investigations. Contact with the international regatta community w i l l also be maintained in future in order to acquire access to possible test yachts.
References
[1] Baader, J.: Segelsport, Segeltechnik, Segelyachten; Delius & Klasing, Bielefeld, 1976
[2] Reinke, K., Liitjen, L., Muhs, I . : Yachtbau; Delius & Klasing, Bielefeld, 1976
[3] Phillips-Birt, D.: Saihng Yacht Design; Hutchinson & Co. Ltd., London 1980
[4] Gerritsma, I . : L i f t en weerstand van dreegvlakken met kleine slankheid; Report No. 260, TH Delft [5] Kerwin, I.E., Mandei, P., Dean Lewis, S.: An
experi-mental study of a series of flapped rudders; Jour-nal of Ship Research 1972, pp. 331 - 339
[6] Marchaj, C.A.: Aerodynamik und Hydrodynamik des Segelns; Delius & Klasing, Bielefeld 1982 [7] Marchaj, C.A.: Seetiichtigkeit - Der vergessene
Faktor; Delius & Klasing, Bielefeld 1988
[8] Ebbutt, P.W: Aerodynamics in the 1977 America's Cup; The Naval Architect, March 1978, p. 54 [9] M u l l , G.W: Strength requirements for sailing
yachts; 7th Symposium on Yacht Architecture; Amsterdam (HISWA) 1981
[10] Hausen, J., Bayer, C , Heinemann, U., Willich, G.: Vorversuche zur Ermittlung der dynamischen Beanspruchung von Rumpf und Rigg moderner Segelyachten; Final report of DFG research pro-ject Ha 1260/2-1, Aachen 1981
[11] Schultz, H.-G. Heinemann, U.: Experimentelle Er-mittlung der dynamischen Beanspruchung von Rumpf und Rigg moderner Segelyachten im Grofi-versuch; Final report of DFG research project Schu 173/15-1 and 15-2, Aachen 1986
[12] Heinemann, U., Talvia, J.: Zur Beanspruchung von Segelyachten im Seegang; Final report of DFG re-search project Schu 173/21-1, Aachen 1988 [13] Talvia, J., Wiefelspiitt, R.: Experimentelle
Er-mittlung der Belastungen von Segelyachten; Final report of DFG research project Schu 173/21-2, Aachen 1990
[14] Talvia, J., Wiinning, J.: Belastungsmessung am Rigg der Segelyacht VAGUS; Final report of DFG research project Schu 173/26-1, Aachen 1991 [15] Talvia, J.: Belastungsmessung am Rigg der
35-m-Kutteryacht ONYX; Final report of DFG research report Schu 173/27-1, Aachen, 1991
Dr.-lng. Jiirgen Hausen is a senior assistant in the marine engineering department at Aachen Technical University (RWTH), FRG and lectures in naval architecture and ocean engineering. Dipl.-Ing. Jukka Talvia and Dipl.-Ing. Richard Wiefelspiitt are both members of the scientific staff in the above department, specializing i n naval architec-ture and ocean engineering. Dipl.-Ing. Uirich Heinemann, a former member of the scien-tific staff, now works as a freelance yacht designer.
