Difficulties are very often encountered in selecting proper

material for making stern bearings for fishing boats. Conventional materials such as white metal, bronze etc. wear out too fast also causing considerable wear to the propeller shaft. Lignum-vitae, a natural wood commonly used for this purpose, is not available on a commercial scale in our country. Results of trials carried out to find out suitable substitutes for lignum-vitae from among indigenous timbers are presented in this paper

Sivadas, T.K.

Velu, M.

Aquatic Commons

INDIGENOUS SUBSTITUTES FOR "LIGNUM VITAE, FOR 
MAKING STERN BEARINGS FOR FISHING BOATS 
M. VELD & T. K. SIVADAS 
Central Institute of Fisheries Technology (Craft & Gear Wing) Cochin-5. 
Difficulties are very often encountered in selecting proper 
material for making stern bearings for fishing boats. Conventional 
materials such as white metal, bronze etc. wear out too fast also 
causing considerable wear to the propeller shaft. Lignumvitae, a 
natural wood commonly used for this purpose, is not available on a 
commercial scale in our country. Results of trials carried out to 
find out suitable substitutes for lignum-vitae from among indigenous 
timbers are presented in this paper. 
INTRODUCTION 
Selection of piOper material for stern 
bearings for fishing boats presents certain 
special problems because of the peculiar 
operating conditions, to which these 
bearings are subjected. The bearings are 
lubricated by water, which is often full of 
suspended material lib fine sand, as the 
boats operate mostly in shallow waters. 
Stern bearings made of conventional mateG 
rials like white metal, bronze etc are found 
to wear out rapidly and cause considerable 
WF.:ar to the propeller shaft. Lignum~ 
vitae(Guaiacum offlcianale Linn.) a natural 
wood, is widely used for this purpose but 
this timber is not grown in India on a 
commercial scale. 
Systematic investigations were there-
fore undertaken to evaluate the wearing 
14 
qualities of certain Indian timbers which 
were suggested as probable substitutes* for 
lignum-vitae. 
METHODS AND MATERIALS 
Laboratory trials: Specimens of the 
following timbers were prepared and 
subjected to both laboratory and field 
trials. 
1. Guaiacum officinale- lignum-vitae: 
Control. 
2. Acacia chundra 
- red cutch 
3. Mimusops littoralis - Andaman 
bullet-wood, 
4. l-1ardwichia binata anjan 
Laboratory trials were conducted in a 
~-~--------------------
* Probable substit,utes were suggestflcl by the 
l<'oresl:; Res:arch Institute, Debra Dun. 
FISHERY TECH!\OLOGY 
DETAILS, AI 'A' 
Fig!. ARRANGEMENT SHOWING THE TESTING OF THE §.P.ECIME!'J IN THE LABORATORY 
tank of size 120 X 105 X 90 em in which 
the specimens were subjected to wearing 
by loading them with weights suspended 
on a lever as shown in the Figure 1. 
The shaft was driven at 500 rpm, 
which is approximately the propeller shaft 
speed in fishing boats. Sea water was 
filled in tank so that the level was 50 mm 
above the specimen to be tested. One 
litre of fine sand was admixed in the sea 
water to simulate as far as possible, the 
actual working conditions. The specimens 
were taken out after 250 hours of running 
and maximum depth of wearing was taken 
as a measure of the wearing quality of the 
wood. 
Field Trials: In order to confirm the 
results of the laboratory trials, the timbers 
were subjected to field trials. Stern 
bearings made of these species were fitted 
to boats and their performance under 
actual working conditions was observed. 
After about 500 hours of working the 
bearings were taken out and the extent of 
wearing determined on the basis of the 
difference between the initial and final 
internal diameters after drying them to 
original moisture content, under shade. 
TABLE I MAXIMUM DEPTH OF WEARING OF SPECIMEN IN THE LABORATORY TRIALS. 
Maximum depth of wearing in mm. 
Specimen First Second Third Fourth 
trial trial trial trial 
1. Guaiacum officianale 1 ign urn-vitae 8.3 3.1 6.2 6.1 
Control 
2. Mimusops litforalis Andaman bullet wood 5.7 4.0 5.8 7.2 
3. Hardwickia binata an jan 4.4 3.4 4 2 8.2 
4. Acacia chundra red cutch 6.0 2.7 8.6 7.5 
YoL VI No. 1 1969 15 
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-I:J:l ::r: 
ti:I 
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..., 
ti:I (") 
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0 
r-' 
0 
0 
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Sl. 
No. 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
8. 
TABLE II: WEARING CAUSED ON THE BEARINGS UNDER FIELD TRIALS. 
Inner dia- Wear on 
Inner dia- Inner dia- meter of the bearing 
Size of boat No of Rated RPM Ma~erial of mete_r of mf'te~ of bearing along the Wearing Species of (Length over- hours of the ro eller bearmg beanng along diameter caused Resiste-bearing material all in metars) the boat propeller P /: ft before the after the the dia- ('alcu1ated on to the ranee to se 
worked shaft 8 a trials trial'l meter du- for 500 hrs prope- water 
ringactual of running Her shaft 
(mm) (mm) 
working 
(mm) (mm) 
Mimusops littoralis 9.12 (30') 461 400 Bronze 58.58 59.55 0.97 1.05 *** 
} Good 
9.76 (32') 558 750 -do- 51.80 52.85 1.05 0.94 *';'* 
" 
Hardwickia binata 9.76 481 750 -do- 51.62 51.96 0.34 0.35 ,;,,;'** 
} Good 
9.12 520 400 -do- 58.42 59.58 1. I 6 1.12 **** 
" 
Acacia chundra 9.76 480 750 -do- 49.02 50.10 1.08 1.13 ** 
} Good 
9.12 472 400 -do- 58.49 59.41 0.92 0.98 *~~ 
" 
Guaiacum o{ficianale 9.12 522 400 -do- 58.25 59.27 1.02 0.98 * ) 
9.76 420 750 -do- 51.54 52.50 0.96 1.14 * 
I V.Good 
" 
* Very little wearing ';'* ) 
> Wearing within the allowable limits. 
*';'** Wearing a little above allowable limits. *** J 
For practical reasons all these bearings 
could not be made of uniform size and the 
time interval between fixing the bearings 
and taking them out for inspection also 
could not be kept constant. 
RESULTS AND DISCUSSION 
The results of the trials are presented 
in tables I & II. 
Analysis of the results of the labora· 
tory trials showed that all the indigenous 
timbers tested have wearing propertie<> 
which compare favourably with those of 
lignum-vitae. All the timbers are more or 
less affected to the same extent. These 
results were also confirmed in the field 
trials. 
Though the laboratory trials indicated 
that these timbers possess wearing qualities 
almost equal to those of lignum-vitae, 
field trials clearly showed the latter is supe-
rior to these timbers as it causes too little 
wear on the propeller shaft compared to 
that caused by the indigenous timbers. It 
is, however, beyond the scope of the 
present investigations to go into the basic 
properties which contribute towards better 
wearing qualities. The diagonal and 
oblique arrangement of successive fibre 
layers, silky texture, self lubricating pro-
perties, resistance to salt water and great 
compression strength (CS IR 1956) besides 
the presence of a waxy resin in the timber 
appear to be the contributory factors for 
its superior wearing qualities. 
The undermentioned indigenous tim· 
bers which were subjected to the trials can 
however be recommended for use as stern 
bearings in fishing boats in the following 
order. 
1. 
2. 
Mimusops littora/is - Andaman 
bullet wood. 
Acacia chundra 
3. Hardwickia binata 
Red cutch. 
an jan. 
VoL VI No. 1 1969 
Clearance to be allowed in the finished 
bearings. 
It is necessary to provide correct 
amount of clearance between the propeller 
shaft and the bearing as also between the 
bearing and the stern tube. Too much 
clearance between the bearing and the 
stern tube will cause the bearing to work 
loose in the stern tube with the resultant 
vibration of the propeller shaft. Insuffi-
cient clearance between the propeller shaft 
and bearing will make the bearing too 
tight on the shaft when it absorbs water 
and swells. Experiments were therefore 
carried out to determine the cle1rances to 
be allowed. It was concluded that the 
bearings with an initial moi.,;ture content 
of 12-20%, which is normally the moisture 
content of air dried timbers should be 
machined as a ''push fit" in the stern tube 
and a free "sliding fit" on the propeller 
shaft. 
AcKNOWLEDGEMENTS 
The authon express their thanks to 
Shri G. K. Kuriyan, Director-in-charge, 
Central Institute of Fisheries Technology, 
Erna kulam, for going through the manu-
script and for kindly granting permission 
to publish this paper. They also ack-
nowledge with thanks the co-operation 
extended by the President, Forest Research 
Institute, Debra Dun who suggested 
probable indigenous substitutes, the Forest 
Utilization Officers, Poona and Baria and 
the Chief Conservator of Forests, Andaman 
Nicobar Islands who supplied the speci-
mens for trials. 
REFERENCE 
C. S. I. R. 1956 
The wealth of India. 
Raw materials-IV PP 266-67. 
17 


Indigenous substitutes for "Lignum vitae" for making stern bearings for fishing boats

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Indigenous substitutes for "Lignum vitae" for making stern bearings for fishing boats

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