Jan. 14, 1905.] 
FOREST AND STREAM. 
37 
Typesjand Measurements of 
Propellers. 
BY A. E. POTTER. 
Primarily there are two types of solid screw propellers, 
the one designed for speed and the other for power, 
although frequently a combination of the two is attempted. 
The high speed autoboat and the narrow fast yacht need 
surely a different design from the snorting, puffing harbor 
towboat. There are cases where power boats may be 
occasionally used for towing, and in such cases it would 
be hardly appropriate to equip her with a speed wheel. 
Another appfication of power needing special_ study and 
selection is for the working or pleasure sailing craft 
equipped with small auxiliary power. Arrangements need 
to be made so that when the power is not being utilized 
the wheel will be the very least drag to the boat under 
sail. AH these cases need separate attention, and no fixed 
rules can be made or formulated whereby success can be 
guaranteed the first time. At best the first wheel appHed 
to a power boat is largely a matter of experiment; still 
two boats of about the same power and of the same gen- 
eral build will usually give about the same results with 
the same wheel. Where boat builders put out a standard 
model of boat or stock models, the difference is usually 
not very marked. 
It is quite evident that no wheel can be, or ever has 
been, designed that will give j ust as satisfactory results in 
a heavy small powered as in a light heavy powered boat. 
I think that you will agree with me that for a gasolene 
engine manufacturer to send out the same wheel with 
every engine of a certain rated horsepower, is decidedly 
improper, to say the least, and is not in keeping with ad- 
vanced ideas in construction. In itself it is a confession 
of apathy, ignorance, carelessness, or unwillingness tO' 
study the propeller question intelligently rather than by 
"rule o' thumb." 
Often it is that an engine is condemned as wanting 
power when the whole trouble may lie in a poorly de- 
signed or machined propeller wheel, and its sale materially 
reduced, while some other engine of much less actual 
power shows remarkable speed results just because the 
propeller was a little better suited for the boat. In the 
latter case the usual result is that the poorer engine gets 
the business that rightfully belongs to the better make. 
Such cases are extremely frequent, but manufacturers do 
not seem to realize the importance of suitable wheels. 
It certainly seems that they should pay more attention to 
the detail of this important part of the equipment, and in- 
stead of putting out a wheel that costs the least, find out 
what the requirements are and meet them. 
Power boat owners occasionally, not very often, experi- 
ment with their wheels; usually, of course, with no 
knowledge of what the pitch of either wheel is, or how 
true the blades are to each other, and yet occasionally 
get good results. In order tO' do this intelligently, they 
should be able to measure the pitch of the wheel which 
comes with the engine, find its imperfections, if any, study 
out what ought to remedy existing trouble, and be sure 
that a change of wheel will result in an improvement be- 
fore going to the expense of a new wheel. In order tO' do 
this, it is necessary to be able to carefully and accurately 
measure up propeller wheels. 
But before proceeding in this operation, one should 
bear in mind that there is usually decidedly more slip in 
small launches than in large steamers, which in itself 
.shows imperfect design or application. Find first just 
what your boat engine and propeller will do- before try- 
ing any experiments. Take two points convenient for the 
purpose, two obstruction or other buoys, and accurately 
time the run in both directions, both with and against the 
tide. Note the number of, revolutions of the engine. 
Next, on a large harbor map, accurately measure or scale 
the distance in statute miles. The speed of your launch 
or boat can be found by multiplying the mean time, or 
one half the sum of the elapsed time in both directions, 
by the distance between the two points, and divide by 
sixty. Having now the rate of. speed in miles per hour 
and the number of revolutions, you are ready tO' measure 
your wheel. To do this accurately will take considerable 
time, but the results will usually pay you. 
Take six or eight pieces of hard wood planed accurately 
to one-half inch thick, ten inches wide, and fifteen inches 
long. Fasten them together by means of two or more 
v/ooden pins or dowels at a point two and one-half inches 
from one end and half way betv/,een the sides ; under a 
drill press in a machine shop bore a three-fourths-inch 
hole all the way through, after having described a straight 
line from the center where the hole is to be bored to the 
upper end of an arc of a circle, the radius of which is 
exactly 12^ inches described on the opposite end of the 
built-up pieces, with one foot of the dividers at the center. 
Next describe another arc with a radius of three and one- 
half inches. With a fine band saw cut carefully the whole 
length of the straight line between the two arcs of circles 
described, also the long or outer arc, the one farthest 
from the center. Take the several pieces apart, and, using 
the same center, accurately describe on each piece, top and 
bottom, arcs of circles with radii of 4, 4^, 5, 5^2, 6, 6^2, 
7, 7V2, 8, 8^, 9, 10, 10^, II, wVi, and 12 inches. 
In order to find the center after the hole has been bored, 
insert a piece of wood three- fourths inch diameter in the 
hole and find a center, which you need not be so very 
particular to locate. Next saw along the 3^-inch or 
inner arc of all the pieces hut one, and you will have a 
simple but accurate instrument for measuring propellers 
up to 25 or 30 inches diameter. 
To use this, turn a mandrel of hardwood or any 
material handy to just fit the taper of the wheel with a 
hub that will fit the three-fourths inch hole in the long 
piece. The wheel is right-handed if the top of the fly- 
wheel of the engine in its ahead motion goes from port 
to starboard, and left-handed if from starboard to port. 
Place, the wheel with the forward side of the hub; down 
and the flat or drive side of the wheel against the sawn 
corner with the mandrel in the three-fourths inch hole. 
Arrange the other pieces on fop forming a regular set of 
steps,, the "outer sawii edges together, and spaced as regu- 
larly as possible. Clamp the blocks together and also 
hold the wheel by clamping it from above. There will be 
seme place along the sawn edges where the wheel will 
touch several of the pieces. If at each one of the arcs, 
say 4 to 12 inches radius, the face is equally distant, 
measuring horizontally at each step, the wheel is true 
pitch, or the same the whole length. The pitch can then 
be found by the following rule : Measure the distance that 
the edge of the topmost piece is horizontally away from 
the edge of the bottom piece at its outer end. Multiply 
the diameter 25 inches by 3.1416, divide by the distance 
found, and multiply that by the number of short pieces 
used, and divide by 2. The result will be the pitch at the 
poiiit or points of contact, or where the surface is equi- 
distant at that particular diameter. It is usually cus- 
tomary to measure the wheel at the widest part of the 
blade, and take this as a basis of further computation. 
In case the wheel does not conform to the straight line, 
set the blocks so that each one will be equidistant from 
the face of the blade, always measuring horizontally, and 
at the same distance from the other end of the measuring 
blocks. Flaving now satisfied yourself that the pitch is 
true at a certain diameter, carefully remove the wheel, 
swing around until the next blade is in place, and note 
if each blade is of the same pitch at that diameter. If it 
is desired to measure the wheel at different diameters, 
should the pitch be irregular, prepare a table something 
' like this : 
Number 
of block. 
Gin.rad. 
Sin. rad. 
9in. rad. 
lOin. rad. 
llin. rad. 
1 
1 1-2 
2 
0 
2 
1 5-S 
2 1-16 
1 3-4 
1 3-16 
3 
6 '5-8 
1 3-4 
2 1-8 
1 5-8 
1 1-8 
4 
0 5-8 
1 3-4 
2 3-16 
1 5-8 
1 1-16 
5 
0 5^8 
1 5-8 
2 1-4 
1 3-4 
1 
6 
1 5-8 
2 5-16 
1 7-8 
15-16 
.- 7 
, 1 1-2 
2 1-2 
2 
0 7-8 
; Number the blocks from the bottom, and carefully note 
the distance at the different radii. If the distance in- 
creases, the pitch is more, if it decreases, the pitch is less 
than at the place where the pitch was true. In the above 
table the pitch was true at 12 inches diameter, more at 
18 inches diameter, and less at 22 inches. The way to 
measure the pitch at 12, 18 and 22 inches would be as fol- 
lows : Providing the distance at the outer edge of the 
blocks is 8 inches and six short blocks are used, by the 
formula we would get 25 x 3.1416 8 x 6 x y^" — 29.45 
inches pitch at the 12-inch diameter. At 18 inches the 
formula would be , 
25 X 3.1416 ^ (8 ■ 
) X 6 X ^ = 82.25 in. 
Another formula reducing the diameter to 18 inches 
would be , 
18 X 3.1416 -5- ofS — X 6 X K = 82.26 in. 
the same result. At 22 inches the formula would be 
25 X 3,1416 (8-1- (— X 6 X K = 28.5 in. pitch. 
\Lt 10/ 
By the other formula, 
22 X 3.1416 (— of8 ^- ~\ x5X2i^ = 28,5 in , 
\to 16 / 
the same result. 
At 8 inches radius, or 16 inches diameter, the wheel is 
considerably dishing on the driving side, while at 10 
inches radius or 20 inches diameter the surface is con- 
siderably crowning. Here are some points which it is 
well to know with reference to dishing and crowning 
driving surface. No condition should warrant the use 
of a crowning face except the engine is. to exert more 
power backing than going ahead. A slightly dishing wheel 
is sometimes allowable, but that dishing, to get the best 
results, should decrease as the speed increases. The 
measurements in the table would show to the experienced 
designer that the wheel was considerably hooking, and 
would give as a result rather poor result when going 
astern. Hooking like this is thus allowable in high speed 
work where speed astern is not essential. The higher 
the engine speed, however, the less the necessity. 
Now, having measured your wheel, and finding that 
there is quite a variation in the blades with respect to 
each other, say as much as i^ or 2 inches, nothing un- 
likely, that the pitch is fairly true the whole length of 
the blade approximately 30 inches, that the engine speed 
is 325, while the manufacturers rate it at 350 tO' 375, you 
find your slip is as much as 30 to 35 or even 40 per cent. 
The deductions would be in such a case that a large pro- 
portion of the power of the engine, from the irregularity 
of the blades with respect to each other,- was absorbed in 
the frictional resistance or dead water carried around by 
the wheel. The engine is . not developing as much power 
at 325 as it would at 350 or 375. An improvement in 
your wheel could be made by substituting a wheel of 30- 
inch pitch, true the Avliole length of the blade, and true 
one blade with another of about the same blade surface. 
Take another case where the pitch is fairly true, blades 
are nearly or practically alike, speed is below that rated 
by the manufacturer and slip is 30 to 40 per cent. This 
case would need a v/heel of considerably less pitch and 
more blade surface. In case the speed was above the limit, 
blades regular and nearly true and slip excessive, it would 
need increased blade surface anyway, possibly slightly re- 
duced pitch. What is usually found to be the trouble with 
power boat v/heels is imperfect wheels first ; next, too lit- 
tle blade surface and too much pitch. 
Queries on Marine Motors. 
J. E. C, Baltimore, Md.— 1. Will you explain, so I can under- 
stand, how the spark coil in a make-and-break engine increases 
the intensity of the spark? 2, What is the principle of the mag- 
netic igniter? 3. Is this an -economical form of igniter? 
Ans. — I. The spark-coil consists of a core of soft iron 
wire inclosed in a spool, outside of which is wound several 
pounds of well insulated soft copper wire. When the cir- 
cuit is completed and the positive and negative currents 
start in opposite directions, they come to- this obstruction, 
which they try to overcome, taking an appreciable length 
of time tO' do so, magnetizing the soft iron core. On 
breaking the contact, it seems the nature of the electric 
current excited by the coil to- object to a disruption of its 
free passage and it leaps across the intervening air space, 
heating the air to incandescence, forming the spark. 
2. The iron core by induction becomes a magnet, and it is 
this principle that is adopted in the magnetic igniter. An 
armature is connected to one end of a Vocker shaft. This 
armature is kept away by a light spring or other means, 
and when the two electrodes are in contact_ and the cir- 
cuit is completed, the magnetizing of the iron core at- 
tracts the armature, which in turn opens, the circuit, caus- 
ing a spark; opening the circuit releases the armature, 
which in turn closes the circuit. A spark is produced 
every time the electrodes separate, the opening and clos- 
ing sometimes occurring several hundred times a minute. 
This is very similar in its action to the ordinary electric 
bell or buzzer from which the idea was undoubtedly ob- 
tained. 3. We ha^e never had occasion to- test the con- 
sumption of electrical energy or make any practical com- 
petitive tests, but from its action should say it would not 
take a great deal more or less than the make-and-break. 
A. O. H., New York. — Is it not better to- have the gasolene 
tank in the stern of a power boat, as there would be less rocking, 
if the tank can be set high enough to drain to carbureter, the 
engine being set amidship? 
Ans. — For several reasons it seems best to- locate the 
tank forward ; with a tight bulkhead aft of it is the only 
safe method of installation. Another reason would be 
that under headway the boat will usually settle astern, 
and the flow of gasolene somewhat lessened. It would be 
impracticable to locate the tank aft and inclose it in a 
tight compartment with water surrounding it, for this par- 
ticular reason, as the trouble usuallj' is to prevent undue 
settling which the additional weight would cause. Where 
the engine exhaust runs through the stern, it would have 
a tendency to heat the gasolene, and as boats have been 
known to take fire, from overheated exhaust piping, it 
would always remain there as an element of danger. 
W. H. W., Fall River, Mass. — 1. Why is bronze usually the metal 
employed for stern bearings in small launches with bronze shafts, 
while lignum vitje bushings are used in large propeller stern 
bearings? 2. Could not some other metal be used better than 
bronze running in bronze? 
Ans. — I. Bronze is generally used in small stem bearings 
for the sake of economy. A bushing of lignum vitse 
would be much better, but it would make auite a bulky 
stern bearing. Many experiments have been made, but 
this wood gives the very best results. 2. Sometimes stern 
bearings _ are babbitted, but the electrolytical action of 
two dissimilar metals in salt water tends to waste away 
the metal more easily attacked. For this reason zinc 
plates are frequently attached to steel hulls to prevent 
electrolytical action of bronze propellers on the iron work 
of the ship and rudder. In fresh water there is no elec- 
trolysis, and steel shafts are frequently employed on 
launches, and will last years. 
B. E. D., Cincinnati, O.— 1. What is the fire test of gasolene? 
2. Which is the lighter, gasolene or benzine? 3. How is naphtha 
"washed" ? 
Ans. — I. There is no fire test to benzine, naphtha or gaso- 
lene. 2. No product of petroleum lighter than kerosene 
has a fire or flash test. Kerosene of 150 degrees fire test 
usually has a gravity measured by the Beaume scale for 
liquids lighter than water of about 46 degrees. Naphtha 
runs from 69 to 76 degrees, while gasolene proper runs 
from 86 to 90 degrees. 3. Naphtha, when treated with 
steam, is called deodorized, and when agitated with dilute 
sulphuric acid, or "washed," is called acid-treated. 
W. H. R., New York.— 1. What is the usual speed of two-cycle 
-engines when used for marine work? 2. Why can four-cycle en- 
gines be operated more rapidly? 
Ans.~i. Two-cycle engines up to 6 degrees stroke usually 
run from 300 to 400 revolutions per minute. At higher 
speed they do not seem to develop a great deal more 
power, owmg, no doubt, many times to low crank case 
compression, improperly designed and proportioned ports, 
mertia of the explosive mixture, etc. 2. Four-cycle engine 
valves operate but once to every other stroke, giving more 
time to open and close. Some two-cycle engines, using an 
mlet port opened and closed by the piston, show very 
much better speed results than the older tvpe with -check 
valve controlled inlet. 
J. W. B., Babylon, L. I.— Which is the safer to operate, a two- 
cycle two-cyhnder engme, or a two-cyhnder four-cycle? 
Ans.— If your two-cycle engine is started by turning" the 
hywheel entirely over, there would be very little differ- 
ence. If, however, you start it by rocking the flywheel 
back and forth, you are liable to get an explosion in the 
after cylinder and get hurt. Probably more people have 
been hurt by the "starting" pin than any o-ther part of 
gasolene engines. 
Numerotts Entries for the Ocean Race* 
Last week in an editorial we urged American yachts- 
men to enter their yachts in the trans-Atlantic race foi 
the cup offered by the German Emperor. We are now 
able to announce that eight entries are assured, and of 
that number all but one will positively start. 
The first entry was that of an English vessel, Valhalla, 
owned by the Earl of Crawford. The second entry was 
Apache, another large square-rigged auxiliary owned by 
Mr. Edward Randolph, New York Y. C. Utowana, an- 
other auxiliary owned by Mr. Allison Armour was the 
third entry, while Ailsa, the English-built yawl owned 
by Mr. Henry S. Redmond, was the fourth. 
The most gratifying news of all is that Mr. Wilson 
Marshall's three-masted auxiliary schooner will be among 
the starters. There has been some doubt that Atlantic 
would be a participant. Now that it can be definitely 
stated that she will make the passage, it ought to have 
a beneficial effect on other owners. 
The other boats that may be included in the list of 
starters are : Schooner Endymion, owned by Mr. George 
Lauder, Jr., and the schooner Thistle, owned by Mr 
Robert E, Tod. , , : ' 
Mr. C. Oliver Iselin is very anxious to start Constitu- 
tion, and It is considered quite probable that she may be 
numbered among the contestants. If she makes the trip 
It will be under a ketch rig. The last named vessel is 
perhaps the only doubtful .one among the eight. Tt is to 
be hoped that her owners will make arrangements 'for her 
to start, as it would greatly add to its interest; ■ - ^ 
From the :entries assured, it -will be seen the list is a 
representative one, and that it includes all classes- of--v6s- 
sels from the large ocean-going auxiliary tlown to the 
modern first-class racer twice a contestant for America's 
Cup honors, , ' ■ 
