April 13, 1907.] 
FOREST AND STREAM 
583 
Resistance to Launches. 
It is very mystifying to some yachtsmen why 
a boat that can go four and one half miles with 
100 pounds push or thrust on her propeller, can¬ 
not make nine miles with double the power or 
200 pounds pushing her. That the boat will not 
make double the speed by doubling the power 
too many of us have found out by actual ex¬ 
periment, which has sometimes proven a very 
costly one for those trying it. Let us see about 
how the power does increase as the speed goes 
up by taking the actual performance of some 
boat that has been tested over a measured mile 
and the results all accurately compiled. 
times with and against the tide and her speed 
per hour computed from the average time of the 
various runs. 
It showed that on the first trial with least 
power, 153 pounds push she made 5.75 miles an 
hour. On the second power with 231 pounds 
push she only made 7.05 miles. On the third 
power with 310 pounds push she made 7 93. On 
the fourth 380 pounds gave 8.61 miles. The fifth 
456 pounds gave 8.97 miles. 
1 
Pounds 
Miles 
Push. 
Difference. per hour. 
Difference. 
153 
.78 
5-75 
231 
7-05 
1.30 
310 
.70 
7-93 
.88 
380 
.70 
8.61 
.68 
456 
.76 
8.97 
•36 
Putting these into a 
table we find 
that for 
SHAPE OF MOULDS OF BOAT TESTED. 
The boat in question was a 28ft. good, big, 
common sense launch fitted with electric storage 
batteries and motor which made the test all the 
more accurate as the horsepower or “push” could 
be increased to five different powers by means 
of the controller. On a gasolene engine this is 
not obtainable as the power is not so easy to 
vary. 
To simplify matters let us reduce the push or 
thrust exerted by the motor and propeller into 
pounds push. This we do by multiplying each 
horsepower by 33,000 (as a horsepower is sup¬ 
posed to be capable of lifting a weight of 33,000 
pounds one foot high in a minute) so we find 
how many pounds the horses our engine repre¬ 
sents can lift. As this power is to be utilized 
by means of a propeller, whose pitch multiplied 
by the revolutions it makes in a minute gives 
the distance during a moment’s time when that 
power is to be applied that the propeller would 
move ahead, we can easily find how many pounds 
push there is for each foot of distance by divid¬ 
ing the pounds (represented by the horsepower 
of the engine) by the distance in feet, and the 
push, or thrust, for each foot is given. Now 
that shows you how the power of your engine 
exerts its push against the water. 
How fast did this particular launch go with 
the push as measured by the five steps on the 
controller? She was run back and forth four 
The space mef between 
these show bow speed 
fe/s /ess and Jesb . 
t; R: ^ s! 8 
MILES 1 234 5 6 ’ 6 w 
CURVE SHOWS HOW RESISTANCE INCREASES AS 
SPEED INCREASES. 
E D 
11 
10 
12032 lbs. 
—©- 
At six miles an hour an old style round fan 
tail boat runs perfectly natural and level, but 
when it gets up to 10 miles she is nearly under 
water aft. A modern flat stern launch like a 
modern speed boat does not swamp herself aft. 
She leaves the water so suddenly at the square 
stern it shoots by clear of her before it rises in 
a long billow astern. But nevertheless she crowds 
out forward. The fastest hulls like the Chal¬ 
lenger model you will find lift up the least. 
increase of 78 pounds push on the first trial 
she made a difference in speed of 1.3 miles, while 
for 76 pounds push on last trial she only in¬ 
creased her speed .36 or a little over one-third 
of a mile. By plotting these figures on a sheet 
of cross section paper, letting each square on the 
vertical column represent 10 pounds push (see 
plate No. 1), and every one across the bottom of 
the paper represent a quarter of a mile. Make 
a cross where 153 carried across meets 5.75 
carried up, and do the same with all the spots. 
A curve drawn through these spots will show 
clearer to the eye and in such a way that the 
mind can comprehend at a glance just how the 
resistance is increasing as the speed increases. 
Now as to what causes this increase of re¬ 
sistance let us throw aside all theoretical cube 
of the speed ideas and look at it as if we were 
hayseeds standing on the bank with a straw be¬ 
tween our teeth. Cannot anyone see how a 
launch, when speeding up fast, crowds up so her 
bow lifts out of water and her stem drags down. 
I- 
As soon as the bow lifts up a farmer can see 
that the hull is crowding; the bottom presented 
at an angle against the water. If she is too 
heavy to slide up, raising some of her weight 
out of water, she has to push this immense wave 
bodily aside and that takes a lot of power. It 
is not using the length of the boat as a wedge 
to separate it, but is pushing, trying to crush 
the water under it as the hull is pushed on 
against it. 
In extremely fast boats where the speed gets 
up to near 30 miles, the speed curve, as some of 
the speed boats designed by Mr. C. D. Mosher 
have demonstrated, goes over a series of humps. 
The resistance increases as shown in the first 
curve until the hull is driven so fast the crest of 
the wave she rolls up is crowded aft so it is 
aft of the middle of the launch. Then as you 
might say she is traveling down hill. _ Her bow 
shoots out ahead of the wave, so _ instead of 
climbing up it she is like a boat running through 
the surf with the following wave helping her. 
*— - - r rrtmfc 
You will find she now makes several more 
miles per hour for a very little increase of push, 
until her bow enters what may be called the 
second wave: then the resistance increases 
rapidly and the speed refuses stubbornly to be 
increased to any-extent. 
So you see by this curve that the boat’s posi¬ 
tion in the water, the angle at which she is push- 
IV 
III 
IT 
£ 
FORE AND AFT LINES OF LAUNCFI TESTED TO FIND RESISTANCE AT VARIOUS SPEEDS. 
