372 
FOREST AND STREAM. 
[May io, 1002. 
square feet by 64. This is the total weight or displace- 
ment, as it is called, of the launch and everything in 
her, when the line was marked, showing how deep she 
sets in the water. 
For all general purposes this can be determined before 
a boat is built or even designed, if a man wants to know 
about how much weight she will float, by multiplying the 
length on the waterline by half the width on the water, 
and this product by half the depth; this product will 
represent the cubic feet contained in the launch. This 
will be. as I said, approximately, the displacement or 
weight the boat will carry and float at the waterline as 
marked, and when multiplied by 64 will give the total 
displacement in pounds. 
Length X ^ breadth X H depth X 64 = f^S* 
Because in whittling out the shape of the launch from 
a square block, just one-half of both width and depth is 
cut away in nine cases out of ten. 
Take the launch that we have described how to build 
for an example. Her length on the water is 17.75ft., l /i 
her beam 4.75 = 2.375ft. and half her draft ift. = .5, 
therefore 1775 X 2.375 X -5 = 21 X 64 = i,344lbs. 
She will float a total weight of i,344lbs. There is one 
item difficult to determine, and that is how much of this 
weight the hull represents. Suppose we say the hull will 
weigh 70olbs., the whole can be figured up as follows : 
Hull 700 Displacement 1,344 
Two people at 150 300 
Engine 250 , . i i ; 
Oil, fuel 80 
1,330 7 " ~ ' ' ' r ' 
Miscellaneous 14 < 
1,344 ^344 
The difference between a man who can draw a plan 
and figure displacement and build a boat from them, and 
the man who builds his launch from a model whittled out 
of wood, is this : The latter may build just as good a look- 
ing launch, but he doesn't know when he puts her into the 
water how deep she is going to set into it, and how much of 
it he will have left above water; whereas, the man with 
a plan can figure out just the number of cubic feet con- 
tained in the boat below the waterline, long before she 
is built. This tells him just how many pounds the boat 
will float and come to her designed waterline. He knows 
by computing the weight of his engines, coal, water, tanks, 
anchors, chains and everything going into her just how 
many pounds he has to float, and if the weight of water 
his plans shows him his boat is going to displace does 
not equal these figures, he increases the size of the boat 
below water until he has enough buoyancy to float the 
required weight ; or if he has more boat than he needs he 
can cut down the bulk below the water, so when she is 
launched she will float just to her required waterline. 
Having shown how you can assure yourself the launch 
will float at the proper depth, the question now comes up. 
"Will the launch set even on the water when her engine 
is in?" In this we will start again with the assumption 
that in most launches the center of buoyancy is in the 
middle of her length. 
Looking sideways at the block of wood which repre- 
sents cur model, Fig. 2, if it were square its whole length 
" 
it would float even and a weight placed exactly in the 
middle would cause it to sink as much at one end as at 
the other. In other words, to be technical, we would 
say its center of buoyancy is in the middle of its length. 
If the launch is cut away as much forward as aft the 
bulk of wood will remain equal at each end and the 
center of buoyancy remain in the middle. This we have 
assumed to be the case in our launch, and so if the en- 
gine, tanks and people were all put in the center, she 
would go down equally at each end. But as this is not 
practical on account of the engine being placed aft, some- 
thing else must be put forward to balance it. If not, the 
launch would be tipped down at the stern end. 
Any one can figure out this problem. The grocer does 
it every day when he weighs out to you a pound of 
butter. He knows just how far out on the scales to 
put a small weight to balance the pound. This is all you 
have to do. If the engine (Fig. 3) weighs 30olbs. and is 2ft. 
— 2' — 
FIG -3 
over, while the square block would hardly be affected 
by his weight. Fig. 5. 
aft of the center of buoyancy, to balance it you must have 
some weight that multiplied by its distance forward ii the 
center will equal (300 X 2 =) 600. A tank holding 
25gals. of naphtha 4ft. forward would balance this as at 
61bs. per gallon 25gals. would weigh isolbs. X 4ft. = 6oo 
foot pounds, as it is called So the weight forward and 
aft are equal so far as any tendency to tip the boat goes. 
The naphtha, like the small weight of the grocer's scales, 
equals the larger weight of the engine by being at the end 
of a longer lever, Fig. 4. 
FIG -4. 
«-*2'- — -¥'- * 
! 
5C - ' 
J 50 
There is another point worth consideration before we 
leave this subject, and that is the stability or power of 
the launch to resist being upset. 
This can best be illustrated by a ball and a square box. 
Suppose both were setting on the table. You could turn 
the ball over with a slight touch, but the box resists being 
turned over, and the wider and flatter the box the harder 
it will be for you to turn it over. This resistance to up- 
setting is what we call "stability" in boats. And, as the 
ball and box have illustrated, stability is greater in the 
flat boat than it is in the round one. 
Put the two in a pan of water and you will find that a 
fly, if he should alight on the ball, would cause it to roll 
This is "initial" or "natural stability." We now have 
what is called "artificial" stability." That is, if we 
should take the ball and tack a piece of lead to it we 
would sink the ball deeper and the lead would always 
be acting as a lever to hold the ball upright. Several 
flies could then alight on it and not roll it over, but if 
enough of them could land on it to more than equal the 
weight of the lead the ball would again roll over. Its 
tendency would be to alwa3's have the greatest weight 
hanging underneath. If the flies, to prevent getting wet 
as the ball rolled over, should fly off, the lead would again 
become the heavier and bring the ball back to its upright 
position again. I bring this subject up mainly as a warn- 
ing to the inexperienced not to get the motor and other 
heavy parts too high up in their launch. Launches are 
not made perfectly round like a ball, but are flattened out 
to more the shape of an oval, Fig. 6. 
If they were circular, the point upon which they would 
turn or rotate would be just where the axle is in a wheel — 
in the center. Yacht designers call this by the technical 
name of meta-center. 
Perhaps a clearer explanation of this principle is 
afforded us in the old-fashioned baby cradle or a rocking 
chair. Fig. 7. 
Two lines, A and B, squared up from the ends of 
the rocker, meet at a point C, which- is the axle of what 
would be a wheel if the rocker were carried on around. 
This is the meta-center, and so long as the weight of a 
man's body is below this point, the rocker will not 
upset. 
The lower the man's weight is below C the safer he is 
from a capsize, and the nearer he gets to C the easier it 
is for the rocker to tip over. 
This is just the same in a launch, and it is good policy 
to keep the weight of engine, etc.. as low as possible. 
The narrower the launch the lower her meta-center is, 
and therefore the lower must be her weights of ma- 
chinery, etc. A man can upset a canoe by standing up 
in her, but he can stand up with safety in a flat-bottomed 
skiff the same length. 
As the propelling power has so much to do with the 
engine, it is not worth while going too deeply into the 
subject. The propeller has been found the most efficient 
means of turning the rotar}' motion of the engine into 
forward push. The power exerted by the engine is not 
all realized in push. Some is lost in friction, turning the 
shaft in its bearings, and some at the propeller. If 
it were some solid substance instead of thin water the 
propeller were turning in, the propeller would turn ahead 
whatever its pitch or angle of the blades were. If the 
pitch was 12m.. the propeller for every revolution would 
cut its way ahead I2in. But water gives away so easily 
when a pressure" is put upon it that there is quite a lot 
of energy or power used in driving a current of water 
against the surrounding water. The faster the boat goes 
ahead the qtticker the screw cuts into clear water and less 
power is wasted in driving a current of water astern. 
The difference 'between the distance the propeller should 
move ahead according to its pitch and the distance it actu* 
allv does drive the boat ahead, is called the slip. 
Boat building, as any one who has ever tried it has 
soon found out, is a trade distinct in itself. It is unlike 
house building. wagOn building or any of those trades 
principally in the fact that every square -foot of the boa,t 
is of a different shape from the part next to it. It is 
nothing but a variety of bevels from one end to the 
other. But, like house building, wood-working tools are 
used, and, like the blacksmith or wagon maker who has 
special contrivances for bending the tires for the wheels, 
boat builders have special wooden moulds or forms over 
which to bend the frames for the boats. 
A man, having a knowledge of wood-working tools 
and how to use them, has a big advantage over one who 
must learn all that in building his first boat. It is quite 
enough to learn the art of boat building after the car- 
penter trade has been mastered without trying to learn the 
two at once. But as the individuality of the man has so 
much to do with -this, we will suppose the novice to be 
a "handy" man with tools, and simply give a list at the 
end of this article of what tools are necessary for boat 
building, and not attempt to explain all that is to be 
learned in carpentry. Court plaster is the best reminder 
of lessons learned by practice. 
It would be almost an impossibility to try and explain to 
the novice at boat building the hundred and one various 
ways different builders go about the building of a launch 
by what is called the "rule of thumb" method. The 
method by which they build a boat without plans, "just," 
as they tell you, "framing her by eye." But the more 
exact method of building from a plan admits of a very 
complete explanation, because the boat, instead of being 
an imaginary form in the brain of her builder, is drawn 
out to an exact scale from which measurements can be 
taken and work laid out before any timber is cut. 
There is no such thing as guesswork in building from 
a plan; the designer does all that when he drafts out 
the plan. It is your place to follow them. It's a very 
nice piece of work to take a set of plans, drawn on a 
sheet of paper, which merely represent the curves the 
designer wants the outside of the completed hull to 
assume, and lay them out full size in such a way that you 
can tell the exact shape and bevel of every piece of wood. 
Yet when you understand it, like everything else in this 
world, it is simple enough. I will not go so deep into 
this subject as to change the nature of this work into 
the theory of design rather than an explanation of how 
to build a launch. A certain amount of insight into the 
subject of plans, etc., is necessary, in order to under- 
stand and follow the work as I explain it. 
The hardest part of such a work as this is for me to 
realize that many of my readers are not just as con- 
versant with buttock lines, waterlines, etc., as myself but 
I will try and go back to the clays when such names were 
Greek to me and explain in such a simple manner that 
any novice may follow my explanation of how to build a 
launch. 
[to be continued.] 
Prisca. 
Prisca was designed by Mr. C. O. Liljegren and built 
by the Gothenburg Mechanical Works in 1900, for Mr. 
Henric Pripp, a Swedish yachtsman who has had con- 
siderable experience in yacht sailing and racing. He 
wanted a racing boat that would not leak after every race, 
no matter how hard she might be driven, as wooden boats 
of light construction often do, although well built, and for 
that reason steel was chosen as the material for Prisca. 
Prisca is probably the smallest steel sailing yacht that 
has ever been seen in the pages of Forest and Stream. 
but we live in an age of steel, and every year will see 
more and more yachts built of this material For cruising 
yachts this material cannot be beaten, and with ordinary 
care there is little or no danger from deterioration, as 
has been borne out by actual facts, Her dimensions are as 
follows : 
Length — 
Over all 48ft. gin. 
L.W.L 30ft. o:n. 
Breadth- 
Breadth — Extreme 10ft. oin. 
Draft — Extreme ' 6ft. gin. 
Displacement n,7oolbs. 
Ballast on keel 4,20olbs. 
Sail Area — ■ 
Mainsail 1,000 sq. ft. 
Jib 350 sq. ft. 
Total 1,350 sq. ft, 
As has been stated, the hull is built of mild steel, except 
deck, deck beams and cabin trunk, which are of wood. 
Frames are 1.^x1 ^xs-itnn., plates 3-i6in. in keel, i-ioin. 
in bilge and sides. Deck of iin. white pine, and cabin 
trunk entirely of mahogany, %in. thick. The hull is spe- 
cially strengthened under the mast by heavier plates and 
angles, in addition to mast stool and stringers. 
For a small yacht, a hull of steel is, of course, heavier 
than one of wood built for racing only, but it is certainly 
not heavier than a wooden hull strong enough to stand 
rough weather without leaking. But in spite of her heavy 
hull, Prisca has shown herself very fast during two sea- 
sons of racing, especially in her windward work, having 
repeatedly beaten larger yachts sailing against heavy sea 
and wind. 
The accompanying photo shows her raeing in her first 
race. Her sails were made by an American firm, but in 
justice to the sailmakers, it must be said that the sails 
were not properly stretched, as time did not permit; later 
they have given entire satisfaction. 
Although designed for racing, Prisca has very good 
accommodation under the cabin house, with four berths in 
cabin, a toilet room and two berths in forecastle. Aft 
there is a large self-bailing cockpit, with seats on deck. 
Mr. Liljegren was born in Gothenburg, Sweden, and 
was the son of a well-known ship owner. He was gradu- 
ated from the Royal Technical College at Stockholm as 
a naval architect. He afterward spent several years study- 
ing in the largest shipyards in England, France and Ger- 1 
many. Mr. Liljegren has been in this country for over 
five years, and during that time has been employed by the 
Herreshoff Mfg. Co., Bristol, R. I. ; the Newport News 
Shipbuilding and Dry Dock Co., the New York Ship- 
building Co., Camden, N. J., and is now president and 
general manager of the Standard Shipbuilding Co., Perth 
Amboy, N. J. He is a regular member of the; Society of 
Naval Architects and Marine Engineers. 
