How To Build Catamarans

Craig O'Donnell of the famous Cheap Pages has generously sent this article from an old Scientific American.  Click here for more about Craig.



Scientific American
Supplement, 1878.

Note: it was popular to write using a pseudonym back in the 1870s and 80s and at the moment I don't know who "Paddlefast" may have been.

IN the following are full particulars and dimensions how to build a fast, safe, and cheap boat, of the plainest make, or a lighter and more elegant one, as desired, from 25 to 30 feet in length. Technical skill is not necessary, as there is no bending of ribs nor fitting of planks.


The only successful principles in the construction of the catamaran are extremely sharp hulls, and all possible lightness. Hence it is not surprising that a length less than 25 feet is not advisable for these boats, and that the catamaran about to be described will not carry more than two passengers, without detriment to speed and safety. The speed of this boat, under favorable circumstances, is about 14 miles per hour. The approximate cost of materials is $60. This boat can be used to advantage only in well protected waters.

The two hulls are precisely alike, and much time will be saved by duplicating every stick and board as it enters the hull first built, and laying them systematically away for the construction of the other. The entire craft is constructed of pine, except the "aprons."

The keel of each hull is of the rocker kind, i.e., curving upward at each end, like the bottom of a surf boat. Placing the keel, about 25 feet long, 2 inches thick, and 5.5 inches deep, in proper position, with an upward curvature of 1 foot, we set uprights at both ends, to temporarily represent stem and stern, which are both to be near the vertical. Next, a pair of thin strips, or combination of strips, are tacked to bow and stern and made to describe the curves of the hulls shown in Fig. 4.

There are to be 17 frames in the boat, about 16 inches apart. The curve of the strips should be regulated, so that the deadflat, or widest part, is at the seventh frame from the bow. This should also be the lowest point of the keel. The beam between the strips at deadflat is 36 inches. The vertical height from bottom to keel at deadflat to upper edges of strips should be 35 inches. At about a yard from both ends of the boat, the strips may be slightly pressed apart to make a better curve. The sheer given to the strips should not be more than 4 inches.  Fig1


Fig. 1 gives the deadflat section of the boat. The ribs B are perfectly straight, and are nailed directly to the keel A, one a little forward of the other, so that one rib is secured against the aft side of the deck beam C, and the other rib on the forward side.

Now, by placing a stick in the position of B, the lower end against the keel, the upper end against the gunwale strips, and shifting this stick both forward and aft, we can mark out just how much is to be beveled off the lower edges of the keel. By the aid of the strips it will not be difficult to determine the shapes of all the bow and stern pieces, which will be put together as previously described in SUPPLEMENT 30.  fig2

Fig. 2 shows a section of the boat near bow or stern, wherein the keel fails to afford sufficient nailing room for the ribs B. In this case, however, the bow or stern deadwood, A', is extended to nearly a third of the boat's length to afford a good backing for the ribs.

The temporary framework may now be taken down, the lower edges of the keel properly planed off, and the bow and stern pieces shaped. From this point the builder has no difficulty in replacing the keel in position, setting up bow and stern pieces, which are very much alike, consisting of stem or stern piece, apron (of oak), and deadwood, constructing the deadflat frame, Fig. 1, and the other frames in turn, being guided, as before, by the gunwale strips. As the frames are made they are connected by the diagonal braces B', Figs.1 and 3, the latter being a side view of boat, the vertical lines representing the frames. fig3

The deck beams C, to which the crossbeams D are to be bolted, should be about 1.75 x 3 in., the other deck beams may be 1 x 3, or less. The ribs A should be 1 x 2 in.

After nailing on the deck, composed of 5/8 inch strips, 2 inches wide, and inserting the bolts intended to secure the hull to the cross beams D, Fig. 1, the boat is planked up outside with 7/8 stuff, caulked and pointed. It must not be forgotten to provide a hatch in each deck, as is shown in Fig. 4, large enough to bale from. fig4


Click here for larger GIF.  

The hulls, when thus far completed, are set perfectly parallel, 11 feet apart from keel to keel. Then to frames 1, 7, 8, 10, and 13, counting from the bows, where the bolts have already been provided, are bolted the five crossbeams D, 2x6 in., shown in Fig. 4. On the crossbeams are set edgewise the diagonal pieces E, 1.5x6 or 2x5 in., with end piece E' extending from hull to hull. The space thus enclosed is floored over with 1/8 in. matched stuff, after securing the supplementary floor beams F, 2x2 in., to the pieces E.

The mast G is stepped midway between the second and third crossbeams, and surrounded by three heavy blocks about 8 inches high, which enclose the end of bowsprit also. The port and starboard blocks are connected by two horizontal bolts, one abaft the mast, passing through the end block there, and one forward of the mast, crossing just over the bowsprit. The two side blocks are bolted to the crossbeams beneath, also.

The bowsprit H is secured to the first cross beams by the iron strap H'. The bobstays I are attached to the cutwaters at the waterline.

The mast is supported wholly by the jib stay and the shrouds I'. The shrouds are carried well back to obtain a firm spread, consequently they are attached to the hulls by blocks and running lines operated from the deck, so that either shroud may be loosed when necessary, to afford sufficient swing to the boom.

The tiller J is about 3.25 feet long forward of the pivot, and about twenty inches aft the pivot. The port rudder is several inches higher than the deck, to afford nailing room for the lever L. There is the usual traveler on the stern piece E' with double blocks, as shown.

The best jib is that used on the "John Gilpin" and the "Tarantella," SUPPLEMENT No. 105. The jib-boom projects about 18 inches beyond the end of the bowsprit, to which it is attached by a swivel.

The jib stay runs to the end of the boom, but is quite independent of the jib. The jib, instead of sliding up the stay with hoops, is simply hoisted by the halyard with double blocks, and hauled taut.

The end of the peak halyard is attached to the gaff well up toward the peak; the halyard is then run through a double block at the mast head, then through a single block on the gaff, again through the double masthead block, thence to the deck.

The stays and shrouds should be wire rope 3/8 in. diameter. The anchor should weigh 30 lbs. The mast should be tapered considerably from the middle downward. The foot needs but very little more diameter than the head.

We will again run over the principal dimensions:


Length of hulls over all 25 ft.
Beam on deck 3 "
Draft 17 in.
Height out of water, at X 18 "
Distance of hulls apart, center to center 11 ft.
Length of boom 23 "
Length of bowsprit 16 ft. 9 in.
Length of jib-boom 18 ft.
Hoist of mainsail 16 "
Diameter of mast and main boom 5-1/4 in.
Diameter of bowsprit 5 "


fig_5sm.gif (25978 bytes)



Front view.


Joint at heel of bowsprit.


8: End of L to deck beam.

9: Aft support E' to crossbeam.



The deck of the previous boat is but 18 inches above the surface of the water, and besides a great bulk of hulls is exposed to the action of the sea; consequently it would hardly do for bay sailing. The boat that we have now to do with is intended to be safe, fast, and serviceable in almost any waters, and the combination of simplicity and lightness in construction is unequaled.

This boat, like the previous, is not expected to carry a much greater weight than two persons. The maximum speed will, doubtless, be 18 miles per hour, as that rate has been reached by the "Amaryllis," a catamaran 5 feet shorter, but of lighter construction.

The cost of materials, including sail cloth and rigging, will be about $80.

The hulls are made just as before, save that the length over all is to be 30 feet, and the height out of water, 12 inches, instead of 18 inches, or, in other words, the vertical depth at the deadflat frame, from the upper edge of the deck-beam to the lower edge of the keel, is 29 inches. The beam on deck is thus reduced to 30 inches. The frames are still 16 inches apart.

As our isometric scale drawing, Fig. 5, shows, the boat consists of two low hulls connected by crossbeams at intervals, and a passenger deck or car swung midway between the hulls at a higher level than the decks of the latter. This car consists mainly of the sides E, 1.5x6 in., to which the 7/8 in. flooring is nailed. The flooring is strengthened by the middle piece F, beneath, 1.5x6 in. The car is supported at the stern end by the vertical pieces E', which rest upon the rearmost cross beams.

Fig. 9 shows a good method of bolting E' to the cross beams. The car at the bow end hangs from the junction of the pieces L, which serve somewhat the purpose of shears.

As Fig. 7 represents, the shear-pieces L are attached to the rear end of the bowsprit H, by a joint that allows free vertical play. The bowsprit is squared for about 4 inches at the end, and bound by two iron straps carrying rings to which the pieces L are hinged. The forward of these two straps also binds the pendant piece R securely to the bowsprit. This latter is firmly nailed to the end piece of the car on the outside, and a narrow portion of R continues down and is secured to the middle piece F, as Fig. 6 shows.

The first cross beams D is bolted to the second frame from the bow. The shears L come over the eighth frame from the bow; the second cross beams D over the ninth frame; the deadflat and the mast also are at the ninth frame. The third cross beams comes over the eleventh frame; the backstay I'' is attached to the thirteenth frame, and the fourth cross beam is at the nineteenth frame. The car overhangs the fourth cross beam by about 2 feet 6 inches.

When cruising, the sail may be furled at night, the boom supported at convenient height by the throat halyard and a crutch, and a piece of canvas thrown over the boom and attached to the sides of the car, making a snug tent.



Length of hulls overall 30 ft.
Beam on deck 30 in.
Depth at deadflat 29 "
Draft at deadflat 17 "
Sheer or rise of keel 12 "
Frames apart 16 "
Beam from center to center of hulls 13 ft.
Shear pieces, L 3x3 in.
Crossbeams, D 1.5x6 "
Middle piece, F 1.5x6 "
Side pieces E 1.5x6 "
Diameter of bowsprit 5.25 "
Diameter of mast and boom 5.75 "
Length of bowsprit 21 ft. 6 in.  
Length of boom 30 ft. 4 in.  
Hoist of mainsail 19 ft. 3 in.  
Weight of anchor 34 lbs.  


may be built by attaching the deck beams, car, etc., of the preceding boat to full-modeled hulls, i.e., hulls of curved cross-section thereby obtaining a handsomer and faster boat. This involves an increase in cost of materials of about $15. The principal dimensions would be the same. The best form of section of hulls for speed has been determined by numerous experiments, and persons desirous of building a full-modeled catamaran may obtain a "body plan" from the writer at reasonable price.

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Craig O'Donnell
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