Minimost

Five evenings building - $50 for materials – and

then comes the real thrill of owning and running
your own 8-ft. outboard sports hydroplane.
By William Jackson

Minimost was designed as a low-cost playmate for the Minimax, Science & Mechanics most popular
sport boat (Craft Print Project No.
255). By using stressed-skin
construction and advanced under-
hull design, however, it’s been
possible to wring more speed out
of the same size hull. When fitted
out with any of this year’s light-
weight outboard motors in the 10
hp class, you’ve got speeds well
up into the 30-mph range.

Needless to say
Minimost’s record low cost and
construction time means more
hours and money that can be used
to show off your new hydro.
Unlike most boats you
have built or read about,
Minimost’s construction begins
with the shaping of the bottom
plank.
First layout the 24”
radius (Fig. 3A) on one of the two
sheets of ¼” fir exterior plywood,
using a beam compass or pencil
and string.
Then place measure and
lay out the slit and bow cutaway,
using measurements on bottom
plank.
 Shaping the Bottom Plank (plywood sheet). After cutting the gore(slot) then cut the circular front ,
set the plank with its poorest side up on a pair of sawhorses, positioning the forward horse at the aft end of the
cutaway. Then set a 1x2 prop between the ceiling beam of your shop and the aft end of the cutaway directly
over the forward horse. This arrangement will hold the plank while you bend the plywood ends up slowly and
evenly on each side of the slit and wedge a shorter piece of 1x2 stock between the floor and plank. When the
slit is closed, secure the joint with a galvanized metal tie plate and stove bolts (fig. 2)

The hull framework consists of a transom, and a single frame, connected by the keelson, bottom
stringers, and chines, made up as a subassembly and fitted into the pre-shaped bottom plank (Fig.6).

Begin the framework by transferring a pattern of the transom to 3/8-in. plywood (Fig. 4B). Then cut
the motor board and framing from ¾-in. stock. After checking the fit of each part, coat the contacting surfaces
with glue and assemble the framing on the plywood transom, using 1-1/2-inch ringed nails spaced 2 inches
apart. When the glue has dried, saw a 15-degree bevel along the bottom edge of the transom and set it aside
while you make up the frame.

Lay out and cut the parts for the frame (Fig. 4A), using 1x6 stock for the top and bottom members and
1x4 stock ripped to 3-inches for the sides. Assemble the frame members over the full size pattern and secure
with glue and two 1-1/2-inch ringed nails to each joint.
 Framework Subassembly. Next make the keelson by ripping 2x4 stock
as in Fig. 5F. The remaining piece will be used to make the chines. Now set the
keelson with its center-line on the center-line of the bottom plank and shape its
lower for end to fit the upward sweep of the bow. Be sure the lower edge of the
keelson stays in full contact with the plywood. When finished, cut the transom

knee (Fig. 5G) from 2X6 stock and,

locating as in Fig.5A, attach this with plywood gussets, glue, and 1-inch ringed nails. Now attach the transom
to the knee as in
Fig 5C.

Next position the frame on the keelson (Fig. 5E), applying glue to the contacting surfaces of the frame,
keelson, and two tri-angular glue blocks. Then secure the assembly with 1-1/2-inch galvanized wire nails.

Set the framework upside down on the sawhorses while you notch the frames for the chines and bilge
stringers (Fig. 4). Bevel the stock for the chines 30-degrees before assembly. Cut the notches in the frames by
running a handsaw between the stringers and the edges of the notches. Then coat the contacting surfaces
of the chines, stringers, and frames with glue and secure with on #8x 1-3/4-inch FH screw to each joint.

Assembling the hull. Now set the frame-work right side up in the shaped bottom plankfit the chines
and stringer to the plank (Fig.
6), and drill a ¼-inch hole 2-
inches aft of the frame. Insert a
carriage bolt on the hole from
the underside of the plank and
tighten the bolt to draw the
framework into position. Check
the fit of each part marking the
assembly where adjustments
are necessary and trace the
outlines of the framework on
the plywood plank.
Remove the framework
to drill 1/8-inch nail locating
holes in the center of the
outlines and connect the holes
with the bottom of the plank.
 Mix glue and fine sawdust to the consistency of paste and coat the contacting surfaces of the
framework and plank before reassembling them with hold-down bolt and clamps. After turning the hull upside
down, drive 1-inch ringed nails through the locating holes and drive additional nails at 2-inch intervals along
pencil lines.
Cut and bevel the anti-skid rails (Figs. 5a and D) and, after securing the plank to the bilge stringers
with nails, attach the rails to the plank at the stringer centerlines, using glue and #8X2\3/4-inch FH screws 5-
inches apart.
Now turn the hull right side up again to install the sheer clamps (Fig. 5A), notching them into the
transom and frame and clamping them in place while you mark the taper where the clamps meet the chines
(Fig. 2B). Secure the clamps in the same way as you did the bottom stringers and then install a ¼-inch
plywood gusset at the inside of the clamp-chine joints.

Rough out the stem (Fig. 5H) from 2x4 stock and shape its curved fore edge
to fit the plank along the slit. Use a rasp to trim away the wood and check your progress to obtain a close fit.
When finished, coat the stem, keelson, and plank with glue and install the stem with 1-inch ringed nails driven
at 1-1/1-inch intervals through the plywood on each side of the slit.

Deck Framing. When the glue is dry, fit the fore end of the deck batten (Fig. 5C) into the stem notch,
clamping it temporarily while you cut the deck strut to fit 14 inches forward of the frame. Start with a 12-inch
length of 1x2 stock for the strut and cut it down until the deck batten makes a smooth curve when bent over it
and fitted in a notch in the frame. The actual length will vary with the stiffness of the batten stock.
When the strut fits, install the batten with glue and #8x1-3/4-inch FH screws driven into the stem,
frame, and strut. Then attach plywood tabs where the strut meets the batten and keelson. Next rip a 48-inch
length of stock along in centerline to make carlins to support the side decks (Fig. 5A).

When finished, lay a

6-foot length of ¾-
inch square stock over
the framework in
several positions to
check for high spots
and trim with rasp or
plane so the deck
can be applied
smoothly making full
contact with the
framing.

Side planks
and Deck. Clamp the
stock for the side
planks (Fig. 3B) in
place, trace the
outlines of the frames,
sheer clamps, and
chines, on them, and
remove to drill
locating holes for nails as you did when attaching the bottom plank. Coat the mating surfaces with glue and
attach the planks with 1-inch ringed nails space 2-inches apart.

When glue has dried, trim the planks flush at the sheer and chine lines. then cut and attach the transom
outer frame (Fig.2), using glue and #8x1-3/4-inch FH screws space 4-inches apart and trim the aft ends of the
plank. Give the interior of the hull a coat of flat white enamel, avoiding areas to be glued.
Before installing the fore deck, cut and attach the brackets for the steering wheel mounting board (Fig.
7), fastening them to the frame with two #8x1-3/4-inch FH screws to each bracket. Then make up and attach
the mounting board as in Figs. 7 and 5C.

Next clamp a straight edge of one of the fore deck panels (Fig. 3B) to the centerline of the deck batten
so there is enough stock aft of the frame to trim flush. Then bend the plank in place, clamp, and trace the
edges of the bottom plank, and framing on it. Remove the plank and cut it out, allowing 1/8-inch beyond the
traced lines for trimming after assembly. The remaining piece of each panel will be used as one side of deck
(Fig. 2).

When the deck halves are cut to shape, paint their underside, except for the joints at the edges and
centerline, before permanently installing them. Use a wood rasp to cut a ¼-inch flat where the panels meet
(Fig. 5B) and then a putty knife to insert a bead of glue mixed with fine sawdust. Secure the seam with 1-inch
wire nails clinched in the underside.

Finally attach the side decks to the carlins and sheer

clamps and when the glue is dry, trim and sand the plank
joints. Then sand the entire hull with medium sandpaper, and
apply thee coats of well thinned semi-gloss enamel, trimming
with a contrasting color.
 Materials List-Minimost

Plywood
2 ¼”x 4x 8 fir Ext. AB or AC bottom planks, decks
1 3/8” x 14” x 60” Fir Ext. AB or AC plywood Transom

Lumber
1-(2x6) 15” Fir Transom knee
1-(2x4) x 10’ Fir keelson, chines, stem
1-(1x6) x 10’ Fir frame members, clamps
2 (1x4) x 10’ Fir transom frame, bottom stringers, carlines, deck batten, anti-skid rails.

Fastenings
4 #12 x 2-1/2” FH woodscrews
12 #8 x 1-3/4” FH woodscrews
24 #8 x 1” FH woodscrews
1 lb. 1” Maze ringed nails 12 1-1/2” wire nails 1lb. Weldwood plastic-resin glue powder