Big Twin R/C Outboard Motorboat
May 1957 American Modeler
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Airplanes
and Rockets visitor Kevin B. requested that I scan and post this
article on the "Big Twin" R/C outboard motorboat model. It appeared
in the May 1957 edition of American Modeler (AM). AM was
one of the forerunners of today's Model Aviation (the official
AMA publication), and was more all-encompassing in regards to modeling
as it included model boats, cars, rockets, and trains. It also was
known to occasionally have articles on full-size aircraft. Anyway,
the Big Twin is 32" long and is built of traditional model boating
materials like mahogany plywood and spruce. This model's claim to
fame is the use of balsa planking on the hull - which is much easier
to form than spruce - and then a layer of fiberglass is laid over
it for strength and waterproofing. An
Allyn Twin outboard motor
is specified for power. Big Twin R/C Outboard Motorboat
Bill Baughman
"A.M.'s" West Coast Boating Whiz Gives
You an Honest-to-Goodness Man-Sized R/C for K&B-Allyn's Outboard
Twin; Overall Length, 32"
With
the advent of large displacement motors and remote controls there
has been an increase in the number and types of full-size outboard-cruisers,
This new form of safe, economical family boating has proven very
popular in recent years.
Now that the Allyn Twin miniature
outboard motors are on the model market, it is only natural that
model builders try their hand at this type craft; the ease of power
installation and the running cleanliness make it ideal for model
use.
Ours is a composite model in that it uses features
found in many different full-size cruisers adapted to model operation.
If you will take a closer look at the plans, you will note Big Twin"
is designed primarily for radio-controlled operation. However, if
you are not an R/C bug yet, don't let this throw you, "Big Twin"
can be built as is and operated as a free-running craft very nicely.
Too, if you would rather have inboard power, the simple matter of
closing in the outboard motor rear deck and transom cut-outs and
the installation of motor mounts at Frame #4 will make for an easy
conversion.
With the preliminaries out of the way, let's
get to building. To assure a true hull of this size when completed,
the removable jig shown in plans is used; take care in its construction
and adhere absolutely to the measurements given as the jig will
determine the final result of your building.
After cutting
out all ribs, transom and stem from the designated plywood, these
are screw-fastened to jib-board blocks, blocks in turn screw-fastened
to jig-board (see photo #1). Now spring in all sheer and chine stringers,
bottom and side battens, fastening with Weldwood, Elmer's, or similar
hardwood glue. It is good practice to fasten to chine and sheer
pieces to stem with a miniature screw at each stringer if these
fastenings are available locally (photo #2 shows construction at
this stage).

Big Twin underway.
Photo #1. With all ribs, transom and stem piece
cut out and set in place on jib-board, actual hull construction
gets started.
Photo #2. Horizontal stringers in place, hull
frame sanded and faired, ready for planking. Keep careful check
on rib spacing.
Photo #3. Ease of applying sheet balsa planking
obvious here. Although Bill applied both sides and bottom at
same time, better practice is to follow article and apply sides,
true up, then the bottom. Pins and Scotch Tape hold everything
while glue dries.
Photo #4. Finishing final bottom planking "course,"
note penciled marking along forward part of chine. As you apply
bottom planking at this point, change from over-lap to butt
joint between bottom and side planking due to increasing "V."
Photo #5. Other than one last covering board
and monkey rail for port side, hull is complete. Monkey rail
is placed flush with outside edge of planking, sheer rub rail
extends beyond.
Photo #6. Using rough-cut parts, removable
cabin unit is built in completed boat hull to assure accurate
fit. Cord keeps lines correct as roof beams are glued in place.
Using sandpaper fastened to a flat board approximately 12" long,
carefully fair up hull framework by sanding each side and bottom
section full width at one time. Working in this manner will give
you the necessary bevels on the frames, at the chine and other necessary
fitting spots, will take out all humps and hollows and allow the
planking to lie flat on the framework.
Having a large quantity
of pins and Scotch Tape handy and using the 1/16" sheet balsa, start
the planking procedure by gluing random lengths with grain running
vertical along each side, working both sides simultaneously. You
will find this will go very quickly and when completed and with
glue dry it is a simple matter to sand the side planking to conform
to the bottom contour.
After gluing exterior keel in place,
the first course of bottom planking can now be fastened down, working
both halves at same time as with the sides (see photo #3). Sand
off random bottom lengths to side contour and give entire hull a
solid sanding with a medium paper to secure a good, true base for
the final course. This is applied in the same manner as the first,
laying the balsa strips with grain running lengthwise with the hull,
however (see photo #4). As all ribs have a straight bottom configuration
rather than a developable curve, it will be necessary to use fairly
narrow strips to plank the forward one-quarter of the hull bottom.
Now a final dressing down of the planking, sanding the exterior
keel piece flush with planking, and hardwood false stem piece installation
completes this portion of construction.
When placing the
false stem, you may find it necessary to steam the wood or make
numerous small saw-cuts on inside edge to obtain required bend.
If you will take the time and effort at this point to fibre-glass
your hull from chine to chine with Berkeley "Fib-Res" and model
weight glass cloth supplied by this concern, applying same according
to their directions, you will have a boat that should be as strong
as any hardwood job and many times lighter.
The hull may
now be removed from the jig by the simple process of loosening all
screws. This is a good time to build a supporting cradle to hold
boat while completing construction and later when running and storing
it. Anything will do, the one shown in the photos was made from
scrap ply and an old round handle; supporting surfaces well padded
with heavy wool material.
With hull right side up in the
cradle, it is apparent that sheer line is automatically determined
by designed rib tops and sheer stringers; a careful trim sanding
down to the sheer stringer line will bring the extended edges of
side planking down to a nice straight sheer. Before applying the
front deck and covering boards, it will be necessary to fashion
and glue the front deck beams to Frames #1 and #2. The outlines
for these two parts are shown at top of these two frame patterns
in plan.
Cut out the two half sections of front deck and
glue into position; these will be slightly oversize to allow trimming
to your hull when in place. Small clamps, nails or miniature screws
will undoubtedly be needed in this fastening operation. As the monkey
rails will cover the section of the deck at sheer edge where application
of temporary fastenings will be necessary, the removal of these
fastenings after glue has dried will not mar the final result.
The side covering boards come next and cardboard patterns
can be taken roughly from the plans. As all hulls will vary slightly
in construction, check pattern thus made to your hull before cutting
the wood. Glue on covering boards, sand outside edges of these and
front deck to angle of side planking and then apply the mahogany
sheer rub rail at this point flush with top edge of deck and covering
boards. The center rub rails can now be installed where shown and
the chine splash rails along the chine line.
If you have
fibre-glassed the hull, use the resin as glue in this operation,
otherwise regular glues are okay. The making and installation of
the two monkey rails on forward deck completes actual hull construction
(photo # 5 shows this stage). So that the R/C hound
will have room enough to work with both hands on the control gear,
the entire rear deck, cockpit area and cabin are built into one
removable unit. While patterns are given for the parts of this unit,
to assure a perfect fit it is best to build the unit in your hull.
Cut out the cabin sides, rear deck, seat and seat back, removable
section of cockpit flooring and rear cabin bulkhead as a start.
Test fit these main parts and the reinforcing pieces of hardwood
used at bulkhead, rear of front deck section, and rear seat area.
Glue up these parts, being careful not to glue to hull. You will
undoubtedly find use of light cord necessary to hold side sections
in position while drying (see photo #6). The two cabin
roof beams come next and before gluing in permanently, lay a straight
edge across the bulkhead top and the two roof beams to check proper
height. The cabin front supporting brace is glued to reinforcing
hardwood piece at rear of front deck, after which the two cabin
front sections can be fitted and glued. You will find it necessary
to bevel all four edges of these two parts if a proper fit is to
be secured - the patterns provide enough extra material to do this.
The cabin front patterns show the front ports per the pilot
model; should you not want ports at this location, or perhaps something
different, now is the time to decide and make your cut-outs before
gluing these parts in place. The 1/8" balsa roof can be applied
by gluing on strips approximately 3/4" wide. Use sandpaper to trim
the front, side and rear bulkhead contours. From mahogany
scrap pieces, make up the flying bridge windshield brace and cross
pieces and glue in place. If desired, plastic or light-weight glass
can be attached to rear side of windshield frame as well as all
cabin port openings later. Use of blue-tinged material helps create
large-boat illusion and hide radio gear placed in cabin.
Before finishing the detail on the cabin unit, it is best to install
the two permanent cockpit flooring pieces and the two permanent
rear bulkhead pieces. These must be fitted to the hull and removable
unit while in place and then glued to hull. Small scrap gluing pieces
may be utilized where necessary in this step. Build the pilot seat,
cut out steering wheel or rustle one up, make kick strips, but do
not install until boat is painted.
You
should now be able to separate the cabin unit and hull and be ready
for the painting. A boat of this size, type and use is best treated
as a full-size craft when finishing. Stain and fill
all parts to have a natural wood finish with a good grade of mahogany
filler-stain and follow the directions on the can explicitly. If
you have used the woods designated, the complete decking, cabin
sides and front, seat back, windshield framework, and covering boards
can receive this treatment. Using a good spar varnish, give this
stained wood at least five thinned coats, sanding with fine paper
or cutting with fine steel wool between coats. When
the varnish has reached a glass-like finish and has set up - and
be sure this is the case - mask it all off with masking tape and
newspaper and start with the paint. If a little of the stain and
varnish has run over onto the area to be painted, don't worry as
the paint will cover. Now if the hull has been fibre-glassed, get
the special fibre-glass primer-sander available at all boat shops;
if you are using the wood hull only, regular auto primer-sander
for auto synthetic enamel is fine. Use a spray gun if at all possible.
On hull exterior, with the first application of the
primer-sander, it is sanded with wet-or-dry sandpaper of fine grade
and the alternating painting - sanding process continued until a
perfectly smooth base is obtained. The finish glossy auto synthetic
enamel coats are next applied, two with very light sanding between
should do fine. Please note that the paint referred to is auto synthetic
enamel, not ordinary house enamels. The hull interior does not need
the sanding but be sure enough material is applied to protect against
fuel, oil, etc. Painted portions of the cabin unit receive same
treatment as hull exterior, giving a glossy finish when completed.
MATERIALS LIST FOR HULL

Miscellaneous small fastenings called for in plan, Weldwood
or Elmer's Glue, etc. If hull is double-planked with sheet balsa,
some 1590 square inches of 1/16" material will be needed, single
planking would call for approximately 1/2 that amount. To fibre-glass
this hull, secure the regular Berkeley Model. glassing kit consisting
of 1 square yard of glass cloth and sealed can of enough resin and
hardener to process that quantity of cloth.
As to color
scheme, let your conscience be your guide; the pilot model has white
hull inside and out, red bottom to chine line, buff cockpit flooring,
blue seats and cabin roof, red steering wheel. Naturally masking
tape should be utilized for any color change.
To complete
the model, the mahogany kick strips, pilot seat, and steering wheel
are glued in position. Commercial fittings such as bow lite, flag
pole socket, bits and cleats as well as numbering decals can be
obtained from your hobby dealer, if desired.
Other than
providing for the radio installation in the design, it is futile
to attempt to cover that in detail here. The R/C builder will have
his own ideas anyway! It should be pointed out, though, that the
arrangement of this boat contemplates the placement of radio gear
and batteries inside cabin area of hull, with either a servo unit
or escapement placed in rear deck area to turn rudders or motor
itself. With the rib cut-outs, the servo can be installed up with
receiver, operating a torsion bar arrangement leading under cockpit
flooring to motor area under rear deck, turning motor itself.
MATERIALS LIST FOR REMOVABLE CABIN UNIT

Material for Jig board, together with screw-fastenings for
same, usual pins, Scotch Tape, glue, knife, finishing materials
(preferably auto synthetic enamel paint), mahogany wood filler-stain,
spar varnish, decals, trim hardware, race car steering wheel approximately
2" in diameter, material for boat stand.

Big Twin R/C Outboard Motorboat
Plans Sheet #1

Big Twin R/C Outboard Motorboat
Plans Sheet #2
Full size plans for "Big Twin" are on Group Plan #557 from Hobby
Helpers, 770 Hunts Pt. Ave., New York 59, N. Y. ($1).
{Note: These are no longer available}
Posted January 23, 2013
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