draft at DWL:
weight at DWL:
specifications are approximate and subject to changes in function
of the mood of the designer and the skills of the builder . .
XF20 was designed in collaboration with one of our Phantom builders.
Ken Owens, an experienced flats fisherman talked for years about
a tunnel hull. He was persistent and we resisted: see our message
board archives for some memorable arguments.
Tunnel hulls have drawbacks but many fanatic flats fishermen are
willing to accept them to reach those elusive shallow areas where
the big redfish hide.
Ken Owens approached us with a intriguing design idea: he had
seen an interesting flats boat on Florida's West Coast. This wide
and light 20' skiff, built by a fishing guide, captain Henderson,
had a tunnel common to many flats boats and looked like an enlarged
version of our GF16. The boat had a great reputation.
suggested to enlarge and widen our GF16 then install a tunnel
seeing pictures of the Henderson skiff, I decided that we could
do better in
several areas: lighter, better built composite hull and better
hull shape. An enlarged GF16 would be easier to pole and behave
better at speed. It also looks better than some of the hasty copies
of the Henderson skiff that began to appear locally.
During our emails and drawings exchanges, we first called her
the Cool Boat and what came out of our collaboration
is really a cool boat. We kept the rod holder location and designed
a poling platform like the one seen on the Henderson skiff.
This new design is the best possible no-compromise extreme flats
like the light weight, quiet hull and low freeboard for reduced
windage are common to many flats boats but here we have a hull
that can be built with a small budget and perform better than
most carbon fiber machines costing 30K.
The XF20 is light and will run fast with a small outboard. Don't plan
on anything larger than 50 HP. Keep the weight and draft low, the trim
reasonable. She will top at 25 mph with a 25 HP, 30 with a 30 HP.
This boats transom is designed for a standard 20" shaft. The transom
can easily be modified to accept other shaft lengths.
The XF20 hull weighs 450 lbs as designed. At a displacement of
1,000+ lbs, equal to two or three fishermen, engine and gear,
the draft will be only 3". The buoyancy and stability are
impressive: it takes 413 lbs to increase the draft by one inch.
drive, catamaran tunnel and some theory:
are different ways to design a flats boat with minimal draft.
Some catamarans claim to have less draft than monohulls but that
is simply not possible.
Let's demonstrate by comparing the geometry of two simple hulls.
The sketch below shows the waterplane area of a monohull compared
to a typical catamaran hull, each simple rectangles but the comparison
works just as well for more sophisticated shapes.
can see that the catamaran hull as less foot print than the monohull:
at equal draft, the cat displaces only half the water of the mono
and therefore, at equal boat weight will have two times more draft.
Even if the tunnel is narrower, the cat hull will always need
more draft but there is more: a cat structure is more complicated
and the hull area is larger, therefore heavier.
A cat type tunnel will always have more draft because it has less
waterplane area and a heavier structure.
is for static draft but what happens when running?
There again the monohull is superior.
The water between the hulls of the cat is turbulent. All kind
of steps and other contraptions installed between the hulls have
been tried to reduce that problem but despite the claims of some
cat manufacturers, the prop runs in aerated water and is not as
efficient. The prop has to be lowered to run in "hard"
water but this increases draft.
is where the tunnel is clearly superior. Not only does the water
coming out of the tunnel rise higher than along the hull sides
but it is compressed by the tunnel shape: the prop will cavitate
much less than between the cat hulls.
The correct name for our type of tunnel is pocket drive. The theory
is well known: the forward part of the tunnel is higher than the
exit and the aerated water is compressed before it reaches the
prop. At the transom, the water makes a hump and that is the level
of the cavitation plate at planing speeds. For the hole shot,
the prop must be deeper but once the pocket drive is "primed"
the engine can rise on a jack plate allowing the boat to run in
4 or 5" of water.
A pocket drive is not perfect: there will always be a turbulence
at the mouth of the tunnel. There is simply no way around it.
At planing speeds, the water going through the pocket has to cover
a longer distance than the water running under the straight parts
of the bottom. This will create cavitation but we can reduce its
consequences with the shape of the tunnel. The tunnel is not a
plain box: it is tapered in profile and all corners have large
radii fillets. It is heavily reinforced to withstand the pressure
The pioneer of this type of tunnel is Uffa Fox: he designed the
first ones for the British Navy boats during W.W.II. Today, we
know what the ideal shapes and proportions are and that is how
the XF20 pocket drive was calculated.
XF20 is strong but very simple, she will go together easily and
The construction is epoxy-fiberglass-plywood composite, a second-generation
stitch and glue system.
This building method combines the ease of stitch and glue (plywood-epoxy)
with the strength, lightweight, longevity and low maintenance
of a high tech composite hull.
The bottom material is a fiberglass sandwich with a plywood core.
It will resist abrasion, grounding and IUS (Unidentified Underwater
The hull is built like a large flat bottom skiff with a few frames
and a pair of stringers. The tunnel is built separately out of
the hull, then dropped in the hull.
The tunnel unit core is made of plywood panels. The inside angles
are rounded with wide fillets and the tunnel is fiberglassed with
Bottom and transom are cut to the tunnel size and the whole boat
is fiberglassed inside and outside.
pictures of the assembly method here.
In this boat, we deviate from our usual plywood frames. The frames
are made of plain 1x3 boards.
XF20 is one of the easiest boats to build. The plans include all
dimensions to cut all the hull parts flat on the shop floor.
plans give complete layout freedom to the builder: we show a simple
hull but there is plenty of room for baitwells, fish boxes and
The stern transom can take any jack plate and the bow transom
can be cut lower if it interferes with the trolling motor bracket.
plans include a simple removable poling platform drawing with
from our BOM)
BOM list materials based on our Sole Option and includes a 15% waste
factor for resin and fiberglass. For plywood, we use standard sheets
4' x 8' (122 x 244 cm). Please read the building notes and see the plans
for detailed specifications.
We worked hard on the plywood layout to keep the total cost down:
boards, linear '
The hull can be build in 30 hours but a finished boat will require
80 hours or more depending on the level of detail and the skills
of the builder. This boat can be built in a few weekends if kept
Visit our message board, help pages, tutorial pages and read our
FAQ: most questions are answered there.
detailed drawings with all dimensions required to cut the side
panels, bottom panels, bulkheads, seats and all parts from flat
plywood sheets: no lofting, no templates required.
Nesting drawings for the best plywood layout with numbered parts.
drawings showing assembly with notches, parts numbers for most
of the hardware such as hatches, fuel fill, inspection plates
Plan and Profile, Specifications
Plywood nesting for all parts
Construction drawing with plan and profile sections. Typical fiberglass
lamination detail. Frames and bulkheads dimensions. Detailed notes.
Developed dimensions for all hull panels (flat), seat tops, stringer,
and outboard clamping board.
Detailed drawing for the construction of the tunnel.
Detailed drawing for the construction of the Polling Platform.
Detailed drawing for seat locker lid with framing and assembly view.
Typical Small Boat Electrical
building notes for this boat
files reference list and more.
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7485 Commercial Circle,
Fort Pierce, Florida USA