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XF20 modified Pita Orlando FL.

Came in at 18'10" and 420 Lbs. Runs great and fishes great.


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Specifications:
LOA: 20' 4" 6,20 m
Max. Beam: 8' 2,44 m
Hull draft at DWL: 2" 51 mm
PPI at DWL: 413 lbs 188 kg
Hull weight at DWL: 450 lbs. 205 kg
HP 25 max 50
Material: Stitch & Glue .

All specifications are approximate and subject to changes in function of the mood of the designer and the skills of the builder . . .

The 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.
Ken suggested to enlarge and widen our GF16 then install a tunnel aft.
After 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 boat.

Features 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.

Pocket drive, catamaran tunnel and some theory:

There 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.


You 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 hull

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.

That 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.


This 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 variations.
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.

Building method:

The XF20 is strong but very simple, she will go together easily and fast.
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 Stumps).
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 biaxial fabric.
Bottom and transom are cut to the tunnel size and the whole boat is fiberglassed inside and outside.
See pictures of the assembly method here.
In this boat, we deviate from our usual plywood frames. The frames are made of plain 1x3 boards.

Required Skills:
The 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.

Options:
The plans give complete layout freedom to the builder: we show a simple hull but there is plenty of room for baitwells, fish boxes and storage.
The stern transom can take any jack plate and the bow transom can be cut lower if it interferes with the trolling motor bracket.

The plans include a simple removable poling platform drawing with dimensions.

Bill Of Materials:
(Excerpts from our BOM)
The 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:

Plywood 4x8' (122x244cm)
3/8" (9mm) 14  
1/2" (13mm) 1  
Framing wood
1x3 boards, linear ' 81  
Fiberglass (totals)
Biaxial tape 250 yards 228.6 m
Woven tape 77 yards 69 m
Biaxial fabric 35 yards 32 m
Resin
Epoxy, total 15 gallons 56.8 liters

Labor:
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 simple.

More:
Visit our message board, help pages, tutorial pages and read our FAQ: most questions are answered there.

Plans Packing List:

8 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. Construction drawings showing assembly with notches, parts numbers for most of the hardware such as hatches, fuel fill, inspection plates etc.

Drawings list:

  • B233_1 Plan and Profile, Specifications
  • B233_2 Plywood nesting for all parts
  • E233_3 Construction drawing with plan and profile sections. Typical fiberglass lamination detail. Frames and bulkheads dimensions. Detailed notes.
  • D233_4 Developed dimensions for all hull panels (flat), seat tops, stringer, and outboard clamping board.
  • B233_5 Detailed drawing for the construction of the tunnel.
  • A233_6 Detailed drawing for the construction of the Polling Platform.
  • B225_c Detailed drawing for seat locker lid with framing and assembly view.
  • B221 Typical Small Boat Electrical
  • Specific building notes for this boat
  • Bill Of Materials
  • Help files reference list and more.

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Fort Pierce, Florida USA