Home | Articles | Links | Projects | Columns | Designs | Boat Index

by Jeff Gilbert  jgilbert@dynamite.com.au

jeff.jpg (3473 bytes)

Jeff invites comments thru the Duckworks forum on any aspect of this design, and in particular the unorthodox hull build method suggested. 

See these other Jeff Gilbert designs:
Ketchup                Scarper Flo               Ebben Flo

line.gif (878 bytes)


Gumboots is a 28ft Offshore capable Cruising Catamaran.  Intended as my retirement boat, its my happiest design yet for its sheer simplicity in all departments.  Construction is by introducing a planing triangular dory bottom into a simple Veed catamaran hull. Slamming is minimised by only one ninth of the bottom area being in the forward third of the hull, plus a high bridgedeck clearance.  The sheerline is generated by the factory ply cut combined with slightly concave forward & convex rear sections, with the rectangular transom just clear of the waterline. A plumb bow maximises WLL.  A possible pod for the bridgedeck is drawn but not recommended: for ocean crossings Id forget it and decorate the bridgedeck with one of my 'Squeezebox" seven foot sailable ply liferafts!  I hope you enjoy looking over this design,

Yours Sincerely
Jeff Gilbert

Jeff Gilbert Design #C105 C105 Doc01
28ft Offshore Catamaran

A uniquely simple sheathed ply design for garage builders.

This boat resolves a standard planing hull form into a very simple structure, built traditional dory style without frames, stringers, strongbacks or moulds. The build uses standard sheet ply sizes with few cuts and hence minimal lofting, all basic.  A traditional dory build is shown in Woodenboat Magazine #145 p76.

This write-up refers to the basic Hulls-only structure: -a person of average stature can complete all work from the shed floor. Adding a demountable pod to the bridgedeck introduces clambering , vastly increasing the build time.

The design is a way to obtain a capable offshore cruiser within the boundaries of a wage-earning life. "Capable" means this catamaran can make shorthanded passages in safety and reasonable comfort without the crew returning to work a week late.

Reaching_small.gif (9891 bytes)

Hull Study
Pod Study

Constraints met by design
1/ buildable by one person in a year part-time.
2/ buildable in garage or rentable area with flat floor.
3/ cost bearable as you build by disposable income from award-level wage.
4/ buildable with basic tools, skill and confidence levels.


Initial Compromises...

Minimalisation in all design decisions. (ref Bob Oram)

No expensive spinnakers, gennakers, prodders.(ref "Red Jacket",Hobart 99)

Build without the demountable pod, which is a social area only
- bow -indoor social dining with panoramic view (sitting HR)
- stern -outdoor -fishing/dinghy launching area

Ppassage making is safer without it anyway, dispersing water through the boat between hulls. (see Wharram)

sitting HR only, but with full leg-room. (Across-hull seating is possible but cant stretch out)

Use a second hand rig.

Heavy CL plank crossing beams enables rig position tuning.


Other design notes/specs:

Not trailerable but demountable. Will fit trailer limits by reversing & overturning one hull. Difficult (350 kg/hull, curved decks -need lowloader).

Waterproof hatches and portholes only. No companionway = dry interiors.

Daggerboards 8’x 2’ 25mm shaped Marine ply held flat against inside face of hulls at CLR by rungs and sacrificial blocks.

Fast run-off Bridgedeck(slats, net or tramp).

Cockpits 2' deep with 1.5-inch drains and slope (slight) to assist.

Main hull layout flexible, see attached schedule.

Cockpit bulkheads slope up to rear doubling as saloon/cockpit seat backs.

Note stern utility spaces -head, double berth, raft, anything bulky & light.

9.9HP OBs were intended auxiliaries. If cost permitted a 10hp electric inboard in one or both hulls would suit, retaining a small OB for a RIB. The payload covers six batteries which, apart from usual uses, would be handy for trimming if adjustably mounted in a low angle iron rack under the berths.


Ply: Sides, bulkheads 12mm, decks 2x6mm, furniture 6-12mm.

Dory hull bottom-soles, daggerboards 25mm Hardwood or marine ply.

Total ply is 1 cubic metre or 640kg.

Dry weight one ton without crew, cruising two.

Dimensions (ft) 28/28 x 5/18 x 0.8 @ 2tons.

HR max 53 in.

Suggested fractional rig with 100% FT on 30ft woodwing rotating mast with
swept back shrouds to outer of hulls and forestay to front beam. Roachy fully battened main. Total 500 sq ft.


Build Method

Cut and assemble main 4 bulkheads and transom from 12mm ply and 50 x 12mm HW. These are flat bottom. Sides 25mm flare in (no 1), straight (#2), curve out 25mm(3,4) and the transom is a rectangle 3ft wide and one 9.9HP long-shaft OB leg high. Deck profiles are circle sections curved on various radii very close to 3ft 6in (see station dwgs) and easily lofted with pencil and string. Highest is 15ins above sheer @ Saloon/Cockpit interface (NB no companionway to "Galley" Saloon, maybe to "Nav" Saloon. This is not a design you want full of water, as slatted floor is the only bilge in saloons.

Scarf and cut 25mm bottoms 28ft long triangle, stern 3ft triangle base. Plane edges from vertical stem (12mm wide tip) and stern using bulkhead angles.

Scarf 12mm sides, cut just over 28 ft long. No other cuts. Tack temporary chine logs on sides to position rocker, which is flat under saloon rising 10in. to bow/stern.

Clamp and glue 75x12 stem-piece between sides leaving a 28ft long 4ft high plywood Vee shape (viewed from above) self-standing upside down on the floor.

Fit the bottom into this, positioning down onto temporary chine logs, and minimally tack sides to bottom planed edges (2x28ft of join).

Fit transom. Remember we are building sailing way-up.

Fit bulkheads, checking that this will true up any twist. If not brace the twist out. Here high quality ply will pay for itself.

When satisfied with true run thin epoxy into all joins. Repeat when dry.

Fit permanent chine logs between bulkheads.

Measure cut and fit extra bulkheads 9inches from 1,2 and 4. Fix by sandwiching 9 inches of straight (Xbeam) lumber running sheer to sheer with the fixed companion bulkhead. You now have your 3 crossbeam slots.

Fit beam-lashing cleats (with backing plates), even if you think you are going to bolt crossbeams (see below). You’ll use them (tie up dinghy, tie up boat, raft up, tow log, jury rig, tie down/craning point for demounting.

Fit furniture - seats, tanks, battery rack under berth, & berth base(38 x 96in). Put in cockpit floor/seats with slope to boat CL and 40mm drain..

Roll over hull with ceiling beam block and tackle. NB max girth < 16ft.

Trim excess sides to bottom , sand and fair, glass bottom & sides, paint, roll back. Glass carefully around cleats to strengthen. Glass cockpits.

Deck it all (except 5 x 5ft cockpit & the 9in crossbeam slots) Stringers, 2 layers of 6mm ply will take the curve. Crossbeams will break up your eye if deck is less than perfect and you can even throw extra height in the saloon, which is isolated visually at deck level (between beam & cockpit).

Build crossbeams. Note their size is a bit arbitrary, the main one is 18 in deep and you may want it 12in wide instead of nine.

Carefully glass beam slots, it’s a rot point. Glue hard rubber mats in bottom.

Crossbeams can be bolted through hull bulkheads with one big SS bolt (flex) or 2 smaller (rigid). Use ply washers to spread the load, lots of epoxy, and wall the holes with SS or plastic pipe. Reckon this will leak slightly. If you cant live with that don’t do it, lash the beams (semi-rigid) to either side of each hull using multiple loops of 6mm high quality nylon round cleats mentioned above.

Bolt 24ft long 12"x 1" plank in position up centreline. This is for tramp fixing, walking forr’d, and is your mast-step, enabling you to rig the boat any way within reason including 2 masts. The mast-step plank provides the flexibility to use a much cheaper 2nd hand rig. Fitting this plank will have to be incorporated in design as the crossbeams vary in depth, the main one tall enoghto provide some safety and shelter for the bridgedeck.



Cruising/Racing: Bruce # 1.36/1.66 SA/D 29/43 D/WLL 91/51

500sqft or 10 BHP o/b indicates 8kts @ cruising disp 2 tons (Race = 2500lb)

Calculations indicate an absolute maximum hull speed of 16 knots.

A stripped racing version souped up to 600sq ft of sail and weighing 1 ton indicates a base speed of 13kts, at 2600 lbs 12 knots.


Intended changes from current plan…

Lengthen the six-foot long saloons to seven.

The 6-footers have 2 seats with 17"sq base pads that 6'6" people can sit side by side on with their backs straight and fully supported.   Replacing the companionway and lengthening the room a foot enables you to repeat the arrangement giving 2 pair of opposing seats. Seats can be fitted to sit crossways if facing fore and aft causes back discomfort, a problem in monohulls. However whilst the hull side is a good backrest, you cant stretch your legs out.  A 3ft minimal galley unit on one wall (cooker, sink, chopping board) has a fold up table in front, a fold down seat opposite for the cook who faces outboard.  There is always one seat free for stepping down or looking out from the overhead inboard hatch.  In the other hull you can have a similar setup, Id be inclined to make it a master stateroom including the instruments & large desk/navigation table.

To gain the additional foot of length in the stateroom(s) you can

1/ move the beam/ bed forward (this narrows berth 1.3 inches unless one increases the main b/head beam, which also widens seats.)

2/ move the cockpit back

3/ lengthen the boat to 29ft without needing more ply for sides.

4/ Slope the rear cabin bilkhead to the rear at the top. This creates both a backrest angle for the cross-seating in the saloon, and a foot-space so one can stand up to the windscreen in the cockpit-in effect it gives an extra 6 inches in both cockpit and saloon.

Option 1 appeals as it is best structurally, plus simplest.
Option 4 would also definitely be adopted by the author.


Pod Option

Dimension OA 9x5 Min, could be bigger (widen boat) Rec only, sailing from hull cockpit.  Front under-cover half -roomy table/seats for 6 with panoramic view.  Rear half -for fishing & launching tender it’s for coastal stuff - id lose it for Ocean Crossing.



Design C105 is a good result against Dick Newick’s well known

Designer’s aim of comfort & high performance at low cost.

It should be remembered in the assessment of intangibles that the

folk this design are intended for are satisfied with the basics of food,

warmth and shelter rather than a glut of fashionable rubbish.

Though comfort is relative, performance and low cost are absolute

and this boat delivers both, especially with labour evaluated as a cost.


Jeff Gilbert Design

July, 2000

line.gif (878 bytes)

Hullstdy2.gif (24903 bytes)

line.gif (878 bytes)

podstudy2.gif (40090 bytes)

line.gif (878 bytes)


Home | Articles | Links | Projects | Columns | Designs | Boat Index