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An original design by Jeff Gilbert jgilbert@dynamite.com.au

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My Friend Jeff Gilbert drew "Ketchup" for the Practical Boat Owner Amateur Design Competition.   This is a serious, well thought out boat design with a number of interesting features.  Look over the documentation, and if you decide to build, I think Jeff would let plans for the prototype go pretty reasonably.  See also Jeff's Design Philosophy






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Ketchup is a 23 ft-7.0m trailerable Catamaran/Tunnel-hull, suitable for home-building from readily available plantation plywood. She offers a practical blend of performance, stability and accommodation for a small group or family. While attractive, fast & fun, this boat has been designed primarily to be simple to build. To this end there is maximal use of right angles with minimal cutting & no torturing of ply.

Ketchup has been designed for Australian conditions. As these include cruising everything from placid inland lakes to fighting the sudden squalls and steep-pitched waves of the South-East Coast, she would be an able coastal cruiser anywhere in the world. Ketchup is not intended for major Ocean crossings, but because of her speed potentiaI she would be happy Island-hopping in favourable conditions.


Ketchup is an asymmetric catamaran, maximizing the keel to keel "track" while staying within the 2.50m (8' 2.5") legal trailerable limit. The 8' track is the same as that of a conventional cat 11' wide. The outside of the hulls are vertical and a straight line from bow to stern, meaning that sections are fitted into a right angle between the hull and the bridgedeck. The 10 station frames start with a quarter circle at the stern(stn 10), with the radius of curvature increasing exponentially forward until by the front of the bridgedeck(stn 3) the arc has become a straight line at 45deg.(see Drawing 04, 'HULL SECTIONS') This angle is held through to the bow. This gives her a fine entry with load carrying ability increasing aft. All arcs are 4' - the width of a plysheet. This shape also builds in 3.5" of rocker, though this isn't visible in profile because the side/keel piece is left protruding to its full 4 ft width. This provides a mini keel running 16 feet from 2" depth @ station #3 to 5.5" @ the stern(#10), giving a little more bite to windward without even using the daggerboards. It should be remembered that the straight outer hulls also resist lee drift a lot more than conventional hull shapes, a feature employed to great effect in the early Pacific Proas.

Frames start at width (&height) 30.5" at the transom (station10 @ 23ft, bow=datum) and increase to 34" at station 3. Increments are half an inch/station.

Station spacing is 28inches with 2ft left between 9 & 10.( A supplementary station should be added @ 1' 0", between datum & station 1),Hull width is increased 32mm to account for the keel piece, which is made of laminated ply or hardwood, to form an on-edge plank 200mm wide. It is shaped right up to the bow, protruding an inch or so beyond to allow for beveling off. Below the waterline it extends half an inch below the sides..it is not shaped but has a 50mm aluminum or brass strip screwed to it to take the bottom for grounding, beaching, transport etc. (Note that the fact that the rocker is hidden within the boats sides means that it will sit firmly and level along its two 16ft side/keels, and means it can be transported on a flatbed trailer, truck, railway wagon or ship's deck. It would even be possible to build a detachable axle to these keels making the boat its own temporary trailer.

Battens are run along the inside surface of the keel plank & the underside of b'deck to hold the inside sheathing ply curve while epoxy sets. The deck is adequately supported as the tunnel "ceiling" is 2' 6'' Max width. Joins are strengthened with strips of glass before a final complete layer of glass, epoxy then paint is applied across the sides, bottom/tunnel & outboard pod. Painting should include Awlgrip or equivalent on the tunnel "ceiling".


Requires no strongbacks or jigs, but simply a flat area, a few sawhorses and the sort of tools most households already have. Necessary tools are a power hand drill, jigsaw, sander, power saw & a dozen or more clamps. In harsh climates much of the boat can be cut out and finished in a spare room, then outdoor assembled in a few days. The heart of the boat is the 16' x 8' Bridgedeck made from 8 standard 9mm sheets of ply in two layers (one laid athwartships, one bow to stern) edge glued with longitudinal stringers under and crossways stringers on the ends of the deck. Ply bulkheads(12mm) are then stapled & epoxied to the underside of the b'deck -some are missed out or cut out depending on the accom plan decided upon,. Those staying are epoxy-filleted in.

Each hull side takes 3 standard 9mm sheets, with 150mm scarf joins reducing the total length to the desired 23ft. Keel pieces (32mm Hardwood or Layers of ply for taking the bottom) are fitted to the inside lower edge of each flat outer side panel before attaching to the bridgedeck & bulkheads. Where the frames are removed for berth/stowage purposes, the floors put in place adequately compensate, especially as the entire hull structure is made from ply 50% thicker than that used in many similarly sized production cats.

Sheathing is simple as the only cutting required is in the front sheet. Before bending this first sheet out to meet the sides, fit the front plywood "box beam" to prevent bending the keel outwards and giving your cat a disastrous splay-footed look! This beam provides emergency flotation (about 160lbs!), supports the front positioning block/clamp which takes the removable wood or alloytube bowsprit/prodder, and lastly supports the front net. This mesh is a great place to get a cooling splash on hot days, and makes a comfy hammock & even bed unless there be ferocious mosquitoes! The twin foredecks have generous overhangs to prevent pitch-poling...these are drilled as fixing points to lash on the net.

Fitting the inside-of-hull ply cladding should, if possible, be left until the cuddy accommodation plan has been decided upon, as bunk floors & storage areas need to be installed, involving alteration/removal of some bulkheads.

Also flotation bags & tankage are now installed, & if required, access holes in b'deck positioned and cut, head partition installed etc. etc. All this can be done after turning the hull, its just more difficult! It may be covenant to build the boat over two winters...in this case all these decisions could be left & the boat sealed up & sailed as a large (and fast!) beach cat for the intervening summer season.

Flotation bags are air-filled 4 litre wine casks in all flotation cells. Even if a compartment is holed it will take little water...to prevent the casks popping out of a holed hull section they are encased in old fishing net...see your local trawler fleet!


Cuddy: Best use of this is 2 permanent berths, a fold-up eat/cook/navigation table with seating for 3, and a dedicated head with separate entrance & ventilation. The cook sits on the boat centerline at b'deck level, with feet in the port hull under the table & probably resting comfortably on the auxiliary quarter berth which extends under the cockpit! (This berth is only available if the table is folded up against the side window). The cook and/or diner(s) use the head wall as a padded backrest. Most cooking will be done by placing the camp stove(metho 2-burner -no gas bottle hassle) on the "cook box", under the boom tent. But under way one needs to cook out of the weather and spray. A large flat-bottomed stainless steel bowl is the sink - sea water can be heated for wash-up/showers directly over the stove. A SS bucket is used for retrieving sea water.Flexible fresh water tank is in the bilge below the Q-berth....hand pump mounted on the rear cuddy wall.

The Navigation table is a flat oblong box with a hinged lid which can be temporarily set at any angle. It can be used anywhere on the boat. Within are charts, drawing gear, torch, GPS, ship's log,. It slides into a rack on the inside of the rear cuddy wall. From this position the map taped to its top is flush against the porthole in the rear cuddy wall, & thus the helmsman can refer to it from the cockpit.

Windows indicated in drawings are in fact tinted perspex covers across two non-opening portholes each side. This ensures that there are no leaks. The portholes siting and size are optional - the important thing is to preserve as much "meat" as possible in the cabin sides to retain the integrity of the monocoque structure. If ventilation thru hatch/doorway proves in adequate a rear-facing "Dorade" vent can be installed in the foredeck of either or both hulls.

Berths: Two arrangements possible:

(1) For max sitting headroom, two single berths are placed forward in the hulls, with their foot extending under the foredeck.

(2) For couples a double berth across the beam is more suitable..this is structurally stronger than (1) , and frees up massive storage forr'd in the hulls, but at the cost of headroom above the bunk, which must be at bridgedeck level. This berth forms an excellent 3rd seat forward of the table. Headroom problem can be alleviated a little by a stargazer hatch above the bed-head, though foredeck access is best achieved via the foot-wide side decks.

Heads: 3' x 3' 6" Shower has nearly 5' HR -the head is forrd to give max HR over the shower. Shower choices are to sit on a low bench or use a hatch swinging up with a rose attached under its highest point. This connects to a hand pump on the wall via hose, another hose from the pump is plunged into the water bucket sitting on the floor or toilet, wherever it fits. Boiling a jug on the stove then mixing with sea water is enough to give a nice hot shower. A camp shower (canvas water bag + rose) would also work well from under this hatch (also from the boarding step, hanging the camp shower from the wing)

Head/shower floor is high enough above the sea to self-drain via a bung.

There is room for a toilet holding tank low in the hulls. Porta potty/bucket & chuckit are options depending on local laws. (A simple hole in the bridgedeck level is tempting as activity is hidden between the hulls! )


General: The cockpit is 8' x 6' 3", and is great for sleeping, eating and general socializing. Rear of the cockpit a bench seating 4 extends the width of the yacht. A large hinged table splits the seating; under this is the nacelle holding the elec. outboard. Over the table is the tiller, controlling a large kick-up rudder. This is fixed to the 12mm ply seatback-transom. The port seat back is free from the other seats, and hinged at deck level. When the seat-back panel is unclipped & swung back thru 180 deg. the underseat becomes an access step between the bridgedeck & the rear boarding step. Heaps of storage is available in the starboard hull(& port if Q-berth not used). below the cockpit...hinged "lids" can be 45deg. bevel-cut as access. There would even be room to slide a slightly deflated RIB in from a door in the transom-cum-seatback, launching it off the rear boarding step.(NB -an 8ft RIB could be carried from the wing). It would also be possible to hinge the entire backrest & drop it into the water as a boarding ramp.

Boom tent: This waterproof canvas tent is stored in the huge locker accessed by lifting the hinged front half of the rear wing top. The tent needs no special poles. It extends over the boom (raised 18") & lowered mainsail, and is laced each side of the boat thru a row of small holes in the daggerboard which form the boom tent-cum-cabin walls. Either end the tent is cut to slope into the cuddy roof and rear wing...it can be fixed to a row of domes along the back of the wing, meaning the storage box remains indoors & useful. Windows are provided by the daggerboard handles! (see Dwg # 01). In certain warm climes it may behoove one to frantically deploy the mossie net before and underneath the tent to avoid the crew being eaten. This alone (quite apart from all your eating irons, tools & spare chain winding up on the seabed) is a sound reason to forget slatted decks.

Side-decks : These are 12mm ply, a full foot wide ,& may be lowered in two sections when sailing. They are supported by chains from the cabin-top & wing. When stored for towing they fold up against the boat side below the window (front section) and become a cockpit coaming (rear section). If racing the rear section may be used for hiking out on a trapeze. Both are coated with grip-paint (as are all bridgedeck/cabintop surfaces) Side decks are further supported by hinged triangular braces which flip out & lock from flush with the outer hull sides. When side-decks are in place a 3-ft by 1in wide leather-lined slot cut out of the inner side-deck edge guides & constrains the......

Daggerboards: these slide down flush against the hull sides...the other fixing point is just below the waterline - a wooden rung which will serve as an anchor point for inflatables/dinghies/visitors, and is handy for swimmers . The front rung support is streamlined, while the rear support is built light so the daggerboard will blast out the back without compromising the hull should the board strike a submerged object (such as the bottom!). Note that this is made possible by the side-deck slot being a foot over-width - this last foot can be filled with lightly glued-in polystyrene foam or other crushable material. Boards have a top lip (outer side only) of ply to prevent them falling through.

At anchor the boards are used as solid boom-tent sides. They lock into position exactly above the raised side-decks and between the angled sections of cuddy sides & wing side support (see dwg # 01). Holes in board sides are for lacing tent roof. The boards are fashioned from epoxy-saturated 18mm ply, tapered to leading and trailing edges, but shaped on the outer face only. A cut-out serves as both daggerboard handle and tent side window in its dual roles.


  • Rigged as Ketch-Cutter. (But could also be classed as a Schooner)
  • Foremast 21' with
    • 100 ft2 fully battened square-topped loose-footed Foresail, with see-thru panel.
    • 37 ft2 Jib/staysail on three-quarter forestay with bridle to bows and furler above this.
    • 107 ft2 Masthead semi-Genoa (extends 1ft behind mast) on furler. Attached to demountable 11ft prodder clamped into seats on front beam & foredeck forr'd of mast. This sail is not necessary, but it or an asymmetric spinnaker would be great in light airs.
    • For downwind racing the 2 jibs can be replaced by an asymmetric spinnaker of 285 ft2 (490ft2 tot)
    • Full spinnaker seems futile as the two mains running wing & wing downwind would rob it.
    • Twin backstays clip to chainplates ss-bolted thru outer hull sides with 12mm ply backboards.
  • Mainmast 23' (constant diameter) carries
    • 105 ft2 fully battened square-topped main with 8' boom doubling as tent ridge-pole.
    • Twin backstays run to eyes bolted thru the bridgedeck (again mountings strengthened by ply backing plates).
  • All stays on the boat have spring-loaded clip-ons, and screw adjusters are replaced by tensioned springs.
  • Both masts are aluminum sections with sail -track and halyard pulley built in below welded-on mast-head containers each housing a standard automotive airbag pack. (See below)

Masts are stepped in tabernacles comprising a 2ft alloy pipe with diagonal braces to deck plus an upper leather-lined collar clamp braced to the cabin roof. Beneath the pipe an equivalent diam.hole is cut in the bridgedeck...a hardwood block or alloy pin prevents the mast falling thru the deck (more on this below)


Anti-Capsize Features: The boat is unlikely to go past 90 deg as when the mastheads strike the water they set off masthead air-bags (pinched from automobile technology). The bags are packed into a special aluminum masthead containers welded above and independent of the halyard pulley. They are identical to the car version except they remain inflated once triggered. Two low aspect mainsails instead of one (high-aspect in most cats) produces a lower Centre of Effort (Encircled "x" in Sail Plan -Dwg # 01) 260lbs of batteries low in each hull will be extremely effective ballast. These would be removable for racing.

Recovery: Beneath the tube holding the mainmast foot is a hole in the bridgedeck large enough to push the mast thru. Standing on the deck side (1ft wide), (prob up to your knees in water), drop the main & loosen the mast clamps. Pass the main right thru the boat till the air-bag container stops you. Tighten the tabernacle clamp just below the air-bag housing (large wing nuts with welded on extensions) and get the crew swinging on the wrong-way mast plus dagger boards. As a last resort you could tie a few batteries on the end of the main, but they are pretty helpful where they are. Dropping all other sails would help. The foremast could be freed to float about on its airbag at the end of a line. The really serious type could fix pipes into the angled sections of the cabin side and wing for someone to swing on! The spar from the cabin would make a great hand-hold while leaping up & down on the slippery daggerboard.

Storms: If caught out and in trouble with windage in the bare rig the main mast could be dropped through the cockpit deck to act as a bizarre 18-foot keel.


Single large spade rudder mounted on the transom-cum-seat back. Kicks up for beaching. SOL-Z electric outboard acts as an emergency rudder(it is designed for this) Also rudder control may be linked to the motor. Anchor and drogue can be carried one each hull in the compartment between stations 1 & 2. There is adequate room for chains & warps. Deployment is via a pulley in the centre of the triangular box forebeam. No winches are needed as no sail is over 10 sq. meters. The usual 2:1 or 4:1 sheeting as used on beach cats is adequate. All sail lines are led back thru blocks to the centre rear of the cuddy roof. Rope ends disappear into a large canvas bag between the doors.The mainsheet is left aft with the helmsman, who can cleat it off or use it for more effective sailing. The main is also controlled by a vang plus an 8' radius curved traveler to the rear of the flat top surface of the wing.


The SOL-Z 8hp electric outboard was chosen for:

  • Efficiency (as much thrust as a 15hp conventional internal combustion unit)
  • Reliability
  • Running cost. Twenty percent of diesel inboard or petrol outboard.
  • Environmental reasons (zero emissions + very quiet -this has a huge effect on fish/bird sightings).
  • Size, shape, design. It is small (easily built into a pod) and light (28k), Best of all the long shaft is blade shaped it not only steers well it would make a fine back-up rudder.
  • Practicality. Most marinas have shore power.
    • Also batteries can be charged by wind, solar power (try cells on cuddy& wing) or towing a prop unit (I've ruled out petrol generators as they defeat the Principle!)

SOL-Z make even more compact inboard/saildrive units but they are unsuitable for beaching.
The SOL-Z 8hp electric outboard was NOT chosen for the weight of its power source - 4 Lifeline 4D batteries in sealed plastic boxes. However these are small enough to mount low in the hulls (2 in each) giving 260lbs ballast in each hull. These don't need much attention but would be accessible from beneath the bunks. Further, a lot of weight in this yacht would be carried aft. This is countered in part by the hull sections, but final trim for unusual circumstances can be achieved by mounting the batteries in a long wooden trough with a clamp each end, and sliding them fore & aft to fine-tune the trim. Setting up for a weekend cruise, this would be worth the few minutes it takes.

The SOL-Z was NOT chosen for its price! Landed in the USA it is $3900 plus $840 for batteries(see below), however this does beat $7000 for an equivalent Diesel. Cheaper electric motors are available, but the SOL-Z has the right combination of reliability & size for this design. It is manufactured in Switzerland, so in Europe it should be cheaper than the above quote.


items include foredeck railings, liferings in cockpit and there is plenty of space to carry a liferaft in the cockpit, on the foredeck, on cuddy roof, or in the starboard hull.

To prevent pitchpoling we have high bows, devoid of storage and sealed off as dedicated buoyancy areas. The 45 degree hull line plus the 3" overhang of the deck further discourage plunging. The underside of the forebeam is also angled as the bows to force its way upwards. The 3/4 staysail provides plenty of lift.

The 4 batteries for the SOL-Z , radio, stereo, Nav lights etc. are seen as a danger. I've specified Absorbed Glass Matting batteries by Lifeline -these are guaranteed to have the following qualities: sealed, nonspillable, install in any position, submersible without damage, maintenance free, 2xnormal life, fast recharge with no voltage limit, deep cycle, 3% per month unattended discharge, shock & vibration resistant.

For storms a set of reefing tapes is sewn on each main to drop them 40%. The flexibility of the sail plan is of great help in changeable conditions.


LOA .27'
LOD .23' 
LWL .21' 
BEAM 8' 2" 
WORKING SA 349 ft2
LIGHTSHIP 1100 lbs 
RACING 1500 lbs (2 crew) 
DESIGN .2220 lbs (2 crew, batteries &ob -weekender setup...b'deck 23" clear of water) 
MAJOR CRUISE 3180 lbs (kitchen sink etc -drops the bridgedeck 3" to 20" clearance ) 
IMMERSION 310 lbs/inch (increasing each inch) 



Sail Area:Displacement 33 / 27 (Design WL / Cruise) 
Disp:WLL 107 /153
Expected Speed (15k wind) 9 kts
Expected Speed (SOL-Z) 10 kts cruise, 15 max



PS: -a final question:

Q: The Design Brief stresses simplicity & ease of handling. Yet Ketchup has up to 4 sails?


  1. The sail plan divides the sail area into manageable chunks which need no winches; masts are easy to rig.
  2. The centre of effort is low, but more importantly almost identical for the entire plan & the forward main alone, thus...
  3. The beginner can sail with foremast alone, the boat will sail the same as with all 4 sails (just more slowly!)
  4. In sudden changes it is easier to drop a sail than to struggle losing an equivalent area reefing.
  5. A sudden gybe, or badly torn sail is no longer a disaster!

On the other hand one will never experience the thrill of dropping the only winch handle overboard!

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