KD 650

By Bernd Kohler - The Netherlands

Listening to my customers, as I always do, I sensed a demand for a small trailer, coastal cruising catamaran.

Looking my files over I stumbled over some photos from our old “Butterfly”. I designed this boat 1983. Construction toke us three mounts spare time. We have had a lot of good sailing fun with this boat. We used it also for holiday cruises along the Dutch coast. Here some pictures of the boat.


My wife at the helm

click images for larger views

And in the port

As can be seen on this picture the boat sports already a biplane rig with a bent gaff. The boat was very fast and stiff. I do not remember the weight of the boat anymore but it was light. When we where lying on a beach at low tide and wanted to stay a bit longer I was lifting the boat under the fore beam and tracked it a bit higher on the beach.

Some Technical Data

Length overall - 6,5m
Beam center line to center line - 3,0m
Draft - 0,25m
Draft dagger boards - 1,4m
Sail area - 2x 12 m2
Outboard engine - Yanmar 6 HP

This boat was my test bed for new ideas like the biplane rig, sails sleeved over the mast. The sleeve could be opened with a zipper for easy of removal. The gaff was made from carbon fiber. An absolute novelty for this time. As can be seen on the picture the dagger boards where on the inside of the hulls. For a perfect trim the could candle about 12 ° forward and backward. This was my first boat with an trapezoid cross section. The hatches could slide and turned up.

Starboard hull looking aft towards the “pantry”. Watch the water container at the right. A nifty construction. The could be folded when empty. We have had provisions to hang them up and have had“running water”. But I never found them again.

Starboard hull looking forward. The folding table and the mast box at the right can be seen

I was thinking about this boat as I got the ideas for the KD 650. It is a trailerable, beach, costal cruiser racer. It is a mouth full ;-) But this was the idea.

The KD 650


LOA hulls - 6.50 m without rudders
Beam c/c - 3.00 m
Beam catamaran - 3.50 m
Beam on trailer - 2.00m
Draft hull/rudders - 0.33/0.54 m
Weight empty - 340 kg
Weight CWL - 760 kg
Sail area - 18.6m2
L/B hulls - 1 : 16.6
Prismatic coefficient - 0.568
L/B ratio center line - 50 %
Construction time about - 500 hours
Engine, outboard - 4 HP

The main features of the boat:

  • The hulls have an asymmetrical hull shape with a 1/3 , 2/3 asymmetry.
  • Construction is plywood/glass cloth/Epoxy composite.
  • Because of the asymmetrical hulls no dagger boards or keels are necessary.
  • The rudders are dagger board rudders with automatic kick-up system when an obstacle is hit.
  • A hull weights about 90 kilograms ( 198 lbs). Good to handle under construction and when setting the boat up.
The seat position on the aft decks and the feet resting on the trampoline will make long sailing trips a lot more comfortable.

The KD 650 - top view and lay out

The top view shows a possible lay out. The “Butterfly” was 84 cm wide, the KD 650 is 92 cm wide. This 8 cm makes a big difference. It is now possible to have a L-shaped seating arrangement. See next picture . The port hull sleeps 2 and has a toilet under one of the bunks. The starboard hull has a seating area with a folding table and a small pantry with cooker and sink. The seat area can be converted to an other bunk. By the way, the pantry is an option, I personal prefer to cook on deck. The wet lockers can be used for ropes, the anchor and other deck gear not in use. Under the bunks are huge storage areas. The compartments under the wet locker and the aft bunk under the wet locker and the aft bunk respectable the lower area under behind the pantry make the boat unsinkable.

Inside starboard hull

The hulls are set up upside down. The boat is built on 5 bulkheads, including the transom. I stick to the normal building system on stringers. The reason is simple that a first time boat builder will be secure in this way to have enough references to make fewer mistakes compared to other systems so he will end up with two similar hulls. The drawings are all CAD drawings scale 1 : 1.

The costs for the materials for plywood boat are in general reasonable. A commercial rig can cost as much as the whole boat again. So I designed a Gunter gaff rig people can build by themselves. An impressive example is this very old “Sonderklasse” boat with it's beautiful Gunter rig.

German J class boat with Gunter rig

Through a different approach the main sail is set with one halyard only. The gaff is connected with two batten sliders to the mast. As can be seen in the picture the upper sail part has a sleeve like a wind surf sail.

The mast is free rotating. No boom, less costs and more safe. No mast will stick out from the trailer. No back breaking maneuver to set the 4.3m (16 ft ) long mast. This sail will be as efficient as a very good Bermuda rig. Through the direct connection of the gaff to the mast it will also point as high as any other modern rig.

The beams are standard aluminum tubes. Besides cutting them to length and trilling two holes nothing to be done. Fastening the beams will be quick with the Eccentric fastening/release system on the inside of the hulls and four nuts at the outside. More time will be spent fastening the trampoline than anything else.

The boat has a maximum dynamic stability of 1300 kgm. Even at a wind speed of 25 knots the rest stability (recovery stability) at 50° is still 500 kgm. Calculated with a reserve of 90 % ! Our speed calculation shows that the boat has a good speed potential. The boat will have a speed of 17 knots at a wind speed of 21 knots ( this is the wind speed a hull starts to lift of the water surface) and a speed of 12 knots with a wind speed of 12 knots. Not bad for such a small boat.

Best regards
Bernd Kohler

The web page with more information about this boat can be found at:

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