Tuesday 5 May 2009

The new KD 650 trailer catamaran

This is Bernd Kholers new design,its about to be built by him as well he says,its ideal for towing due to its narrow beam and light weight,some words on the design from Bernd,kits will be available to your order.

General description

The KD 650 is an other approach for an affordable trailer catamaran for amateur construction. The boat is designed for fast coastal sailing.
One of his main features is the aerodynamically efficient Gunter rig.
Every effort was made to end up with a practical, cost efficient and fun to sail boat.
It starts with his low overall weight of only 340kg. The width of only 2m ( 6.6ft ) ensures easy trailing. Only a medium size car is necessary. No mast is sticking out when the boat is on the trailer.
Setting up will take only about one hour instead of days as with many trailer catamarans. A hull weights around 90 kilograms ( 198 lbs). Setting the boat up is quickly done with the all new Eccenter fastening systems for the beams. Only four nuts have to be used.
Setting the short mast ( 4,3m long, 14,3 ft ) is easy done. The mast weights only 9 kg ( 19,8 lbs ). So no back breaking action here.
and the sail can be set. The re-designed Gunter rig gaff has only one halyard in contrast to a normal gaff rig which has 2. So setting the sail is comparable to a usual sail. The jib can be a roller reefing jib.
The boat is a perfect coastal sailing boat with more comfort as the usual beach cat. For instance you do not seat on the trampoline but on the aft deck while your feet are resting on the «cockpit sole» (trampoline).
This makes a big difference when you sail a longer stretch.
In one of the hulls is room for a toilet. Your female company will appreciate that (and you self to). In the cabin the height above the berths is 920mm. This berths can be used as seats over the day. If you like to have a small pantry with cooker and sink in the starboard hull for instance, there is still room for three to sleep. Personal I prefer to cook on deck. So no drawings for the pantry in the drawings.


Like my other boats the KD 650 is a plywood/glass/Epoxy construction. In most countries these materials are easy to obtain. I have included the list of materials at the end of this study plan. With these the costs for the materials can be easy obtained. I was going even so far as to use for all stringers and framing only one size of wood. This makes it easier for your supplier, which can safe you in some cases money. The extra weight is negligible.

To avoid the high costs for the rig i.e. mast and rigging the boat sports a modified Gunter rig. The plan shows how to build the mast from an Aluminum tube and some hardware. The gaff is made from clear fir, UD- carbon and glass cloth. When the sail is set the height of the sail is 7,2m. The sail uses a heavy batten instead of a boom. Much saver to sail.

The beams are made from ready available Aluminum tubes. Besides cutting them to length and drilling some holes nothing to do here. This will shorten the construction time
compared to boats where wooden beams have to be build ( in some cases till 40 % ! ).
The set up time once you at your favored sailing ground is short. To make sure of this I designed a quick beam fastening system. Only four nuts have to be fastened to fix the beams. The mast is only 4,3 m long and weight a mere 9 kg. To set up the boat will take 2 persons not more as about one hour. This will be not different on a beach or in a port.

A relative small trailer will be sufficient to tow the boat. A middle class car is big enough to tow the boat. An other important cost saver over the long run.
Experience sows that plywood/glass/Epoxy boats need less maintenance than boats built with any other material. Example our PELICAN: even after 11 years of use the boat needed not even a new coat of paint.
The hulls have an a-symmetrical cross section. The drawing above is a small version of the original 1 : 1 drawing.

A-symmetrical catamarans need no keel or dagger boards to go to windward. The KD 650 is a homage to the early famous designers of asymmetrical catamarans from the Pacific like Rudy Choy and Woody Brown. See figure 6.

The hulls are build up side down on 5 bulkhead ( including transom ). All bulkhead drawings as most of the drawings are to scale ( 1 : 1 ). After the bulkheads are set up the hulls are planked with 6mm plywood and 8mm plywood at the bottom. After the «knuckle» stringers at the out side are glued to the hulls, the planking till the sheer line is added.
The hulls are build on stringers as shown. This construction system give by them self sufficient measurable references to make sure to end up with two equal hulls. I find this very important for best performance. Besides it will give the first time boat builder confidence that he (she) will do a good job.
Now the hulls are turned right side up and the inner cabin sides, sheer line parts are build.
The sloped outer cabin structure is made.
Now all inner structure like bunks, superstructure for the beam Eccenter etc are build.

For easy and fast construction Al. tubes are used as beams. Besides cutting to size and drilling of two holes in each beam nothing to do. If you use mainsheet rails and rails for the trampoline on the aft beam these have to be mounted also.

Mast and rigging is one of the most cost intensive parts. A solution to prevent this high costs is to build also these parts by your self. The drawings show the most cost efficient way to build your own mast ( Al. tube diameter 90/84mm, length 4,30) by your self including mast rotor and mast step. The gaff is made from spruce or clear fir with some carbon fiber. The result will be a very efficient and easy to handle rig.
The hulls are covered with glass cloth and ( I advice ) a two pot polyurethane paint system. This will result in a long lasting boat with almost no maintenance.
As rudder system we use our well known dagger board rudder system. The rudder will kick up when an obstacle is hit. The rudder blades are made from 4mm plywood some carbon fiber and spruce. The are build in a jig to make sure to end with a profile which resembles the NACA 0012 section.
Total construction time will be around 500 hours. This depends of course on your experience and tools on hand.


The boat was designed with a CAD program. All technical drawings are DXF file. The table of offsets is delivered as an Excel file. For list of drawings see following page.
With the list of materials you can calculate the costs for the boats by forehand.
Small parts like for example Aluminum sheets for the Eccenter and the stainless steel shafts for these are not shown in this list but on the relevant drawings.
Plan costs including description how to build US $ 238,00. The plan can be ordered direct on the KD 650 web page (PAYPAL). After I received the money I send the plan as attachment to two emails.


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 5 HP maximum

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