Entry 15

Clipper - a two sheet ketch
 Design by Horst Werner  horst.werner@sap.com

Loa 3.92 m (w/o rudder)
LoD 3.20 m
Beam 1.06 m
Total weight empty 40 kg
Loading capacity 160 kg
Draught20 cm
Total sail area 5 m2
Main 3,1 m2
Jib 1 m2
Mizzen 0,9 m2

(Click on the image to see the VRML model)

This little boat is meant for teaching sailing to (small) children. Typically this will take place in very shallow waters and light winds. The boat has enough loading capacity to carry an adult and two kids, but it's small enough to be handled by two kids alone. The beam of only about 1 m indicates a small stability and 5 square meters of sail seem much in relation. But both the small beam and the big sail area help getting the boat moved in calm conditions - and that's its purpose. When the wind freshens, simply dropping the main will reduce the sail area to less than 2 m2 without changing balance.

To keep handling of the sails easy, the mizzen sheet is led to an eye directly above the rudder's hinge, then to a second eye mounted on top of the tiller axle and then to the clamp at the front of the longitudinal tiller. However, in light winds handling three sails might help to keep the children from getting bored.

The boat features a full-length keel which is not very popular today. Of course you can't expect an outstanding (or even average, to be honest) windward performance, but on the other hand there are a number of advantages:

  • The high center of lateral area together with the low center of effort of the ketch rig keeps the lever of the heeling momentum small, which is important since the stability is comparable to a sailing canoe.
  • You don't have to care about retracting daggerboard and/or rudder in shoal water.
  • At very slow speeds a keel delivers more lateral resistance than a daggerboard/centreboard.
  • Furthermore it significantly stiffens and protects the boat's bottom.

In the following pictures, all parts made from plywood are yellow, whereas the parts made from lumber are red. The thwart in the middle is not to be used while sailing. Its main function is to stiffen the hull. It can be used when the boat is rowed (oarlocks are not in the picture but meant to be fitted) or anchored.

(Click on the image to see the VRML model)

The front bulkhead is composed of four pieces of 1" lumber and also serves as a "butt plate". Since one inch is not much width for a butt plate, it may be wise to join the side and bottom parts by overlapping a strip of fibreglass tape on the outside before bending them over the bulkheads.

The masts are put in mast steps on the bottom through holes in the decks. The main mast is secured in place by the forestay, the mizzen mast by the halyard which will be clamped to the rear bulkhead. Shrouds should not be necessary due to the hull's limited stability.


Building Clipper

Construction is stitch and glue, I estimate that about 1 kg of Epoxy and 50 cable tiers are needed. About 16 drywall screws will fix the bottom to the keel (Epoxy fillets between keel and bottom help transmitting the bending momentum). The material is sufficient to make the keel up to 10 cm deeper than specified if desired.

Here's the plywood sheet layout. Note that the aft parts of the sides can be cut together due to the symmetry:

The 1" lumber is cut up like this:

Note that the total weight of the boat can be reduced to about 34 kg if keel, stem and rudder are cut to a thickness of 20 mm, the aft bulkhead to 12 mm and the thwart to 18 mm. This weight is based on exact calculations assuming a density of 0.5 kg/l for all wooden parts and a total weight of 2 kg for epoxy and paint. As for the 2"x4" beams, one is intended to be cut into 8 parts of about 20 x 20 x 2400 mm. These are used as follows:

  • main boom (1600 mm)+ mizzen boom(800 mm)= 1 part
  • main gaff (1450 mm)+ mizzen gaff(680 mm)= 1 part
  • 2 rubrails scarfed from 2 parts = 4 parts
  • longitudinal tiller: 1860 mm = 1 part
  • left: 1 part (might be used to make a pair of small oars/paddles)
From the other beam, a length of 1 m is cut to 30 x 45 mm, forming the bowsprit. The following pictures demonstrate that the sails can be easily cut from the given polytarps. Of course, in reality the layout will be a bit more complicated.


Horst Werner - short biography

I'm a mechanical engineer and have been reading about boat design as a hobby since 1998. Currently I'm building a 4.70 m folding tri of my own design in stitch and glue (See Duckworks Magazine's projects page). I use a self-written design system ("Ligo") for modeling and calculations. (Unfortunately this system is too buggy and complex to be used by anyone else, so please don't ask about it.)

Information about VRML

VRML (Virtual Reality Modeling Language) is a format to describe 3D-Models. You can display VRML models with your Web browser, if you add an appropriate plug-in like Cosmo Player. The cosmo player can be downloaded at
https://cosmo.sgi.com

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