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The 50' Ketch


50' Ketch - SONJA - Kasten Marine Design, Inc.
Preliminary Sail Plan | Traditional Pilot House - Side | Traditional Pilot House - Aft | Sloped Pilot House Concept

Copyright 2006 Michael Kasten

The Concept

Planned for fast and sea-kindly ocean voyaging, the 50' SONJA prototype design has been configured with a skeg hung rudder and a relatively shoal draft fin keel with a ballast bulb. Steel or aluminum would be very suitable construction materials. The single chine shape is easy to achieve with any panel material, so it would be simple to build a mold in order to construct the hull in fiberglass.

At 50' on deck, the boat is large enough be able to eliminate the trunk cabin and have a flush fore deck. This produces an extremely strong deck structure that is far simpler to build. Due to the added freeboard, it also provides a much greater range of positive stability and reserve buoyancy than the usual trunk cabin arrangement.

The Interior

An interior layout has been sketched, but has not as yet been formally drawn. A variety of interiors would fit here, but so-far it is planned to have sleeping for six, a generous galley / saloon, and a private owner's cabin, as follows... Right forward would be a large and private sleeping cabin. Aft of that would be a head to starb'd and shower / bath compartment to port. Aft of that would be a settee to each side with a pilot berths outboard and a large table on center. Aft of this 'saloon' area would be a large galley, half to port, half to starb'd. Aft of the galley, a hanging locker to starb'd and chart desk to port, then right aft would be a pair of quarter berths that extend below the bridge deck.

This provides a simple and open interior, especially so since the cabin top extends the full width from bulwark to bulwark and there is no 'deck' to cut into the wide expanse of headroom.

The Hull Form

As with the majority of our designs, a single chine hull form has been chosen on the basis of its simplicity, economy of construction and for good performance under sail. Particulars are: 50' on Deck x 42' WL x 14.5' Beam x 6.5' Draft, and a design displacement of 45,000 lb.

The keel is configured with a bulb in order to concentrate the ballast as well as to provide an 'end-plate' effect for the fin keel. The skeg hung rudder in combination with this kind of keel will provide excellent windward ability and also accurate tracking at sea.

The Rig

The ketch rig was originally chosen for its ocean cruising benefits, to yield a generous sail area, to provide a variety of well-balanced sail combinations, and to keep the sails to more manageable sizes. The mains'l - the largest sail - is well inboard where it can be handled with the greatest safety. Our Preliminary Sail Plan shows a target sail area of 1,720 sq ft.

It would be possible to arrange a cutter rig as well. In so doing, the boom would be longer and the mast would only be slightly taller. This would encourage a deeper keel, and would provide just a bit better windward ability.

Being a prototype, there are any number of combinations that would be equally possible, not only with the rig but also with the hull form, the interior, the preferred materials and so on... This kind of flexibility is the key benefit of working with a prototype or new design.

Alternate Configuration

We have also worked out a preliminary model of this design that does not have a flush fore deck and instead has a traditional cabin trunk arrangement. The rig would be the same, although as with the Sonja 50 a sloop, and as with the Sonja 50 a cutter rig would be equally possible. Yet another variation is to make use of a full keel for the ultimate in terms of strength and tracking at sea. The 'trunk-cabin' version can be seen in the following links...

Trunk Cabin & Fin Keel | Trunk Cabin & Full Keel

For more information, please inquire.