Catspaw Dinghy

A 'Plank on Frame' version of the Catspaw Dinghy shown below is Joel White’s interpretation of Nat Herreshoff’s classic Columbia Dinghy is a real classic design. I guess you could even say that 'They do not build them like that anymore'.

The rowing version weighed no less than about 150 pounds while the sailing version will weighed no less than 220 pounds considering the addition of the Centerboard case, Centerboard, Mast, boom, Sails, and Rudder.

Plans are still available for this 'Plank on Frame' constructed Catspaw Dinghy.

Katspaw Dinghy

Using 'Approximate Development' to fabricate a True Round Steel or aluminum hull is a relatively a new concept. 'One Of' Builders would rather default back to building a Hard Chine design before even considering building a True Round design' let alone building it by a method such as 'Approximate Development, and is understandable.

To overcome the above I designed the Katspaw Dinghy. This is a classic design that has real value at completion, because it is a classic.

Building this true round aluminum version of the Katspaw Dinghy will hone your skill before taking on a larger 'Approximate Developed' hull design. This would be true no matter your status be your a Professional or Non-professional Builder.

From a design prospective, the only common between the 'Plank of Frame' classic version of the Catspaw Dinghy and the 'Aluminum' version are the approximate overall length and weight, rig design, and design intent. Difference are listed below.

The original design had a Plumb Bow, the aluminum version has a Spoon bow.
Original version had a centerboard, the aluminum version uses a Daggerboard.
The original design seating arrangement had two rowing stations configured in a typical arrangement, while the aluminum version has a seating arrangement that has the feel of a cockpit seating.
While the original version has a bit of tumblehone, the aluminum version has a bit more tumblehone.

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Section Line Drawing

Static Hydrostatics

Floation

Being constructed of aluminum the boat has no possibility of floating in a swamped condition, therefore the aluminum version of the Katspaw Dinghy incorporates flotation, which is neatly placed under the Fore deck, Helm seating, and aft deck.

According to my calculations the minimum floatation required, in cubic feet, to support a swamped Catspaw Dinghy and 400 pounds of crew is 3.878 cubic feet.

The floatation designed into the hull under the Fore deck seat is 0.649 cubic feet, under the Helm is 3.5230 cubic feet, and under the Aft seat 1.234 cubic feet.

Notably there is 1.528 cubic feet of reserve flotation. The image below shows these void spaces.

Center of Gravity Calculations

I am eLines Drawing - Static Hydrostaticsxpecting that the base rowing aluminum version of the Catspaw-Dinghy to come in at 175 pounds while the Sailing version at about 225 pound. A bit more than the 'Plank on Frame' version.

Construction

Framing System

The aluminum version of the Katspaw Dinghy will have no Transverse frames, exactly opposite the 'Plank on Frame' version which has no Longitudinal Frames.

The below rendering shows five (5) longitudinal frames between the sheer longitudinal and Center Vertical Keel - CVK, and the six transverse hull formers.

Initially the longitudinal frames will be supported by six (6) transverse hull Formers which will be removed after the shell plating is tack to the longintudinal frames.

The following drawing show the angle tabs that are thru-bolted to the wood formers positioned to allow the aluminum longintudinal frames to be tack welded in the proper position.

As per 'Bezier Design and Construction' the longitudinals will be marked where it crosses every Former.

Since the longitudinal frames are the only point of reference to align the shell plating they will be held securely held in place by an aluminum angle clip thru-bolted to the hull former and tack welded to the longitudinal frame.

StrongBack

The Strongback shown below in plan. profile, and section views is constructed from 3.500" x 1.500 and 5.500" x1.500" common lumber. Other materials can be used if you like as long as you keep the distance's between Formers as shown.

Remember that the accuracy of the Build all starts here! Everything that follows depends on the accuracy of the Strongback.